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Jonathan
09-04-2011, 02:39 PM
I've been thinking about how to attach bearings and a timing pulley to a ballnut such that it can be rotated with a stepper motor, instead of rotating the screw.

I've already done this on my router with an M20 screw since the long X-axis means that screw whip and inertia would have limited the speed significantly:

3913

This is, I think, the most promising design I've come up with for rotating a ballnut. The chosen ballnut is the standard RM2510 ballnut from linearmotionbearings eBay seller (among others). The diameter of the ballnut and screw would makes angular contact bearings quite costly. At first I drew it with the ballnut inside the bearings, however the following design allows for smaller 6007 bearings and also gives the option of putting 2 ballnuts in to eliminate backlash. One ballnut attached to either side of the shaft.

Here it is:
(I'll draw it in 3D if necessary)

3914

Black is ballnut/screw/bearings
Red is shaft, steel or aluminium - probably aluminium since lower moment of inertia.
Orange is thingy that fits on the end of the shaft and attaches to the ballnut via the 6 holes in the ballnut.
Green is some aluminum plate to hold every thing. One plate will be extended to accommodate the stepper motor.
Brown is timing pulley. I could machine the pulley on to the shaft, however that means making a new shaft if I get the number of teeth wrong.
Blue is a collar bolted to the aluminium plate to preload the bearings.

Any comments/ideas/criticism welcome!

m_c
09-04-2011, 09:20 PM
Have a look at my Harrison conversion thread for pics of how I done it -http://www.mycncuk.com/forums/showthread.php/277-Harrison-Mill-Conversion
Everything is big, but that's only because it needs a big screw. I should still have the solidworks files somewhere, which I'll have a look for shortly.

Jonathan
09-04-2011, 10:28 PM
Have a look at my Harrison conversion thread for pics of how I done it -http://www.mycncuk.com/forums/showthread.php/277-Harrison-Mill-Conversion
Everything is big, but that's only because it needs a big screw. I should still have the solidworks files somewhere, which I'll have a look for shortly.

Nice design, if you could find the solidworks file that would be much appreciated. I had drawn something similar previously, but with two angular contact bearings. I've now redrawn it based on yours with a single 5207 bearing. It's going to cost about 32 for two of those bearings, so quite a lot more than the previous design. It also means the pulley has to be at least 60T (assuming XL pitch), which is getting a bit large if I need a bigger pulley on the stepper motor. I've drawn two versions with the pulleys mounted using different methods.

3927

I'm not sure if where the force from the tension of the timing belt is ok?

The problem for me with the above design is making the thread / nut. It's not something I've had much luck with before. Unless I spend 20 on a die and can buy the nuts:

http://cgi.ebay.co.uk/35mm-35-x-1-5-Metric-Right-Hand-Die-M35-x-1-50-Pitch-/120698220928?pt=LH_DefaultDomain_0&hash=item1c1a2cb180

m_c
09-04-2011, 11:18 PM
The main reason I ended up with my design, was it was the best way I could find to fit everything into the available space, plus I was aiming for a pretty large ratio, so pulley size wasn't really an issue. Everything was made from hot rolled steel, due to it having to carry the full weight of the knee.
http://www.mycncuk.com/forums/asset.php?fid=3530&uid=158&d=1302383608

That was my 3rd revision of the design. Prior ones used two single row angular contact bearings, but they were too tall to fit in the available space.
Once I get the Conect lathe up and running, and get the Triumph manouvered into place, I'm hoping to get back to the milling machine.

If you want the actual solidworks files, PM me your email, and I'll send them over.

Jonathan
09-04-2011, 11:26 PM
The main reason I ended up with my design, was it was the best way I could find to fit everything into the available space, plus I was aiming for a pretty large ratio, so pulley size wasn't really an issue. Everything was made from hot rolled steel, due to it having to carry the full weight of the knee.

I see, so out of the designs I've posted so far which do you think is the best option? Space is no problem at all for me.

I'm just redrawing the latest one to use a 40mm bore angular contact bearing since that means I can bolt a sort of collar on to the end of the shaft so that I don't need to do any thread cutting.

m_c
10-04-2011, 12:26 AM
Personally, if space wasn't an issue, I'd take your top design, replace one of the normal bearings with a pair of angular contact bearings, and put a lock nut onto the shaft to adjust play out the bearings. That way endfloat is adjustable, the normal bearing provides extra support for the belt tension, and it allows for smaller pulleys.

I got my bearing and locknut from www.simplybearings.co.uk (http://www.simplybearings.co.uk) (for locknuts, click bearings on the front page then scroll down).

As for thread cutting, sharp tool (I bought a HSS insert one from greenwood tools, but I see chronos/glanze have them listed now), slow speed, and remember don't disengage the leadscrew feed once you've made the first cut when doing metric! Reverse the lathe back to the start.

Jonathan
10-04-2011, 12:57 AM
Personally, if space wasn't an issue, I'd take your top design, replace one of the normal bearings with a pair of angular contact bearings, and put a lock nut onto the shaft to adjust play out the bearings. That way endfloat is adjustable, the normal bearing provides extra support for the belt tension, and it allows for smaller pulleys.

By a pair of angular contact bearings are you referring to something like 2 of 7207, or a single 3207? Either way that's getting a bit expensive, plus cost of threading tool though I would use that for other things. I don't want to compromise on the design due to cost, but there are limits. Another thing with that design is since the bearing supports are separate pieces I've got to get the centre heights very accurate, and mount them on a good flat surface for the bearings to be concentric. Bear in mind I'm probably going to be machining this on my mini lathe, which isn't great.

How much end float can I expect in a 5207 bearing if I went for the other design? These are the cheapest I can find:

http://cgi.ebay.co.uk/5208-ZZ-DOUBLE-ROW-BEARING-40mm-x-80mm-x-30-2mm-/360274866150?pt=LH_DefaultDomain_0&hash=item53e20e2fe6

This is the other design modified for no thread cutting:

3929

Thanks for the link to the lock nuts, I'll get them if necessary.

Decisions decisions...

Jonathan
10-04-2011, 01:09 AM
I could use the original design but with tapered roller bearings - 32007 or L68149.

m_c
11-04-2011, 12:30 AM
The bearing I used is a 3208 double row angular contact.
Normal bearings, which includes double row ones, aren't really suited to applications with lateral loads. Deep grove bearings are better, but still not ideal.
However, most common angular contact bearings are the same physical size as normal bearings, so you can always switch to angular contact at a later date.

Jonathan
11-04-2011, 12:41 AM
The bearing I used is a 3208 double row angular contact.

Almost the same as I drew in post #7 then.


Normal bearings, which includes double row ones, aren't really suited to applications with lateral loads.

Yes, however I looked up on the SKF site before using those and is say that the bearings I chose for the initial design are OK for half the radial load rating in the axial direction. That's 5100N, which is far higher than I will get on the router - unless I crash it, and the force is shared over 4 bearings so my thinking is that that might be the way to go as they're so cheap and, as you say, I can either replace with angular or the same if they wear out. I wouldn't be able to fit double row in though.

Is there a reason why you have not mentioned tapered roller bearings? They would fit well.

m_c
11-04-2011, 12:57 AM
Is the 50% figure for static or dynamic loading?

You only really need to use taper bearings when dealing with high loads. For the majority of applications, ball bearings work fine, they have less resistance, and are more tolerant should dirt get into them.

Jonathan
11-04-2011, 01:00 AM
Is the 50% figure for static or dynamic loading?

Yes that's static, forgot to mention that! I can't find a rating for dynamic, just interesting explanations about how the bearing responds to axial load.


... less resistance, and are more tolerant should dirt get into them.

Fair enough, those are both clearly important for this application.

According to the SKF site the axial internal clearence for a 40mm bore double row angular contact bearing is 11um. So that's like having 11um end float on the screw if I used the design with the single bearing. I'm not sure how bad that amount is - it doesn't sound like much. C2 clearance bearings only have 2um clearance.

Jonathan
11-04-2011, 11:51 PM
4th, and hopefully final, design:

3934

That's one double row angular contact bearing to take the thrust load, and one standard bearing. Stepper will bolt on to the 15mm plate. I can use 4" aluminium bar for the bits on the other side, and 2.5" for the rest purely because those are the sizes I've found cheap!

Edit: Disregard that, not thinking... this one still does not preload the bearing properly. I'll add another angular contact bearing, but that's going to cost too much :sad::sad: Now I'm a bit stuck, perhaps a thrust bearing would do.

m_c
12-04-2011, 02:01 AM
If you're using a double row angular contact bearing, then it doesn't need preloaded, as they come preloaded.
Provided the orange and red bits tighten together onto the inner race, and the blue and green bits clamp the outer race, then that design will work fine.

The only purpose the second bearing has, is to provide support to the shaft to stop it being pulled/deflected by the stepper/belt.

Jonathan
12-04-2011, 12:45 PM
If you're using a double row angular contact bearing, then it doesn't need preloaded, as they come preloaded.

Are you sure about that, I was worried about what I posted here:


According to the SKF site the axial internal clearence for a 40mm bore double row angular contact bearing is 11um. So that's like having 11um end float on the screw if I used the design with the single bearing. I'm not sure how bad that amount is - it doesn't sound like much. C2 clearance bearings only have 2um clearance.

Here's the link:

http://www.skf.com/skf/productcatalogue/jsp/viewers/tableViewer.jsp?tableName=1_3_t5.tab&maincatalogue=1&lang=en

http://www.skf.com/images/cat/images/1/1_3/010_0303.jpg

On a related topic, do you think 2 ballnuts like I drew in #13 is sensible for a router? It should enable backlash, and endfloat from the above, to be eliminated I think.

m_c
12-04-2011, 02:51 PM
You'll have far bigger sources of play than 11 microns in a bearing!

Whether you use two ball nuts or nut, will depend entirely on how much play you're willing to allow (or money you're wanting to spend!), and there will also be an increase in friction using two preloaded ballnuts.

Jonathan
12-04-2011, 03:33 PM
You'll have far bigger sources of play than 11 microns in a bearing!

I was thinking it's best to eliminate as much play as I can. In that case it's either the design #13 or #7. The latter, with one bearing, is easier to machine and cheaper, but less stable and bigger pulley. I'm worried about getting the centre height in the design with two bearings accurate.


