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Reducing dobsonian bearing friction

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#1 mconnelley

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Posted 17 April 2019 - 02:58 AM

Hello:

  

  I put together the dobsonian that I've been working on (glacially), and noticed that the motion in elevation is quite heavy.  Right now there's no bearing material in azimuth.  In elevation, I have aluminum riding on teflon.  The first thing I'll try is cleaning off the teflon, and polishing the aluminum.  I think this will help, but maybe not enough.  My other scope has aluminum on teflon and doesn't feel this heavy, so I'm a bit puzzled.  The geometry of the mount is set.  The aluminum was chosen in part because I hope to have a drive system on it eventually.  So let's consider those fixed.  

 

  I've heard about folks trying a range of things from Armor-All to bath soap.  What have people found to work well?  What have people tried that didn't work?

 

  I've attached a rather old photo of the scope, but it still (unfortunately) pretty much looks like this today.  

Cheers

Mike

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  • 1m_scope_astroday.jpg


#2 Stevegeo

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Posted 17 April 2019 - 03:58 AM

Clean both surfaces throughout.. an try it... If that isn't sufficient,  buy a small spray can of lock lubicant (silicone type)

Not the moly (mol will discolor plastic and is messy).

I have used spray silicone on many locks, slide mechanisms, and even my convertible top sliders on my smart car...

Works very well, clear, non greasy, and does not pick up dirt ...



#3 Jon Isaacs

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Posted 17 April 2019 - 04:48 AM

I have a Dob with aluminum bearings riding on Teflon.  

 

I use the same thing I use on my other scopes:  Bar soap.  

 

For the altitude bearings I just wipe them clean with a damp cloth or paper towel. 

 

I normally start with the scope near horizontal 

 

I then wet the soap and rub it over the exposed aluminum on both sides.  I give it few minutes to dry.  I then raise the scope to that azimuth and repeat.  

 

It seems to works very well, stiction free, smooth.  It seems to work for others as well.  I have tried everything I could think of, everything I have read about.. This is the one that works for me.  The others, so-so.

 

Jon


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#4 ngc7319_20

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Posted 17 April 2019 - 05:39 AM

Various articles discussing formica products running on teflon discuss getting the PSI (pounds per square inch) correct.  Is the PSI different for your two scopes?  Maybe you need larger (or smaller?) teflon pads?   Some sites describe using PTFE instead of Teflon.   At least with the laminates, a rough surface seems to be preferred -- not sure polishing the Aluminum is the way to go.

 

I used Turtle Wax car wax on my Teflon / Aluminum altitude bearings and it helped, but needs to be re-applied often.

 

Some info here:

https://www.cloudyni...earings-tested/


Edited by ngc7319_20, 17 April 2019 - 05:40 AM.


#5 Bob4BVM

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Posted 17 April 2019 - 01:12 PM

I have a Dob with aluminum bearings riding on Teflon.  

 

I use the same thing I use on my other scopes:  Bar soap.  

 

For the altitude bearings I just wipe them clean with a damp cloth or paper towel. 

 

I normally start with the scope near horizontal 

 

I then wet the soap and rub it over the exposed aluminum on both sides.  I give it few minutes to dry.  I then raise the scope to that azimuth and repeat.  

 

It seems to works very well, stiction free, smooth.  It seems to work for others as well.  I have tried everything I could think of, everything I have read about.. This is the one that works for me.  The others, so-so.

 

Jon

Another vote for simple bar soap. Goes on easy. Easy to clean off if necessary. No oily mess. Works just great.

CS

Bob


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#6 MiguelStrongEye

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Posted 17 April 2019 - 01:16 PM

Polishing will increase stiction.


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#7 dddileo

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Posted 17 April 2019 - 01:56 PM

 

Some sites describe using PTFE instead of Teflon.

Same thing. Teflon is the DuPont/Chemours brand for the generic.

From Wikipedia: 

    "Polytetrafluoroethylene (PTFE) is a synthetic fluoropolymer of tetrafluoroethylene that has numerous applications. The best-known brand name of PTFE-based formulas is Teflon by Chemours. Chemours is a spin-off of DuPont, which originally discovered the compound in 1938."


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#8 Pezdragon

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Posted 17 April 2019 - 02:16 PM

I’ve found running bare polished aluminum against Teflon can cause problems. It seems the Teflon strips off and embeds bits of the aluminum then the embedded aluminum starts galling the main bearing. This happened to me when I was first building my sled focuser with Teflon pads against the aluminum tubes....I changed to woven nylon and all was well.

