Wednesday, April 3, 2013

Upgrade Losmandy GM-8 Wormgear

This week, I upgraded the DEC and RA gear assemblies to Losmandy's One-Piece Worm Blocks (OPWB).  These assemblies allow the worm-gear engagement to be adjusted without disturbing the alignment of the worm-gear bearings.  In addition to getting better tracking and guiding performance,  I also want to simplify the process of adjusting the gears in the field as I seem to need to do this on every camping trip.

The new parts from Losmandy are accurately machined, as are all of their components.  Even better, they come with absolutely no instructions to curb ones engineering creativity!  ... Humm, fortunately, I was able to find instructions posted by Michael Siniscalchi for upgrading the G11 mount.  Though the kits are not identical, these instructions were of enormous help.

The first step was to remove the old worm-blocks, motors, and gears.  I think that this required every standard sized hex key from 0.05" up to 5/32" SAE.  Removing the motor from the gear box would have been much easier with a ball-ended hex key.  The only parts that are reused are the servo-motor, gear box, gear-box cover, and the original high-precision brass worm gear.

Using the new kit parts, I inserted the worm gear into the bearing housings, and mounted these back on the bracket.  I then attached the Ruland coupling to the worm-gear, leaving the 0.01" gap as suggested by Siniscalchi.  Finally, I mounted the gear box mounting plate to the end of the bracket.

The next step was to screw in the "stud screws" as shown below using the 0.05" hex key from the worm gear side until flush.

Up until this point, everything had gone without difficulty.  The next step, however, was more troublesome and where my steps deviated from those outlined by Siniscalchi.  
  • Inserting the gear-box onto the coupling took a bit of jiggling.  I suspect the issue was with the shaft of the old gear box, not the new coupling.  
  • Originally, the nylon washer shown below would have been placed between the gear box tabs and the mounting posts.  With the new mounting plate, the gear box did not sit flush on the mounting plate causing the tabs to get bent and the gear box to sit cock-eyed.   I moved the washers below the tab, raising the gearbox up off of the mounting plate.  This seemed to work better.  
  • The "centering" trick described by Siniscalchi was very important.  However, I was never satisfied with the uniform smoothness of the gear motion.  It seems that the gear box continued to wobble slightly side to side as the worm was rotated.  Short of assuming that both worm gears were slightly bent, I could not figure out where the extra wobble was coming from.  In the end, I found the best compromise placement of the gearbox and tightened down the new hex mounting posts on to the ends of the "stud screws".   
  • My kit came with two sets of mounting posts.  One set was very long, approximately 2" -  I could not figure out what these were for. The other set was shorter than the original posts. Using these, there was a large gap between the top of the posts and the surface of the gear box cover which they are meant to support. However, the provided screws were just long enough to reach down to the posts. 

After remounting the gear box cover and the servo-motor, I attached the entire assembly to the mount and ran tests to make sure the controller could slew the mount smoothly over the whole range of each axis.  The motors seemed to purr smoothly and the controller did not issue any "lag" error messages.

This process highlighted one final problem.  In an earlier post, I described how I created a DEC axis puck which raised the telescope saddle plate up 3/4".  This allowed my larger, dual-style saddle plate to clear the RA axis motor assembly.  Unfortunately, as shown in the image above, the new OPWB kit moves the servo-motor out further from the mount ... just enough that my puck is no longer tall enough ... I think it needs to be 1" tall now. While I decide whether to manufacture a new puck (uhg), I have put back on the stock Losmandy saddle.

I am looking forward to some clear skies to see how the upgrade improves tracking and guiding.  It has certainly made adjustment a breeze.


  1. Hey Ed, I came across your post looking for reviews of the OWB on a GM8. What is your opinion of the upgrade? Did you do high precision gears on each one or just your RA? Do you feel it was worth it?

    Thanks! Love the blog.

    1. Thanks Christopher, I still think the upgrade was worthwhile. I upgraded both axes, but used my old worm gears since they were already the higher precision brass ones. I still think that the main benefit for me has been the fact that the separation between the worm and the main gear can now be adjusted without messing up the alignment of whole block.


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