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LX200GPS clutch slips when weight added.

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#1 Sirius395


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Posted 15 May 2019 - 06:29 PM

Good evening forum members. I have a 10” LX200GPS that has been working great. There have been no issues in polar aligning when the scope was set to polar home or anytime else when slewing to different objects. I then decided that it was a good time to get into AP. I researched and the common advice I found was that the 10” should be able to handle around 20 pounds. So, I started to order things to put it together. First to arrive was a Losmandy D series plate with the counterweight. I installed it and with the weight attached, all things worked well. No perceivable issues seen in slewing, but I have to admit, and I haven’t had clear skies to test outside. With plans to piggy back an ED80 APO I purchased another Losmandy D series plate to mount on top to hold the APO. Once it was installed, I did another quick indoor test with both plates attached but without the counterweight. That’s where things went bad. I set the scope in polar home and powered up, and Smart Drive started its small RA slew. About halfway through that slew the RA clutch started to slip, and I had to powered off to stop it. I checked and the RA lock was tight and no other equipment was attached. I tested it twice and the clutch slipped both times. I removed the top plate only and tested again, and things worked as it should and without slippage. I can’t believe that adding a 4 pounds plate to the top overloaded the clutch. I estimate the with the scope, both plates, rings and eyepiece the total weight I am looking to add is around 18 pounds max. So now for my questions.


What is the estimated weight can I add?
Is there an adjustment that I can make to the clutch?
Is there a clutch mod that can be done?
Do I need forget about mounting the APO on my 10” an just buy another mount?



Many thanks and any ideas/help I would greatly appreciate.

#2 carolinaskies


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Posted 15 May 2019 - 08:21 PM

I think you need to adjust the RA clutch lever-to-shaft position.  To do this loosen the allen screw holding the lever thumb plate to the shaft. Rotate the thumbplate about 1/4" clockwise and tighten the allen screw.  Now check and see if when the lever is engaged enough friction is applied to hold the added weight. 

The LX200 should, when properly balanced, be able to handle about 20lbs additional weight.  If your declination starts to slip badly, Peterson Engineering offers a clutch kit for the DEC axis.  

#3 Sirius395


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Posted 16 May 2019 - 06:29 AM

I most likely described the issue wrong. With the 2 plates installed and the scope in polar home, I turn on the power. As the smart drives slews the scope it will start out fine then it will start slipping with a lot of clicking noise. With only the bottom plate attached, all seems to be good. I assumed and described it as the clutch was slipping. It could be gears slipping or something else. I have never been in the guts of the RA so I really don't know and don't know where to start to fix it.

#4 Peterson Engineering

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Posted 16 May 2019 - 01:31 PM

Hi Sirius,


Balance is the key here.  IMHO 20# is quite excessive.  Both balance and inertia are critical as the drive motors are pretty small.


RA "slipping" is usually corrected by readjusting the lock lever position.  But the "clicking" sound you're hearing is almost certainly associated with the worm lifting off of the aluminum worm gear and chewing the aluminum gear. 


A work-around would be to reduce the scope maximum slew speed, as the large mass of the loaded scope puts a big strain on the drives with start and stop.  Or you can try opening up the scope and figuring out the adjustment.  Or, since you asked about availability of a "clutch mod" you can look into Buck's Tune-Up, which addresses a myriad of potential scope problems as if refines the drives.


The mounting plates you're using are probably as heavy as the piggyback scopes.  You might look for a work-around.  I've got a set of scope rings from Scopestuff that are mounted directly to existing female threads in the ends of my Meade 14" so even with counterweights there's very little inertial mass.  Of course, this is a permanent setup.


Theoretically the scope should be just a tad nose heavy and just a tad biased toward the east side to insure that the RA gears are always engaged as the mount tracks the motion of the target.


Clear skies,



#5 junomike



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Posted 16 May 2019 - 04:34 PM

How much counter weight are you using?

I find I need 11 - 13 lbs for a 12 - 16 lbs OTA.

#6 Sirius395


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Posted 16 May 2019 - 06:37 PM

Thanks to all for all the input. It has helped me a lot to get on the right track. At the suggestions of balancing, I went back over my set up and started from scratch. I loaded up everything on the scope and in checking my DEC balance first, I found that it was off, still being tail heavy with the OTA at 45. I adjusted the counterweight and got it to balance. Once I pointed it at zenith, the OTA wanted to drift north so I adjust the counterweight away from the OTA but determined that I will need more weight to get it to balance at zenith. Current have only the 3.5 pounds of weight. Using a trigger pull gauge to estimate, 4-5 more pounds should help the zenith balance. I'll be ordering a 5 pound to add to it. As for RA balance, there is a slight roll to the West. I'll work on that to get the slight East roll. With just the 3.5 pound on, I did a quick test and was really great to see that even though it wasn't perfectly balanced it performed the smart drive slew and a quick slew to a object without any gear slippage. Now it will stay off until I can get the additional weights. No sense in pushing my luck. Additionally I will also look into Buck's or Peterson clutch mods. I have to say, there's nothing like have a group to fall back on for help!

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