hans wrote: ↑
Wed Sep 23, 2020 8:31 pm
PeteM wrote: ↑
Wed Sep 23, 2020 4:54 am
A plus with some of the silicon greases...
Any opinion on the concerns regarding silicone grease near optics that seem to be repeated various places around the web . . .
Hans, I’m not an expert in greases, so take this with a grain of salt:
1) There are oil based, synthetic oil base, and silicone (a specific synthetic) oil-based greases. The helicoid grease I’ve been using is a fairly stable synthetic with tiny PTFE (Teflon) particles and an old standby (lithium) as thickeners. It has worked OK, once the old gummed up grease has been removed: https://www.amazon.com/MicroLubrol-Heli ... B00D0HOLO6
I’ve also used the Nye product - also OK but mine (would have to check the #) a bit too much damping IMO. It’s probably an NLGI #2 rather than the lighter NLGI #1 consistency. Note on the following link that many Nye damping greases are based on synthetic oils and thickened by silica:
Many of the Molykote products are also PAO synthetics.
“Super Lube” is another synthetic grease with tiny PTFE particles – this one a medium thick NLGI grade 2 grease.
2) One of the pluses of many synthetic and most silicone greases is their ability to resist outgassing and high temperatures. This should also lead to longer stability and less chance they might run (and migrate) from something like the heat of a 100 watt illuminator? Silicone greases do have low surface tension so they’re likely to spread over the surface they’re lubed with. I don’t see them “jumping” to adjacent glass, though.
3) I have heard (but can’t verify specific instances) of incompatibilities of various mineral and synthetic bases and thickeners. Hence the advice to thoroughly clean off the old stuff. I do know that gear oils with EP additives (to handle pressure) like sulfur will attack brass gears and slides.
4) Silicone greases do stick pretty well to glass if you screw up. However, an added factor is that they aren’t soluble in alcohol (the main solvent besides water in many lens cleaners), so that may be the source of some concern about cleaning them off should they get on glass. They are soluble in solvents like mineral spirits, toluene, hexane, petroleum ether, etc. etc. So they should clean off with the right solvent if there’s a mistake and a smear?
Don’t know about the effects, if any, of various greases and oils on various coatings. The fatty oils from our fingertips seem to come off easily enough. I’m personally more worried about greases and oils dripping, outgassing, or oxidizing over time than screwing up and smearing them on a lens, though that is of course a risk.
5) Silicone greases are often used as a water-proof lubes for rubber parts (o-rings etc.) – even with underwater cameras -- but aren’t good with silicone rubbers.
6) All of which suggest to me that synthetic oil based greases are likely dominant today in optics – but silicone synthetics are to be used with some caution.
7) All that said, I’m amazed how well some fifty year old Japanese microscopes (Tiyoda an example) sometimes still function with old oil-based greases – apparently with little or no maintenance over the years. I came across a couple small tubes of two greases – still good decades after the company ceased US business.
The link you posted does seem to have a lot of good info. However, some of the advice such as never trying to repair an objective or condenser would have left me short of several now-repaired objectives and condensers.