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Bronze bearing lubrication?

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#1 orlyandico

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Posted 28 November 2012 - 04:31 AM

I have been having trouble with declination guiding on my old AP600 mount, particularly with a heavy load (C9.25 + 18lb of counterweights) I can actually hear a "grunting" sound when the declination moves in tiny increments.

Obviously there is massive stiction and getting round stars with the C9 is a big challenge.

I decided to see what I could do to improve the declination axis - it runs on plain bearings (i.e. bushings).

There is a lower bushing made of PE or some similar plastic, and an upper bushing that looks like its bronze
Posted Image

And this is the declination axis..
Posted Image

Co-axial forces on the declination axis (e.g. when the scope is pointing close to the horizon) are carried by a steel sleeve and some hard plastic washers... like so:
Posted Image

I was able to significantly reduce the friction (and stiction) of the thrust surface by sandwiching some super-thin PTFE film between the white plastic washers. Although a better solution would be to replace the washers with PTFE equivalents (they are 2.5mm thick).

However the challenge is, when the scope is pointing close to zenith, a lot of the weight is carried by the bronze bushing on top and the plastic bushing at the bottom of the axis - and there's a very significant amount of stiction.

I tried wrapping the PTFE film around the dec shaft - the film is 1 mil thick (0.025mm) but it's too thick, the axis basically got stuck.

So what would be an alternative solution to reducing the friction? I think if even the top - bronze - bushing could be improved, the declination guiding performance would be much better.

I have tried superlube, white lithium, and silicone grease prior. None of these solutions was very good - the PTFE film on the washers is the best solution so far, but it doesn't help when the scope is pointing up.

#2 okieav8r

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Posted 28 November 2012 - 06:23 AM

I would consult with Roland Christen and see what he says.

#3 orlyandico

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Posted 28 November 2012 - 07:16 AM

AP recommends lubriplate 105.

I have followed all the instructions here..
http://www.astro-phy...00EGTOMaint.pdf

Haven't tried lubriplate though.

#4 verycoldtoday

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Posted 04 December 2012 - 03:49 AM

I don't know how well it will work with plastic, but most machine tools use an oil called "Waylube 32". I believe it is made by Shell. I use the generic equivalent at work. It is available in heavier weights but you shouldn't need them, as they are quite viscous. Waylube is designed to prevent the stick and release cycle for slow and heavy moving parts such as milling machine tables.
Perhaps a local machine shop or aquaintance could give you some.

#5 piaras

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Posted 04 December 2012 - 07:15 AM

Be aware that there are 2 types of ways lube. One for manual machines and the other for CNC. The difference is not in the oil but the additive package. I would suspect that in this application there would be not real difference as the speed of movement is minimal. Mobil Vactra 2 and Mobil Vacuoline series, there are other makers naturally.

#6 John Jarosz

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Posted 04 December 2012 - 07:33 AM

It sounds ridiculous (to me), but have you measured the bore and the shaft? If you have .002" clearance (almost!), maybe you are not getting the benefit of the full bearing contact. Maybe you are getting line contact and the high contact forces associated with it. It seems odd for this to happen because I certainly wouldn't expect wear to occur on such slow moving parts - but you never know. I'd also vote to see what AP says.

Just a thought,
John

#7 Gordon Rayner

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Posted 04 December 2012 - 01:06 PM

Washers and sleeve bearings can be made using Delrin-AF, which has stick-slip absence approaching Teflon, because it is loaded with Teflon particles, yet is easily machined , much more easily than either Teflon or PE. I have experience with all three.

But Delrin-AF costs $$$, about the same as Teflon. Ready-made polymer sleeve bearings equivalent to Delrin-AF are available from www.igus.com . Type J (?) or (?) M-250.

Or replace the PE( ?) sleeve with self-lubed bronze sleeve or tee-sleeve ? McMaster-Carr, Reid's, etc.

#8 orlyandico

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Posted 16 January 2013 - 08:50 AM

Resurrecting this thread.

I got this anti-stiction Kendall 80W90 gear oil and used that. It has done nothing for the stiction.

I did notice that I am getting line contact on the bearing. Found this out because I tried polishing the bearing by putting metal polish on it and fitting the two parts together. After turning the bearing a while I noticed that the polish had been removed from a line around the bearing.

I really should not be futzing around with this mount because I got a Mach1, but it seems a waste to have it just sitting there, and selling it is out of the question due to very high shipping costs.

#9 piaras

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Posted 16 January 2013 - 04:32 PM

What about a spindle oil? Something along the line of Mobil Velocite line. We use #6 for the sleeve bushings in the spindle of the cylindrical grinder. It is better in our case as it is thinner then the gear oil that you are using, the machine does not pump the lube, it is just sitting in a bath with splash lubrication.
Pierre

#10 stmguy

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Posted 16 January 2013 - 04:54 PM

You could replace your plain thrust washers with thrust needle bearings for not a lot of money

http://www.vxb.com/M...ode=bearings...

Not sure what might work best on the bronze bushing , my SB lathe uses a Teflon grease in the cone assembly
Norm

#11 seryddwr

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Posted 16 January 2013 - 05:36 PM

You could replace your plain thrust washers with thrust needle bearings for not a lot of money

http://www.vxb.com/M...ode=bearings...

Not sure what might work best on the bronze bushing , my SB lathe uses a Teflon grease in the cone assembly
Norm

Cool! I just ordered some.

#12 orlyandico

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Posted 16 January 2013 - 07:41 PM

Wally from AP strongly recommended not to replace the sleeve bearing with a needle bearing.

It would require machining and would permanently modify / disfigure this classic AP mount.

What I've done (slightly less heretical) is ordered a scraper and some Prussian blue. :tonofbricks: hopefully I can scrape off the high parts of the bearing sleeve.

Let's see where this goes...

#13 ohioalfa64

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Posted 17 March 2013 - 03:59 PM

If your PTFE is not extruding under load it is not overloaded. UHMW does provide lower friction factors in cold temperature versus PTFE. Some PTFE has too high a plastic content and has about 2x friction over other standard PTFE.

#14 don clement

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Posted 17 March 2013 - 06:40 PM

I have had good luck with using Turcite X and Turcite A http://www.boedeker.com/turcax_p.htm for self-lubricating bearing surfaces. Turcite does not have the negative cold flow characteristics of PTFE. The trick with successfully using Turcite as a close tolerance bearing was not to use as a full contact bearing as the differential CTE between the housing and bearing meant that tolerances had to be large in order to accommodate the full contact bearing.

Don Clement






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