Fixin' Stiction

[Guest HowieG]

I've had an Orion XT6i Intelliscope dob for years and have used it more or less happily.  I have never been intensely thrilled with the azimuth bearings, however.  The motion was heavy and sticky.  I tried the usual small tricks over time such as ArmorAll and Turtle wax.  These kind of worked, kind of temporarily, but I always had more important things to do, so I never delved deeply into a better solution.  Now that I am retired and have some time to piddle around, I thought I would look at the situation and come up with a solution.

The standard azimuth bearings on this dob are three round PTFE pads of approximately 1 square inch each.  The weight of my upper mount/OTA is approx. 23 lbs without an eyepiece installed.  That gave a pressure on the bearing pads of more than 7 lbs per sq. inch.  This seemed with a bit much.  This dob is an intelliscope, which means it has a magnet wheel and encoder board mounted in the center of the base plate with the center bolt going through them.  This precluded installing the usual "milk jug" washer solution.

After looking over the situation, I decided the way to go was to install some extra bearings as close as possible to the center, but far enough out to miss the encoder board.  I also decided to keep the factory installed round azimuth pads.  I measured this pads and came up with the unusual thickness of 0.262 inches.  One of the pads was actually 3 or 4 thousanths of an inch over that dimension, but a little work with a flat file got it level with the other two.

I then obtained a block of virgin PTFE from Ebay that measured 1" X 1.4" X 4".  If you have never handled bulk Teflon, this stuff is dense!  My little block weighed a half a pound.  I decided power was the best way to cut the stuff and proceeded to set up my radial arm saw for the job.  I used a fairly thin 80 tooth general purpose blade and a wood block for a stop.  After measuring where to clamp my stop, I proceeded to cut my PTFE block up into slices of the proper thickness ( I hoped!).  I actually ended up with a range of slices that were plus or minus of my target dimension, but three were the 0.262" I was looking for.

I pilot drilled my three new bearings and counter sunk the holes with a 1/4" diameter countersink bit.  I then screwed the bearing in place on the base board in the chosen locations using #6 countersunk phillips head wood screws.  Please see the attached picture of the finished product.

Calculations showed that the bearing pad pressure is now just a touch over 3 psi, and the telescope now turns beautifully.  I think the comment generally used is "like butter".  In retrospect, if I were to do this again, I think I would simplify things by removing the factory bearing pads and installing six new pads of 0.250".  Obtaining PTFE in that dimension is easy and I doubt the encoder wheel and board would mind the small change in dimension.

I hope this helps some folks out if their intelliscope dob stiction is bugging them.