Gearbox Backlash Is this to be expected?

[Cutter99]

I hope you guys don't mind me asking these questions here...

I have some older, but not overly used PK296A1A-SG7.2 Vexta steppers that I am using on my old PR Shopbot. I am still using the old SB controller so no Geckos.

I noticed I can wiggle the shaft back and forth slightly with my fingers when they are powered up or off. This is not the gear moving on the shaft but the shaft itself wiggling slightly.

How much of this is acceptable? Is there a way to tighten it up if I take the gearbox appart? How about some automotive transmission repair additive?

IS that why people went for the belt drive gearboxes?

I also have some PK296A1A-3G3.6 motors but they don't have as good a resolution as the others (At least that is what I think...). They don't seem to wiggle as much. Should I use them and live with the drop in resolution?

Does this happen from wear? I don't have electrical stops on the rails. To stop the Y from going off the table I put some bolts on the end of the machine.
If the carriage hits these bolts and stops the carriage, what happens to the force of the stepper turning?

Thanks,

RB

[Richards]

Assuming that you have a 1.25" pitch diameter spur gear (25-tooth), if you installed Gecko G203v stepper drivers, the 3.6:1 steppers would have a resolution of 0.000545" per step and the 7.2:1 would have a resolution of 0.0000272" per step. So, the 'slop' in the gear box would be greater than the resolution. A properly adjusted belt-drive has no 'slop', but the 'slop' in a factory drive is not usually a big enough factor to cause worry - in actual practice.

The SGxx in the part number indicates a Spur Gear, which also indicates that there is no adjustment possible inside the motor to reduce backlash. A TH (tapered hobb) type gearbox can be adjusted (at least in theory) by moving one gear on the shaft towards the other gear.

I don't have the data sheet for the stepper drivers from Shopbot for your machine, so I don't know how the motors are configurered (200 steps per revolution, 400 steps or other), but you would get significantly better resolution if you installed Gecko stepper drivers that have 2,000 steps per revolution.

Stepper motors can hit the stops without excessive damage because the motors are driven by magnetism. (It would be better to NOT let them hit the stops.) If they hit the stops, the machine will lose its absolute positioning, so you'll need to re-zero the axis before you continue cutting.

[Cutter99]

Thanks you for responding to my dilemma.

Would I get better resolution today by changing to my other set of motors the PK296A1A-3G3.6 with much less loose movement in the shaft?

Is there any automotive additive that might help reduce the backlash?

Thanks,

Richard

[lumberjack_jeff]

It wouldn't hurt to disassemble and use some new, high quality grease.

[Gerald D]

Also see this thread:
Backlash in the gearhead of the Oriental Motor PK296A2A-SG7.2 (http://www.mechmate.com/forums/showthread.php?t=887)