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RE: Why Not Teflon!

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Posted by tom@digitalmoon.net on September 20, 2000 02:18:56 UTC

rick, i see it is you again. let me see, the question is "how can the guy fix his telescope mount so that the mechanism will still operate with the heavier load placed on the bushings which are known to be nylon, after he has loaded it up with excess, but ballanced baggage? " ok, nylon although in very common use as a smoothe sleeve bearing material, is not of really low friction coefficient when run on a steel shaft. first, i assume that the manufacturer made the shaft reasonably smoothe at the bushing location. it is probably best to leave this alone. the modifications which can be made in order of ease of doing are:
shoot the trouble spot with a little wd-40
take it apart and grease it
take it apart and spray dry film lubricant on it
remove the nylon bushings and replace with teflon bushings
remove the nylon bushings and replace with sinthered bronze oilite bushings
remove the nylon bushings and replace with antifriction (ball/tapered roller) bearings

my suggestions were to try some dry film lubricant, or to replace with teflon bushings.

nylon will absorb both water and oil (swell) a little bit, and the person who`s telescope it is should be aware of this in case the thing tightens up by the applicaion of lubricant no matter what kind.

in my opinion, the imbedding of the dry film lubricant into the nylon matrix would be just what the doctor would want, it would give the substance a chance to do exactly what it is meant to do.
if he is to use a regular lubricant, grease/oil, i would recommend oil rather than grease, unless he has all kinds of technical data on the grease. grease is a matrix of soapy paste and oil, and if the oil is squeezed out, he may end up with a pasty poor lubricant over time.
bottom line, nylon will respond to any of the above treatments with reduced friction as long as the swelling does not get out of hand.

what you are dealing with here is "stick-slip" action. a telescope drive is so slow that with a sticky bushing in the mount, the rotation is really a series of jerks as the static coeficient is overcome, the drive slips to a new position, then sticks again until the drive torque overcomes the friction again. the problem is that the slip portion occurs under the sliding coeficient of frction, which is much lower, but this eventually stops, and places the drive in the static friction mode again. to conquer a thing, you must understand it. who said that ?
anyway, on the subject of teflon, here is a story.
in about 1969 i noted a thing in the paper that said that telescope making classes would be given at a certain time and addres in mill valley, ca. this was near me, so i went. the guy who showed up was a 50 year old hippy with a poney tail, and a ravishing brawless hippy girl helper. he was all enthusiasm, a very inteligent,well educated and amiable guy. he said that he had just escaped from the catholic ministry or something like that, and proceeded to tell everybody how easy it was to build a really viable astronomical telescope. first of all, go to market street radio surplus in sanfrancisco and buy a couple of port hole glasses, and some chunks/strips of teflon. you could choose your porthole glass from 6 foot high stacks of the stuff from about 6 inches to 24 inches in diameter, you choose. then you buy a small amount of grinding/ polishing/pitch from him, and you are on your way to getting a sonotube, etc. this guy was john dobson, they called him" telescope john " and lots of people were inspired by him to do something real for the first time in their lives. i had peered with fascination through telescopes on the streetcorners of sanfrancisco as a boy, and i suppose he had also. john dobson was responsible for creating the "sanfrancisco sidewalk astronomers" and now down in l.a. market street radio surplus was one of my favorite hangouts on saturday mornings, and i had seen the porthole glasses and teflon chunks before.

anyway, the story above was inserted to tell people a bit about telescope john , he is a really neat guy, that they might not not have known, and to show that teflon can be had in chunks.

teflon is available in sheets up to inches thick by 4 feet square, and in round rod up to about 6 inches in diameter, and lots of shapes in between. it has an unbeatable friction coeficient ( 0.04 on a smooth metal surface), so good that lubrication does not help. it can be machined easily into about any shape desired. although "virgin"teflon has a very low bearing strength compared to most other bushing materials, and is rarely used that way, (only good for 500 psi bearing load), if the application is low stress/low speed, it performs well, as in telescope john`s mounts. i am aware of one solid teflon toilet seat. because of its low load bearing capacity, teflon is usually found in a fabric/ other plastic/bronze matrix. it is strong enough for this application tho. i have been cut off again, must be time to stop. point of interest, teflon has no known solvents. tom leech

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