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Pointers on low Periodic Error worm gear drives

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#1 dansawyer

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Posted 01 April 2020 - 02:06 PM

The project is to convert a Meade LX50 to OnStep GOTO. The issue is there is visible periodic oscillation in the worm gear. I removed the original clock drive shaft and measured it to be 3/16. I replaced the shaft and left about 1/2 inch to connect to a flexible coupler. The stepper drive motor was mounted on an adapter plate, which was in turn mounted about an inch from the housing. Adjustments were build into the adapter / motor mounts such that the motor shaft may be be aligned to the gear shaft. This appears to be in proper alignment. 

First observation: The total the connection is: Gear - shaft - 1st coupler - intermediate shaft - 2nd coupler - motor.

Second observation: The motor - 2nd coupler - intermediate shaft all rotate smoothly. 

This issue is the gear - shaft - 1st coupler have a visible in / out oscillation once per revolution. 

My speculation is the replacement worm gear shaft is not holding the worm square to the gear, thus producing a wobble. 

Does anyone have experience with this? 

I have a modest machine shop and am willing to create a improved housing / mount for the worm gear. Does anyone have access to a best practice implementation for worm housings? Dan 

 



#2 photoracer18

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Posted 01 April 2020 - 03:05 PM

Even if it holds it square worm gears have periodic error which is where PEC correction comes from. Its something that, if I understand correctly, is part of the machining method used to create the gears. Its big in badly machined gears and small in precision machined ones, but is never completely gone. In the days of eyeball guiding there was a significant part of the worm cycle that was error free, so that was when you ran to the bathroom or did other things. What PEC control did was allow you to record a cycle of the work gear and play it back to eliminate most of it. Of course with autoguiders  and now absolute encoders it has became less important.

The smaller the gears and the less teeth the error is greater generally but it is still based on how precision the gears are cut (and lapped). Which is why some mounts have precision gears available. Its the worm that determines that.


Edited by photoracer18, 01 April 2020 - 03:09 PM.


#3 dansawyer

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Posted 01 April 2020 - 08:51 PM

The couplers are 'springs'. If I carefully adjust the motor to position the worm slightly away from the gear and then use a screw adjustment to retention the worm into the gear I can remove most of the worm gear wobble. I will put a dial indicator on the worm case and measure. Thanks for the advice on the gear. 



#4 dansawyer

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Posted 02 April 2020 - 11:19 AM

Follow up question:

Most of the RA worm gear mechanisms I have seen use a spring to position the worm against the drive gear. Has anyone evaluated the opposite; i.e. keeping the worm a fixed distance from the gear? I am considering using a spring to tension the worm away from the drive gear an then a screw in opposition to the spring to position the worm a fixed position the drive gear. 

I would appreciate anyone's experience with this. Dan 




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