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Subject: [M]: RE: another lx200 bearing modification
From: Michael Hart
Reply To: mapug@shore.net
Date: Mon Mar 16 00:33:53 1998

On Sunday, March 15, 1998 3:43 PM, Bobby Middleton wrote:


First, I want to thank Bobby for providing details of his experience
with this project. I tried to anticipate what an average skilled
do-it-yourselfer would face, however, I realize that certain skills
I take for granted may need to be addressed in greater detail.

Where I use the words "your" or "you", I'm speaking as if I'm
responding to a comment or question one might receive from a
group during a speech or lecture, rather than directly to Bobby.
The group is, of course, MAPUG.


> I did the Michael Hart bearing modification to my 10"
> lx 200 yesterday. Initial results look good and I believe
> it has solved my dec retrograde motion problem. I have been able to
> reduce my backlash compensation from 85 to 30. I have not had a
> chance to check what my alignment will be like and it may be
> a few days according to the weather forecast.
> The conversion was not without incident; however, and I will
> tell my experiences for the benefit of others considering the
> modification.

> The first problem I encountered was the removal of the 2
> Allen machine screws holding the inner clutch plate. I found that the
> recommended time of heating the 2 screws was not enough. I was
> using a small propane torch (for soldering copper water lines). It took
> about 20 secs of heat before the 1st screw gave way. The second screw
> did not give after about 25 sec of heat and of course the Allen wrench
> slipped in the Allen head, rounding out the corners.
> This will probably be the most common problem in this modification.
> My screws were really held tight by the lock-tight.

I used a very hot torch inner blue flame, if you heat a screw or bolt that is
secured with Locktite correctly, it will soften the Locktite and allow screw
removal. Experience will help with knowing how much force to use and
how much not to use and when to use more heat. Heat the screw too hot,
and the screw will expand, making removal difficult. Let the screw cool a
bit before attempting removal if you find considerable resistance. The
Locktite bond will remain soft, but the screw metal will return to optimal
strength and size.

For those will limited mechanical experience, practice on a similar sized
screw secured into long threads tapped in a piece of metal, finally primed
and secured with high strength Locktite (red). The shear strength of this
Locktite will be very high. Tighten a few up until damage occurs to
learn how much torque will shear the screw head or round the Allen flats.

If you have a driver drill with a clutch, you can set this to act a mini
electric impact wrench to remove a stubborn screw. I find high a high
quality Allen wrench pressed firmly and squarely may be all you need.

Still, an out of spec Allen head screw or one that was damaged or over-
torque on installation could produce Bobby's results in spite of all his
efforts, we may need to refine the text on this based on his experience
so that others can minimize the possibility of having problems.


> I wound up going to a machine shop and drilling out the offending
> screw and re-tapping the screw-hole. In retrospect, and after examining
> the scope more closely, I believe the best thing to do if this occurs would
> be to remove the scope/west fork arm from the base and weld a small
> nut to the frozen screw head. The scope must be removed from the
> electronics in the base to eliminate the voltage danger to them.
> A wrench can then be applied to remove the screw.
> Or would it be possible to braze weld a nut on the stainless steel
> Allen screw? If so, base removal would not be a concern. I'm not a welding
> expert so I'm not sure if there is a usable method.

The flat head Allen machine screws are stainless steel. It is difficult
to weld so small a surface with common techniques. I believe your
machine shop did the best thing- extract the offending screw.


> These Allen screws are common ones and
> can be found at most hardware stores. They are 8-32 3/4"long
> countersunk. They can also be found with torx ends instead of the
> Allen heads; I believe the torx ends may stand up to pressure better
> w/o giving way.

You are correct that the torx head screws will handle more torque.
Once existing Allen head screws are removed, torx head screws
could be substituted if you anticipate future removal. Torx head
screws are usually made of high carbon steel that is more resistant
than stainless to rounding. If the existing screws are removed, only
moderate torque is needed upon installation as primed Locktite
easily retains these screws and the rear Dec clutch plate.


> One other option would be to just drill off the Allen screw head so
> as to allow clutch plate removal. then once the OTA is off the forks, the
> dec shaft plates can be switched. They are identical except that the east
> plate shaft has no screws in it, only the center bolt which holds the dec
> circles.
> Switching these 2 plates allows one to put the west plate with the
> unremovable
> frozen screw on the east side where the 2 outside screw holes are not
> needed.
> The east plate moves to the west side with all 3 screw holes available for
> use. A potential alignment problem is created by doing this and one needs
> to score a mark so as to re-mount them in their exact location. Be cautious
> when you re-tighten the 2 Allen screws down into the slots of the dec shaft
> plates. It's pretty soft material and mine had been pulled through to some
> extent just by the factory tightening.
>
> The next concern in the rebuild came as the new bearings were mounted on
> the shaft. I was quite surprised and concerned (actually scared
> you-know-whatless!)
> to see there was a small amount of play with the bearing and shaft.