Whether you use two ball nuts or nut, will depend entirely on how much play you're willing to allow (or money you're wanting to spend!), and there will also be an increase in friction using two preloaded ballnuts.

I've worked out that linearmotionbearings only charges 18 per RM2510 ballnut (and about 31 per meter of the screw), so it's not much more to have a second nut. Friction is a good point since the whole idea of this is to make the machine run faster.

m_c
13-04-2011, 01:52 PM
It's always best to eliminate play, but there are costs to that, as you're discovering.

Most accurate way to machine bearing bores so they're in perfect alignment, is ideally do them in one go, but for that design wouldn't be too easy.
However, as the double row bearing gets clamped in position, you could machine it's bore to allow the bearing to move around a bit, so when you finally assemble it, the bearings get lined up via the shaft, and it's just a case of clamping the doublerow in place.

Andrew Wilding
13-04-2011, 07:05 PM
Hi Jonathan,
If you would like some ideas or a rotating nut you can look at my mill build log. I have just uploaded some pictures showing the assembly of the rotating nut. I used 2 AC bearings and preloaded with a nut. I bought a double AC bearing from EBAY but was not happy with the amount of clearance between the balls and the race so I resorted to 2 seperate AC bearings. The nut is not a lock nut but the pulley below has a grub screw over the key. The nut stack is preloaded then clamped with the grub screw. If this causes problems the nut is thick enough to slit radially and add an M3 clamp screw. Hope this gives food for thought:rolleyes:

Jonathan
13-04-2011, 08:45 PM
Hi Andrew


If you would like some ideas or a rotating nut you can look at my mill build log.

That did get my attention - I was going to post but hadn't finished thinking!


I have just uploaded some pictures showing the assembly of the rotating nut. I used 2 AC bearings and preloaded with a nut. I bought a double AC bearing from EBAY but was not happy with the amount of clearance between the balls and the race so I resorted to 2 seperate AC bearings.

It is interesting what you say about the bearings. I was indeed intending to get them from eBay, two of these to be precise:

http://cgi.ebay.co.uk/5208-ZZ-DOUBLE-ROW-BEARING-40mm-x-80mm-x-30-2mm-/360274866150?pt=LH_DefaultDomain_0&hash=item53e20e2fe6

I'm hoping if I press fit the bearing onto the shaft that should expand the inner ring, and tighten the bearing up nicely. Similarly with the outer ring, except if I follow m_c's excellent suggestion of boring it slightly oversize and clamping it then that's not going to happen. Did you get the single row angular contact bearings on eBay, and what size are they?

The problem I've got is that I don't have the constant force applied to my bearings due to gravity, unlike on the various milling machines that have a rotating nut.

I'm really not at all sure what to do now. I've said already two single row angular contact bearings is a lot of money...and yet it looks like that is certainly the best way to do it. Having said that there's someone on CNC zone who has done it with two standard deep groove bearings.

If I could get away with a 16mm. 10mm pitch, ballscrew then I would save so much on the screw, that I could easily afford the cheaper smaller bearings. However I would prefer to have the 25mm screw just in case it fails completely and I have to spin the screw.

Andrew Wilding
13-04-2011, 10:23 PM
Here is a rambling collection of my thoughts some maybe relevant most probably not!

The bearings I purchased were from Bolton Bearings and had a 30mm bore (too small for you?) and were about 8 quid each. As I am sure you are aware that bearing is not an ac bearing and probably not a deep groove bearing. Deep groove can take some axial load but a normal bearing is quite limited in this respect. The bearing shown will not have any preload but as you say you could acheive this by pressing the bearing onto the shaft (nut). You will need a fairly close tolerence shaft to control the fit and you will need to haver a fair idea of the clearance in the bearing to work out how much of an interference fit you will need. If you are pressing up to a shoulder than you will not be able to remove the bearing. Mine are a push fit (couple of tenths interference) which can be removed without wrecking the bearing. Compressing externally would give you more control and would allow you dismantle the assy.

The real solution is to try to avoid using a rotating nut, particulary when you are talking about 40mm bore bearings and all the associated pulleys etc that go with it. I expect this is a last resort as you have already looked at other design solutions.

I would be surprised if you are getting anwhere near the design axial loads of a suitable AC or deep groove 40mm bore bearing in your arrangement. The load figures may be based on L10 life rather than ultimate limit, or the bearing you were looking at is not designed for axial loads.

The worst thing for bearing life is not having enough load and the balls sliding rather than rolling and so preload is sometimes used for this reason as much as combatting lash.

Jonathan
13-04-2011, 10:43 PM
I've machined plenty of parts with bearings before, so I'm not worried about getting the required accuracy. The lathe I've just bought should help there too :)

30mm is too small I think, on the drawing currently I've bored the shaft 27mm which does not leave much for the bearing.

How do you tell if the bearing is classified as 'deep groove', or is it somewhat arbitrary? The standard bearings I had selected were the ones with a bigger outer diameter (72mm), though not the biggest. I've got 2 good quality, FAG and SKF if I recall correctly, 45x85mm bearings.

I'm not entertaining a belt or rack and pinion drive so the only other option as far as I'm aware is to tension the ballscrew sufficiently to stop whipping. That does not help with the inertia of the screw though, which for 2000mm is similar in magnitude to the other forces.

Andrew Wilding
13-04-2011, 11:51 PM
deep groove is usually in the description of the bearing. On closer inspection of the diagram that you linked the bearing you have shown probably is a deep groove. These are the norm but still not ideal for use in an application where relative to radial load the axial load is significant.

Some more thought (sorry if all this has been covered in other threads)

Moment of inertia is proportional to diameter squared so I am surprised that there is much difference between the shaft and nut? I am sure you have done the sums.

Is the moment of inertia significant when compared to the mass of the 'carriage' (when calculating acceleration)?

If you have to tension the shaft to raise its critical frequency then will have to start considering beefy shaft support bearings, I can now see why the rotating nut idea is appealing!

a 2000mm leadscrew poses a lot of design challenges even when cost is not an issue (which I assume is, if you were as poor as I was when I was a student!) have you considered other methods with feedback or your real accuracy requirement? One solution maybe to use a rack and pinion/belt and calibrate the travel using a dial indicator and blocks of a known dimension. The smaller the increments the better idea you would have of variation along its length. Even a crude survey (100mm steps?) would give you significant improvement in accuracy. Obviously this would require a decent home switch.

I look forward to seeing your solutions. This is not an easy project too keep in budget but will be satisfying when cracked.

Jonathan
14-04-2011, 12:04 AM
Moment of inertia is proportional to diameter squared so I am surprised that there is much difference between the shaft and nut? I am sure you have done the sums

The problem is mass is proportional to the radius squared, so you end up with 4th power. Hollow ballscrew would be nice!


Some more thought (sorry if all this has been covered in other threads)

Keep thinking! It's good to have it all in one place.


Is the moment of inertia significant when compared to the mass of the 'carriage' (when calculating acceleration)?

Yes it is, I'll post the numbers when I've verified them.


If you have to tension the shaft to raise its critical frequency then will have to start considering beefy shaft support bearings, I can now see why the rotating nut idea is appealing!

Just what I was thinking. I suppose tapered roller bearings would do the trick, but then you're adding friction.


For a rotating nut have you considered how much the timing belt tension would deflect the nut on a 2000mm length <25mm shaft and the effect of out of balance forces of the nut on the shaft?

Surely the bearings are going to take the radial load and stop the screw deflecting, or am I misunderstanding you? So as long as the ballnut is held on centre I should be ok. That could be an issue if the flange on the ballnut isn't concentric. Another issue I've found is that at sufficiently high rpm with a standard ballnut centripetal force will stop the balls rolling properly.


a 2000mm leadscrew poses a lot of design challenges even when cost is not an issue (which I assume is,...calibrate the travel using a dial indicator and blocks of a known dimension....survey (100mm steps?) would give you significant improvement in accuracy.

I think that method of measurement would have a cumulative error, which is especially significantly over this distance. It could maybe be done with a digital calliper and carefully fixing to consecutive points along the bed. Theoretically the process could be automated.


Obviously this would require a decent home switch.

Easily done with a cheap laser pointer I reckon.

Andrew Wilding
14-04-2011, 12:15 AM
Jonathan,
You can ignore the stuff about critical frequecy/deflection of the screw with the spinning nut. I had a dim moment and forgot you were supporting the nut with bearings. Its late or at least that is my excuse!:cry:

Agreed with the 4th power bit but that would make the nut inertia even larger than the screw. How does moment of inerta or rotating bits compare with mass of linear bits when you look at a good old newton second law equation?

going tp bed now.

Jonathan
14-04-2011, 01:17 AM
If I was to spin the screw the unsupported length would be 1800mm and overall length about 1900mm, so the calculations below are based on that.

J = m*pitch^2/(2*pi)^2

J = 45*0.01^2/4/pi^2 = 0.114 g-m^2

For the screw:

J = 0.5*m*r^2
Steel --> 7850Kg/m^3
Therefore:
J = 0.5*7.85*pi*r^4*l

So for mine:
J = 0.5*7.85*pi*0.0125^4*1.9 = 0.572 g-m^2

That makes the moment of inertia of the screw 5 times that of the gantry.

I will estimate J for the ballnut assembly.

Jonathan
14-04-2011, 11:13 PM
Not much to add to the actual design here, just a 3D drawing which clarifies the bearing mounts:

3950

Solid Edge got a bit carried away with the reflections here:
3949

Jonathan
31-05-2011, 07:03 PM
You can ignore the stuff about critical frequecy/deflection of the screw with the spinning nut. I had a dim moment and forgot you were supporting the nut with bearings. Its late or at least that is my excuse!:cry:

Interestingly I've since found that when you rotate the ballnut the critical frequency still has an applies - the end fixity is just better which, in my case, gives 1250rpm. So 12500mm/min. I think I'll be lucky to get that much out of my motors anyway so I'm not going to worry.

I just got prices for the screws:

2 of RM1610-2000mm with nut and end machined(both ends machined same as A type )
238 usd
1 of RM1610-900mm with nut and end machined(standard end machining)
72 usd
Sub-total
310 usd
Air express shipping
127 usd
Total
437 usd
-----------------------
Offer tow:

2 of RM2510-2000mm with nut and end machined(both ends machined same as A type )
269 usd
1 of RM1610-900mm with nut and end machined(standard end machinings)
72 usd
Sub-total
341 usd
Air express shipping
168 usd
Total
509usd

I'll go for the second option - the price difference is not as much as I expected. The smaller screw is for the Y-axis. I've decided to get both ends of the big screws machined to fit the BK type support ... that should enable me to tension the screw using the nut on either end.

