Sticking "O" rings on rotating seal

savarin

savarin

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On my quad start helical focusser I have an "O" ring clamped at the top of the fixed tube that has the thread cut full length.
The rotating tube that fits inside and carries the eyepiece has only a short section threaded with the remainder reduced in size so it doesnt touch the threads.
The threads are greased so the main purpose of the "O" ring is to seal the grease in and stop it contaminating the reduced section as fingers can touch it there.
It works as designed very well with very smooth action.
BUT
When left for any time longer than 30 mins the "O" ring seizes onto the rotating tube locking it in place whereupon it requires a hell of a lot of torque to getting it to move smoothly and easily again.
Can anyone suggest the best material to use for the "O" ring so it doesnt seize up but still keeps the grease inside.
 
If the lubricant isn't the complete fix, I would be certain that the OD of the moving tube has a very fine surface finish. If the tube is polished, the O-ring won't have any surface features to "grip".

Another thing to consider is to reduce the radial compression of the O-ring on the moving tube.
 
The tube is polished because I dont want it to grip.
The radial compression is very slight and cannot be reduced any further.
I'm convinced its the o ring material, I dont know what it is as they came in a plumbing selection with no other info.
 
I know that some petroleum-based greases will swell o-rings.
So much so that they may change shape not fit their grooves anymore.

I like to use silicone grease on them instead; it does not affect the material.
It is also available in the plumbing aisle.
However, it does seem to change viscosity with temperature and might be stiff on cool nights.

Brian
 
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I think it has more to do with the grease than O ring material. Synthetic grease such as silicone or PTFE should not cause such problem unless the O ring is compressed excessively. Bear in mind that Silicone is not good for metal-to-metal contact so PTFE is recommended.
 
I was having a problem with a very fine oring. It would not go into the bore (screw in fit on the male part), it would bunch up, even with a food grade teflon grease.

I went to a food grade mineral oil USP. And that's what solved my problem... yours... well you are going to have to try different things.
 
extropic said:
If the lubricant isn't the complete fix, I would be certain that the OD of the moving tube has a very fine surface finish. If the tube is polished, the O-ring won't have any surface features to "grip".

Another thing to consider is to reduce the radial compression of the O-ring on the moving tube.
Click to expand...
I disagree, polished may be the problem.
There is more surface tension and grip on a polished surface than one that has a brushed finish.
 
If the problem is not caused by the use of wrong lubricant, it must be caused by the O ring having excessive contact area with the metal which suggests that the O ring is likely to be relatively fat and soft. If the metal surface is polished, the problem will be more sever as there is no room for accomodating the lubricant in the contact area.

One way to reduce the contact area is to use thinner and harder O rings. Plumbing O rings are relatively soft as they are designed to conform to the shape of the gap to be sealed. The hardness is typically 70 duro or lower. For moving parts, it's better to use 90 duro O rings. I ordered O rings for my airguns from this store : https://www.oringsandmore.com/buna-90-orings. These O rings are very common so I am pretty sure you can find similar suppliers in Australia.
 
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That sounds right and the way I was thinking.
Didnt know what the duro business meant, thanks
 
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