Sponsored By:

Subject: Re: Declination retrograde
From: Brandon Jones
Reply To: mapug@shore.net
Date: Wed Dec 13 03:30:48 1995

> While disasterous for astrophotograhy, the effect is so minute
> mechanically that is is hard to measure and repeat exactly,
> making a conclusion difficult. However, I have become convenced
> the problem is slop in the declination motor gear-train assembly,
> possibly in the shaft and bronze bearings of the third (of four)
> gears which is connected to the slow motion mechanical knob.

Recently I have had my old 10 inch apart with both boxes out.I found
that they were over a very wide spec relative to slop in the gear
train.One was good the other was aceptable.
I think that even if you had a new one it would not be
necessarily any better.I have hand made boxes like this using a
milling machine and a dividing head in the past and it is not
possible with gears of this type to remove the backlash unless they
are of the split spur gear type that are spring loaded(not practical
in a box of this size).In the case of my two yes I can confirm that
part of the reason was due to the lousy fit of the shaft in the
central bronze bearing.However a lot of backlash can be removed by
not allowing the gear shafts to slop in their bearings either.This
was evident in some of the gears because you could waggle them
about.With the bronze bearings they should be force fitted into the
box when made so that they are compressed slightly.This makes them
shaft tight.You then run the shaft after oiling it until it polishes
the bearing to an exact fit and runs properly without slop.Then end
float which they adjust and lock by painting the bearing end is
something else entirely.I do not think that the people who make these
boxes do that.Remember the box is part of a clock in effect and the
gearbox has to very efficient in power loss in the box hence straight
cut gears.I found no real problems with the worm assemblies ie
no backlash as long as the spring tensioner was set correctly and the
worm centered properly.I disconnected the motors and ran them off
a power pack and watched what happened on reversal even at voltages
as low as one and a half volts, the motors ran quite fast running the
whole scope easily.I found that the revs increased on different worm
positions and adjusted it for the best ie lowest load on motor.The
gears on reversal can bunch up slightly for a short period but the
pulses from the control circuit should alleviate this effect by the
tapping action of the motor being pulsed.

ok the only conclusions that I got out of this was that the spur
gears machining quality could have been better for the metal gears
and more effort was needed by whoever made the box to make the gear
pinion bearings better.My experiences are from clock making and
making my own drives in the past.Not being a mechanical engineer
can any body else comment on this.

Brandon S. Jones