I have also acquired a pair of 7206 FAG bearings cheaply, so I'm hoping I can use them instead of 7207. The obvious problem is that the bearing bore is only 30mm, and with the screw passing through it being 25mm the shaft will be very thin - about 1.5mm.

Jonathan
17-06-2011, 03:22 PM
The ballscrews arrived today - I paid for them on 08/06 and I requested different end machining (longer portion for pulley and same machining on both ends of screw) so I think that's good service. He also charged me $447 in total even though I ordered the 25mm screws, so $72 less than originally quoted. The ballnuts backdrive easily and they seem nice and smooth.

I will start working on the rotating ballnut mount soon. I'm currently doing one last 3D drawing. I will use 5/8" aluminium for the plates that hold the bearings with some aluminium 'posts' to hold the plates together parallel at a fixed distance.

I made a mistake with the bearings and bought open not shielded so I'm going to have to put some sort of enclosure round them. I got them on eBay, one SKF and three SBC 7207.

Robin Hewitt
18-06-2011, 08:25 AM
I made a mistake with the bearings and bought open not shielded so I'm going to have to put some sort of enclosure round them. I got them on eBay, one SKF and three SBC 7207.

Don't bust a gut searching for single row angular shielded, I don't think it exists :naughty:

Jonathan
19-06-2011, 03:55 PM
Don't bust a gut searching for single row angular shielded, I don't think it exists :naughty:

That explains a lot!

I've just estimated the moment of inertia of the rotating bits of the assembly, so pulleys shaft, ballnut and bearings. Surprisingly it comes to slightly more than the inertia of the 2085mm, 25mm diameter screw! That's annoying since one of my main reasons for doing this was because I thought this value would be much lower. I've not included the inertia of the bearings I would require to rotate the screw, and I suppose I would use pulleys with it anyway. The inertia of the ballnut and shaft is still about two thirds of that of the screw.
This is going to make a big difference to the rapid speeds I can get since the torque required to move the 50kg gantry, due to the low coefficient of friction of the bearings, is very small.

Now I'm going to try modelling it with the stepper at different ratios - not 1:1, and see what happens.

Jonathan
30-06-2011, 03:48 PM
I have at last started making the rotating ballnut assembly. I've started with one stepper motor mount, it's 100x245mm:

4207

4206

The reason for having such a long cutout for the motor is to allow me to use any reasonable size pulleys without changing the belt, which will obviously be difficult. I CNC milled all except the bearing bore which was bored on the lathe. I had to remove the gap bed on the lathe to do it - the join was painted over so it looks like this might be the first time it was removed! The six holes closest to the bearing will be used to push a ring against the outer ring of the bearing to preload it. The 4 bigger holes are for posts which link this to the other bearing mount.

I'll make some more bits when I've found my caliper :whistling:

Jonathan
02-07-2011, 11:37 PM
I've now made all 4 bearing/stepper mounts and one shaft. I will machine the pulley (30 tooth) directly on to the shaft.

When I machined the shaft on the lathe I got only small pieces of swarf, not the long stringy stuff you normally get - see picture below of drilling it for what I mean. It was almost like cast aluminium...but it's not. Must be a strange grade?

Lots of photos, hopefully pretty self explanatory:

4213

4220

4212

4208

4216

4219

4215

4209

4218

It got 0.01mm bigger after drilling...so now 34.99mm which fits the bearing nicely:
4214

4217

4211

4210
(Hopefully nobody without broadband is trying to view this!)

wiatroda
03-07-2011, 03:50 PM
Very nicely done

Jonathan
03-07-2011, 04:19 PM
Very nicely done

Thanks :). I hope it works well after all this... I modified irving's spreadsheet to include pulleys and used it to model this setup. Looks like, with the 3Nm motors I'll get at best 7500mm/min rapid and 2000mm/min with 50N cutting force. Both of those are a lot lower than the prediction for the Y-axis, 18100mm/min and 8200mm/min. All of those figures are with a 3x safety factor on the torque, so fingers crossed it might be better!

I'm currently making a mandrel from 35mm steel bar to hold the shaft in the rotary table/4th-axis to mill the pulley teeth on to it.

wiatroda
04-07-2011, 08:24 AM
Thanks :) .... mill the pulley teeth on to it.
What belt profile? How wide pulley going to be ? I had to buy 3M HTD pulley recently , but I would give a try to cut my own .

Jonathan
04-07-2011, 10:40 AM
I'm using XL belts, purely because I already have lots of XL timing pulleys which will enable me to experiment with the ratios. HTD would be better in theory.

I will use a form tool to cut the pulleys. The profile is a simple trapezium which I have ground on to a piece of HSS. You can make HTD pulleys accurately with a ballnose cutter ... or some people use a drill but I think that's a bit crude.

If you do decide to try it you might find my program here useful:

http://www.mycncuk.com/forums/showthread.php/3051-Some-Gcode-generating-programs

Jonathan
05-07-2011, 12:12 AM
Today I cut the pulley teeth into the first shaft, and whilst that was running made the second shaft.

Pictures:

4226

The belt fits (yay!):

4227

I will cut the pulley teeth into the second shaft tomorrow, then I just need to make the part that connects the shaft to the ballnut, and the ring that pre-loads the bearings...and maybe some bits to act as bearing seals.

Jonathan
06-07-2011, 01:27 AM
I have now cut the second pulley, and made the remaining parts for one of the assemblies except the bearing pressure plate. I immediately stuck it in the milling machine vice, put a stepper on and tested it:

4228

Will post a video tomorrow. With me just holding the 2092mm ballscrew in one hand it did 14m/min. Almost certainly more - I didn't try.

Might be while until I finish off the other one since the tip on my parting tool became an Unexpected Flying Object. It might still be ok, but my lathe is buried in so much swarf I can't find it. It took me 5 mins to find the 60mm diameter part I had parted off, let alone a tiny tip...maybe I'll tidy it tomorrow.

AdCNC
06-07-2011, 05:53 PM
Looking good fella!

Jonathan
06-07-2011, 05:59 PM
Thanks for the kind words...

I just mounted one ballscrew to the frame with a couple of G-clamps and some wood, so not the best but it holds. So with one ballscrew and 3nm motor I'm getting at least 12,000mm/min! It might do more but things are wobbling a bit as I have not fixed the X-axis linear rails down to the frame.

With the previous rotating nut setup (see post #1) using M20 threaded rod the best it would do reliably was about 2500mm/min.

The gantry hasn't got the spindle and Y axis screw etc mounted, so it's only about 40kg not 50kg, but still looks very promising. It's odd since for this motor and pulley (28T on motor, 30T on screw) I calculated 7300mm/min.

Will post a photo in a few mins.

Jonathan
06-07-2011, 06:38 PM
Video here:

http://www.youtube.com/watch?v=mSNFD9zx7fE&feature=youtube_gdata

Whipping caused it to stall at 15m/min. That be less of a problem when everything is mounted properly.

Some pictures:

4229

4230

At 8m/min I couldn't stall it by pushing on the gantry. Above that I
can, but only by bracing myself against the wall and pushing with both
hands...otherwise it just pushed me along the floor.

After the video I tried adjusting the acceleration. It went up to 3m/s^2 quite happily.

Swarfing
07-07-2011, 12:53 AM
Jonathan interested to know what cutter you are using to cut the teeth on your pulleys?

Jonathan
07-07-2011, 12:57 AM
Jonathan interested to know what cutter you are using to cut the teeth on your pulleys?

It's just a trapezium shaped form tool. I ground it by hand on to a piece of 5/16" HSS. I mentioned it in post #37. Can post a picture if you want, but there's really nothing to it...

Swarfing
07-07-2011, 01:41 AM
yes please

AdCNC
09-07-2011, 12:48 AM
any more news on the ballnut jon?

Jonathan
09-07-2011, 08:57 PM
any more news on the ballnut jon?

Not a lot. I made one out of 4 aluminium mounts for the ballscrew from 90mm aluminium bar. Now the blade on my metal bandsaw has broken (don't really know why :sad:) ... I thought all was lost until I tried parting the bar off on the lathe. Quite unnerving having a 500mm long length of 90mm diameter bar spinning round on the lathe, but it went well and actually parted off very easily with my 2mm parting tool. I sawed through the last 10mm or so by hand for obvious reasons. Coolant made a bit of a mess ... it's actually a faster and more economical way of doing it than the bandsaw.

I used dial indicator to get the ballnut as close to on centre as I could. This completely eliminated the whip I had before. Now using 42T pulley on motor (shaft still 30T) instead of 28T I got 18m/min. There was no significant vibration or whipping at this speed, which is 50% higher than the critical speed of the screw in a fixed-fixed configuration (1180rpm)...which would require a pair of angular contact bearings at both ends (or similar). I would have tried more but I was limited by the kernel speed in mach3. That's with the motor on 1/4 step ... I'll try it on half and see what happens.

In the end I will probably stick to just (!) 12m/min since that gets me better resolution and is plenty fast enough. It probably doesn't do the ballnut any good spinning round too fast? Centripetal force may cause the oil to be flung outwards?

I'd love to try this with a 25mm lead ballscrew and really see how fast it would go...or something like this:

http://www.zappautomation.co.uk/r2020-ballscrew-p-444.html?osCsid=5266365704808afeef347140d1aaa921

Would probably be measuring the feedrate in meters per second with that... would need an encoder to get useful resolution though.

Jonathan
09-07-2011, 09:18 PM
I've just found this quote from John S. a long time ago:


One area where spinning ball nuts come into their own is on a CNC
lathe. ... a spin off is if you also fit a bevel gear to the nut block you can then drive this with another bevel mounted on a disengageable handwheel. This way you can
get to use a CNC lathe in manual mode without having to rely on
standing at one end turning the ball screw.

(Source: http://groups.yahoo.com/group/CAD_CAM_EDM_DRO/message/70038)

Backdriving could still be an issue, but you could add some friction to the handle and/or a lock easily enough to combat that. That's tempted me to convert my lathe ... I could probably still use my design for the router, perhaps with better (tapered roller maybe) bearings as the forces are likely to be greater. The tricky bit will be fitting the ballnut into the cross slide.

