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Plenty of spacecraft seem to have moving parts. Space shuttle doors open, robot arms manipulate things, etc.

This is perplexing, when we consider that lubricating oil probably won't remain liquid, and even solid-on-solid contact risks vacuum welding, which can (and apparently does) freeze up joints. If lubricating won't work, and not lubricating won't work, it seems like we've argued ourselves into a corner.

Robotic arms often seem to be covered in a fabric. Maybe they're just "canned", like a space suit so that the joint itself isn't exposed to vacuum? Excluding those that are canned, are there any types of joints that are actually used in space with the rotating part exposed to vacuum?

Shuttle arm

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    $\begingroup$ Most lubricating oils used terrestrially are inappropriate for space environments. Slip joints are common, with dissimilar materials used to prevent cold welding. Teflon or other similar low-friction coatings are used as well. I'll let a mechanical engineer write a real answer. $\endgroup$ – Adam Wuerl Sep 26 '13 at 17:35
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This is actually quite a serious issue, and one that has been solved in a variety of ways. The method I'm most familiar with is Reaction Wheels, and I'll try and cover some of the other key points as well.

There are basically two kinds of reaction wheels. The first kind is those that are sealed up, done to make it easier to manage the lubrication. The second is those that use lubrication made specifically for a vacuum in a zero g environment. The second is considered the more reliable way, although there are a fair number of missions which choose the first, as it is made much easier in the end. Hubble chose the second, and while I can't read the paper about it, I can tell you that the reaction wheels used a grease qualified for space.

There are also a fair number of dry lubrications out there. The linked paper details how it was done, but the end result was that it worked, although the maximum speed was reduced over other methods.

As far as why many mechanical joints have cloth, no doubt it is to help keep the thermal situation in better control. Direct light could heat the joint up, causing it to not work optimally, but the cloth works an an insulator. A great insulator isn't required, and it is required to move flexibly, cloth would work ideally for this situation.

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