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Evaluating CGEM motors

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#1 GoldSpider

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Posted 18 December 2018 - 07:23 AM

I've got an old CGEM that has spent a LOT of time working under the stars (mostly for previous owner) that has resisted most efforts to get guiding smoothly.  I've relegated it to planetary imaging and visual observing, but I'd like to take another crack at it to steer a heavier-than-I-expected scope I recently picked up.  Hypertuning has helped, I've swapped the RA and Dec. motors, spent a lot of time tweaking the worm gear mesh tightness, and slowed the custom slew speed to try to take some stress off of them, but they still sound like hell. 

 

I suspect that these motors are just worn out from high mileage, but I'd like to know if there is a way to objectively test this before investing in replacement motors.  

 

Thanks.



#2 DuncanM

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Posted 18 December 2018 - 10:24 AM

As long as you're getting accurate gotos and sidereal tracking then the motors are fine. 



#3 Geo.

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Posted 18 December 2018 - 10:36 AM

The main problem you will see with old motors are bad brushes and a worn commutator. As the Igarashi motors are not designed to be serviced, replacement is the only option. That being said telescope mounts are not very demanding on their drives. For example consider the sewing machine. They run at high RPMs most of the time and can be used for much more than a few hours a month. I rebuilt a 40 year Bernina for my daughter. Aside from paint the motor needed new brushes. 

That as a replacement motor was $500, I sure had motivation!



#4 choward94002

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Posted 18 December 2018 - 10:59 AM

I've got an old CGEM that has spent a LOT of time working under the stars (mostly for previous owner) that has resisted most efforts to get guiding smoothly.  I've relegated it to planetary imaging and visual observing, but I'd like to take another crack at it to steer a heavier-than-I-expected scope I recently picked up.  Hypertuning has helped, I've swapped the RA and Dec. motors, spent a lot of time tweaking the worm gear mesh tightness, and slowed the custom slew speed to try to take some stress off of them, but they still sound like hell. 

 

I suspect that these motors are just worn out from high mileage, but I'd like to know if there is a way to objectively test this before investing in replacement motors.  

 

Thanks.

You normally do a torque test for this, where the motor with a pulley wheel is attached to a known weight on a string and turned on ... if it can lift the weight in a known time then it passes.  Unfortunately for order motors this is a destructive test ... it's like speed testing your '67 VW bug ... you can well verify that it can still go 90mph, but at the end of the test you've basically killed the VW ...

 

As mentioned, as long as it is able to properly track sidereally and slew to goto's it's likely good.  Make sure your scope balance is absolutely perfect (forget the "east heavy" thing), that there's no way something can get snagged or interfere with the mount motion and set your slew rates as slow as reasonable (no more "degrees per second" setting) and you'll likely be OK ... if you smell something funny, or if it has trouble maintaining sidereal tracking, or it gets hung up on even a slow slew then it's probably replacement time ... the noise is likely just dust in the brushes which sounds bad but isn't fatal ...




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