Jonathan
13-07-2011, 01:34 PM
I have now mounted both X-axis ballscrews, like this:

4231

Interestingly with the second motor the feedrate decreased from 12m/min to 10.5m/min. I think this might be due to me not yet being able to get the second ballnut on centre which is causing the ballscrew to vibrate a bit.

Jonathan
23-11-2011, 02:01 PM
Update:

Can't really fault the rotating ballnut mounts. They're rigid and get a plenty high enough feedrate from quite small motors. The oil does keep flying out of the ballnuts, as predicted, so I occasionally put more in and they seem fine.
I should have incorporated dust covers as swarf does land in them. I did cut some aluminium covers on the router to protect the angular contact bearings:

4957
4958

More recently I have made two more mounts, this time for 20mm or 16mm screws and with polycarbonate covers, HTD pulleys, and just generally neater as I made them on the router which gets a better finish than the milling machine:

4961
4955
4956
4959
4960
4954

wiatroda
23-11-2011, 06:18 PM
looks really good
What's the gear ratio between motor and pulley/ballnut??
Is 10mm screw pitch?

Jonathan
24-11-2011, 02:07 PM
It is 1:1 and 10mm pitch, but clearly one could use it with any pitch and to change the ratio just change the pulley on the motor. The mount for the motor is long enough to accommodate different pulleys without changing the belts. It will go faster with a bigger pulley on the motor, but with a 10mm pitch screw there's not much point ... you're just loosing resolution.

andrewbond
29-02-2012, 03:56 PM
So I know I may be over simplifying and havnt researched this much but...

Would it be possible, and much simpler to use a hollow shaft stepper/servo and use feed the screw through the motor and mount the motor directly to the ballnut with a flanged hub

Andrew

JAZZCNC
01-03-2012, 12:14 PM
So I know I may be over simplifying and havnt researched this much but...

Would it be possible, and much simpler to use a hollow shaft stepper/servo and use feed the screw through the motor and mount the motor directly to the ballnut with a flanged hub

Andrew

Ye that would work great.!! . . . . Now You try finding an affordable motor with a 25mm hollow shaft.!!!!!!

Jonathan
01-03-2012, 01:46 PM
Would it be possible, and much simpler to use a hollow shaft stepper/servo and use feed the screw through the motor and mount the motor directly to the ballnut with a flanged hub

Yes that method is used on some expensive commercial laser cutters. They use large servomotors with a hollow shaft containing the ball-nut running on a tensioned ballscrew. They get feedrates measured in meters per second.

There are few stepper or servos available with hollow shafts. The common ones have too little torque or the bore is insufficient. Anything above 16mm would be useful. It may be possible to dismantle an existing motor, machine a new shaft with the hole through the middle and use bigger, angular contact, bearings. For the size of my machine (1.7*0.74*0.4m) it's just not worth it as the 3nm steppers can already run it more than fast enough.

The advantage with using a standard motor, other than simplicity, is the timing belt allows you to change the drive ratio and reduces resonance. Currently mine is on 1:1 ratio, however it does go faster with a larger pulley on the motor. I elected to leave it at 1:1 however as 10m/min is plenty and I prefer to keep the resolution than get 15m/min+, but at least I have the option which wouldn't be the case with a hollow motor. In short I'm saying you have to be very careful with the selection of the motor, which makes it even more difficult to find a suitable one.

Gary
01-03-2012, 08:29 PM
This is something we are developing, but it will not be for hobby use.
It will start with a servo system and the 2525 and 3232 ballnut,
We will then be moving to the 4040 and 5050 ballnut.

The only steppers that could have been used for this design were the IOS motor from IMS, but these are not made anymore.



Yes that method is used on some expensive commercial laser cutters. They use large servomotors with a hollow shaft containing the ball-nut running on a tensioned ballscrew. They get feedrates measured in meters per second.

There are few stepper or servos available with hollow shafts. The common ones have too little torque or the bore is insufficient. Anything above 16mm would be useful. It may be possible to dismantle an existing motor, machine a new shaft with the hole through the middle and use bigger, angular contact, bearings. For the size of my machine (1.7*0.74*0.4m) it's just not worth it as the 3nm steppers can already run it more than fast enough.

The advantage with using a standard motor, other than simplicity, is the timing belt allows you to change the drive ratio and reduces resonance. Currently mine is on 1:1 ratio, however it does go faster with a larger pulley on the motor. I elected to leave it at 1:1 however as 10m/min is plenty and I prefer to keep the resolution than get 15m/min+, but at least I have the option which wouldn't be the case with a hollow motor. In short I'm saying you have to be very careful with the selection of the motor, which makes it even more difficult to find a suitable one.

georgetheforge
29-03-2012, 05:50 PM
Hi Jonathan,

really nice design on these- they still working a treat?

i came across these badgers http://www.drives.co.uk/fullstory.asp?id=3388- but as i say on my little build log thing- bet they're bleedy expensive-!

i'm tossing up rack and pinion vs this type affair for my long axis- (2.5m) so lurking about tying to decide to my way forward!

George

Jonathan
30-03-2012, 01:17 AM
really nice design on these- they still working a treat?

Thanks... yes they're still working very well indeed. Same performance as in my post a month ago. I wouldn't hesitate to use them instead of rack and pinion on a 2.5m axis. You'll get better accuracy, longevity and it'll work out cheaper. With a RM2510 (or even a higher lead) the machine will be plenty fast enough with good acceleration.


i came across these badgers http://www.drives.co.uk/fullstory.asp?id=3388- but as i say on my little build log thing- bet they're bleedy expensive-!

They do look good, but as you say they wont be cheap.

georgetheforge
30-03-2012, 09:08 AM
that would indeed be interesting Jonathan- do you want to work it out and PM me- or post it's up to you-!

i'll work out the mounting of them- i was thinking of tucking them out the way under the x rails- so to minimise dust/ crap getting onto them-

thanks again

G

D-man
25-07-2012, 10:13 PM
Hi Jonathan, any chance you could knock 2 of these units up for me?

Im running the 2020 ballscrew and need someway of using a screw of 3000mm length without whipping and this looks ideal

Jonathan
26-07-2012, 11:42 PM
Hi Jonathan, any chance you could knock 2 of these units up for me?

Im running the 2020 ballscrew and need someway of using a screw of 3000mm length without whipping and this looks ideal

Yes I can, depending on how soon you need them.

A rotating ballnut would be ideal with the a 2020, 3000mm long screw. Since (I gather) this is a woodworking machine, you would probably want to gear it 2:1 so the gantry moves 10mm for every motor revolution.

Here's one I made recently, version 3 I suppose. The significant change is using sealed angular contact bearings (turns out they do exist) and threading the shaft to enable a pair of locknuts to pre-load the bearings.

6831682868306829

Will post a video when I have time.

D-man
26-07-2012, 11:51 PM
I would like it to cut Ali is there a reason you think it's just a woodworking machine? What have I done wrong lol?

If you wanted to test the R2020 screw and nut I don't have a problem sending some over as long as you send them back when your done :-)

JAZZCNC
27-07-2012, 03:12 PM
If you wanted to test the R2020 screw and nut I don't have a problem sending some over as long as you send them back when your done :-)

Sure he will but in 5yrs time then they'll be bent and worn out. . lol

boldford
27-07-2012, 11:04 PM
. . . . .All of those figures are with a 3x safety factor on the torque, . . . . . .Was the 3x figure a guestimate or based on based on "real world" experience? Your reply may help me determine if some unlabled steppers I have will work reliably in my first router.

Jonathan
29-07-2012, 12:17 PM
I would like it to cut Ali is there a reason you think it's just a woodworking machine? What have I done wrong lol?

Er, not sure really... maybe the MDF bed. That wont stop it working, but my router bed is currently MDF and it is rather limiting.


If you wanted to test the R2020 screw and nut I don't have a problem sending some over as long as you send them back when your done :-)

Thanks for the offer, but it wouldn't really be practical/worth it .. I'll just wait and see how it goes on your machine.


Was the 3x figure a guestimate or based on based on "real world" experience? Your reply may help me determine if some unlabled steppers I have will work reliably in my first router.

I used 3x because that's what's in irving's motor calculation spreadsheet (http://www.mycncuk.com/forums/showthread.php/1524-What-size-stepper-motor-do-I-need). In reality I got a lot better than that - if you include the inertia of the pulleys in the calculation it's quite a lot more accurate.

D-man
29-07-2012, 12:50 PM
The mdf bed is only 5mm lol it's only there to rest all my junk on lol I'm hoping to get some small extrusion to give it a nice solid bed. However this is in the future wood and plastic is its need for now.

boldford
29-07-2012, 05:27 PM
I've just found this quote from John S. a long time ago:



(Source: http://groups.yahoo.com/group/CAD_CAM_EDM_DRO/message/70038)

Backdriving could still be an issue, but you could add some friction to the handle and/or a lock easily enough to combat that. That's tempted me to convert my lathe ... I could probably still use my design for the router, perhaps with better (tapered roller maybe) bearings as the forces are likely to be greater. The tricky bit will be fitting the ballnut into the cross slide.

Perhaps interestingly, the George Taylor milling machines used a similar arrangement. The lead-screw being turned by the hand-wheel and the nut being turned by the self-act. http://www.lathes.co.uk/taylormiller/

boldford
29-07-2012, 05:29 PM
I used 3x because that's what's in irving's motor calculation spreadsheet (http://www.mycncuk.com/forums/showthread.php/1524-What-size-stepper-motor-do-I-need). In reality I got a lot better than that - if you include the inertia of the pulleys in the calculation it's quite a lot more accurate.Many thanks for the pointer. That's really useful information which may save me some grief.

jcb121
19-09-2013, 04:36 PM
I'm just wondering, that timing pulley you used? was it around 26 teeth 5mm pitch?

Jonathan
19-09-2013, 04:41 PM
I'm just wondering, that timing pulley you used? was it around 26 teeth 5mm pitch?

