NexStar GPS Azimuth Motion Rough
Cleaning the Roller Bearings
On the NexStar 8/9.25/11 GPS mount, the upper base
rotates on three roller bearings consisting of steel wheels riding on an
aluminum track in the lower base. The inside (toward the center of
the mount) edge of the wheels needs to turn slower than the outside edge of
the wheels (and incrementally in between) for both edges to actually be
rolling along the aluminum track. That is physically impossible; some part
of the wheel will always be sliding along the aluminum, not rolling. For
example, if the outside is rolling, the inside is being forced to move
faster than it should and it is sliding on the track. Steel sliding on
aluminum will scrape a little aluminum off the track as time goes along and
it accumulates as rough spots on the wheels and track resulting in a rough
"rumble" that you can feel when you rotate the scope with the clutch
disengaged. This also inhibits smooth tracking and manual slewing with the
hand control. The engineers came up with a bearing that is rock solid, even
on a wedge, but they neglected to think through this little detail.
 Both
the wheels and the track should be smooth and clean. Fortunately, cleaning
the wheels and track is not difficult. Release the altitude clutch, move the
optical tube perpendicular to the fork arms, and reengage the clutch. Using a
3.5mm or 9/64 inch hex wrench, completely loosen the five screws securing the
plastic base cover - the cover with the Aux, Auto Guide, and PC ports. It is
not necessary to remove the screws as we will only be lifting the cover, not
completely removing it. If you do remove the screws, note that the one in the
back is shorter than the rest.
Carefully lift the base cover. Wires run from the cover to
the base and only allow the cover to be raised about 2 inches (5cm). Put a
rolled-up sock in both gaps between the cover and each fork arm to hold it in
place. Dampen a lint free cloth (not tissue!) with alcohol. Do not make the
cloth so wet that you can squeeze liquid out - the excess alcohol could rinse
away the grease on the bearings.
Start by
cleaning the wheels. Release the azimuth clutch. Hold the damp cloth to one of
the wheels and rotate the base. Be careful to prevent the cloth from catching
between the wheels and the base. After you clean the wheels, clean the track.
Find the largest opening leading to the track and clean the track while rotating
the base to gain access to the entire path.
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