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Subject: RE: [M]: LX200 Dec Rebuild- Performance Results 3 Months Later
From: Michael Hart
Reply To: mapug@shore.net
Date: Mon Mar 30 01:31:45 1998

On Sunday, March 29, 1998 8:05 PM, Philip Perkins
[SMTP:philip@astrocruise.com] wrote:

> Michael Hart wrote:
>
> > Those that are satisfied with their current drive performance and/or
> > who have limited mechanical skills may want to postpone this project
> > or seek assistance. In addition, the significant friction reduction
> > requires careful attention when the Dec clutch is released.
>
> Be aware that mere binding of the Dec clutch can lead one to a strong
> belief that the Dec axis is binding and lead one unnecessarily into this
> fairly major construction job. For the 'mechanically challenged' such as
> myself who do not relish the prospect of unnecessarily tearing down their
> Dec drive assemblies let me relate what happened:
>
> For more than a year after acquiring my LX200 I worried increasingly about
> the large amount of binding in the Dec axis. It was very tight, and I became
> convinced that I had a major problem on my hands. Then, almost a year ago
> today, my Dec motor started playing up - it would intermittently stop during
> slews. Since I was in the middle of an astrophotography expedition in the
> south of France I had no option but to dig into it myself (I was totally on
> my own 800 miles from home).
>
> The first thing I did was to remove the Dec motor assembly to carry out a
> close inspection. As soon as I removed the Dec motor assembly I was almost
> shocked by what happened. The OTA suddenly swung free. It became
> incredibly free, as though there was no friction at all in the Dec axis. I
> checked the OTA throughout its range of travel, and it was the same.
>
> (I managed to patch up the Dec motor by extremely careful adjustment of the
> fixed encoder mask in the reduction gearbox. Through wear in the motor
> shaft I think that the rotating encoder was 'wobbling' on its axis and
> therefore intermittently misaligning with the fixed one. This was after
> three years of hard use).
>
> As soon as I replaced the Dec motor, the large amount of friction in the Dec
> axis returned. In other words, the friction that I was experiencing was
> *entirely* due to the clutch binding and had nothing to do with friction in
> the actual Dec axis.
>
> There is a very interesting conclusion to this:
> Friction caused by a binding clutch is *cosmetic* only. The only purpose
> of the clutch is to lock the motion of the OTA to that of the worm wheel.
> The clutch has no effect whatsoever on the inherent freedom of the Dec axis
> or the ability of the Dec motor to drive the dec axis. Bear in mind that
> during normal operation the clutch is locked in any case.
>
> The simple moral is this: before becoming convinced (like I was) that you
> have a major problem with a binding Dec axis, do one simple test: disengage
> the Dec motor from the worm wheel. You may well find (like I did) that you
> have no problem at all.
>
> Regards,
> --Philip
>
> Philip Perkins -- philip@astrocruise.com
> 51 27'N 1 36'W -- Wiltshire, UK
> 43 54'N 5 32'E -- Luberon, France
> Astrocruise -- http://www.astrocruise.com
>


Philip's experience shows the importance of understanding the mechanical
relationships of various components. If one does contemplate the Dec
rebuild procedure, the very first step in this procedure after obtaining parts
is to remove the Dec drive assembly which removes the effects of the Dec
clutch. My factory nylon bearings had optimal clearances, lubrication and
align bore with the optical tube shafts. My Dec clutch with roller bearings
require 9 grams of unbalance to displace the tube. If one removes the Dec
clutch contribution by removing the Dec drive assembly what is left is
friction/stiction attributed to the Dec bearing surfaces. However, if a more
viscous material was used on your clutch faces, the Dec axis may require
much more unbalance to displace the optical tube. This makes the bearing
contributions feel quite low by comparison with the added clutch resistance.
What feels as quite low may not be so.

Now is the time to measure the unbalance needed to move the tube with
the Dec drive removed. This can be compared with the RA axis to gauge
the potential for improvement on your particular scope. Measuring this
requires careful initial balance, then adding weight until an unbalance
state occurs. A laser attached to the optical tube projecting against a
wall is a good indicator of when the unbalanced state exists. Bearing
stiction can cause the needed unbalance weight to increase with the
time it takes for the bearing surface to become deformed. When bearing
stiction is virtually eliminated and the Dec clutch attached, adding a few
grams of weight will start an extremely slow tube movement as the
Dec clutch grease shears between the clutch plates which may result
in what first appears as inadequate weight.

Doc G has quantified Dec friction/stiction reduction in an earlier post about
his 10" in which he measured the force required to displace the tube with
an unbalance was 10 grams as compared to 150 grams before the mod.
If you are satisfied with this amount of friction/stiction, you may wish to go
no further at this point. I believe heavier scopes such as the 12" loaded
with accessories are more likely to exhibit bigger improvements as stiction
is reduced. A scope that produces excellent results on film with reasonable enlargements is likely to be entirely adequate for that purpose. When that
same scope is used as a 3000 mm platform for CCD imaging, results may
be less spectacular.

The Dec clutch should exhibit resistance between the clutch faces that
are coated with grease. In fact, as a safety measure, those that are
concerned with the very low friction that will exist after the mod may want
to consider replacing the Dec clutch grease on both faces with a more
viscous material up to and including a mixture of silicone grease used in
fluid head camera mounts to achieve controlled resistance.

There is no end to extent of the modifications one can do to a telescope.
I believe that if certain problems are traced to specific causes, one has
but two choices- live with it or fix it. It is easy to spend considerable time
perfecting equipment that works fine as is, however it is also easy for
nagging problems to effect the consistency of results. For many of us,
available observatory time and/or ideal imaging conditions are limited.
There are those whose primary objective is improving or perfecting the
mechanics or optics. For others, taking pictures or visual work is desired.
If a perceived problem is quite cosmetic and does not effect your results
or minimize or eliminate an important variable, I believe the effort is best
directed at your primary purpose for owning a telescope.


--
Michael Hart
Husen Observatory
mhart@netexpress.net