Suddenly No Holding Torque

[scotttin]

I have been registered at this sight for years, and this is my first post because I have been able to find ALL kinds of information. This question I can not find, but I am hoping it is not a dumb question.

Over the years I have assembled a simple bench top mill with three stepper motors on the axis. I have built everything on my own, so it is likely I am at fault here.

Upon finishing my mill I ran about a thousand lines of code producing a part that was accurate. When I recently ran the same code the part was not cutting right. The X axis was loosing steps. I quickly found that this motor had no holding torque. It did before, and now it does not.

My drivers are gecko 203v and they are powering the keling stepper motors. I have double checked my limiting resistors and I have moved the drivers to different axis to eliminate possible problems with drivers, wiring, and anything else.

The problem is in the motor. It gets power, and all the windings in the motor have continuity. It travels in both directions accurately, but it will not hold in place when stopped.

What could cause this?

If I replace the motor will this just happen again? Or do motors go bad, and still test with a meter as being OK?

Thanks Guys!

Scott

[scotttin]

As a follow up....

This was my fault!

I hate to admit this... I checked every wire five times. But I never noticed the pulley being loose on the shaft of the stepper motor. This should have been the first thing I looked for!!!!!

Oh well, at least I found it.

[vger]

I'm glad I'm not the only one that has missed a loose pully! LOL

About 20 years ago while working as a tech at a glass plant... There was a large machine (one that blow molded 200 bottles a minute) that had three motors that were syncronized by computer controlled VFD drives. One motor drove a reduction gear box which turned a large cam to control movement of 3 chutes that directed the molten glass gobs into the machine. There was a single prox detector for sync on this cam. The machine would not stay in sync and using an oscilloscope I could see the timing of the cam prox pulse advancing in relation to the referance pulse. 3 days, 6 technicians, 2 engineers, and 2 factory reps later, It was determined that the key on the shaft that turned the cam had sheered allowing the spring tension on the cam follower to force the cam to move just a bit each revolution. Nobody got in trouble cause the plant manager had originally stated "There is nothing wrong with that cam box, we just rebuilt it"

Steve

[scotttin]

Thank you for your reply.


When I begin the day I tell myself..."assume nothing!"

At the end of everyday I tell myself..."why didn't I check that first?"

Sooner or later I'll get it.

[RomanLini]

Do you have a flat surface on the shaft for the grub screw to bite into?

Stepper motors produce very high peak torques so it's important to have a flat surface so the coupling won't rotate over time.

[scotttin]

It has a flat on the shaft, but my set screw is only a cup point. I am going to pick-up a few with a "dog point" (the ones with serrated edge)

Plus a drop of red loctite will hopefully hold it in the future.

I have also adjusted my acceleration down in Mach3. Maybe this will help with the torque you spoke about.

Thanks!