Losmandy G11 colleagues,
I recently acquired a "vintage Celestron by Losmandy G11 mount with the Gemini System computer".? Mechanically, the mount needed some TLC.? I decided for a complete end to end take apart and rebuild to fully understand how it was built.? This email is to highlight and share how I solved an interesting problem.? This was that the unloaded motion of the Declination shaft when rotated liked to slowly cork-screw either up or down depending upon clockwise or counter-clockwise shaft rotation.? Additionally the level of hand applied torque was also somewhat uneven when rotated, fully free at times and then "binding", then free again, this was even when the end clutch was removed and the shaft flipped 180 for general checkout, something was just not OK with the behavior of the shaft and bearings.? Eliminating possible causes included a close visual inspection of the shaft and bearings for dings, nicks, dents, etc.? Then I verified the shaft was acceptably straight using a granite surface plate and two Starrett # 278B V-Blocks and and a ~0.00005 resolution dial indicator.? I rotated and measured shaft at bearing point areas and in the center (either side of the shaft hole) and satisfied myself the shaft mechanical run-out was OK.? I then checked the overall state of the shaft diameter using a 1 to 2 inch carbide tipped micrometer with vernier (good for no better than 0.00005 (visual interpolation)), multiple locations of the bearing tracks, in general this shaft measured 1.2500 +/-, did not pick up any signal requiring intervention.??
Next for a more sensitive diagnostic test, I realized I needed to see what was going on between the shaft bearings and the shaft interface while the shaft was rotated.? For this I cleaned all the residual grease off the shaft, and used a black indelible marker (Sharpie) and made a continuous thin uniform band of ink around the perimeter of the shaft, slightly in excess of the track of each of the two shaft bearings.? I then inserted the shaft back into the bearings, and rotated it by hand about 50 turns, pulled out the shaft and visually inspected the wear pattern results.?
One end of the shaft was tracking reasonably perfectly with the bearing surface, evenly, continuously around the full diameter of the shaft, ink was uniformly removed. The other end (clutch end) had three "high" visual wear points on the shaft where the ink was worn away, one wear area was about 1/4 inch diameter, the other two wear areas were about 3/8 inch ellipse with long axis of ellipse parallel to the length of the shaft.? The wear points shown by Black Sharpie ink worn away and surrounding shaft remained black.? Using the micrometer I checked the overall diameter at these locations.? The diameter was about 0.00027 larger than adjacent shaft area, this was the source of the shaft binding.? I used 4,000 silicon wet and dry cut paper into a 1/2 inch strip, about 10 inch long with a slight amount of WD-40 oil on the surface and lapped the shaft at these high points.? This was initially done as a spot polish application about 10 strokes (measured after 5) using a very flat piece of highly polish tool steel ~0.4 x 3 inch as a backing to the paper, this was followed with a total of about 20 lapping strokes (measured after 10) holding the strip tight with thumb and forefinger of each hand and moving back and forth keeping the polishing paper tight.? I cleaned up the residue and placed the shaft back into the bearings.? I have had success, the shaft now tracks very smoothly and uniformly now with even application of torque.? I hope that this helps others who may be experiencing this type of defective behavior.? It also shows that 4,000 grit silicon carbide paper (3-M) is helpful to the level of precision needed.? Time to enjoy the G-11? Cheers John
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