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In Spacesuits, especially in newer ones like the Exploration Extravehicular Mobility Unit (xEMU), there are a large number of rotating joints at different portions like wrists, arms, torso, etc.

According to my logic, between a rotating part and a stationary part, there must a small gap to allow rotation. For example, consider a conventional DC Motor, there is a small circular gap between the rotating shaft and the non-rotating motor body. Now coming to space suits, if there are gaps between rotating and stationary parts in a spacesuit, it leads to decompression, but in reality, it doesn't happen. Further, I could think of gas-tight spacesuits with rotating joints with limited range of rotation about its axis, but the new Artemis Generation Spacesuits have the full degree of rotation in arms especially.

How do engineers tackle the problem of gas leaks or decompression due to the rotating joints (bearings) when the suit is pressurized?

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    $\begingroup$ Sealed rotating joints are at least as old as the first ship with a screw propeller. Which is not to say an air-tight joint is as simple as a water-tight one. $\endgroup$
    – Violet Giraffe
    Commented Oct 21, 2019 at 16:21
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    $\begingroup$ I would prefer gas-tight instead of air-tight. There are no spacesuits filled with air. $\endgroup$
    – Uwe
    Commented Oct 21, 2019 at 18:15
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    $\begingroup$ Something to remember about asking any engineering question is that there is always tolerance involved. For "air tightness," there are acceptable leak rates. No seal is perfect, but many may be good enough. $\endgroup$
    – Erin Anne
    Commented Oct 21, 2019 at 23:39

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One of several solutions is the use of O-ring seals. A seal does not need to be perfect gas-tight. As long as the leak rate of the suit is small compared to the oxygen consumption of the astronaut it is ok.

There are lubricated O-ring seals between the moving parts. The elastic O-ring is seated in a precisely machined grove and slightly compressed during assembley. There should be no nicks, burrs, or scratches at the metal surfaces over which the O-Ring moves. The gap between stationary and rotating parts is filled by the O-ring. The gas pressure should move the O-ring closer to the gap.

See the images at the bottom of page 6 and 9 of this O-ring handbook PDF.

The EMU Maintenance Kit for the Apollo Suits contains a tool for O-ring removal and pads for lubrication:

enter image description here

From the Apollo Operations Handbook EMU.

But there is no information about O-rings used for rotating joints.

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    $\begingroup$ For the suits in question, how do you know this is true, or is this just a guess? There are alternatives. $\endgroup$
    – uhoh
    Commented Oct 22, 2019 at 4:57
  • $\begingroup$ @uhoh, I've heard that O-Rings are used in Cupola's Shutter windows' mechanical handles to prevent depressurization. So it works. But, I'm not sure they use the same here. Having an O-Ring of that large radius might cause some additional trouble. So most probably, it must be something else. $\endgroup$
    – Vishnu
    Commented Oct 22, 2019 at 10:10
  • $\begingroup$ @Intellex I've just asked the possibly-related question Are ferrofluidic seals used in spaceflight? Main applications? Human-rated? $\endgroup$
    – uhoh
    Commented Oct 22, 2019 at 10:23
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    $\begingroup$ This iopscience.iop.org/article/10.1088/1757-899X/187/1/012014/pdf seems to be a summary of current and proposed future designs and suggests lip seals are used (more motion tolerance than a basic o-ring) but vague on material details. Scye bearings seems to be a useful search term $\endgroup$
    – GremlinWranger
    Commented Oct 22, 2019 at 11:58
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    $\begingroup$ Shuttle/ISS EMU suits did use o-rings in the bearing seals but also lip seals. The arm and waist bearings were called "Dual Seal [joint name] Bearings" $\endgroup$
    – Organic Marble
    Commented Oct 22, 2019 at 12:50
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These joints are absolutely amazing. Not only do they seal gas pressure, they move with minimal friction, independent of axial pressure on the joint. Any conventional joint design would “lock” due to pressure. Even the 4.5 psi pressure in an EVA suit would put over a half ton of axial pressure on a torso joint. Providing low friction movement under these loads is a much bigger challenge than sealing. Used on atmospheric pressure diving suits, joints are certified for use at 1000ft. That produces about 125,000lb axial compressive force on a torso joint.

The key feature which allows rotation under pressure is a ball bearing race, visible as item 134 in the patent drawing below and mentioned in https://www.americanscientist.org/article/the-past-and-future-space-suit

There are numerous seal cross section which are proven performers. “O-rings” are the most common, and is the design used in the patent application below. "X-rings" are similar to O-rings" but have an "X" cross section so they have4 sealing contacts instead of 2.

The joints have a double seal, with no load on the second “back-up” seal. So if the primary seal fails, the secondary seal is “virgin”. Detailed patent drawing available at https://patents.google.com/patent/US6725464B2/en

enter image description here

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    $\begingroup$ Is this diving suit design applicable to EVA suits? $\endgroup$
    – Organic Marble
    Commented Jan 14, 2022 at 19:12
  • $\begingroup$ Nasa has designed a "hard shell" space suit using these joints. See flashbak.com/nasas-number-1-space-suits-model-419940 for action shots. $\endgroup$
    – Woody
    Commented Jan 14, 2022 at 19:27
  • $\begingroup$ @OrganicMarble ---- The OP specifically asked about "rotating joints" as opposed to "fixed joints". EVA suits are a combination of hard shell and soft shell components joined by both fixed and rotating joints. The dive suit is all hard shell segments and rotating joints. $\endgroup$
    – Woody
    Commented Jan 14, 2022 at 22:24

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