For the one in post #61 I used a 30T HTD 5m pulley. Clearly it depends on the ratio you require, but if you can keep the pulleys small that helps a lot with reducing the moment of inertia of the rotating assembly.

jcb121
19-09-2013, 05:12 PM
yeh, i was thinking of using a 24 tooth one as that would give me a lot of factors to change the gearing.

but it has the same amount of factors as 30 so either will be good in my design :)

Jonathan
19-09-2013, 05:27 PM
But factors aren't the only factor...
Ideally you don't want to be using less than a 15T pulley as that bends the belt a lot and gets lower tooth engagement, so if you need a 1:2 ratio then you've got to go with at least 30T. However if you're happy with ratios from 15:24 and you can fit the ballscrew shaft through the 24T, then use it as it's inertia will be about 60% less than the 30T pulley. That will help get high acceleration, although you need to consider the system as a whole not just the pulley.

Boyan Silyavski
22-11-2013, 11:34 PM
Hi Jonathan,
have you seen these (http://www.ibc-waelzlager.eu/dmdocuments/en2009/IBC_Ball_Screw_Support_Bearings.pdf) bearings, which are meant for the purpose?

Jonathan
15-12-2013, 01:35 PM
Hi Jonathan,
have you seen these bearings, which are meant for the purpose?


No, I hadn't spotted that. I see they've used bearings with a higher contact angle than most angular contact bearings, as that allows the bearing spacing to be lower for the same stiffness. I don't think this is too big a concern with the rotating ballnut bearings, as they are oversized compared to the bearings one would use if spinning the screw.

I've just finished making three more rotating nut mounts for someone who spotted the video on YouTube who's making a large machine with two SFE2525 ballscrews and one SFU2010 ballscrew. I simplified the design by changing the housing for a block machined from 4" square aluminium bar, and a separate piece cut on my router for the motor mount.


Some pictures:

10994109951099610997109981099911000110011100211003


A video, feedrate displayed assumes connection to 25mm pitch ballscrew:


http://www.youtube.com/watch?v=kmIqQqngLaI&amp;feature=youtu.be

Drawings:
1100411005
11006


Feel free to copy or modify the drawings and use them to make your own or get the parts machined by your favorite company... I don't mind.

Clive S
15-12-2013, 02:01 PM
Very nice indeed Jonathan, I can see a lot of work has gone into those. ..Clive

paulus.v
15-12-2013, 02:41 PM
Hi Johnathan,

nice work! I have a question: How did you fasten the pulleys on the rotating ballnut shaft and motors?

Jonathan
15-12-2013, 03:17 PM
Very nice indeed Jonathan, I can see a lot of work has gone into those. ..Clive

Thanks, yes they take forever, or at least it feels like forever when you're only able to machine things at weekends. It's been three months since I agreed to make them.


How did you fasten the pulleys on the rotating ballnut shaft and motors?

M5 and M4/M5 grubscrews respectively. So long as you threadlock them it works. The reason I didn't use a clamping method is that tends to require a larger diameter pulley, particularly on the ballnut shaft, which would increase the moment of inertia.

Boyan Silyavski
15-12-2013, 04:55 PM
Basically from what i understand 24 tooth pulley / OD 37.08 at the lower point at the tooth/ can not be fit, had to be machined on the shaft directly? So the next in line is the 26 tooth pulley which is OD 40.24 at the lower tooth point, so that will give 40-35=5/2=2.5mm wall, which seems quite thin. Then 28 tooth pulley is 43.42-35=8.42/2=4.21.

So it seems if not machined directly, the minimum pulley will be 28 tooth? Or am i missing something?

Jonathan
15-12-2013, 05:03 PM
The minimum pulley size that I could fit on the shaft was 30T, so that's what I used. By machining the pulley teeth directly into the shaft, as with my first version, a smaller pulley could indeed be used, but it adds a lot of time to the machining.

The shafts are 35mm diameter, so it's a bit thin even with 30T as you can see from the previous one I made, here:

11008

Boyan Silyavski
15-12-2013, 05:26 PM
Thanks.

Now wouldn't this exact RM2510 (http://www.ebay.com/itm/NEW-RM2510-RM-2510-BALL-SCREW-NUT-/281220584543)ball nut simplify things and lead to another design, even possibly with 2 of them?

Lee Roberts
15-12-2013, 07:04 PM
Feel free to copy or modify the drawings and use them to make your own or get the parts machined by your favorite company... I don't mind.

Thanks for the share JB, ok to put this in the Open Source section?

.Me

Jonathan
15-12-2013, 07:15 PM
Thanks for the share JB, ok to put this in the Open Source section?

.Me

Yes if you want I will. I considered posting it there but decided not to as, if I recall correctly, members can't reply to posts in the open source section?


Now wouldn't this exact RM2510 (http://www.ebay.com/itm/NEW-RM2510-RM-2510-BALL-SCREW-NUT-/281220584543)ball nut simplify things and lead to another design, even possibly with 2 of them?

I considered using threaded nuts, but decided not to risk it in this case as I don't know how accurate the thread is. If the thread is slightly wonky/'drunk' or not concentric then that's clearly going to cause a problem that's hard to fix...

Boyan Silyavski
15-12-2013, 08:43 PM
Hi Jonathan,
While you are contemplating if you would ever find the time to make me two of these beauties, i am trying to make a detailed drawing in Sketchup which i could share here with your permission.


Would you throw some light on few details please:

-distance from motor shaft center to pulley center? nema 23 yes?
-exact belt length for 30:30 ?
-did you make the 2 holding nuts or buy them?
-thickness of inner and outer race ring of the bearing? It is the 7207 2rs bearing from here (http://www.ebay.es/itm/111131790701?var=410197121294&ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649) yes?

Jonathan
15-12-2013, 08:50 PM
Nema 23, 5M HTD belt 340mm long which enables the motor pulley to be anything from 15T to 30T without changing the belt length. Center distance 90-115mm approx.

I bought them - KM5 locknuts:
PACK OF 5 KM Series KM5 Locknut 25mm x 38 mm x 7mm FREE P&P | eBay (http://www.ebay.co.uk/itm/PACK-OF-5-KM-Series-KM5-Locknut-25mm-x-38-mm-x-7mm-FREE-P-P-/330955422355?pt=LH_DefaultDomain_3&hash=item4d0e7afa93)

That's the correct bearing. You should be able to get the rest from the SKF website. Interestingly the seller specifies that the bearings are 15 or 25 contact angle, but the ones I got have the suffix 'B' which (going by SKF's nomenclature) means they're 40. Excellent if true...

Boyan Silyavski
15-12-2013, 11:11 PM
Jonathan, THANKS!

Great work and info!

As this rotating ball nut idea consumed my weekend and the whole last week, and i am sure much much more of Jonathan's time, here is the Sketchup file for all to use, if this is alright with Jonathan:



rotating ball nut Jonathan design MYCNCUK.rar (http://silyavski.com/storage/cnc/rotating%20ball%20nut%20Jonathan%20design%20MYCNCU K.rar)


Notes:
1. Every part is labeled , so can easily be found in internet
2. The bearings are recessed 0.5mm in the casing
3. The aluminum is 20mm thick and the bar can be 100x100, 100x90 or as originally 5"
4. Everything is tight fit / I still don't get it how the bearing casings are centered from both sides but hey, i am not a machinist yet :-)/
6. The clearing between the 2510/25.. nut and casing is 1mm
7. The clearing between rotating shaft and the ball screw is 1mm
8. The 2 retaining nuts actually are PACK OF 5 KM Series KM7 Locknut 35mm x 52 mm x 8mm FREE P&P | eBay (http://www.ebay.co.uk/itm/PACK-OF-5-KM-Series-KM7-Locknut-35mm-x-52-mm-x-8mm-FREE-P-P-/221253125044?pt=LH_DefaultDomain_3&hash=item3383b6c3b4) as the thread will be 35mm
9. There could be small corrections of the lips that hold the outer and inner bearing racing rings, will update if necessary.
10. The motor is from Zapautomation 3Nm SY60STH86-3008B NEMA 23 STEPPER MOTOR, which i assume most will use for this job 8mm shaft


1101111012 11013

Jonathan
15-12-2013, 11:25 PM
2) Optional
3) Note that on the drawing I posted the motor mount is 88mm wide, not 90mm which is the height of the main block. I intentionally left a 1mm gap so that when the block is bolted to a flat surface, you wont end up with the assembly partially resting on the motor mount (if the motor mount isn't attached accurately). The gap is on both sides since I put mounting holes on both sides of one block so they can be mirrored.
4) Very carefully. You use a 4-jaw chuck and a dial indicator.
7) Same, that's fine.
10) Or CNC4you (http://www.cnc4you.co.uk/index.php?route=product/product&product_id=65) if you want a cheaper option.

Minor point - on your drawing the 6 tapped holes that fasten the ballnut to the mount go all the way through the flange on the shaft. I made them blind holes so if you accidently try to use too long a bolt, the bolt doesn't go all the way through and damage the bearing.

EddyCurrent
15-12-2013, 11:54 PM
I know you had the housing made from several pieces originally but they looked a bit complex so is it feasible to make the rectangular housing from 4 or 5 just rectangular pieces bolted together with the motor mounting plate being one side ?

Jonathan
16-12-2013, 12:19 AM
Is it feasible to make the rectangular housing from 4 or 5 pieces bolted together with the motor mounting plate being one side ?

That's pretty much how I did it earlier, see posts #50 (http://www.mycncuk.com/forums/linear-rotary-motion/3340-rotating-ballnut-design-ideas-2.html#post25863) and #61 (http://www.mycncuk.com/forums/linear-rotary-motion/3340-rotating-ballnut-design-ideas-2.html#post33232). One thing to bear in mind is it's harder to maintain the accuracy of the bearing mounts if they're separate parts.

EddyCurrent
16-12-2013, 12:25 AM
Damn, the very pages I must has skipped :stupid:
Yes I realise about alignment, it just seemed easier for someone without a decent machine shop.

Boyan Silyavski
16-12-2013, 12:42 AM
3) Note that on the drawing I posted the motor mount is 88mm wide, not 90mm which is the height of the main block. I intentionally left a 1mm gap so that when the block is bolted to a flat surface, you wont end up with the assembly partially resting on the motor mount (if the motor mount isn't attached accurately). The gap is on both sides since I put mounting holes on both sides of one block so they can be mirrored.

I see, will correct it.

There are 2 minor changes from the original design:

- i made the motor mount from the wider side, as many will like maybe 34 nema and it looks more sturdy. I wanted to make a combined nema 23 and 34 mount, but i feel sleepy now. when ready will see if it will stay like this or follow your original design.

