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Possible Bent Shaft?

2875 Views 13 Replies 6 Participants Last post by  MEBCWD, 
L
oops, my apologies.

I thought I was in the CNC section when i posted this.

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L
Hello everyone

We've been having a problem for some time now with one of our servo motors not riding along the rack "nicely".
I won't go into all the details right now as I think I may have found the problem and was hoping for some feedback on my findings.

Attached is a photo of the rack.
As you can see there is an almost crescent like shape or partial circle shape on the rack.
This shape re-occurs at regular intervals along the rack.

I thought it might be how the motor was mounted to the machine...perhaps it was crooked....but if that was the case it would have a continuous line or pattern all the way along the rack.
Then I thought it might be the rack itself was out of wack...but if that was the case the patterns would not be exactly the same at regular intervals.

My final thought is this....the shaft that connects the motor to the gear (the gear that rides along the rack)...the shaft is bent just slightly.
I'm thinking this would cause the gear to connect perfectly with the rack, then at an angle, then perfectly, then at an angle....thus causing the pattern we see in the photo.

So I'm wondering...do you think I'm right or is there something else I'm missing?

Any feedback wound be great.
Thanks
Ryan

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Cherryville Chuck
I moved it to the CNC forum for you.
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BalloonEngineer
I think you are probably correct. Does the recurrence match up with the diameter of the pinion (same number of teeth apart as on pinion)?
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DaninVan
This is getting confusing...there are now two threads with the same title(?).
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L
Yes...it appears to be one complete rotation of the gear.
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Cherryville Chuck
DaninVan said:
This is getting confusing...there are now two threads with the same title(?).
Click to expand...
John deleted one of them.
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MEBCWD
One other thing to check would be where the drive gear connects to the shaft, it might be shifted so it is not consistently square on the shaft. The shaft or inside of the gear may be worn.

I think I would pull the drive motor so it is free of the rack and the drive gear is clearly visible. Then run the motor at a very slow feed rate so you can view the gear and see if you notice any variance in the operation. If you see that there is a problem then you could pull the drive gear and repeat the slow feed rate test to see if it is indeed a bent shaft.
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L
Excellent idea Mike (MEBCWD)
Thanks
I'll give that a check
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L
Mike (MEBCWD)

I was talking with a few people over the weekend about all this. One person expressed concern that if I disengage the motor from the rack to watch it run, that it will become out of sync with the other motor.
The reason, our gantry runs along the Y-axis and is driven on either side by servos. Y1 and Y2.

So they're saying in order to watch Y2 (the problem servo/gear) then I will have to disengage it from the rack but keep it connected to the controller.
Y1 would only be disconnected from the controller but not disengaged from the rack (because it's running perfectly along the rack right now).
This person seems to think doing this will cause the two motors to become out of sync. with the controller because Y2 is moving and Y1 is not.

I have no idea. Does this make sense? Would this happen or are they wrong? :)

Thanks
Ryan
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Pro4824
I see their point but....
Once you restart your controller and re-home your machine, you'll be good to go.
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MEBCWD
You would disconnect Y1 from the controller so it will not move then pull Y2 and do the testing.

As Joe says, you will re-home the machine before you use it after testing/repair so they should be in sync.

You might also need to replace the rack because of the wear or it might end up causing a problem.
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L
I gave it a try.
Unfortunately our controller wants to home the machine right after turning it on. So I turned it off and moved the gantry right to the trigger point indicating home in the hopes it would fool the controller...but it didn't work.
The controller rotates the Y2 motor one direction for about a 1/4 turn and then back...then it stops and gives an error on the Y-axis.
After this the controller basically freezes up. Only option is to turn it off.
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MEBCWD
landmarksign said:
I gave it a try.
Unfortunately our controller wants to home the machine right after turning it on. So I turned it off and moved the gantry right to the trigger point indicating home in the hopes it would fool the controller...but it didn't work.
The controller rotates the Y2 motor one direction for about a 1/4 turn and then back...then it stops and gives an error on the Y-axis.
After this the controller basically freezes up. Only option is to turn it off.
Click to expand...
There is probably a way to turn the homing off so you would be able to test the motor but I'm not sure where in the software that setting might be.
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