If you measure the shaft and get close to 1.000", the bearings I
recommended will be optimally sized to allow the clearances needed to
prevent seizure. Don't get picky here. Production machining can and
does vary a bit. The softer aluminum shafts MUST have adequate
clearances to allow for expansion and lubricant. With the nylon factory
bearings, a lack of clearance will not result in shaft damage rather, it
will stress the Dec motor and electronics. If you damage the optical
tube shafts, all is not lost, just have them sleeved or turned down
to the next bearing size.


> I used the recommended bearings from Mr. Hart's write up and had
> the hole on the fork arms bored to right at 1.311" for the 1.313
> OD bearing. It was a very tight fit and I was even concerned with the
> cast aluminum fork arm standing up to the pressure of the press fitting,

Installing a bearing that is 0.002" over bore size may need some help
via bearing cooling/slight heating of the casting, a bearing press or
have the machine shop do it for you. For our application (extreme low
speed), you do not need so tight a fit. If the methods don't appeal
to you, buy a $15 brake cylinder hone and open up the bore a bit, prime
and set with meduim strength Locktite which will fill any small gaps and
act as a lubricant during bearing insertion.


> so I don't think my holes were over-bored.
> Would there be bearings available with a slightly smaller ID (few thous")
> so as to fit the shafts tighter? I would guess not.

Yes, there are smaller bearings, but you may need to turn the shaft
down to the next standard size- 0.984". However, for this application,
tight tolerances are not needed. I removed 0.008" clearance from my right
fork nylon factory bearing, leaving .002". Pointing was not improved which
was somewhat expected because the optical tube weight usually maintains
pressure in one direction- at the angle of the forks to the RA axis. You
may well have decreased Dec bearing play from what was allowed at the
factory. However, for the thrust bearings, you WANT no backlash as this
WILL effect pointing. This is easily provided through the forks exertion of
pressure on the thrust bearings.

When removing the factory nylon bearings, the 0.040" on each fork is lost
by their removal and needs to be replaced by thrust washers that I described.
If you use LESS than 2 X 0.040" or MORE than that, you should know
why you are changing from factory adjustments. DON'T adjust the
thrust bearings to barely remove backlash as normal fork flexure will open
the backlash up. When adding or replacing thrust washers, replace them
in PAIRS to keep the optical tube centered between the forks. Remember,
that final optical tube alignment depends on an optical tube that is fairly well
centered in the fork.


> After complete re-assembly the dec axis seemed tight and
> no play was noticeable, but I cannot keep from being concerned
> with the play that was there between the bearing and shaft. I felt that
> there might have been enough room for a very thin nylon bushing to
> be inserted. It would be similar to the original one but have to be much
> thinner. This would probably re-introduce a little extra friction but I
> would think that the bearings would still give a big improvement.

If your shafts measure quite close to 1.000", your roller bearing
clearances are quite close to optimal. If your shafts are quite
far from 1.000" or perfection is critical, have the shafts sleeved,
or turned down to the next bearing size- 0.984". However, I doubt
your meticulous efforts probably make a difference in pointing
and tracking.


> One last thing I noticed immediately was an increase in
> vibration. It could be possible that my mount will now
> have problems in some wind that it didn't have beforehand;

I did not notice any increase in low frequency vibrations, however,
I might speculate that higher frequency Dec drive vibrations and
sounds during higher speed slews may be more readily transmitted.
In other words, I'd expect a noisier Dec motor during a slew. For
me, the decrease in stiction and resulting oscillations had a greater
effect on improving tracking than high frequency vibrations or sounds
from the Dec motor and gears. Bottom line- my images are better.

I don't follow your reasoning as to how vibrations and wind
are related, unless your referring to what you believe is
Dec bearing play and the wind, which we have addressed
earlier.

I will defer to Doc G for further comment as he has done extensive
vibration analysis on the LX-200.


> this will remain to be seen. Michael, Doc, and others,
> have you seen this effect after the modification?
> Did you have any play in the bearing to shaft mating?

My shaft was 1.000"- yes, I had play. I have a piece of 1.000"
round stock and a 1" caged roller bearing. If I rock the bearing
ends in opposite directions, I feel what could seem like a lot of
play. If your gauging play this way, it is easy to be fooled to reducing
needed clearances to the point of bearing seizure. Roller bearings
feel different than sleeve bearings because the rollers float in their
cage.


> Also, would you have additions to my means of taking
> care of the frozen dec clutch screws?

See above.


> Bobby Middleton
>
>
> astropics at: http://www.webwide.net/~bobbym/astro1.htm
>
>


--
Michael Hart
Husen Observatory
mhart@netexpress.net