-i believe i shortened the rotating shaft by making its wide part go through the motor mount plate. It is now 2mm to place the ball screw nut in its place,10mm for the ball screw nut holding thread, 2mm ring to push only the small bearing ring and 90mm the rest.

I think that all can be further shortened and lightened :
-2mm protruding thread instead of 5mm , when the nuts are tightened
-6mm /shortening the whole assembly/ eating from the clearance near the shaft pulley
-The area where the ball screw nut fastens could be recessed 8mm with diameter 40mm and some more holes could be done / 45 degree following the pattern/

This could lower considerably rotating mass

11014

Boyan Silyavski
16-12-2013, 01:04 AM
Damn, the very pages I must has skipped :stupid:
Yes I realise about alignment, it just seemed easier for someone without a decent machine shop.

Actually if Jonathan or sb from the forum doesn't have the time to make the parts for me, i have to go to my local guy, who is way off with the 0.01mm, he works in the 0.5 zone definition :hysterical:
So i would update the drawings with extra drawing , made from OD100 and OD80 round bar. That way it would be more easy for him not to make a mistake an find the center. Further benefit will be that it could be mounted in cut in 2 pieces 85mm or 100mm spindle mount/not perfect but will work/ . anyway, i will mount it directly on its face, not using the bottom holes.

paulus.v
16-12-2013, 05:14 PM
One grubscrew is enough for clamping the pulley to the motor shaft or should I use two at 90 degrees? Are there other simple methods that I could use to be able to change the pulley (ratio) more easily?


Here are my machined parts, still not assembled and tested:


http://i285.photobucket.com/albums/ll63/bau_x3/CNC/2013-12-16-1358m_zps771483c3.jpg

http://i285.photobucket.com/albums/ll63/bau_x3/CNC/2013-12-16-1359m_zps9e5c7455.jpg

GEOFFREY
16-12-2013, 07:22 PM
Machining looks very nice. G.

Boyan Silyavski
17-12-2013, 06:15 PM
For the sake of experiment, found some providers of Servo motors with hollow shaft where a ball nut can be mounted. Usually the 3nm motors accept only16xx sizes, for 25xx sizes a 9Nm servos are offered.

I wanted to make sure Hybrid Closed Loop Stepper driver + motor+shipping + this assembly will not equal a proper servomotor price.

No prices on the sites/ means "in the thousands" to me/ . Well , one called me , wow, i got even more suspicious about the price. Now received the quote.

Wow, again. 1600eur the motor and the driver and cables. driver costs 420, so rest is the motor.

Boyan Silyavski
18-12-2013, 04:11 PM
Guys, now the most stupid question: How to order the ball screws machined at both ends? Simple thread, long enough for 3 nuts? One inside, then mount plate then another plus locking one?

Jonathan
18-12-2013, 04:28 PM
Paulus - I like the way you've designed it such that the housing is one part, however I'm slightly concerned by the apparent small spacing of the angular contact bearings. According to NSK, you need to have the angular contact bearings spaced by 1.5 times their external diameter, but it looks like in your design they're almost touching, or a double row bearing which wont qualify? I expect you'll still get a reasonably high critical speed from this setup, but not as high as it could have been. The reason the bearings need to be so rigid is that the ballnut will inevitably not be perfectly concentric to the screw. Ideally the error will be small, but any error will cause energy to be transferred from the ballnut to the screw causing them both to oscillate. Both the nut and screw need to be rigidly held to suppress this.



For the sake of experiment, found some providers of Servo motors with hollow shaft where a ball nut can be mounted.
[...]
Wow, again. 1600eur the motor and the driver and cables. driver costs 420, so rest is the motor.


Make your own.


Guys, now the most stupid question: How to order the ball screws machined at both ends? Simple thread, long enough for 3 nuts? One inside, then mount plate then another plus locking one?

I got mine machined the same as the standard end machining for the angular contact bearings at both ends, in case the rotating nut didn't work. All you really need is a cylindrical portion and a thread on both ends. On one end simply use the thread to clamp the screw, ideally into a hole the same diameter as the cylindrical portion so it's held rigidly. Something similar on the other end, except you can use the thread to tension the screw a bit if your frame is strong enough. Also, design the end mounts in two parts so you can adjust the ends of the screw to be concentric with the bearings.

If you look earlier in the thread you'll see the end mounts I made are very simple - essentially just a block with a clearance hole. They work, but having a hole the same diameter as the screw instead of using it for adjustment will be better.

Edit: Idea for silvaski - to make the housing easier to machine, instead of boring the bearings from both sides you could bore from one side almost all the way through, then machine a spacer tube the right thickness to go in between the outer rings of the bearings.

Boyan Silyavski
18-12-2013, 04:42 PM
Paulus - I like the way you've designed it such that the housing is one part, however I'm slightly concerned by the apparent small spacing of the angular contact bearings. According to NSK, you need to have the angular contact bearings spaced by 1.5 times their external diameter, but it looks like in your design they're almost touching, or a double row bearing which wont qualify? I expect you'll still get a reasonably high critical speed from this setup, but not as high as it could have been. The reason the bearings need to be so rigid is that the ballnut will inevitably not be perfectly concentric to the screw. Ideally the error will be small, but any error will cause energy to be transferred from the ballnut to the screw causing them both to oscillate. Both the nut and screw need to be rigidly held to suppress this.


I knew this is important. I was sleepy the other night when i made the Sketchup drawing and wondered why you separated them so much, but overlooked it. I even gave ideas how to be made shorter :stupid:

paulus.v
21-12-2013, 01:38 AM
Paulus - I like the way you've designed it such that the housing is one part, however I'm slightly concerned by the apparent small spacing of the angular contact bearings. According to NSK, you need to have the angular contact bearings spaced by 1.5 times their external diameter, but it looks like in your design they're almost touching, or a double row bearing which wont qualify? I expect you'll still get a reasonably high critical speed from this setup, but not as high as it could have been. The reason the bearings need to be so rigid is that the ballnut will inevitably not be perfectly concentric to the screw. Ideally the error will be small, but any error will cause energy to be transferred from the ballnut to the screw causing them both to oscillate. Both the nut and screw need to be rigidly held to suppress this.


Thanks Jonathan for your comments,

I needed a simple design as I don't have access to precision machining. I wanted one part housing without the need of bearing pair alignment. So I chose a double row? -or how it's called- bearing. Will see how it will perform.

http://i285.photobucket.com/albums/ll63/bau_x3/CNC/Double-Row-Angular-Contact-Ball-Bearings-FAG_zpse4777ca1.jpg

Your last design is great, I will be able to modify mine if necessary :)

http://i285.photobucket.com/albums/ll63/bau_x3/CNC/rotxv2design_zpsc67d5470.jpg

paulus.v
15-01-2014, 01:38 PM
Jonathan, from your experience with rotating ballnut, what is the best way of lubrication? As continuous flow lubrication is excluded is it better to use grease instead of oil to last longer? What type of grease/oil do you recommend?

thanks

cnctom
10-03-2014, 12:57 AM
No, I hadn't spotted that. I see they've used bearings with a higher contact angle than most angular contact bearings, as that allows the bearing spacing to be lower for the same stiffness. I don't think this is too big a concern with the rotating ballnut bearings, as they are oversized compared to the bearings one would use if spinning the screw.

I've just finished making three more rotating nut mounts for someone who spotted the video on YouTube who's making a large machine with two SFE2525 ballscrews and one SFU2010 ballscrew. I simplified the design by changing the housing for a block machined from 4" square aluminium bar, and a separate piece cut on my router for the motor mount.


Some pictures:

10994109951099610997109981099911000110011100211003


A video, feedrate displayed assumes connection to 25mm pitch ballscrew:


http://www.youtube.com/watch?v=kmIqQqngLaI&amp;feature=youtu.be

Drawings:
1100411005
11006


Feel free to copy or modify the drawings and use them to make your own or get the parts machined by your favorite company... I don't mind.



Very nice design what size pulleys did you use i'm looking into building this myself but want to be sure to order the right pulleys and belt thanks for your time and engineering.

vre
22-03-2014, 01:34 PM
Very nice the idea of rotating nut!
I want to convert a conventional lathe to cnc and Z axis leadscrew has 1900mm length 30mm diameter.
If i use L1900mm 3210 rotating ballscrew with 2 bk25 end supports i will have 1300rpm citical speed & column strength 28477 newton
The critical speed is ok to me because i will get 10m/min rapids with 1000rpm in ballscrew i have concern about column strength because 28477 newton (2900kg / Z axis servo can give 3400kg ) isn't to big force and also i have fear about sag to 3210 ballscrew and runout at rotation that will do a lot of vibration.

In my situation would be better a rotating nut design like this to overcome these 2 problems (column strength & ballscrew sag/buckling) ?
Also with rotating nut design can i use a lower diameter ballscrew without having whipping ?

mutzy
04-09-2016, 09:41 PM
Jonathan and all of the determined mad scientists and hobbyists of this forum. This is great work. I really appreciate you sharing the setbacks and successes of hard work. Keep up the great efforts...I learn so much from this. Thank you again......Mutzy

mutzy
07-09-2016, 01:21 AM
Jonathan, how are you?
1. Where did you get the pulley that is associated with post 79? Was it machined out with a larger bore to fit the shaft or store bought? Where can i get one?
2. I see that there are two different shaft configurations with the pics. One the shaft where the ballnut attaches sits kind of deeper in the cupped out area, and the other sits flush with the whole face. what is the reason for this.
Thank you so much for your time.

Jonathan
08-09-2016, 11:01 PM
Where did you get the pulley that is associated with post 79? Was it machined out with a larger bore to fit the shaft or store bought? Where can i get one?

I used a standard pulley and bored it out to the size of the shaft.


I see that there are two different shaft configurations with the pics. One the shaft where the ballnut attaches sits kind of deeper in the cupped out area, and the other sits flush with the whole face. what is the reason for this.

This shaft where the face is 'cupped out' leaves a surface to locate the ballnut concentric to the shaft. This works well if the ballnut is accurately machined, but I found that you can't necessarily rely on this so sometimes it's better to just go with a face and align the nut with a DTI.

mutzy
08-09-2016, 11:49 PM
Thank you Jonathan. What is a DTI?

Mutzy

Clive S
08-09-2016, 11:52 PM
Thank you Jonathan. What is a DTI?

