Rotating Ballnut - design ideas

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/6/5484/3...ea76f52b_z.jpg

https://c7.staticflickr.com/6/5675/3...d786ee2e_z.jpg

https://c5.staticflickr.com/6/5489/3...080330d4_z.jpg

Thanks,

--Mark

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

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Originally Posted by Boyan Silyavski

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.

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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.

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Originally Posted by Boyan Silyavski

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.

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These are 2788 bearings with 2720 cup. 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/6/5672/3...362ab739e4.jpg

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Originally Posted by Boyan Silyavski

Whats the rating of that bearing? RPM?

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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.

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Originally Posted by Boyan Silyavski

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

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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.

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Originally Posted by SafeAirOne

I didn't realize the bearing and ballnut/screw got significantly hot during machining.

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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.

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$

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Originally Posted by Robin Hewitt

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.

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As these bearings have no seals, I'd likely use grease. Which type of grease is yet-to-be-determined. The Timken catalog 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.

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Originally Posted by Boyan Silyavski

Plus the price of the bearings. How much they cost? The Chinese bearings we use normally are not very expensive. 10-15$

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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 (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.

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Originally Posted by Boyan Silyavski

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.

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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.

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.

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Originally Posted by Boyan Silyavski

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

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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/6/5452/3...0dba0ae4_z.jpg

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Originally Posted by Boyan Silyavski

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.

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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

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Originally Posted by Boyan Silyavski

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

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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.

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.

Attachment 4227
I use HTD 5M often and they do not have sharp edges, maybe you should smooth it a bit just to keep belt happy?
Like the concept, did you consider to use RM2010 instead of 2510?
I wonder what to get RM2510 or RM1010, with smaller one bearing choice is better and they are cheaper as they are smaller.
Thanks for sharing Jonathan

The 2788/2720/QCL7C could work i found.

Reference speed 6700 r/min
Limiting speed 10000 r/min


The ball screws and nuts best is to get them from Fred BST Automation, as he has proven himself these years. 3 weeks ago i bought Hiwin from him for 2x 3d printers and he not only gave me the best price around/ i checked just to see the market/ but i had all in 5 days here in Spain.


As far as the HTD5 rounding, Tom, you are right. I assume most of people will cut them on 4rth axis or indexing head. Anyway below pictures of the absolutely correct profile and size of the 30t puley. I have parametric model that have drawn of HTD5 so if sb say wants a 300T pulley up to specs just drop me aline and will send you a DXF. But here i include the DXF for 30T pulley, the passing hole 27mm and shaft 35mm, needed for the important part in the rotating ball nut. So you can base your drawings correctly.

Attachment 20701 Attachment 20702

Boyan the zip file does not work for me. It might be the spaces in it!!

+1 on that
invalid attachment.
Mike

Re-uploaded the Zip file

Works fine now :wink:

Hello to all. I need some advice. I finally got back to tackling the rotating ballnut. After drawing on turbocad the htd gears on th shaft that fits into the bearing box/block(Shaft with gears machined on it with head piece that attaches to the ballnut) I realized that the 7207 2rs bearings will not fit over the machined gears or the large round headpiece that attaches to the ballnut. If I just find a larger ID like 47 or 50 angular contact bearing with 80 or 90 OD, can I substitute it? Of course I'll make the bearing box accomodate it. Thanks Mutzy

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Originally Posted by mutzy

Hello to all. I need some advice. I finally got back to tackling the rotating ballnut. After drawing on turbocad the htd gears on th shaft that fits into the bearing box/block(Shaft with gears machined on it with head piece that attaches to the ballnut) I realized that the 7207 2rs bearings will not fit over the machined gears or the large round headpiece that attaches to the ballnut. If I just find a larger ID like 47 or 50 angular contact bearing with 80 or 90 OD, can I substitute it? Of course I'll make the bearing box accomodate it. Thanks Mutzy

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Hows that will not fit? Over no, but there is a sequence of pressing the parts together. That's why the pulley is bored from a separate pulley and then pressed together. if you machine right on shaft, then you will have to machine a smaller pulley so the bearing passes on top of it. But then the ball screwe size pass trough hole, so you will have to check if that's at all possible. I believe it was possible but not sure now. Have to check

Attachment 22973Attachment 22974 Can't I just get a larger ID bearing for near the machined on gears to compensate for this?

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Originally Posted by mutzy

Attachment 22973Attachment 22974 Can't I just get a larger ID bearing for near the machined on gears to compensate for this?

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I could explain deeper why not, but isn't it easier to bore 4$ pulley than machine a complex part and buy twice as expensive bearing for which you will need bigger aluminum housing which is also more money?????


I would say generally No. The short answer is: do as i suggest, its best for many reasons or before jumping to buy bigger bearing, just sit down and make a detailed inertia calculation, what snappiness of the machine you need, what will be your maximum speeds and so on. And if that will change the motor you need to bigger!

Why? lets have as an example my machine 8x4 / in reality 1300x2600mm in metric/. If you invest in long ballscrews you generally make the machine right. Which means the gantry will be heavy around 180 to 300kg. If you gantry on that span is less than 160kg, then go with Rack and Pinion as that means your machine is not made to cut aluminum, so save yourself the trouble.


