Formica bearing surface for dobs

[markse68]

That’s very clever! So the inner pads take the load and generate the most friction force but have the longer lever to overcome it as they’re on the smaller diameter but then why is it recommended to use large diameter bearings for the alt axis?

i found this calculator for estimating the forces for given dimensions. I guess you have to enter them in inches and pounds as it’s American. I entered my new design values and they both by chance came out at 1.4kg friction force- within the authors ideal range. Handy tool for playing with dimensions and seeing how they’ll  affect real world results

https://www.bbastrodesigns.com/DobFriction.html

Edited May 25, 2020 by markse68

[Chriske]

I opened the link and closed it immediately, sorry..
There's nothing to calculate at all.
During course(started 1981) we always use the procedure described higher up, there's nothing more to it.
Indeed the inner pads takes all the weight of the rocker and scope. The outer are only to very gently support and at the same time very delicately avoid tilting of the rocker on these central pads.
What I forgot to mention is, the central pads need to be positioned as close as possible to the central bolt..! Sometimes we even use a small ring around that central axis/bolt.

Using large Alt bearing is not really necessary. Using the correct materials is.
But there's a problem with the alt-bearings. Well, not really with the bearings itself.
A real problem to overcome is the balancing issue during eyepiece switch. In the early days we all used the familiar 1 1/4 eyepieces. Not only did they have about the same weight, but removing it to replace it with another eyepiece, it did not disturbed the balance of the scope at all. These days it's almost impossible to do that, without the scope falling or lifting rather quick, loosing the object you were observing.
I myself have a few Nagler eyepieces.
One 20mm --> 1051gr..!! If I'm not mistaken that is about 2lbs..!...😳
I also have 4.7mm --> 180gr.
My old Erfle eyepieces(very good quality btw.!) --> 136gr.

Well that is a problem, 1kg versus 136gr..!
There are many ways to tackle this balancing problem.

Just do a search on balancing a Dob and you'll find a handful solutions. Some use a thick steel chains, others use very long springs, and there's also a solution to use a very long rubber band to keep your scope in balance during eyepiece switch. There's also the old counterweight trick, or clamping the scope against the rocker. And there are more possibilities, I'm sure.

My solution is simple, I do not use these heavy eyepieces anymore. I do have a large set eyepieces and they're all lightweight. I know, silly, but it works. So these (few) heavy eyepieces I own are 'out of a job' permanently.
There's still a difference between all my lightweight eyepieces, but during switch my scope(s) all stay put.

And I know, that tilting problem could be solved by applying more tension to the alt-bearing. But that is something I do not want to do. The movement of alt-bearing should 'feel' the same as my az-bearing.

 

[faulksy]

i didnt use formica for any of my bearig surfaces on my dob build. i used hammerite paint on my ground board and alt bearings. makes the motion lovely and smooth and consistant 😀

[markse68]

20 minutes ago, faulksy said:

i didnt use formica for any of my bearig surfaces on my dob build. i used hammerite paint on my ground board and alt bearings. makes the motion lovely and smooth and consistant 😀

Interesting- has it lasted? Wouldn’t have thought it’d be as durable as Formica but I guess when it’s fully cured it’s pretty tough stuff

[faulksy]

27 minutes ago, markse68 said:

Interesting- has it lasted? Wouldn’t have thought it’d be as durable as Formica but I guess when it’s fully cured it’s pretty tough stuff

when you read my build, didnt you see it 😝

4 years ago and coating is still fine and its used a lot, did use the hammerite special surface primer as well. seriously its ace. well tidy

 

Edited May 25, 2020 by faulksy

[faulksy]

15 hours ago, markse68 said:

That’s very clever! So the inner pads take the load and generate the most friction force but have the longer lever to overcome it as they’re on the smaller diameter but then why is it recommended to use large diameter bearings for the alt axis?

i found this calculator for estimating the forces for given dimensions. I guess you have to enter them in inches and pounds as it’s American. I entered my new design values and they both by chance came out at 1.4kg friction force- within the authors ideal range. Handy tool for playing with dimensions and seeing how they’ll  affect real world results

https://www.bbastrodesigns.com/DobFriction.html

large alt bearings allow for big differences in weight at the uta without the dob dipping or raising. small alt bearings need bias to compensate for eps. i can use a bgo of ethos and the tube stays put

Edited May 25, 2020 by faulksy

[markse68]

1 hour ago, faulksy said:

when you read my build, didnt you see it 😝

4 years ago and coating is still fine and its used a lot, did use the hammerite special surface primer as well. seriously its ace. well tidy

 

I have to confess to only flitting through your build thread- it’s quite long 😉 I’ll sit down and go through it more thoroughly I promise 😂 What I saw looked really great though 👍

I never would have believed that paint would stand up to that much use but I guess it goes to show how low friction ptfe really is! (And how tough hammerite  is!)

