Recently did another rebuild of my G11 (with non-spring-loaded worms) as I wanted to replace the 3§ ID thrust bearings and the 3§ ID thrust bearing washers which were a bit scored. Also, I had a stubborn 80 second error (1/3 worm period) which I could not figure out, so I replaced the current RA worm with an old one I had good results with previously. I replaced the thrust bearings with Koyo NTA-4860 bearings which were 5/64 or 0.078§ thick. For some reason I was unable to find the same 3§ ID washers which Losmandy used which are 0.045§ thick, so I used an INA TWA-4860 with 0.030§ thickness, and a Koyo TRB-4860 with 0.060§ thickness, which together total 0.090§ thick, same as the two original washers.
The RA worm blocks were changed out to another set where the right block was honed out a bit to allow the right bearing to slide back and forth as compressed by the Belville washer. As a new test, the bearings were replaced with McMaster-Carr part 2342K163, which are permanently lubricated, and have a black plastic seal over the ball bearings.
A micrometer was used to measure the various parts positions to try and determine the exact center of the worm axis and center of the ring gear. This issue has probably been discussed before, but I was wondering how close is good enough for best RA periodic error behavior and proper function of both axes. All parts were at approximately 50F, the temperature in my unheated workshop.
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The following measurements and determinations were made:
Diameter of worm shaft inside of bearings: 0.400§
Radius of worm shaft: 0.400§/2 = 0.200§
Thickness of ring gear: 0.300§
Midpoint of ring gear: 0.300§/2 = 0.150§
Thickness of black anodized base plate: 0.329§
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With these values in place, the centers of the worm and ring gear can be determined by measuring the distances from the bottom of the base plate to the top of the worm shaft and top of the ring gear, respectively (see pics). The calculations then are as follows:
Top of worm 每 0.200§ = Worm center position
Top of ring 每 0.150§ = Ring gear center position
For the RA drive prior to disassembly:
Top of worm: 0.856§ 每 0.200§ = 0.656§ at worm center
Top of ring: 0.807 每 0.150§ = 0.657§ at ring center
Centers difference = 0.001§
The worm top surface measurement appeared consistent whether done on the left or right side of the brass worm teeth in the central area, thus no tilt.
RA drive after reassembly:
Top of worm: 0.846§ 每 0.200§ = 0.646§ at worm center
Top of ring: 0.805§ 每 0.150§ = 0.655§ at ring center
Centers difference = 0.009§
Note the top of the worm measurement had dropped by 0.010§, which appeared to be too much. I also saw that my original blocks now in use had a bare aluminum bottom, while the newer blocks I bought later were anodized black on all surfaces. So, it would appear all worm blocks are not identical. To remedy this centers discrepancy, I bought some 0.005§ brass shim stock which I trimmed to fit under the blocks and folded one time to get twice the thickness.
RA drive, second assembly after shimming:
Top of worm: 0.856§ 每 0.200§ = 0.656§ at worm center
Top of ring: 0.805§ 每 0.150§ = 0.655§ at ring center
Centers difference = 0.001§
This difference is now the same as originally, though the worm is 0.002§ lower than before disassembly.
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Now for the Dec drive:
Before Disassembly:
Top of worm: 0.856§ 每 0.200§ = 0.656§ at worm center
Top of ring: 0.806§ 每 0.150§ = 0.656§ at ring center
Centers difference = 0.000§
After Dec Drive Reassembly:
Top of worm: 0.850§ 每 0.200§ = 0.650§ at worm center
Top of ring: 0.805§ 每 0.150§ = 0.656§ at ring center
Centers difference = 0.006§
Here I＊m not sure of the reason for the worm drop, since the same blocks were used. One layer of 0.005§ shim stock was used under the blocks and new measurements were performed.
Dec drive, second assembly after shimming:
Top of worm: 0.854§ 每 0.200§ = 0.654§ at worm center
Top of ring: 0.805§ 每 0.150§ = 0.656§ at ring center
Centers difference = 0.002§
Again, the worm center is dropped by 0.002§ from original position.
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So finally, the question is what difference between worm and ring gear centers really plays a factor? I think 0.01§ off seems significant and needs to be adjusted. After shimming, the worm centers are very close to the ring center but have dropped a tiny bit and are essentially on the other side of ring dead center. During previous mount rebuilds, I had never made these measurements, so I do not know what they were when the mount was new. I would assume the scoring I saw on the thrust bearing washers would drop the ring center a bit, but all seemed to return to the same position with the new thrust bearings and washers.
So again, how close is good enough and will the worm being centered higher or lower on ring gear amount to anything? Since my blocks are fixed, the positions should remain the same. With the floating or spring-loaded worms, I assume the relative positions can vary by some degree. Now waiting for some clear skies to check out the performance of the above modifications.
Any comments appreciated.
Thanks,
John
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