O-ring
An o-ring is a rubber gasket with a round cross-section, and is usually used to create an airtight seal around moving parts, such as pistons and bolts. O-rings are cheap and come in a wide range of sizes. However they need to be mounted in a precisely machined groove for most applications, and a lathe is usually needed to achieve this, so they are not always a viable option.
An o-ring seal, when done correctly, can hold thousands of psi.
For those with a lathe, the process for machining an O-ring groove is:
- Chuck the workpiece well, ensuring that it rotates true.
- Gently measure the actual radial thickness of the O-ring.
- Using a cut-off tool or similar tooling that will produce a deep flat bottomed cut, cut a groove of the proper width to a depth that will put a 5-10% squeeze on the O-ring.
This depth can determined by the formula O*(100%-S%) - 1/2(OD - ID) Where O is O-ring thickness, S% is the percent of squeeze (higher percents make tighter O-ring seals), OD is the outside diameter of the interior part, and ID is the inside diameter of the exterior sleeve.
Putting the O-ring on the exterior sleeve is not recommended, as it is harder to machine and tends to fall out if the interior part fails to press against it.
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