Dan Maloney
@Andy Pugh I was mainly talking gaseous - there are some air bearings and things like that, but like I said they're very niche.
Please tell about bearings lube
Okay, so then most of the solid lubricants act the same. If you look at the molecular structure of graphite, boron trifluoride, molybdenum disulphide, calcite or boron nitride, they all look mostly the same - sheets of atoms with low shear strength.
Solid lubes are good for vacuum applications. Except graphite.
Low shear sheets sounds very use-once?
Boron trifluoride is a gas...?
The choice of which one to use mainly comes down to the operating requirements. You see a lot of Moly used because it has very good oxidation stability - it even gets used in the space industry, where the oxidation stability is very important.
in space, is it about oxidation stability, or about working even without water molecules like what graphite relies on?
@Thomas Shaddack sorry that was a brain fart, I meant boron nitride. Specifically hexagonal boron nitride, because there are a few different isomers that don't have the same lubricating properties.
yup, the cubic boron nitride is a diamond-class abrasive. (handy for ferrous metals that dissolve diamonds at high temperatures at contact zone.)
Why is oxidation stability important in space? Are we talking about the atmospheres in crewed areas?
So in space it's a bit weird - in low earth orbit you ironically need really good oxidation stability because there's not much oxygen up there, but the oxygen that is present is in atomic form. So not O2, just random highly reactive oxygen atoms looking for a friend.
Or about low earth orbits where there are way too many single oxygen atoms, all eager to bond to something? (Yes, space corrosion IS a thing.)
Obviously once you get above that, then you're in the clear -the main thing you need to deal with is temperature variation and like @Thomas Shaddack said, the lack of water that would help graphite work.
In space applications, that an example where the solid lubricant is actually part of the machine surface - which is the same as moly coated piston rings. Generally it's sputtered onto the surface.
I saw eg. plastics that are composites with solid lubricant particles embedded in polymer matrix. I also saw eg. electrodeposited nickel codeposited with teflon particles.
Moly still gives you that really nice lubricity, but in piston rings it also helped with run in because moly is quite soft. We're starting to move away from moly rings because they're not durable enough for really high combustion pressure and sometimes they flake off, which means bad things for the engine.
On the solid lubricants stuff, this video does a better job of visualising it:
Also if you're invested in piston rings and liners then I did this interview with a piston ring specialist: he goes into much more detail that I ever could about some of the new spray-bore technologies.
So out of curiosity, what's the deal with dielectric grease? What's added to give it electrical properties? Or subtracted, I suppose.
do you have video for bearings lubricant?
Great question @Dan Maloney
So - dielectrics are an interesting one - there's obviously a lot more interest in the topic now that EVs are starting to become a thing. Getting the dielectric properties right is about balancing the electrical properties of the base oil and additives.
Could have also answers for improvising high voltage/transformer oil for high voltage hacking.
As a general rule - the less polar, the less charge is carried. So PAOs and Group III minerals carry the least charge, esters and PAGs the most.
And on the additives side, metallic additives (like ZDDP, calcium and magnesium detergents) carry charge but sulphur and nitrogen based ones don't.
And, of course, those lovely PCBs
With most dielectric greases, it's about marrying the conductivity to the intended application. If the grease doesn't carry any charge, that can be a bad thing because you get a buildup of static in the grease (or oil) over time, and once the voltage potential gets high enough, you'll get static discharge.
But carrying too much charge will allow things to short. So there's some happy middle ground.
if it's conductive it wouldn't be a dielectric grease. But there are static dissipation additives for fuels, to prevent sparks during transfer.
Alrighty! Halfway though, hope everyone is feeling ok.
There was a question about bearing lubricants.
Aren't all lubricants bearing lubricants? Otherwise they are not lubricating?
</pedant>
Arcing can however do a job on the bearing races. I think I saw somewhere about failure of gyroscopes in space related to solar storms. The sparks can machine ragged pits into the races.
Well, we usually only go for an hour, but keep going if you like!
So bearing oils are relatively simple. Most of the time it's just a base oil (rolling element bearings love 100cSt) and then some rust and corrosion inhibitors. That's if you're just lubricating a bearing. Then you start adding things depending on the location of the bearing. So say if you're in a screw compressor - the oil acts as a seal for the lobes so you actually bump up the viscosity a little bit to optimize for that. If there's also some gears, you'll start to add more antiwear and EP additives.
So yes @Andy Pugh you're absolutely right, most oil start off as just "bearing lubricants" and then we build up from there.
@Dan Maloney okay! Why don't we do another 15 minutes then?
Works for me!
@Rafe I was actually coming at it from the other side, in that if there isn't a bearing (of some sort) then what you need isn't a lubricant.
Greases. The different kinds, lithium, calcium... why are they different?
do lubricant wears out?
It's also about proritizing what we want to protect. If you look at an engine oil, you need to protect the bearings, pump, gears, chain, cams, lifters, etc. But we choose the viscosity based on the crankcase bearings because they're the most expensive (and annoying) to replace.
Greases - great question.
So, the different types of greases are different for a couple of reasons. Let's start with the soap-based ones. These are the lithium, calcium, and aluminium soaps.
If you think of a soap (for your hands) it's designed to be water soluble, but attracted to oils. This way it can bind to the oils on your cutlery and wash them away.
Grease soaps are the opposite - they're oil soluble, but with polar heads that are attracted to each other. That forms a matrix or web that holds the oil. People usually use the analogy of a sponge holding water - when you put a sponge under stress is releases water. Same thing with the thickeners - under load they release the lubricant, and the lube oil is the thing that is actually doing the lubricating.
Okay, so why then are there different types? Mainly it's to do with other things like resistance to water washout and spray off, as well as high temperature performance.
lithium is the most common because its a good "multipurpose" thing. Aluminium can be made to be food grade.
Would that work also with some polymer matrix? Kind of like candle gel?
Then there are the non-soaps. You've got polyurea for example - more stable at high temperatures because there's no metals to catalyse oxidation.
They get used in a lot of high speed electric motors, and fill-for-life applications.
There's also clays - which are really good at high temperatures. And then there's my personal favourite, the Calcium Sulphonates. Calcium sulphonates are cool because the thickener can also act as an additive - there's calcite particles that act as a solid lubricant, but they're also really good anticorrosion additives, plus you get good high temperature performance and water washout performance.
Historically they've been rare because they were 3x more expensive that a lithium grease, but with lithium shortages around the world and the price of lithium going up, that difference is not more like 50>#/span###
I was surprised to find how hard it was to get NLG3 grease. I suspect that NLG1, 000 and 6 are "how many tonnes should we make for you"?
@Thomas Shaddack exactly - so the polyureas are polymer.
So the di and trivalent ions are better as ionic "crosslinkers" for the grease?
@Thomas Shaddack I think yes? But cross linking is also related to to "complexing". That's the difference between Lithium, and Lithium complex greases. Think of the soap molecules like the two long bits of a ladder - the complexing agent is the rungs. It really helps the stability of the grease under high shear and/or high temperature.
@Andy Pugh yes NLGI 3 and up aren't very common.
Alrighty, I think we're just about done!
@Dan Maloney thanks so much for having me!
You bet, this was great -- so many good questions!
Hopefully everyone learned something!
...and I have a new youtube channel to binge on.
Thanks so much for taking time out to be with us today, I really appreciate it. And thanks everyone for attending!
Thanks everyone!
Great questions!
Yes, definitely some great videos on Rafe's channel, I've been subbed for a while now.
Thanks Rafe!
Transcript coming up - thanks all!
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