15 replies to this topic

[Fivemileshigh]

I'm thinking of trying to use fluid dampened ball bearings for a manual dob instead of teflon/formica. I would like to achieve smoothly dampened motion without much difference between the static and dynamic coefficient of friction (i.e. almost equal force to start and maintain motion), much like a good quality video fluid head.

 

The general idea is to use 4 ball bearings, but add a drag device on each, much like a motorcycle wet multi plate clutch on each one, about the same OD as the bearing itself (roughly 3/4")

 

Has anyone tried this or have any thoughts?

 

Thanks!

 

 

[Astrojensen]

 I would like to achieve smoothly dampened motion without much difference between the static and dynamic coefficient of friction (i.e. almost equal force to start and maintain motion), much like a good quality video fluid head.

This is precisely what teflon and formica can achieve, if the scope is made well. A dobsonian should ALWAYS require MORE force to keep moving (and proportionally more with increased speed), than to start moving. 

 

 

Clear skies!

Thomas, Denmark

[luxo II]

I'm thinking of trying to use fluid dampened ball bearings ... much like a good quality video fluid head.

 

Has anyone tried this or have any thoughts?

Yes, been there done that with a real fluid head ... the downside is it won't stay put because there isn't any stiction (as distinction from fluid drag). 

 

The second problem is that if you do add a clutch of some sort to add stiction, the diameter needs to be big to make this work with negligible backlash - typically 50...100% of the aperture of the scope. In my case the scope was a 6". What worked for me was a CD with stiction provided by a felt pad on an arm pressing against the perimeter of the CD.

 

The teflon/formica combination work so well because it provides both stiction and sliding friction (drag); the stiction being about the same as the friction.

 

Fluid heads have negligible stiction and a lot of fluid drag, the assumption being someone is holding a long handle to aim the camera at all times, while the high drag is to limit any rapid slewing. 

Edited by luxo II, 03 December 2021 - 10:37 PM.

[Fivemileshigh]

This is precisely what teflon and formica can achieve, if the scope is made well. A dobsonian should ALWAYS require MORE force to keep moving (and proportionally more with increased speed), than to start moving. 

 

 

Clear skies!

Thomas, Denmark

Perhaps there's a misunderstanding. I don't think it's possible to have a scope require less force to start moving than to keep moving (at low speeds anyway) using sliding friction bearings. There is no combination of materials that I'm aware of has a lower static than dynamic friction coefficient. Put in other words, the scope always requires more force to start moving than to keep moving, which can lead to overshoot on larger scopes at high magnifications (16-20", 500x +)

 

This is just a thought experiment for now.

[luxo II]

That’s not quite true. Quite a few high-molecular-weight plastics do, and if you put an object on them even at a shallow incline they slow slide off. An example of this is the film used for iPhone screen protectors, another is a type of soft plastic grip on ballpoint pens - anything placed on it creeps slowly.

Using these materials as dob bearings is awkward though, as the scope won’t stay put.

Edited by luxo II, 04 December 2021 - 03:33 AM.

[PETER DREW]

I've used ball race bearings and mating stainless steel plates for large Dobsonians for several years.

 

They are dampened by Teflon pads which can have adjustable pressure applied making it possible to dial up the amount of effort needed to move the telescope. The Teflon has itself to be rigidly fixed to prevent backlash.  

[MitchAlsup]

Damping grease is what you desire.

[don clement]

My unipivot tonearm uses silicone oil as a dampener. I am going to replace the unipivot with a sticion-less taut wire pivot but still keep the silicone oil as a dampener. Both tone arm and a dob mount move too slow for an eddy current dampener. 

 

 

[Astrojensen]

Perhaps there's a misunderstanding. I don't think it's possible to have a scope require less force to start moving than to keep moving (at low speeds anyway) using sliding friction bearings. 

But it is. S&T had an article on this, many years ago. 

 

 

Clear skies!

Thomas, Denmark

[Fivemileshigh]

That’s not quite true. Quite a few high-molecular-weight plastics do, and if you put an object on them even at a shallow incline they slow slide off. An example of this is the film used for iPhone screen protectors, another is a type of soft plastic grip on ballpoint pens - anything placed on it creeps slowly.

Using these materials as dob bearings is awkward though, as the scope won’t stay put.

 

Is this anything like UHMW sheet material on Formica? I have a friend who build a telescope using this combination, and while it worked, the motion is similar to using teflon, i.e. noticeably more effort to get it started than to keep it going.

