Lubrication: Dry vs Wet – Oil vs Grease

by on March 25, 2015

grease gun

One thing that is often looked is lubrication (lube). Changing your lubrication and selecting the proper type can dramatically modify the performance of your system. This change of lubrication was one of the primary reasons MSL was delayed from launching to Mars. Improper lubrication can cause your mechanical parts to wear out and fail. It can also lead to annoying audible sounds, and increased power usage. To figure this out we need to turn to Tribology, which is the study of friction, wear, and lubrication.

Where do you have lubrication on a robot? The simple answer is at any joint that moves. In practice this means near actuated joints, bearings, sliders, chains, and in gear boxes. Lube can also be used to put on metal components to reduce rusting.

There are generally two classes of lubrication. Wet and Dry (or solid). Wet lubes stay “wet” while on the part. This is good for operating in wet/humid conditions. The down side is that dirt can stick to it. Dry lube often goes on wet but then dries on the parts. Dry lube is good since soil particles do not stick, but generally it does not last as long as a wet lube and needs to be reapplied more often, with that said dry lube tends to stay-in-place a bit better than wet lubes. The dry lubes are generally more expensive and have lower performance. In space and vacuum applications dry lubes are usually required to minimize out-gassing.

Another common choice is grease vs oil. This is especially true for motor gearboxes. Grease adheres better than oil and can typically last longer, it has the added benefit of helping to keep dirt out by acting as a sealant. With that said, if a lot of particulate matter gets into the grease it will also compromise the lubricant, using seals to “hide” the lubricant is still a good idea. Grease starts thicker than oil, so in a motor when it first starts to spin it can have a higher current draw which will drop as the grease starts moving, heating up, and forming a light coating on the parts. In the cold, grease can get very hard and not form the effective coating on the parts. Starting grease from a cold start can take a lot of energy to heat up and/or spin a motor. Running with cold grease can be equivalent to running with no lubrication or running with a solid mass in the way. If you need to start from a cold temperature and/or want to reduce your current consumption oil might be better.There is usually an optimal oil viscosity given a mechanical speed and power input, since the viscosity and lubricating effectiveness changes based on the heating effects. Oil often is used at higher speeds due to the lower viscous drag, compared with grease. There is a continuum of various “weights” in greases and oils across the viscosity spectrum that you can look at when choosing your lube. Oil is also good where lube needs to be circulated through something (examples are for automated lube systems or for transferring heat for thermal management). As your part wears away grease can sometimes hold better in the part than oil.

Two plots of different standard engine oil. Showing the viscosity with temperature.

Two plots of different standard engine oil. Showing the viscosity with temperature. (Click for larger image)

People often put too much lube on their system. The lube needs to put a thin coat on your moving parts. The globs of lube above that is generally a waste. For factory systems where lube needs to be applied regularly there are often automated systems that apply lube based on some schedule.

The above is general information. There are many different types of oil and grease; each with different characteristics. When you really start getting into the details there are all sorts of material compatibilities, corrosion properties, organic vs. synthetic lubes, viscosities, temperatures, speeds, forces, fretting, water displacement film layers, etc.. that you must take into account. If you are too lazy to select a lubricant (and I often am), purchasing a “general purpose” grease or oil will probably give you medium performance, medium viscosity, and medium just about every other parameter.

It is important to note that lubricants can sometimes be avoided in mechanisms (I know I am contradicting what I have said above). Low friction bushing materials like teflon, nylon, rulon-J, bronze, and even steel on steel are simple options for mechanisms with low-speed joints that can require no lubrication.

Do you know more about lube? Please leave your comments below.

I would like to thank Nick for helping me edit this article, and for providing information to improve this article.

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