Date: 04 Oct 2006 03:16:54
From: Uncle Bob
Subject: Losmandy GM-8 RA axis issues (longish and technical, sort of...)
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Greets--
Now that I've replaced the focuser on my MN-66... ;-)
It's time to turn my attention to my GM8 mount, which has been giving me
peculiar images (unguided) of stars. Sometimes dumb bells, sometimes
L-shaped. In an effort to find the problem, I completely disassembled the
mount. Took me all evening. Parts spread all over the kitchen.
I concentrated on the RA axis, where I found the main RA shaft (which
carries all the load of the mount) is .006" smaller than the ID of the
needle bearings (nominal 1.25"). This is a lot for a bearing-to-shaft
tolerance, especially when the shaft serves as the inner race for the
needle bearings (which are hardened steel, btw. So why is the shaft made
out of aluminum?).
I put it all back together and noticed that when viewing a star at
medium power, if I tighten the clutch, the star moves in the field of view.
Hardly acceptable behavior in a "quality" mount. In fact, with the clutch
loosened, you can grab the counterweight bar and rock the entire RA
assembly up and down.
At a star party last weekend, I met with another member of our club who
let me check his GM-8 and I found the same apparent issue.
Did a little research on needle bearings and found (from McMaster-Carr)
that they usually have a .0005" (note 3 leading zeros) fit to a shaft.
That sounds more like it. I ordered some 303 stainless in 1-5/16"x12"
round as material and am having a machinist friend duplicate the aluminum
RA shaft but with a 1.2495" outside diameter. This will eliminate all the
rocking. The stainless is more durable than the aluminum (although in my
example, there was no apparent wear on the aluminum shaft).
I'm wondering if this could explain some of the tracking/PE issues
that some users are reporting with these mounts...
In my limited experience, two of two examples exhibited this problem.
If you own one, check to see if you can rock the RA axis up and down with
the clutch unscrewed. What happens with your mount? And are the G-11s
also suffering from this issue? I can't remember if mine did (I sold it a
few years back).
BTW, cost of the replacement shaft is about $30 for materials and $150
for the machining operations.
Worth it if it gets the mount to track well.
Clear Skies,
Uncle Bob
Fairfax, CA
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