 There’s a wonderful article in Sky and Telescope October 1999 about improving Dobsonian motion and the writer found a product superior to Teflon from HIX corporation. I use it on my bottom bearing and side bearings and the motion improved tremendously 



#9 Jon Isaacs

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Posted 17 April 2019 - 02:24 PM

I’ve found running bare polished aluminum against Teflon can cause problems. It seems the Teflon strips off and embeds bits of the aluminum then the embedded aluminum starts galling the main bearing. This happened to me when I was first building my sled focuser with Teflon pads against the aluminum tubes....I changed to woven nylon and all was well.

 There’s a wonderful article in Sky and Telescope October 1999 about improving Dobsonian motion and the writer found a product superior to Teflon from HIX corporation. I use it on my bottom bearing and side bearings and the motion improved tremendously 

With my 12.5 inch which has aluminum bearings, I have not had this difficulty.. The scope is about 12 years old.  I do occasionally scrape the surface of the Teflon to remove embedded dust and grime, I do that with all my Dobs. 

 

5102862-Jstar Balance Cloudy Nights.jpg
 
Jon


#10 ngc7319_20

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Posted 17 April 2019 - 02:31 PM

I’ve found running bare polished aluminum against Teflon can cause problems. It seems the Teflon strips off and embeds bits of the aluminum then the embedded aluminum starts galling the main bearing. This happened to me when I was first building my sled focuser with Teflon pads against the aluminum tubes....I changed to woven nylon and all was well.

 There’s a wonderful article in Sky and Telescope October 1999 about improving Dobsonian motion and the writer found a product superior to Teflon from HIX corporation. I use it on my bottom bearing and side bearings and the motion improved tremendously 

Behavior may depend on what flavor of aluminum you have.  Cast, 6061, 2024, etc, etc.  Anodized?  Some are harder / smoother than others.



#11 Pezdragon

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Posted 17 April 2019 - 05:44 PM

That’s so true. I was talking about standard extruded non-anodized aluminum tubing that I polished in the area of the sled guides. It was surprising how fast it self destructed the finish and smooth travel. I suspect it is a soft alloy.



#12 mconnelley

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Posted 17 April 2019 - 05:48 PM

Hello:

   

  Pezdragon:  I looked up HIX, and found a company that makes food service machines and industrial ovens.  What material specifically do you use, and where did you get it (a link would be great).  When I looked up woven nylon, I got a mostly fabrics and thread (probably not what we're looking for).  

 

  It sounds like polishing may be a bad idea.  I'll give the aluminum a good once over with scotchbrite to make sure it's nice and smooth (without polishing it), then give it a good cleaning.  Then I'll try the soap, and see how far we get.  

Cheers

Mike



#13 Pezdragon

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Posted 17 April 2019 - 06:33 PM

I need to correct myself on the HIX material as it is a form of Teflon though fused into a fabric sheet. It is used in the dry/heat transfer process for printing tee shirts and such. It is quite thin and so it installed on top of .062” double sided foam tape to give it some compliance. I’ve found it completely eliminates stiction as the author of the Sky and Telescope article stated. If you can find a S&T copy from October 1999 it is a fantastic read. Here’s a source for the material.

 

https://www.cutterpr...ries-ptfe-sheet

 

a few dollars worth will do for an average Dob and it lasts for years. Rod



#14 MiguelStrongEye

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Posted 17 April 2019 - 06:54 PM

As a matter of reference...

if you can get your aluminum hard anodized it will still have the same properties when cockroaches rule the earth.

see page 6

http://www.balseal.c...evision_F_0.pdf

 

 

it can also be impregnated with ptfe...if you want to try that experiment.

perhaps running on uhmw pads?


Edited by MiguelStrongEye, 17 April 2019 - 07:01 PM.


#15 gregj888

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Posted 17 April 2019 - 06:56 PM

I'be had luck with the dry film lube in some cases.  Drys, so not messy and doesn't pick much up.  I have,'t used this brand, but looks like the same stuff.

https://www.homedepo...-4A24/100559191



#16 Pezdragon

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Posted 17 April 2019 - 07:13 PM

I forget to state that I’m using the HIX material with lightly textured Formica on the baseboard and phenolic side bearings.

I can’t state how well the HEX would work against bare aluminum.



#17 Jon Isaacs

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Posted 18 April 2019 - 05:45 AM

Regarding polishing the aluminum:

 

I would be careful generalizing this one experience.  In bearings,  smoother is generally better.  The imperfections in the hard material will cut into the soft material.  

