Main Axis Bearings on the Losmandy G11 and GM8

Why update the bearings?

As I have found, the Losmandy mounts tend to have a strange behavior when performing a meridian flip.  I've long noticed that when polar aligning on one side of the meridian, that a flip to the other side can show an error.  This has usually been market up to "cone error."

Cone error is where the RA and or DEC do not follow a rotation axis that is in alignment with their respective dimension.  When they spin around their axis, they show a wobble - or cone - when pointing to a part of the sky.  This is usually apparent with Goto slews.  

What's not readily apparent is that the RA cone error will impact polar alignment.  If the alignment is dialed in to a precise amount on one side of the meridian, it will show a difference on the other side.  I've seen this in logs from PHD2 as well as on the hand contoller.  During GSSP, the error reported on the G11 mounts ranged from 7 to 22 arc minuntes.

In practice, this can be adjusted out by taking half of the error on each side.  Thus the error is introduced on one side, leaving less error on the other side.  Using the Polemaster, this could be done by swinging the RA through the CWD position during the rotation phase.  That way the error is measured on both sides of the mount.

I got into talking with Michael Herman about the situation because of the heavy weight requirements of the 1200 project.  His analysis of the alignment issue is that the cone error is also made worse by the choice of axis bearings i the Losmandy mounts.  Because the bearings are made from pin bearings and not self-centering cone bearings, there can be slop or sag in the axis as it spins.  Additionally, because the pins are loose, they can move relative to each other, possibly binding or going non-parallel which can add other issues.

One solution for this is to replace these pin bearings with cage bearings or to replace the pin bearings with a somewhat higher quality unit.  Losmandy installs a bearing that is 3/4 long.  These are smaller than the max size available (1 1/2 inch long).  Several users have replaced them with the longer version to help with the load and improved handling.

With this in mind, I started working on one of the G11 for the bearing upgrades.  Was able to pull the DEC bearings and install the new ones rather easily once I figured out the best method.  However, I was unable to pull off the DEC ring gear.  Somehow it had become stuck on the shaft.  So, I ordered a gear puller - as the one that I had is not large enough to do the job.  Should arrive after Labor Day.

The next day I was able to pull the ring gear off with some effort using a bearing splitter/puller and some other tools.  However, the center bearing remained stuck on the shaft!  So I had to pull that as a separate item.

I expect the replacement thrust bearings to arrive on Friday or possibly next Tuesday.  In the meantime, I have an older set that I may be able to use for comparison purposes.  

The big worry about the thrust bearings is that because I've used the strap wrench to tighten the axes, I may have damaged these bearings.  From what I can see on the wear pattern, there is no evidence of issues with these bearings.  The races are polished nicely with no pitting or bad wear patterns.  No need to replace the one that I have inspected so far.

To avoid future damage, I will be using Michael Herman's increased friction pads.  These will help with locking up the axes so that I don't need to use the strap wrench.

There was a large issue when replacing the DEC shaft bearings.  Once I'd pressed in the new ones, the DEC bar did not drop down through it.  I chalked this up to a learning experience and pressed onwards.  Grabbing a bigger hammer (the 11 lbs counterweight), I pressed the it all the way down.

Well, that was a mistake.  The rotation was marred by a repeating tight spot, almost as if the rollers were passing over a flat area.  I figured that either the bearing was damaged or I'd manage to damage the bar.

I brought the DEC axis to Michael on Tuesday after work, we discussed it, and collaborated on some possible replacement parts.  That evening when I got back home, I wailed on the axis to get it back out.  Yes, the damage was obvious and clear.

See this video showing how the offending bearing catches on that part of the shaft.  The best I can figure is that it may have been cocked during installation and caused binding.  

To fix these grooves in the stainless metal, I used some emery paper in in the painting supplies.  I rolled the bar in the paper by hand and repeatedly inspected so that the amount of material removed was not too much.  This made it possible to polish out the smallest of the damaged areas.  Only the deepest cuts remained.  Of course, I am concerned that this may introduce a bigger error on this DEC as the lower end could have the freedom to wobble.  I may install a wider bearing at this place to mitigate the problem.  Also, I could note that this mount should not carry the heaviest payloads.

I put back the standard pin bearings and found that the DEC shaft now dropped into place easily.

Next steps: 

Repeat for the RA axis and for the second G11.

The parts from Misumi are expected to arrive today.

Also to be done: RA4 bearing upgrades for the worms with specific belleville washers for these bearings.  This should reduce the backlash as measured by PHD2.

Also to be done: Assembly of belleville stacks to implement a "spring loaded" worm for the DEC on the three mounts.  This should also reduce the backlash as measured by PHD2.