MutzyDial test indicator https://www.machinemart.co.uk/p/cm220-dial-test-indicator/?da=1&TC=GS-040218070&gclid=CjwKEAjwmMS-BRCm5dn51JLbp1wSJACc61tFzsobelwSWhXH7fG72RLzD3Xkn-WyRYliFWZMkPc4FBoCNljw_wcB

Jonathan
08-09-2016, 11:54 PM
e.g:

19159

Black Forest
10-09-2016, 07:23 PM
Hello Jonathan, Do you not accept PM? I would like to contact you.

reefy86
10-09-2016, 08:46 PM
Jonathan do you sell these to other people at all or know anyone specifically who can make the from drawings?

cheers

Ash

mutzy
13-09-2016, 01:12 AM
Hello Jonathan, how are you today? Wow, you certainly created a great thread here. I can't seem to find distributors that have this size bearing to bore out. Do you have any suggestions of who might sell them? USA would be best for shipping. Once again thank you.

Boyan Silyavski
13-09-2016, 08:45 AM
I made it a couple of times. If Johnathan could not, i could do it. Plus i have it from my drawing on my machine and is working perfectly fine.


Though last time i did it, i said i would not do this again for the money i charged then. In short only parts are worth ~200euro, as i have to have them at hand as all is not available locally and shipping costs are included that raise the price,

+answering to 50 emails, 3-4h redrawing for different motors and mounting, 8h to make parts and 2h for 2 people to put them together, 2h packaging, 1h sending . Plus while you are making them you learn from small mistakes.

So if you have 500euro prepared for them, then PM me. Thats the reality folks for bespoke parts. Mind that the cheapest servo motor with hole in shaft is 1500euro plus 400euro for drive, and you need 2x.


What i am saying here is i am just trying to help, not sell. So if sb could do it cheaper, go with him. If not- call me, i will do it.

Jonathan
13-09-2016, 04:07 PM
+answering to 50 emails, 3-4h redrawing for different motors and mounting, 8h to make parts and 2h for 2 people to put them together, 2h packaging, 1h sending . .

I know the feeling....

Having said that, I did start making a few some months ago, but got distracted by another project:

19179

I already have the bearings etc. I'm willing to finish these off and sell them, as I have a bit more time now. Silyavski's price is about right, maybe a little less - fortunately I bought the bearings before the crashed!

Black Forest
14-09-2016, 02:34 PM
What exactly would be included in the units you would sell Jonathan?

Jonathan
14-09-2016, 02:59 PM
What exactly would be included in the units you would sell Jonathan?

That's up to you, but in the past:

19191

So the mount, including pulleys and belt. You just add the motor and ballscrew.

mutzy
22-09-2016, 12:53 AM
Jonathan, i would like to purchase some of the bearings from you. how much are they. 30t 5m bored out to correct size.

Thank you
Mutzy

Jonathan
25-09-2016, 11:08 PM
Jonathan, i would like to purchase some of the bearings from you. how much are they. 30t 5m bored out to correct size.

Is that just the pulleys you're after, or bearings and pulleys? For the bearings, I got the last lot from here:

https://www.aliexpress.com/store/product/bearing-7207B-2RS-angular-contact-ball-bearing-7207-size-35X72X17mm/925746_2053270817.html

[It may help to order 2 at a time to avoid customs charges. Not sure how it works in the USA, but I ordered two with no charges, then got taxed a lot when I bought 10 :(]

Regarding the pulleys, I can get 30t 5m HTD for about 6.30 each. Boring them accurately then drilling/tapping the setscrew holes takes maybe half an hour, so I'd be happy to sell them for 10 each plus postage.

This weekend, I made some progress on the six mounts I'm making:

193111931219312

I'm away racing (https://www.facebook.com/UoNElectricMotorbike) next weekend, but after that I should have time to finish these ... at last.

richienz
12-10-2016, 01:09 AM
Impressive nuts! I just took a look at your e-moto webpage quickly too , wicked. Great project and racing at Isle of Man wow respect that place is nuts.

Hey I have a large router that is running well but I want to upgrade the main screw to a fixed screw rotating nut design. The screw is 40mm dia 10mm pitch and 3m long so its heavy. Running a 940W AC servo and a Grantite Devices VSD-XE servo drive, and 80VDC supply, I'm getting good response now with 8m/min velocity and about 400mm/s/s acceleration. This took a long time for me to work out the servo tuning but I got there in the end. I'd like to remove the rotating mass to improve my accelerations. The gantry it moves weighs around 150KG I estimate.

So I am wondering are you interested in building me a ball nut for this? Happy to pay market rates of course and if it helps fund any of your emoto projects then all the better. Please contact me if this is interest to you.

Below is my final servo response I got recently which I am really stoked about it, but to speed up my jobs I really want to up the accelerations.

cheers
Richard

19421


Is that just the pulleys you're after, or bearings and pulleys? For the bearings, I got the last lot from here:

https://www.aliexpress.com/store/product/bearing-7207B-2RS-angular-contact-ball-bearing-7207-size-35X72X17mm/925746_2053270817.html

[It may help to order 2 at a time to avoid customs charges. Not sure how it works in the USA, but I ordered two with no charges, then got taxed a lot when I bought 10 :(]

Regarding the pulleys, I can get 30t 5m HTD for about 6.30 each. Boring them accurately then drilling/tapping the setscrew holes takes maybe half an hour, so I'd be happy to sell them for 10 each plus postage.

This weekend, I made some progress on the six mounts I'm making:

193111931219312

I'm away racing (https://www.facebook.com/UoNElectricMotorbike) next weekend, but after that I should have time to finish these ... at last.

Jonathan
23-10-2016, 05:26 PM
Great project and racing at Isle of Man wow respect that place is nuts.

You can certainly say that of the riders!


I'd like to remove the rotating mass to improve my accelerations. The gantry it moves weighs around 150KG I estimate.

With that size ballscrew then you do have a problem with the rotating inertia, not so much the 150kg gantry. Can you measure the ballnut - approximate it as cylinders, then we can work out the inertia of the ballnut compared to the ballscrew to see what you could gain by rotating the nut.


So I am wondering are you interested in building me a ball nut for this? Happy to pay market rates of course and if it helps fund any of your emoto projects then all the better.

My PhD studies are taking up a lot of time at the moment (need to catch up after all the racing!), but I do intend to have time to finish some rotating nut assemblies soon.


Below is my final servo response I got recently which I am really stoked about it

What else can you plot from your system - can you plot the motor torque in addition to the speed response? If so, that's another way to work out the inertia and get better insight into how the system will behave.

richienz
25-10-2016, 03:36 AM
You can certainly say that of the riders!

With that size ballscrew then you do have a problem with the rotating inertia, not so much the 150kg gantry. Can you measure the ballnut - approximate it as cylinders, then we can work out the inertia of the ballnut compared to the ballscrew to see what you could gain by rotating the nut.

My PhD studies are taking up a lot of time at the moment (need to catch up after all the racing!), but I do intend to have time to finish some rotating nut assemblies soon.

What else can you plot from your system - can you plot the motor torque in addition to the speed response? If so, that's another way to work out the inertia and get better insight into how the system will behave.

I am too scared to watch the racing almost, its impressive riding nothing else is close really is it, but scary when a rider is badly hurt or killed, seems to be all too common.

I'll measure up the ball nut and screw and also try find the original part numbers and specs weights etc.

The PID loop tuning I struggle with, yes I can measure the currents I think , my Granite Devices VSD-XE servo drives have an inbuilt virtual oscilloscope that I use to measure the response with, to be honest the PID tuning I am very hit and miss with I can;t quite qork out what low pass filters I should use and why.......

I retuned the PID loops on both x and y on the weekend and its better but still I am sure there is room for significant improvement. whip of the screw is a problem too when I get over 600RPM approx. Thats a 10mm pitch per revolution screw (isthatthe correct terminology?) so thats a speed 6000mm/min , any faster and sometimes it whips and that really gives me the shits. I'm finding that small circle cuts say a 6mm end mill cutting a small 10mm dia helix has some error in the hole and I have to crank the speed back to around 1500mm/min for cutting (the timber) rather than running at higher speeds that I'd like to.

Anyway if you have time I would appreciate your guidance, keep on top of your studies thats top priority! Even if I can plagiarise your design and try build locally with some of your engineering input that would be greatly appreciated but if I can commission you to build one at the right price that works for us both then that's even better!!!

Funny I was a bit ambitious the last figures I quoted in my earlier post looked good on the scope but in real life cutting it was terrible!

Cheers
Rich

Boyan Silyavski
05-11-2016, 11:05 PM
This (http://www.ebay.com/itm/Anti-rust-Motorcycle-Bike-Clutch-Tool-Clutch-Lock-Nut-Spanner-Wrench-Socket-Tool-/252005426626?hash=item3aacb1a5c2:g:uiwAAOSw0UdXvAO 2&item=252005426626&vxp=mtr) cheap tool looks good for the KM03 locknut used if you have 20xx screws and using KM03 locknuts both sides

Still looking for an inexpensive tool for the KM04 nuts / 25xx screw and KM05/30xx screws. If sb knows , just post it here. Otherwise most probably you have to make yourself one so you could tighten and adjust tension of your ball screws

1956619567




table of locknuts for reference below, i use 2 each side so total of 4 per screw , locktite on thread, red paint to mark any movement, did not know at the time about washers:

1956419565

SafeAirOne
26-11-2016, 08:43 PM
First post on this bulletin board; I enjoyed this thread...or at least what I had a chance to read using the spotty WiFi signal at work anyhow.

I find myself in need of a couple of rotating ballnuts for a 3000mm-long 2505 ballscrew, and have come across this older thread on the subject. I was inspired by the designs depicted here, and worked to duplicate/improve upon them when I became curious as to why angular-contact ball bearings were used instead of tapered roller bearings.

I'm trying to keep the unit as compact as possible and I suspect that the tapered roller bearings will help in that respect. The force on the hollow rotating shaft will be primarily axial, along the ballscrew axis with barely any radial load being applied by the tension of the timing belt/pulley.

