----- Original Message -----
Sent: Wednesday, October 01, 2003 9:02 PM
Subject: Re: [M]: Re: Bad PEC?


> >
> >.. In the E/W direction, you really want the scope to
> >be slightly unbalanced, and ideally, heavy to the east (by default the
> >scope
> >is slightly 'west' heavy). This normally involves adding about 2.5lbs to
> >the
> >east fork arm. Basically without this, the scope can have a tendency to
> >'run
> >ahead' of the gears to the west, then stop, till the gear tooth catches
up,
> >then when pushed by the gear, repeat the action.
>
> If it's doing this, it means the scope is not unbalanced far enough in the
> _west_ direction, so you can fix this by adding a little weight to the
west
> arm. Then the scope will also be pulling against the gears all the time,
> just in the opposite direction of what happens if you add extra weight on
> the east fork.
> -John
There are two 'sections' to the drive on the LX200 (and most worm gear
driven scopes). The first section, is the spur gearbox, between the
motor/encoder, and the worm, and the second is the worm gear/main wheel
junction. Whichever way you 'load' the scope, it will try to take the slack
out of the worm/main junction, but because a worm, generally does not
transmit drive 'backwards', and because of a little bit of physics regarding
sliding motions (the coefficient of dynamic friction, is generally lower
than the coefficient of static friction - this is why a lot of motions
exhibit 'stick/slide' behaviour at some scale), the overall effect on the
gearbox differs. In the case with the load to the west, the load must be
enough to overcome the tendency to 'stick', of all the bearings, and the
gear, and keep the scope 'running ahead' of the gears to the western limit
of all the slop. However with the load to the east, this is not necessary
(all that is necessary, is sufficient load to ensure that the gears remain
in contact when stopped). The difference is major. On my own scope, I could
get smooth tracking with an weight of about 2.5 to 3lbs, to the east (this
corresponds to an imbalance of only a few ounce/feet, because the west arm
weighs at least a couple of pounds more than the east arm), while to get a
similar behaviour with a load to the west, required nearly as much weight
added to the west arm, resulting in a total 'imbalance' of perhaps 3lb/feet.
The motion was similar, but the power 'bar graph', showed how much more the
scope is having to work in the latter case.
Hence the point is that in the case of the LX200, you can work well with
less imbalance to the 'east', than to the 'west' (though probably slightly
more actual weight is needed when working with an 'east' imbalance). However
in general, the mount must either have enough 'east' or 'west' weight to
ensure the gears remain firmly against one limit or the other during normal
operation.
This differs from some (most!) other mounts. For instance, on a G-11, the
guide speeds, never actually 'stop' the motors, and the slop in the gearbox
assemblies is far less, and the scope will move accurately, with no
imbalance at all.
So I stand by my recommendation to load the scope 'east' for an LX200.

Best Wishes

> Adding the weight on the
> >east arm, results in the scope being _gently_ pulled against the gear
teeth
> >all the time, and often results in much smoother motion (to the point of
> >making a massive difference in how the scope behaves). This is a
> >necessary/worthwhile fix, whether guiding or not. ..
> >Best Wishes

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