 

I also wonder how the aluminum was polished.  One can polish aluminum with ever finer polishes much like a mirror and finishing off with the cotton wadding may polish.  

 

Also,  I wonder how heavy the scope is.  Those large diameter bearings do increase the force required to move the scope. 

 

Jon



#18 mconnelley

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Posted 18 April 2019 - 01:53 PM

Hello:

  

   Currently, the moving weight is about 130 lbs.  It'll be closer to 250 lbs when the mirrors are in it plus all of the other little bits.  

 

    I've used a dry spray at work that was basically shooting powdered teflon with some liquid that quickly evaporates.  I hadn't thought of that...might give it a go.

Cheers
Mike



#19 Pogo007

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Posted 23 April 2019 - 09:06 PM

Hi.  I've never faced this challenge but wonder if Boeshield T-9 might help.

 

https://boeshield.com/



#20 Dick Jacobson

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Posted 27 April 2019 - 06:48 AM

Have you considered replacing some of the pads with rollers?

 

On my most recent scopes, I've abandoned the large circular altitude bearings in favor of small bronze bearings right at the axis. These support the scope with virtually zero friction. To provide friction I use polyethylene or nylon pads against an aluminum or stainless steel plate. This provides very smooth, adjustable friction.



#21 mconnelley

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Posted 28 April 2019 - 05:54 PM

Hello:
 

    I have given some though to adding roller bearings, and I expect to need to do that eventually.  I plan on adding a alt-az drive system, at which time I'll need to add rollers.  Before that, I plan on using the scope as a plain dobsonian while I do a basic shakedown of the scope itself.  There is still a LONG list of things I need to do before I get to first-light even in plain-dob mode, so I would like to address this friction issue as quickly/easily as possible and move on.  Even after first light in plain-dob mode, there could well be big issues to address before I want to tackle a drive system.  For example, I may need to implement an active optics control system for the primary mirror support.  I think I would prioritize that over a drive system, since tracking is really nice with a 120" focal length, but good image quality is essential.  

 

   As a brief update, I did a quick polish on the aluminum altitude bearings, and that helped a lot.  The motion might still be heavier than I'd prefer once the scope is up to full weight, but we'll see when we get there.  At any rate, I think it'll be useable.  

Cheers

Mike



#22 pjoyce

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Posted 03 May 2019 - 11:11 AM

Various articles discussing formica products running on teflon discuss getting the PSI (pounds per square inch) correct.  Is the PSI different for your two scopes?  Maybe you need larger (or smaller?) teflon pads? 

Once of the curious things about the friction formula is that area does not appear anywhere in it. This means that PSI does not matter because, assuming a constant force, increasing the contact area raises the friction resistance, but also lowers the pressure by the same rate. What matters more are the material properties related to softness. Teflon is a great non-stick surface for general use, but under higher temperatures it can soften, and (per Wikipedia) it can interact with aluminum. Most aluminum that we interact with on a daily basis is either anodized, or at least oxidized which gives it a very strong and hard surface. Polished aluminum is quite soft at the surface because the act of polishing removes that oxide layer exposing the soft, reactive aluminum. When used as a bearing surface (particularly under high loads) it has a tendency to gall. This is the likely source of the aluminum self-destruction that Pexdragon experienced.

 

A better bearing surface is likely to be Delrin (aka POM, or acetal). It is significantly stronger than teflon, does not interact with aluminum, and has nearly the same friction properties. It is also much easier to work with than Teflon. I use it all the time at work as a bushing material and it always out-performs teflon in practical applications.

 

 

 At least with the laminates, a rough surface seems to be preferred -- not sure polishing the Aluminum is the way to go.

I do not know 100% why the slightly rougher surface texture works better on formica-teflon bushings, but here are my hypotheses. Firstly, I suspect that the rougher texture prevents fine particles of dirt, dust, and sand from getting embedded between the two plastics and creating a sand paper like surface over time. Secondly, I think that a perfectly smooth plastic-plastic interface will create a small region of vacuum (similar to gauge blocks sticking together) that increases the applied force and thus increases the resistance to motion. At some point in the (far) future I would love to do some of my own friction experiments inside a vacuum chamber to test this hypothesis.

 

 

I used Turtle Wax car wax on my Teflon / Aluminum altitude bearings and it helped, but needs to be re-applied often.

Applying the wax to the aluminum is likely the reason you have not experienced galling on the aluminum bushing surfaces because it provides a barrier between the two materials.


Edited by pjoyce, 03 May 2019 - 11:11 AM.



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