Linear Stepper Motor Leadscrew Runout

[damufunman]

Hi all,

New to forum, have a problem I've come across at work. We have a linear stepper motor with a single side mount lead screw. When the motor turns the free end of the leadscrew is wiggling about 1/16". What could be causing this? The manufacturer is saying that extra play between the nut and leadscrew (to allow for some misalignment) is causing the runout and is normal. I'm not sure this makes sense, and I have my own ideas, but I wanted to see what other people think. See linked videos of wiggly leadscrew.

https://www.youtube.com/watch?v=VYdykmyADYw

https://www.youtube.com/watch?v=6PCl1UuBU78

Thanks!

 

[TSnowden]

It could make sense. On some machine tools - like lathes - there will be a leadscrew/nut combo with built in play, and usually a way to snug up that play (sometimes just by cocking the nut). Typically, the wiggle in the screw isn't a problem as long as the ways are in good shape and properly adjusted, as they are what determines accurate motion. However, I didn't watch the videos, and am not sure what your configuration is.

 

[bdeuell]

you probably know this but ... the runout is a result of the acme thread axis not being in the same place as the axis of rotation. depending on how the threaded shaft was fabricated this could be a result of manufacturing tolerances at the bearing locations or that the shaft was bent. Keep in mind that because the threaded portion is much longer than the motor so any angular misalignment would be multiplied over the length of the shaft.

the adjustments on the acme nuts of machine tools are typically used to eliminate backlash by allowing the axial preload on the thread to be adjusted.

The design issue really come from the fact that you have 3 constraining points on your threaded shaft (2 bearings in the motor and the acme nut). the amount of radial clearance in the came nut is limited so it might be a good idea for the design to allow for some degree of misalignment (although in my opinion from the scale of the device i can see in your video the runout does seem excessive). this can be accomplished in several ways one is by allowing the acme nut to float within a plane and constraining it in rotation and axial directions only. it is also common when mounting a motor to a shaft to use a flexible coupling for similar reasons.

 

[damufunman]

We checked the shaft on our granite table and it is straight.

Even when allowing the motor to float (unbolted it and held it by hand) there was side loading on the motor from the nut-shaft interface. This tells me that there isn't radial clearance, but radial interference... I'm waiting on a report from the manufacturer on their investigation. We're suspecting that the nut was just manufactured eccentric to the bearings, as we haven't had these issues with different manufacturers' motors in the same setup.

 

[bdeuell]

I watched you video again and realized i was looking at a through shafted motor (not a motor with a solid acme shaft) ... so yes you are correct the thread of the nut is eccentric to the bearings.