Thread: Rotating Ballnut - design ideas
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.
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.
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.
Last edited by Andrew Wilding; 13-04-2011 at 11:18 PM. Reason: brain not in gear
Last edited by Jonathan; 13-04-2011 at 11:37 PM. Reason: Schoolboy error,
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.
Last edited by Andrew Wilding; 13-04-2011 at 11:25 PM. Reason: added some bits
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
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.
Last edited by Jonathan; 14-04-2011 at 12:22 AM.
I just got prices for the screws:
2 of RM1610-2000mm with nut and end machined(both ends machined same as A type )
1 of RM1610-900mm with nut and end machined(standard end machining)
Air express shipping
2 of RM2510-2000mm with nut and end machined(both ends machined same as A type )
1 of RM1610-900mm with nut and end machined(standard end machinings)
Air express shipping
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.
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.
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