I came up with the following design (sorry for the seemingly random view angles)--Any comments/input regarding it's viability?:


https://c7.staticflickr.com/6/5484/31113708102_24ea76f52b_z.jpg

https://c7.staticflickr.com/6/5675/31113708062_98d786ee2e_z.jpg

https://c5.staticflickr.com/6/5489/31113708092_bc080330d4_z.jpg

Thanks,

--Mark

Boyan Silyavski
26-11-2016, 10:12 PM
The bearings that we use here are angular contact ball bearings versus the roller bearings you have drawn in picture. Hiwin and other manufacturers use also ball bearings in their design. I dont know if you are aware of the fact, but for example my bearings and ball screw heat up quite seriously especially when i am doing some crazy fast trochoidal tool paths on wood. In the 20m/min region and good acceleration. Moving 200kg gantry obviously helps that.

The angular contact bearings must be separated at least 1x OD between, having in mind they are 45? degree i think. Obviously roller bearings could be much stronger in all directions load.


Whats the rating of that bearing? RPM?



I am asking my self, what are you trying to gain here? Why would you be braking the assembly to pieces? The motor alignment and ability to tighten correctly the belt while keeping that alignment is a crucial part of the design . Have that in mind. Any small imprecision on a 3 meter scale becomes big imprecision

SafeAirOne
27-11-2016, 03:31 AM
I dont know if you are aware of the fact, but for example my bearings and ball screw heat up quite seriously especially when i am doing some crazy fast trochoidal tool paths on wood. In the 20m/min region and good acceleration. Moving 200kg gantry obviously helps that.

Thanks for your input.

I didn't realize the bearing and ballnut/screw got significantly hot during machining. I suspect that this will be less of a concern for me as the type of machining I will do will not include much in the line of high-speed toolpaths and rapid moves in general will be at a minimum. I suspect the concern with heat here is in regard to maintaining cutting tolerances and repeatability, as the bearing and screw materials will handle the heat satisfactorily. Is this your understanding as well?

Of course minimizing the contact area and pressure between bearing and race will similarly minimize the generation of heat, which is where the angular contact bearings excel. There's not a whole lot you can do about the ballnut and the ballscrew interaction though.




The angular contact bearings must be separated at least 1x OD between, having in mind they are 45? degree i think. Obviously roller bearings could be much stronger in all directions load.

These are 2788 bearings with 2720 cup (https://cad.timken.com/item/tapered-roller-bearings-ts-tapered-single-/tapered-roller-bearings-ts-tapered-single-imperi-2/item-29650). I just had a quick look at Timken's tapered roller bearing engineer's guide and didn't see any indication that the effective bearing spread (distance between bearings) was critical. In fact, I didn't even see where they made any spread recommendations for this type of bearing at all. Interestingly, I just saw that they make a Double Row Tapered Roller Bearing unit that is completely self-contained with a spread much closer than my own design:

https://c5.staticflickr.com/6/5672/31119059932_362ab739e4.jpg





Whats the rating of that bearing? RPM?

C90 - Dynamic Radial Rating (90 million revolutions): 5060 lbf / 22500 N

C1 - Dynamic Radial Rating (1 million revolutions): 19500 lbf / 86900 N

C0 - Static Radial Rating: 23000 lbf / 102000 N

Ca90 - Dynamic Thrust Rating (90 million revolutions): 2630 lbf / 11700 N


Not sure about RPM, but surely my servo motor at 1:1 or 2:1 pulley ratio will never be able to exceed the limitations of these bearings, I suspect.





I am asking my self, what are you trying to gain here? Why would you be braking the assembly to pieces? The motor alignment and ability to tighten correctly the belt while keeping that alignment is a crucial part of the design . Have that in mind. Any small imprecision on a 3 meter scale becomes big imprecision

My goal is to keep the overall length of the unit as short as possible. Every millimeter of assembly length is a millimeter LESS travel I get out of this axis. The problem I was running into when designing the bearing case as a single piece with the pulley in the middle of the bearings (like the ones you and Jonathan are using), is that the slot to insert the pulley has to be very wide so I'll have enough room to insert the key into the keyway in the shaft, then drop the pulley in and slide it over the key. This makes the whole unit prohibitively long for my application.

I don't believe belt tension will throw the axial alignment off as the pulley is positioned directly against the bearing, minimizing the lever-arm.

Presuming I do the machining carefully and from one side, the axial alignment of the bearings themselves should be just about perfect as the housing is a single piece of aluminum. The servomotor-pulley-to-ballnut-pulley alignment shouldn't be too difficult to handle with a separate, divorced mount, I wouldn't think.

I think the biggest downside to this is the cost of the bearings themselves. I tell myself that these bearings would be a once-in-a-lifetime investment for this machine. It would really stink if this design didn't do the job and I was left with 4 expensive, slightly-used bearings sitting on the shelf for the next 20 years.

Robin Hewitt
27-11-2016, 12:47 PM
I didn't realize the bearing and ballnut/screw got significantly hot during machining.

The heat comes from the motors. We like to use big macho-Volts to overcome inductance which leads to eddy currents and heat. The problem is, what to do with the heat? You want to dump it in to the machine before your motors demagnetise, but you don't want to get it in the screws because you would be forever worrying about heat expansion.

Will you be oiling those taper rollers? I occasionally run oil down in to my spindle bearings and make a terrible mess. That's a thought maybe it is his choice of lube that is creating the heat.

Boyan Silyavski
27-11-2016, 02:38 PM
I believe in my case heat comes from friction. I use grease on ball screw nuts. When i became aware of the heating was when i was machining 8h per day nonstop. So i greased well separately the ball screws and heat lowered, but still there it was. So now i grease very well these elements very often.

But thats why i think the roller bearing is not very suitable. It will generate much more heat due to much bigger contact area for friction. I love overbuilding but i believe this is not the place to do that. Plus the price of the bearings. How much they cost? The Chinese bearings we use normally are not very expensive. 10-15$

SafeAirOne
27-11-2016, 02:41 PM
Will you be oiling those taper rollers? I occasionally run oil down in to my spindle bearings and make a terrible mess. That's a thought maybe it is his choice of lube that is creating the heat.

As these bearings have no seals, I'd likely use grease. Which type of grease is yet-to-be-determined. The Timken catalog (http://www.timken.com/pdf/10481_TRB%20Catalog.pdf) devotes 8 entire pages to lubrication of tapered roller bearings with a myriad of grease recommendations based on usage and environmental conditions.

Plus, I can just add a grease fitting to the hollow area between the bearings in the aluminum housing and grease them both simultaneously during machine maintenance.

The lubricant seems to be the driving factor in how hot these bearings can get; The point at which the various lubricants break down sets the max operating temperature for the bearing.

SafeAirOne
27-11-2016, 03:04 PM
Plus the price of the bearings. How much they cost? The Chinese bearings we use normally are not very expensive. 10-15$

I was surprised to see the large price range for the exact same bearing, depending on where you shop.

I initially found the bearing on the McMaster-Carr website (https://www.mcmaster.com/#) (which I love because they have free 3D CAD models of most of the products they sell, making design in Solidworks easy). McMaster-Carr wants $48.00USD (45.00EUR/39.00GBP), but a quick search proves that this is a VERY common bearing. It looks like I can find these same bearings/races, manufactured by Timken, for as low as $19.00USD. Perhaps this isn't going to be as hard on the wallet as I originally envisioned.


But thats why i think the roller bearing is not very suitable. It will generate much more heat due to much bigger contact area for friction. I love overbuilding but i believe this is not the place to do that.

I guess this is my question, really: What are the consequences of hot bearings? I imagine the preload will increase due to expansion of the bearing parts, so the bearing life will be reduced (to several million revolutions). Grease life will be reduced (by half for each 10 degrees centegrade of temperature increase!). Other than that, I don't think there will be an impact on machine accuracy or repeatability, unless I'm looking at it wrong.

Boyan Silyavski
27-11-2016, 04:43 PM
Having in mind that we are talking 3000rpm here / though some servos can reach as high as 6000, i dont think thats a design consideration. So i think they will work, assembly would be stronger but wear will be premature. In short i would use the normal angular contact bearings as they are more than strong enough for the job.

Size reduction is impossible as the nut should pass through, so that determines the size.

Size reduction in ball screw is absolutely irrelevant as normally you will buy the 3m ball screw as maximum shipping size from China and anyway you need to stretch the ball screw properly on the machine, so you dont need any gain in sizing there, see my second build from signature. As what you loose there from length is the gantry step spread, which is quite more bigger than the rotating nut assembly length.

So in short- you gain absolutely nothing


About the preloaded pair, you have the same in the normal angular bearing. But how do you fix that so it serves you? I see no way.

SafeAirOne
28-11-2016, 03:03 AM
Size reduction is impossible as the nut should pass through, so that determines the size.

Hmm. In my mind, the nut attaches to the to the shaft flange and extends AWAY from the bearing housing. To have the ballnut sit INSIDE the hollow rotating shaft would make the bearings, and therefore the whole assembly, prohibitively large (in diameter), at least using a 2505 ballscrew/nut.

Shown with transparent regular ballnut, not the anti-backlash ballnut I plan to use:

https://c7.staticflickr.com/6/5452/31143069942_0b0dba0ae4_z.jpg


Size reduction in ball screw is absolutely irrelevant as normally you will buy the 3m ball screw as maximum shipping size from China and anyway you need to stretch the ball screw properly on the machine, so you dont need any gain in sizing there, see my second build from signature.

Yes, I have been reading your 2nd build log--I've read the first page, and then skipped to the end, working my way back to the beginning. Very impressive! I see lots of good ideas in your build log. I imagine somewhere in the middle of the thread, I'll come across the change in the machine's purpose, from a CNC air hammer to a CNC router :D



As what you loose there from length is the gantry step spread, which is quite more bigger than the rotating nut assembly length.

Heh, heh...You know, I hate to say this, but I didn't really consider this. The linear rails that my gantry rides on are 3000mm long. So long as I keep the rotating ballnut assembly shorter than the length of the gantry supports, I'll have EXACTLY the same amount of travel along that axis.

Thanks for pointing this out to me! I think this alone should be cause to re-examine my design; Now that I have all this length to play with, I can explore whether using angular contact ball bearings will allow smaller bearing ODs, which in turn will reduce the distance the stationary ballscrew must be from the machine.

Boyan Silyavski
28-11-2016, 09:47 AM
I meant that the ball screw has certain size and the nut attachment should pass through so this is the smallest possible bore bearing. You are right, the nut attaches to the assembly at side.