Edited May 25, 2020 by markse68

[markse68]

On 25/05/2020 at 09:02, Chriske said:

I opened the link and closed it immediately, sorry..
There's nothing to calculate at all.

.....The movement of alt-bearing should 'feel' the same as my az-bearing.

 

But I don't have your years of experience though

Are you saying the calculator will not give accurate results? If it does then surely it's a pretty useful tool for someone with less experience who doesn't want to build multiple dob bases trying to get the right feel?

I like your method for reducing the AZ bearing friction but in my case (8" dob) the problem I think I'm going to have using that calculator is having too little friction! I could use nylon instead of PTFE but then the friction goes up too much. Based on the results of the calculator I'm thinking I might have to increase the ALT bearing support angle to try to increase the ALT friction to match the AZ friction but then I'm going to use Tufnol for my ALT bearing rings and the COF is a bit of an unknown... AND they're both coming out a bit low and I don't want to increase my AZ bearing diameter too much more

Mark

ps Mel sent me a link to improved calculator  it has choice of units 👍

my calculated values seem rather low 😬

 

Edit it I forgot the link 🤦‍♂️

http://bbastrodesigns.com/NewtDesigner.html#movementFriction

Edited May 26, 2020 by markse68

[Chriske]

Hey Mark,

I'm not saying this online calculator is no good.
Maybe for a less-experienced amateur it will set his alt-pads at a good spot, don't know. There's always that rather important question, what type of formica did you use..?
You did some tests already I suppose(?) That is the way to go, test with the materials you have at hand(or can buy in a local shop). There's no way a online-calculator can replace this way of working.
Lets say you don't have the correct formica or even do not have formica at all, then you could use another slippery sheet of wood(varnish)/plastic in combination with teflon spray.
Anyway, I'd always use teflon blocks.
The only disadvantage using teflon-spray is that after a few months/year of usage you need to clean the surfaces because it catches dust. Yet again Teflon-spray does miracles. A very sticky surface will transform in a smooth one after a spray of teflon.

And again, as I mentioned higher up, there are some alternatives, like that felt I mentioned higher up. Believe it or not it is a very good solution I'm using right now. The scope hanging in that fork weigh about 12kg. I don't even bother to glue the felt in the rocker. The roughness of the wood catches the felt and hold it in place. The bearing sitting in the rocker is nothing more then a 120mm diameter printed disk(PLA). To be clear on the matter I do not use felt pads, but rather long strips of felt. The felt covers almost 1/2 the diameter of the disk. The last bino I made is hanging in a printed fork and a printed disk. In between is a strip of felt. in the fork I added two very small pieces of double sided adhesive.
A cloth or even a blanket/carpet might do the job as well. There are tons of possibilities. I didn't test these last ones yet, but I probably will do for my next scope.

A few year back I made a 10' Printsonian. Well for the AZ bearing I did use 3 ball bearings at 120°. To have the correct friction I added a very simple friction device. That device was nothing more the a flat adjustable spring. Adjusting was done with a small bolt pushing against the flat spring. That spring is in contact with the rocker.
The ALT bearing of that scope is teflon/PLA (150mm printed disk).

My point is : I like to try different methods. It's easy indeed to copy what your 'neighbour' did or what you found on the net, but I myself like to explore other possibilities that works as well(at least) as the classic methods.

Chris

 

[markse68]

40 minutes ago, Chriske said:

My point is : I like to try different methods. It's easy indeed to copy what your 'neighbour' did or what you found on the net, but I myself like to explore other possibilities that works as well(at least) as the classic methods.

Chris

 

I love your attitude Chris I really do- it’s totally in the spirit of John Dobson too 👍 I’m too far into this now-I  just want a “traditional” design that’s better than what I have inherited and using now and to get it as close to right as possible first time

[Chriske]