Axial load on Rotary Stepper Motor?

[okok2009]

Hi,

I might have a dumb question. If a rotary stepper motor is connected to a lead screw system to lift something up, will the pushing axial load be transfered to motor itself? Will the load be supported by the motor partially?

Thanks.

 

[benta]

The motor will usually not support axial loads. The leadscrew should have its own radial/thrust bearing system. Mostly there is a clutch between motor and screw to compensate for slight radial mismatches.

Benta.

 

[tygerdawg]

As Benta says. Leadscrew is (should be) set in its own bearings that take up the axial load. Motor is connected via coupling to leadscrew. Good system design lets motor do its as-designed task which is turning and supplying torque. Also quick replacement if it stops turning.

TygerDawg
Blue Technik LLC
Virtuoso Robotics Engineering

http://www.bluetechnik.com

 

[MikeHalloran]

Typical stepping motors have instrument bearings, which do have a thrust rating, though not a large one. They also typically have a small amount of endplay, over which you have no control.

I was once involved with a product that used a stepping motor to move a couple of pistons using a leadscrew, and transferred the axial load to the motor using a Heli-Cal coupling. The load was mostly due to seal friction.

The motor bearings never gave trouble, but we discovered an unexpected side effect. Heli-Cals change their length as torque is applied, which caused the plastic structure coupling the cylinders and the motor to move, and buzz, loudly, in tune with the stepper.

Retrofitting a thrust bearing in the field population was fairly expensive.



Mike Halloran
Pembroke Pines, FL, USA

 

[benta]

I wrote "clutch", "coupling" is of course the correct term. English is not my native language...

Benta.

 

[okok2009]

Hi,

Thanks for you guys' comments. A stepper motor has been chosen now. We are going to use a power-off brake to lock the lifting position when it is off. But also a manual override of the adjustment is also required once the motor turns off. Is there any clutch or system which can engage with the shaft once it is turning, disengage once it is off to perform the manual knob adjustment?

 

[MikeHalloran]

The stepper itself locks magnetically when the driving sequence stops. With two coils on, it heats up.

When power is removed, other things happen:
The stepper takes half a step.
The stepper still locks the shaft, more weakly.
The stepper controller loses all knowledge of where it is, unless there's an absolute encoder or equivalent in the system.



Mike Halloran
Pembroke Pines, FL, USA

 

[SincoTC]

As Mike said, there are a number of problems when power is removed from a stepper motor. It is best to maintain the power so that it doesn't lose steps, but limit the current to reduce the heating, but still provide enough "detent" holding force so that it doesnt slip down. A lot of stepper drive controls have this facility. I would also use the drive controller to "Jog" the position for manual adjustment, so that the controller doesn't lose track of where the load is, as would be the case if you turned the screw with a knob (unless you fit encoders).

I note that in your OP you said that the motor was going to LIFT something with a leadscrew, so the number of starts and hence the Lead of the thread will have considerable affect on how solidly the motor will hold the weight.