Losmandy One Piece Worm Block w/Ceramic Bearings

   Based on others experience having reported good results with ceramic bearings reducing the '76 sec' error I decided to give it a try and ordered Kit 8421 from VXB. At $39 each and using 3 bearings, it was a $120 experiment. I expected good results after reading some of the details on how these are made and their low rolling friction characteristics but was surprised how bad they performed. Below is only a small sample of the several tests conducted over a few nights to try to make these expensive bearings perform at least as well as their steel counterparts.

Note: PemPro does allows you to fine tune the PEC profile by adjusting drift and delays. I've included drift fitting screen shots to show 'none' and the best fit for comparisons.

Ceramic bearings tested with McLennan gearbox and Ruland coupler.
(Clicking on the images will open a larger version to better see the details)

VXB full ceramic and steel bearings side by side. Ceramic bearings installed in worm blocks
PemPro graph shows a large swing in PE compared to the steel bearings. Notice the scale resolution.
The plot pattern also takes on a sinusoidal shape.
The frequency spectrum clearly shows the 76 sec error curve still present and at 3.6 arc seconds peak-peak.
Also high is the PE coming in at a whopping +13 -13 arc seconds peak-peak.
A magnified view of the 3rd fundamental at 'B' which is the 76 error.
Note: The fundamental at 'C' has been eliminated with the McLennan gearbox that was used for this test.
The unfitted PE curve. Notice the high periodic error values.
The 3rd fundamental value in the FFT Waveform Analysis section shows a value higher than the steel bearings.
This shows the 3 ceramic bearing did nothing to reduce this error but caused a big increase in PE.
Selecting a cubic fitting produced the lowest RMS error The best fit histogram is spread out wide instead of a narrow bell curve.

Worm Block with 2 steel bearings reinstalled in the coupler side block and 1 ceramic bearing on the thrust side.
   This is a test of using only one ceramic bearing on the thrust end (opposite the coupler).
The two steel bearings were reinstalled in the block on the coupler end.

Five passes were take for analysis. Note the high PE but smoother slopes. The spectrum plot still shows the 3rd fundamental near 'B'.
The FFT analysis shows the 3rd fundamental has been reduced but is still higher than the steel bearings. The periodic error is still high.
What is interesting is how just the one bearing that takes the thrust has a big impact on PE performance.

Measurements with increased bearing preload.
   The 2 mounting blocks were moved inwards to preload the bearings more. This reduced the '76' error a lot though PE was still high.

Here the preload was slightly increased and PE measured. With more preload, the PE is still high but the '76' error has been reduced to near seeing conditions. This is a big improvement.
With no fitting applied FFT values show the 76 error at the 3rd fundamental has been reduced significant with more preload. A linear fit produced the best RMS values and did lower the 76 error more but made a slight increase in periods 2 and 4.
PEC profile created and set to a 0.7 sec phase delay before uploading to the mount. Another measurement run with PEC enabled. Total PE is improved but is now at the levels the steel bearings produce before PEC is applied.
The 76 error with PEC enabled is still there but very low. Fine tuning the PEC profile with PemPro by adding the first and second PE runs and uploading the new PEC profile using a best fit Quadratic fitting for the lowest RMS, the overall periods haven't changed much from the previous PEC profile. This is indicative of being near the seeing limits of the sky that night.
With PEC fine tuned the '76' error and overall PE is much lower compared to the initial run that showed very high PE of +12 / -11.5 arc seconds. Fine tuned PE performance shows a more tolerable PE profile using the ceramic bearings. More measurement cycles would have been better but the clouds decided to roll in.

  This was a big disappointment as I had better expectations. I can't recommend these bearings based on the test results shown but to be fair, these bearings had no ABEC identifications so at the least they were ABEC-1. Maybe a higher ABEC rating of 5-7 would improve it more as I suspect it will but those bearings are around $80 each and I don't want to risk another expensive mistake.
What was interesting is two different bearing types and still a small amount of 76 sec error (not occurring at the 80 second fundamental mark) is still present. What do the two bearing types have in common? Both have 8 balls in the race and designed for radial loads. See the section on '76 sec error investigation' for more info.

© 2010 Michael A. Siniscalchi