Radial load on ballscrew

Strostkovy · Nov 30, 2021

I know radial loading on ballscrews is frowned upon, but I've only heard of it in the context of misalignment with linear rails. For some simple projects it is easiest to just have a gantry float on a double ballnut, supported solely by the screws and their end bearings.

Assuming load is light relative to ballscrew diameter such that deflection isn't an issue, is there a reason not to do this? I would specifically use a double nut so that parallelism between the nuts and the screw is maintained, without having to add an additional rail.

Milland · Nov 30, 2021

Light load, low length to diameter ratio for the screw, reasonably centered on the nut(s) so overturning force is minimal - I don't see why not.

I'm not sure what you mean by "specifically use a double nut so that parallelism between the nuts and the screw is maintained", could you expand on that, or find a picture that shows what you're saying?

Strostkovy · Nov 30, 2021

Milland said:
Light load, low length to diameter ratio for the screw, reasonably centered on the nut(s) so overturning force is minimal - I don't see why not.

I'm not sure what you mean by "specifically use a double nut so that parallelism between the nuts and the screw is maintained", could you expand on that, or find a picture that shows what you're saying?

Just by grabbing a ballnut on a screw I can skew it a little bit pretty easily. Sort of like how having a single linear bearing on a rail doesn't provide much stiffness. A double nut acts like two bearings on a rail to improve rigidity.

I was messing around with an incredibly cheap screw that wasn't in perfect shape though. I'd think quality screws would not have this issue.

Milland · Nov 30, 2021

Strostkovy said:
I was messing around with an incredibly cheap screw that wasn't in perfect shape though. I'd think quality screws would not have this issue.

Depending on the rotational speed, I believe you can get ballnuts that are slightly preloaded by ball diameter. That should prevent slewing under light loads.

One thing to consider: some ballscrews may not have a specified concentricity between the screw thread and the axis of rotation. If you can't accept any movement of your carriage except for linear along the screw, that may matter.

How are you dealing with rotational torques? Some sort of outrigger?

Strostkovy · Nov 30, 2021

Milland said:
Depending on the rotational speed, I believe you can get ballnuts that are slightly preloaded by ball diameter. That should prevent slewing under light loads.

One thing to consider: some ballscrews may not have a specified concentricity between the screw thread and the axis of rotation. If you can't accept any movement of your carriage except for linear along the screw, that may matter.

How are you dealing with rotational torques? Some sort of outrigger?

It would be a gantry between two screws. Originally I was envisioning a small fiber laser cutter using this mechanism.

Milland · Nov 30, 2021

Hmm, got it now. But it raises more questions, including increasing the demand that the thread concentricity be quite good, otherwise as you rotate there's a risk of the two screws "fighting" each other and steering the stage askew. You'd also want to map BOTH screw's linear pitch errors to prevent similar distortions.

That's a particular issue if doing something where you really want to control the path, as for a laser. Now that I understand what you're doing, I wonder if you're not better off with a traditional rail setup and one screw. I think you'll have an easier time getting a true path that way. Or one screw and one linear rail, with the laser biased off the rail.

Strostkovy · Dec 1, 2021

Milland said:
Hmm, got it now. But it raises more questions, including increasing the demand that the thread concentricity be quite good, otherwise as you rotate there's a risk of the two screws "fighting" each other and steering the stage askew. You'd also want to map BOTH screw's linear pitch errors to prevent similar distortions.

That's a particular issue if doing something where you really want to control the path, as for a laser. Now that I understand what you're doing, I wonder if you're not better off with a traditional rail setup and one screw. I think you'll have an easier time getting a true path that way. Or one screw and one linear rail, with the laser biased off the rail.

That is a good point.

I got to this point by wanting to make the motion components lighter for better acceleration, which requires high lead screws which are large in diameter, though that still isn't a complete solution. If I already have something like a 65mm ballscrew with an 80mm lead, I think the rigidity would be adequate and at the price of the screws the runout better be very good.

I could get the same result with a smaller screw and a large linear shaft with a bearing, and still only have to support the end for this particular project. This helps with not having to machine and entire table with area for a profile rail along the whole length.

wheels17 · Apr 14, 2023

There is a "balls rolling on pairs of pins" ball nut mount that is designed to prevent the runout and non - straightness of the ball screw from affecting the carriage running on linear bearing rails on lower end 3d printers. It floats in x and y, and is very rigid in z. I couldn't find the link on my phone.

technocrat · Apr 14, 2023

I think you will increase wear inside the ball nut as the balls will be in a different position to a conventional load. The nut design is probably not optimised for this kind of load. The second set of nuts is a good idea for both keeping the screw straight and also reducing load and wear in the nuts.

johansen · Apr 14, 2023

It will work, but others have pointed out all the problems

At a 45 degree contact angle, the axial load of the ballscrew will need to be divided by 2 to 3 to get the rated radial load.

The same math is used when converting between the radial load of a standard ball bearing to get the axial load rating.. except that you start out with half the balls taking the radial load to all of the balls taking the axial load..but at a 4 fold force multiplier for a 15 degree angle. So it can handle half...ish. (contact angle may not be that high)

For a 45 degree contact angle which is a safe assumption, the force multiplier is the same in both loadings.

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