Extreme low temperature lubrication

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NicB

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Hey, Im looking for some oil help as searches have not come up with answers Im looking for and after spending some time on this site I thought this could be the best place to ask!

My needs are a bit of a strange... Im looking for a oil which can perform under subzero temperature conditions. The application of the oil is to be used in a pneumatic piston motor where the large pressure drop in the air results in air temperatures to drop as low as -80C (-112F). As for the temperature of the components within the motor, this has not been accurately estimated but safe to assume it will be quite cold.

Looking for something that still holds it lubrication properties at these low temperatures and still has reasonable pressure resistance and viscosity.

This is all for a University project I'm working on, but it seems to have a lot of people stumped! Any help will be appreciated,

Cheers!
 
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Perhaps a low-temperature compressor oil would be suitable? If the air is going down to -80°C the oil might be staying a bit warmer than that.
 
Quote:
My needs are a bit of a strange... Im looking for a oil which can perform under subzero temperature conditions. The application of the oil is to be used in a pneumatic piston motor where the large pressure drop in the air results in air temperatures to drop as low as -80C (-112F). As for the temperature of the components within the motor, this has not been accurately estimated but safe to assume it will be quite cold.

Looking for something that still holds it lubrication properties at these low temperatures and still has reasonable pressure resistance and viscosity.

This is all for a University project I'm working on, but it seems to have a lot of people stumped! Any help will be appreciated,


Some questions for you....

What are the materials that make up the components?

As Turrtlevette inferred, do you really need a liquid lubricant or will a dry lubricant work?

What do mean by "resistance?"
 
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I was thinking a bit of context about the motor will probably help out here. The motor is basically a off-the-shelf compressor modified to run in reverse. Meaning it turns compressed air into mechanical work, rather then turning mechanical work in to compressed air. Its a piston style motor (2 cylinder and its simply a splash style lubrication system)

As for materials, I don't have the exact properties of the material however the basic overview is as follows, Crank is steel (not cast by the looks), pistons are an alloy, con-rods look to be a carbon steel and the cylinder is made from cast iron. (Components may change if temperatures determine they will become too brittle)

A pre-appology if anything regarding oil/lubricants isn't right or my terminology is off, I'm on a steep oil learning curve here! haha

From my understanding of dry lubricants like graphite, they absorb water vapour which reduces the bonding energy in the graphite, thus lowering the friction when two surfaces are sliding. So with the lowered temperatures I'm hesitant this method would result in the water vapour in the graphite to freeze up.

A quick search of wiki and look about the web shows there are a couple of other products, Molybdenum disulphide, Boron nitride, Polytetrafluorethylene... I'm not sure how deep your knowledge goes is are these common in products? Molybdenum disulphide shows good promise as its not reliant on water vapour and it does seem to out perform graphite in most ways.

From anyones opinion/experience, do you think dry lubricant can work as effectively as a liquid?

As for the actual temperature of the components, Yonyon is right. A boundary layer of the moving air will effectively protect the components somewhat. However temperature saturation to some extent will occur and the maths for this along with the rest of the design aspects, is a bit ambitious for the time I have to complete this haha. So I'm using the very conservative assumption that the components will/could reach temperatures this low.

As for resistance, I think what Im trying to say is pressure resistant property oil..? From my understanding, oils have additives that effect the ability to resist being squeezed out and breaching of the lubrication film under pressure? From my understanding of what I've read, the colder the oil gets the worse it is at this.

And Yonyon, Ive had a look about but nothing has come up with anything which will satisfy so far. If you have any recommendations I'd love to research them! As most of you probably know, oils generally have a base temperature properties then go up, where as I'm looking for base temperature then decreasing haha

Also, am i right to say the Pour Point of the oil is just where its starting to behave like a solid (freezing), but its lost its lubrication properties long before this point?

Thanks for your interest so far people!
 
So you are using compressed air to produce power. The compression of the air will generate some heat, perhaps (I am NOT a thermodynamacist!) enough to offset the momentary -80 temperature due to partial vacuum.

I would think that the simple answer here would be to use a 0wxx synthetic oil for lubrication, and perhaps even add an oil thermometer to monitor conditions. Further, if needed and the oil gets too cold to function properly, you could always add a circulating oil heater. But finding out how cold the oil becomes seems to be the first step. Right now it is only theory based on a momentarily cold air temperature.
 
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"Right now it is only theory based on a momentarily cold air temperature."

Exactly, during operation temperature of the components will even out to something somewhat warmer.

You could well be in the range of a 0w- oil
 
I would consider contacting William Garmier at renewablelube.com. He has formulated solutions for similar problems. It's a small company and you can actually get through to Bill, something that will never happen in a larger operation.
 
Don't over think this. It's not so much the temperature of the air but the temperature of the environment that affects the oil. Your oil will be about the temp of the motor itself. If the operating environment is really cold, maybe something like a 0-10 oil, a cst of 6?

It would be interesting to see the application.

Wayne
 
Thanks again for all your input guys!

I think you're a bit right there Wayne about over thinking! When being surrounded by "theoretical" scenarios all day its easy to get caught up and over think it! Need to remember to take a step back every once i awhile!
Kuato I like you're approach, simplest first. Measuring to see real values will be the best way to go once operational.

Kevguy, they were some great videos. I learnt quite a bit in them and was so surprised to see the large difference in fluid properties with what I would have originally classified quite similar oils. Excellent stuff thanks, it ties in will with what expat, Kuato and Wayne have suggested, 0W - range

OneEyeJack, Thanks for the link. Ive been having a look around on his site, might email him when I get the some more data.

Im just in the process of finishing up the Design Proposal, which part of grading criteria needs me to cover a reasonable range of specifications. I think with the information and advise given so far I will be able to justify the decision with confidence and numbers.

Once operational I'll have some hard data to analyse and make better judgment if needed! (The timeline set by the University, and my deadline is October.)

Again, big thanks to everyone for advise so far and teaching me something along the way! And of course Im always open to learning and suggestions so please continue to post if anything springs to mind, I'll continually be checking this forum
 
I would use something like

2 wt

or similar non- or low-detergent oil which has a
Also, contact NYE Lubricants. They have produced some very low temp lubricants for space applications.

NYE Lubricants

Nye Aerospace Lubricants

The internal mechanical friction of the "reverse" compressor motor will heat up the oil.
 
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Just thought of something else regarding the coupling between the the "reverse" compressor motor and the pneumatic piston motor.

If you can use a material that insulates the the "reverse" compressor motor from the pneumatic piston motor you could use a product similar to (That is, a material with a low "conduction" coefficient):

Redline ATF

which would reduce viscous drag.
 
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Originally Posted By: ac_tc
refridgerator compressor oil.. the same as you fill in your cars ac.


But what viscosity? There are many viscosities of POE and PAG refrigerant oils.

The OP never stated what his minimum viscosiy requirement might be?
 
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