Generally if one is there already, one will use servo motors to move the gantry. But next is even more valid if steppers are used.To obtain the right speeds of the machine, the screw will be xx10 , best is the 2510. No need for bigger screw.

So most probably a gearing has to be used or bigger servo motor. Which means much more money if motor is not Chinese.

Also i have found that with normal servo motor the most desired ration is like on mine machine 20t:30t. fast enough and geared enough. It gives me a machine that has 307.2 pulse por mm ---- 1/307.2=0.003 mm resolution with maximum speed 20m/min . In reality that speed is 30-40 as my servos can spin to 6k rpm instead of the normal servos 3k rpm

Anyway, if you feel like, of course there are bigger bearings. But it seems easier for me to preload correctly 2x the same bearings than different sized ones.

So by attaching the Larger bearing to the smaller shaft/gear/bearing combo, it raises the whole inertia dilemma? If there is a possibility of lowering the height of the machined gears to let the 7207 pass over it might work. But do you still believe it's cheaper, easier to bore out a $4.00 pulley?

Mutzy

thanks for the help

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Originally Posted by mutzy

So by attaching the Larger bearing to the smaller shaft/gear/bearing combo, it raises the whole inertia dilemma?

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No. Changing the gear ratio raises the inertia dilemma.

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Originally Posted by mutzy

If there is a possibility of lowering the height of the machined gears to let the 7207 pass over it might work

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have to check that on some specifications but i think this was not possible. Willlook later at night

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Originally Posted by mutzy

But do you still believe it's cheaper, easier to bore out a $4.00 pulley?

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Or you will have to redraw the rotating part and the aluminum block part/ all the parts/ from scratch. I dont know what to believe, depends how you value your time.

Boyan, were you able to put some brain cells around the idea of keeping the same number of teeth, but lowering the overall height of the machined gear?

Mutzy

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Originally Posted by mutzy

Boyan, were you able to put some brain cells around the idea of keeping the same number of teeth, but lowering the overall height of the machined gear?

Mutzy

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Yes, you made me do it :hysterical:

From table below we see that without changing anything, these 3 sized pulley will be possible to incorporate ion the rotating part without further change of components

Attachment 23016

So from these 3, it seems the best will be the P22-5M-15F . 22t is not so bad, outside diameter of the teeth is 35.01 so basically same as the 35mm of the OD of the part and the ID of the bearing. That will work.

Attachment 23017


Not to have in mind that 400w servos and bigger have shafts starting from 12 if not from 14mm in diameter. So smallest pulley there could be even the 12 teeth, also 14, 16, 18, 20. But in reality could be also 13, 15, 17, 19, as i can draw the pulley with as many teeth i would like to and it suits me.


from my Favourite belt calculator https://www.bbman.com/belt-length-calculator/ having in mind minimum distance from motor shaft to center of pulley is 90mm, and if bigger than 400W servo even 100mm, we could see that:

12t puley is No good, less than 6t engagement

But all puley more than 13t including are ok. Now it should be considered that the bending radius of 13t puley is too small, this for the durability of the belts. And believe me it will generate heat on the belt and pulley. But as belts is 3-4 $ its not of a great concern.


So yes, it could be done. 13t pulley and bigger.

now when you divide 22/13 , 22/14 etc. its not a whole number which i dont like. the first one that divides well is the 22/16=1.375 so nothing has to be rounded, 16t pulley i see as a minimum for a good bending radius. So that would be the one i would have used.


Obviously 2x 750W servos at both sides will do the job and if something extreme is desired even bigger could be fit. having in mind gantry around 180-250 kg and max machine speed at 30m/min with acceleration around 3000m/s2 which is my machine tested and it really rocks. You should do calculations again for the specific scenario but more or less thats it.
Also all time i am speaking of Samsung servos and brand servos in general. I have never tried the chinese servos but i think they will do well also. Plus really they are almost same price bigger or smaller/ servos must be correct size, not bigger or smaller /.

It should be noted that when i am engraving small stuff i lower the acceleration at 1000, talking about aluminum stamps and so. But this is very delicate work which normally is done on a mill.

I think that answers your question.

Boyan

Thanks for your extremely detailed researxh. Now i'll think about what i need to do, i'keep you posted thanks. Mutzy

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Originally Posted by Boyan Silyavski

But all puley more than 13t including are ok. Now it should be considered that the bending radius of 13t puley is too small, this for the durability of the belts. And believe me it will generate heat on the belt and pulley. But as belts is 3-4 $ its not of a great concern.


Boyan

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Remember that changing out a belt is no small project with the rotating nut design. So durability of a belt should be a big concern because of the downtime and hassle of disassembling everything in order to change out the belt. The whole ballscrew must be uninstalled to change a belt. No small task regardless of the size of the table.

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Originally Posted by Black Forest

Remember that changing out a belt is no small project with the rotating nut design. So durability of a belt should be a big concern because of the downtime and hassle of disassembling everything in order to change out the belt. The whole ballscrew must be uninstalled to change a belt. No small task regardless of the size of the table.

