coupler

[janwan]

Need some help with failing coupler. We have rigid coupler between ball-screw and a stepper motor. The coupler failed four times so far. Two times the clamping screw got loose and the shaft came out of coupler but two times coupler was sheared.

This is a custom coupler made of Aluminum and I do not have any spec for it. I do not know what max allowable torque is.

I created model and run FEA with max torque stepper can develop. The maximum stress was 62 MPa that is almost 50% of the shear strength for the weakest Aluminum alloy (in my model I assumed 2104 Al). I need to add that the stepper needs to deliver only 25% of its max torque during this application.

We are doing a lot of short moves less than 1 mm per 0.15 s. This is stop and go application 24/7.
Both sheared failures happened after few thousands moves.

Is it fatigue? Appreciate any comments/suggestions.

Thanks

 

[itsmoked]

Oh yeah tizz fatigue in my opinion. Can you make a coupling out of steel? Or buy one. Why are you bothering to make them? Masochist?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[janwan]

Yeah, I like to torture myself. It came with the systems and yes we are thinking about Stainless Steel one. I am wondering if the rigid coupler is the best choice for this application. Will the flex coupler be a better solution. Anybody has opinion on the subject?

Thanks

 

[sreid]

Two set screws and two flats on the shaft will reduce the likelyhood of the set screws loosening. But they usually still loosen. I prefer collar clamps. The rigid coupler is likely putting large loads on the stepper motor bearings. The loads come from (slight) misalignment of the shafts. This is also the source of the loads that are fatiguing the aluminium.

You should be using a flex coupling. I like the Servo Class couplings made by Zero-Max.

 

[MikeHalloran]

Use Loctite on anything to do with a stepper. Think of them as tiny but very effective impact wrenches.

Ordinarily, I like the Heli-Cal spiral beam couplers, but they produce some axial force when loaded... so your stepper would kill either its own bearings or the thrust bearing of whatever you're driving. Have the t-shirt.

If you can stand the positioning error, use a rotationally soft coupling, like the ancient Acushnet K-Coupling.



Mike Halloran
Pembroke Pines, FL, USA

 

[itsmoked]

What size stepper?
What type of load?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[janwan]

Mike, the comparsion to impact wrench is great....

itsmoked, the load is 6 lb , the motor size 42 (diameter) has holding torque 140 mNm

 

[MikeHalloran]

Ah. A size 42 stepper is equivalent to a _big_ impact wrench.



Mike Halloran
Pembroke Pines, FL, USA

 

[itsmoked]

Yep. Your setup is causing "impulse loading" like little collisions(impact wrench). This likely, rapidly loosens the coupling. Once it's loose the impulses go way up because the stepper motor gets a head start before the load motion occurs.
SNAP!

Too bad you can't use a servo motor as this would probably reduce all aspects of the jerking.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com