 

 

Damping grease is what you desire.

 

I agree, on the "clutch" plates. I forgot to mention I don't want any grease on exposed parts.

 

 

But it is. S&T had an article on this, many years ago. 

 

 

Clear skies!

Thomas, Denmark

 

Sounds like I'm reinventing the wheel   Do you happen to remember even vaguely how they did it? Was it the traditional sliding friction bearings or some type of rolling element?

 

Thanks guys!

[Astrojensen]

 

Sounds like I'm reinventing the wheel   Do you happen to remember even vaguely how they did it? Was it the traditional sliding friction bearings or some type of rolling element?

 

Thanks guys!

Traditional friction bearings. I can look it up, when I get home to my parents, where I have my S&T DVD collection. 

 

I've also tried several dobsonians in real life, that had such motions. Very smooth and no overshooting. All were homemade, if I remember correctly. The biggest was a 30". It took a lot of force to get moving, but didn't overshoot. You could track the Veil Nebula easily at 300x and wander up and down the filaments, which stretched on and on for many eyepiece fields. 

 

 

Clear skies!

Thomas, Denmark

[Cordrazine]

Is this the article? 

https://skyinspector...-Bearings-1.pdf

[Astrojensen]

Is this the article? 

https://skyinspector...-Bearings-1.pdf

Yes. 

 

 

Clear skies!

Thomas, Denmark

[Jon Isaacs]

I'm thinking of trying to use fluid dampened ball bearings for a manual dob instead of teflon/formica. I would like to achieve smoothly dampened motion without much difference between the static and dynamic coefficient of friction (i.e. almost equal force to start and maintain motion), much like a good quality video fluid head.

 

The general idea is to use 4 ball bearings, but add a drag device on each, much like a motorcycle wet multi plate clutch on each one, about the same OD as the bearing itself (roughly 3/4")

 

Has anyone tried this or have any thoughts?

 

Thanks!

 

How big a scope are you considering?

 

Have you considered the ease of assembly and disassembly. Standard large diameter Dob bearings are easy to deal with.

 

Have you spent much time with a premium Dob with the typical large diameter altitude bearings? Thomas mentions 300 x on the Veil.. that's just getting started. 

 

Jon

[Fivemileshigh]

How big a scope are you considering?

 

Have you considered the ease of assembly and disassembly. Standard large diameter Dob bearings are easy to deal with.

 

Have you spent much time with a premium Dob with the typical large diameter altitude bearings? Thomas mentions 300 x on the Veil.. that's just getting started. 

 

Jon

It's a 20" compact/portable, so fairly light, or at least as light as is reasonable without compromising the structure. My current 16" is definitely on the heavy side due to the 3" Paracorr/focuser/EP, and it's also top heavy, which requires ballast in the mirror box. Despite this, the structure turned out quite rigid and the motions at 500x are acceptable. I would love to improve that though.

 

Traditional friction bearings. I can look it up, when I get home to my parents, where I have my S&T DVD collection. 

 

I've also tried several dobsonians in real life, that had such motions. Very smooth and no overshooting. All were homemade, if I remember correctly. The biggest was a 30". It took a lot of force to get moving, but didn't overshoot. You could track the Veil Nebula easily at 300x and wander up and down the filaments, which stretched on and on for many eyepiece fields. 

 

 

Clear skies!

Thomas, Denmark

 

Thank you very much for the article suggestion, and thanks to Cordrazine for the link. 

 

I am looking into PFA suppliers for some testing, so far I've found 6x6x0.75" is the smallest piece, and $250, so I'll keep looking.

[careysub]

 

I am looking into PFA suppliers for some testing, so far I've found 6x6x0.75" is the smallest piece, and $250, so I'll keep looking.

You can get PFA film for not too much:

https://www.mcmaster.com/pfa-film/

As little as $4.16 per square foot.

 

A film should get the surface friction properties you are looking for, and there should be no abrasion in a dob bearing application. How to secure a slippery film to a surface securely - that's a different problem to work out.

 

A material not much used, but has some good reputation by those who have, is PTFE-coated fabric used in fabric design transfers:

https://www.cloudyni...n/#entry9301285

 

It is stuff like this (first Google result, so not suggesting this is the best source or type):

https://www.amazon.c...5294599958&th=1

Edited by careysub, 08 December 2021 - 10:25 PM.