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Exactly, that's why i said 16t as minimum. Everyday 8h machining belts would withstand at least say 2 years. Using it as a hobby machine at least 6-8 years.

Thanks for all of the awesome input. i was preoccupied with other things, but now I can get backt to the design.

Mutzy

Boyan, how are you? sorry it has been a long time. I got the 22 teeth HTD drawn onto the shaft. The shaft is only .05 in thickness. is this too small?
Attachment 23460Attachment 23461

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Originally Posted by mutzy

Boyan, how are you? sorry it has been a long time. I got the 22 teeth HTD drawn onto the shaft. The shaft is only .05 in thickness. is this too small?
Attachment 23460Attachment 23461

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I think its too thin. The clearance from screw has to be no more than 1mm, in fact mine is 1mm. so inside bore will be 25+2=27mm.

So boyan, 22 teeth will not work with the 1 in ball screw. Anything larger will not let the bearing slide over the gears.

There is a reason we do it like we do it. Now you know it.

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Originally Posted by Boyan Silyavski

There is a reason we do it like we do it. Now you know it.

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You should just have designed it for him!
Oops! You Did ;-)

Still working on it Boyan, and thanks for all your help as well as Jonathan. I made the Inner diameter of the RBN to be 1.0787. That would be 2mm of space for the ball screw as you suggested. Then I made the outer diameter of the RBN to be 1.3720 in which is about .006 smaller for the 7207 to fit nicely. Still as you can see, at the HTD gear part, The wall is about .0421 in in thickness. I think since both sides of the gear are attached to 1.3720 it will be strong enough. What do you think. MutzyAttachment 23481Attachment 23482Attachment 23483

Just over a mm wall thickness. You might get away with it in steel, but I'd expect aluminium to sheer.

By the time you consider axial loading due to bearing preload/dynamic loading, then the belt/nut causing some sheer/twist/radial loading, that part is going to be under a fair bit of stress under high loads. And that's before you consider stress points due to not being a smooth shaft.

Thanks for the input M_C. How much minimum thickness do I need in Alluminum vs steel? If I bring the numbers to what Boyan suggested in the previous posts to 27mm ID it would leave the thickness at about .054. not too much more, but it helps. mutzy

Would it make sense to use a slighly smaller Ball Screw?

Please take some important notes:

In reality the screw is 25.00mm. And as all must be lined up to under 0.01mm/ 0.02mm could be felt as tightening at certain point/ so you can use 26mm of the center hole with no problem.


You must not make the parts smaller than the fitted part! |I have made that mistake and is impossible to fit correctly. Make all parts "press fit" 0.02mm bigger That means shaft must be 0.02mm bigger than inner bearing and bearing bed must be 0.02mm smaller


You know what i am saying? This is not a skateboard wheel. All your design is based on "esy to service", but in reality it must be based on "precision"




I dont know why the worry about the thickness. Let's speak in mm as this Imperial system gives me a headache

If i am not wrong ~ = The bearing has 35mm internal diameter. 22t pulley has ~35mm OD and ~31mm diameter on the tooth lowest point/35- 2x~2mm/ . So if the hole is 27mm that gives a 1 mm clearance from the 25mm ball screw, then (31-27 ) /2 = 4mm wall at the pulley thinnest point, so whats the big deal?
Even so where the tooth is its thicker by 2mm so...???

Quote:

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Originally Posted by Boyan Silyavski

I dont know why the worry about the thickness. Let's speak in mm as this Imperial system gives me a headache

If i am not wrong ~ = The bearing has 35mm internal diameter. 22t pulley has ~35mm OD and ~31mm diameter on the tooth lowest point/35- 2x~2mm/ . So if the hole is 27mm that gives a 1 mm clearance from the 25mm ball screw, then (31-27 ) /2 = 4mm wall at the pulley thinnest point, so whats the big deal?
Even so where the tooth is its thicker by 2mm so...???

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I was just going by the drawing, which put the thinnest point at 0.0421", or 1.06mm.
It probably would be ok in aluminium, but as I mentioned, by the time you allow for all the forces likely to be acting on the shaft, it is quite a highly stressed part. I wouldn't be comfortable with it unless a FEA showed a good bit of safety margin, but it seems to be a lot of work just to avoid boring out a pulley.

Happy New year to all coming up. May we all have a great year of designing and learning.

Boyan, I did reset the OD of the shaft back to 35 mm + .02mm on both sides of the gear.
The inner diameter hole was set to 26 mm.

The pulley is 33.899 mm outer, 29.533 at the tooth lowest point
Wall thickness from lowest tooth point to inner dia is ~1.85 mm.

Yes M_C, I was doing this to eliminate the pulley step and as Jonathan posted earlier to lower the moment of inertia. (hope i didn't open another can of worms by saying that.LOL
mutzy
Attachment 23517Attachment 23518

In the grand scheme of things, is the amount of inertia saved likely to make that much difference? Is a couple hundred grams of pulley, going to be that significant in relation to a 30KG+ gantry?