When TeflonŽ pads slide across a surface they are trying to follow the plane of least resistance. The Teflon rides the 'bumps' on the bearing material, usually melamine on most commercial Dobsonians, and as such the force applied can be considered as a tangential to the curve of all of the 'bumps'. If you wish your telescope to perform better in the Azimuth motion, there are two things, or a combination, that you can try. Firstly if you have just the melamine finish, try polishing the tracking area with ArmorallŽ. Not just a swipe like you do to your dash, but a good hard application of two or three coats. The second thing you can do to improve your bearings is to put a disc of Ebony Star Formica (a WilsonArt product) where your melamine surface is (use a contact adhesive like Kwik Grip) and you will have an incredible bearing surface. We talk about this in terms of "sticktion". See Richard Berry's article in Amateur Telescope Making Volume 8 page 26 entitled "How to Control Friction in a Dobsonian Telescope". An excerpt:
"We will now find th azimuth component. Let Rz be the radius of the azimuth bearing. Wz be the combined weight of the telescope and rocker, and A, the altitude angle of the telescope. If L is the length of the telescope between the side bearing and the eyepiece, then Lcos(A) is the moment arm of the eyepiece about the center of the azimuth bearing. Given the coefficient of friction -f - between the materials of the bearing then fWz/3 is the force necessary to move one bearing pad in a straight line, and fWzRz is the torque required to rotate the bearing." If you can follow this then you will really understand the article and how important this "sticktion" is to the ease of 'push'. Great article, sorry the magazine is no longer available. I am prepared to answer any question anyone may have about this! The 27" used to rotate pretty well, jus the vibration of the trusses didn't help!
Ive heard many reports that armorall is about the worst thing you can use, with terrible "stiction" being the result.
"stiction" is the difference between static friction, (force required to start moving) and dynamic friction being the force to keep moving. Stiction causes overshoot when making small movements. You push, the scope jumps and then overshoots.
Never had that problem. Could be that the person using didn't buff the surface enough after the application. Teflon can act as a slippery surface and it is also important to have the pads at the correct size for the weight of your telescope. Yes you are right, the force you apply to the telescope has to overcome the static friction and then the telescope accelerates until a linear velocity is reached that is sufficient to resist the bearing force. As soon as you drop your 'push' pressure the static friction takes over and the whole thing comes to a juddery halt. I quote from Berry and Kriege" The reason Dob buuilders like virgin Teflon and Stardust, Ebondy Star, or glass board laminates so much is that for these particular materials, the coefficients of static and dynamic friction are nearly identical when the surfaces are dry and coating the bearing surface with silicone car wax reduces the differences to zero. There is no 'pop' when the telescope starts to move, only the 'buttery" slide of a true Dobsonian.'
Be aware NEVER use Armorall on your altitude bearings! They will squeak at you all night long!
Last edited by Roger Davis; 21-09-2005 at 01:59 PM.
Reason: Ooops, extra info
Why bother with all this?
Tracking planets and other targets at high magnifications then becomes simple instead of an exercise in frustration.
Another factor rarely mentioned is that its very desirable to match the frictions of both alt and az bearings so one can push diagonally for easy tracking.
Why bother with all this?
Tracking planets and other targets at high magnifications then becomes simple instead of an exercise in frustration.
Another factor rarely mentioned is that its very desirable to match the frictions of both alt and az bearings so one can push diagonally for easy tracking.
Why do we bother with all this?
Simple, you asked for advice from the experienced. Some of us have been doing this for a long time and have been there and done that.
I have lost track of how many Lazy Susans I have pulled off scopes and replaced with teflon and ebony star.
Bearings are fine if you are motorising, the motor acts as a brake, but visually teflon make life easier.
Wait for the first breath of wind with a lazy susan and you will discover why Dobs built that way were dubbed as windvanes!
Lazy Susans also attract dust into the bearings, the edge of the teflon pad acts as a scraper for any dust that can fall "upwards".
Next.....
Another factor rarely mentioned is that its very desirable to match the frictions of both alt and az bearings so one can push diagonally for easy tracking.
As Above post I also did the Alt bearing. Very smooth.
I took Geoffs post as sarcasm.
If anyone can persuade people to improve their dobs motion ,I'm all for it.
I've raised the matter before. To me, to put it bluntly, the base design and alt/az motions on a lot of mass produced scopes are "pretty sucky" and some simple improvements can make viewing a lot more pleasurable.
I took Geoffs post as sarcasm.
If anyone can persuade people to improve their dobs motion ,I'm all for it.
I've raised the matter before. To me, to put it bluntly, the base design and alt/az motions on a lot of mass produced scopes are "pretty sucky" and some simple improvements can make viewing a lot more pleasurable.
I wasnt trying to be sarcastic. Maybe I was stating the obvious but my comment was intended for anyone who stumbled across the thread not knowing why we might bother to do these things to a dob.
Quite right about the movements of mass produced dobs, and maybe many of the people who buy one and then wish for an eq mount, would be happier knowing how to fix their dobs motions.
Loved reading all the info guys (both here and on the GS Dob Help thread). Thanks! (though I'm as confused than ever)
I need to do something about my Dob, because I want the motion much smoother than it is now. But I also want it very steady (not a bumpy ride), so that when motorised it will trace a nice clean arc in the sky. Laminates, vinyl records, lazy suzan bearings, I'll have to look into all of the above.
If you can follow this then you will really understand the article and how important this "sticktion" is to the ease of 'push'. Great article, sorry the magazine is no longer available. I am prepared to answer any question anyone may have about this! The 27" used to rotate pretty well, jus the vibration of the trusses didn't help!
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