Smooth Movement on Dob Bases

[Roger Davis]

When Teflon® 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 Armorall®. 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." :zzz: 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!

[iceman]

Great advice Roger, i've moved it to the equipment forum though where it's more appropriate!

I've used furniture polish on the bottom of my base plate to help make it smoother, will try some armorall!

[ving]

thanks roger! :)
armour all is great :thumbsup:

[Starkler]

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.

[Roger Davis]

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!

[davidpretorius]

this has worked at treat!

[janoskiss]

David, what is it, and where did you get it?

[Starkler]

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.

[slice of heaven]

:thumbsup: :)

[davidpretorius]

steve, a lazy susan bearing $20 from most engineering shops

[Roger Davis]

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.....

[Dave47tuc]

Hello Roger :D
Next....
Well my mount has a ring see below that rides on the teflon pads, very smooth.
For those at Snake Valley camp, see for yourself :D

Similar to what you have advised Roger :thumbsup:

[Dave47tuc]

As Above post I also did the Alt bearing. Very smooth. :thumbsup:

[ballaratdragons]

I did the Mod back in May this year! :thumbsup:

See here: http://www.iceinspace.com.au/forum/showthread.php?t=1959&highlight=super+sliders

Laurie also mentions the use of 'Ebony Star' back then.

[slice of heaven]

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.

[ballaratdragons]

Hear Hear! Well said Slick!

[Starkler]

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.

[[1ponders]]

Rhetorical?

[slice of heaven]

Sorry Geoff , I misinterpreted your post. :ashamed:

[janoskiss]

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. :D

[ballaratdragons]

Your confused Steve? Imagine how the newbies feel! :o :confuse2:

[Orion]

I used Formica (Stardust) and 3mm thick Teflon pads cut to the right size by calculating the weight of the telescope and I have never had any problems. With a little car polish and a bit of armorall it makes the telescope move beautifully!

[Starkler]

I found after working on improving the azimuth motions that the alt needed some work.
The gso dob moves much nicer if you can balance the tube up and go without the springs on the alt hub. I used rectangular lead divers weights, nice and cheap :thumbsup:

[slice of heaven]

I never fitted the springs as I did the same as you and tuned the balance of the OT instead from day 1.
Then I added foam strips to the alt bearing supports to match the movement to the az motion.

[RAJAH235]

I still reckon that 'clean & dry' is the way to go, especially if you have 'Ebony Star' installed. Anything you put on a surface will attract dust & grit.
I also changed the position of the alt bearing teflon, (up in the D), to give a little more stiction, as it was too easy to move. (Cheap Meade plastic bearings/pads too far down?).
One saving grace that Meade did, was to include 50 mm? x 1 mm? Teflon disks for the centre pivot bolt. I believe you all use 'milk bottle washers' for this?
All I can say is, try it. You can always, well mostly always, rectify it if things go wrong.
We can but post, what works for us.
HTH. :D L.

[davidpretorius]

anyone tried wd40 on the teflon on the az??

i did a couple of nights ago and very happy. not sure of long term effects.

If people think it is ok, then i will bring a box of samples of wd40 that have been sittingon our roof at work for years and hand out at star camp if anyone is interested!

[ballaratdragons]

Dave, the problem with WD40 and like products is that they trap dust and eventually turn to Gluck! Dry lubricants are better.

[davidpretorius]

good point, anybody want the wd40 cans for anything else????

could be a flame thrower to light up the area whilst viewing!!!

hmmmmm reminds me of a thread????

[ballaratdragons]

Those cans of WD40 are excellent for anyone that owns a car. In other words almost everyone needs one :lol:

[Starkler]

Why am I resurrecting this thread?

I have done a bit of work to my dob to see if i could improve the motions.
I ditched the rather tall magic sliders I was using and have applied a pimply laminate to the underside of the rocker box, and going back to the stock pads as supplied.

I was playing with milk bottle washers and couldnt get it right, one extra would make the scope wobble but without it still not enough weight taken on the centre to reduce friction to an acceptable level..... until I tried waxing the laminate and altitude hubs with turtle wax.

Wow what a difference :eyepop:

The scope is moving beautifully now :D I just hope that lasts.
The wax is a dry substance so its not going to attract any grit.

[cjmarsh81]

I tried the milk washer mod myself and found the same problems as you. Too many and it rocks and not enough then there is no benefit. I could not get a perfect balance.

Between the two boards there is three pads for the dob to move on. I simply added three more so there is now six evenly spaced around the base. I did this not long after I got my scope and it has worked really well so far. Just cut some old smooth plastic and double-sided tape it down. :thumbsup:

[RAJAH235]

FWIW, if you need 'just that little less thickness', you can always use 'shim brass or coke can' between the MB washers.....:shrug:....:D L.
ps. You're only 'just' supporting the centre..

[astro_nutt]

Hi everyone,
I discovered with both my dobs (8 and 10 inch) I had to give the laminate surfaces a good clean with metho..then applied carwax and polish off..as for the teflon pads..I gave them a very slight buff with steelwool around the edges..in case of burrs on the teflon pads...the blank portion of central bolt has been smoothed down by placing in a drill and using 400 then 1200 grade wet-and-dry to remove any imperfections..finish off with carwax. A ring of medium-density foam sandwiched between two plastic rings have helped to support the centre...I dust in between the boards using a piece of felt wrapped around a ruler every few observation sessions. So far it works well.
Dark skies everyone.