[PSUBS-MAILIST] K-sub hatch o-ring

[Sean T. Stevenson via Personal_Submersibles]

Per ORD5700, stretch should be limited generally to no more than 5%. A 2-471 O-ring (21.955" ID) on a 23.25" gland is 5.90% stretch. A 2-472 O-ring (22.940" ID) on that same 23.25" gland is only 1.35% stretch.

Sometimes it is advantageous to specify an O-ring which must be stretched for installation in order to create friction against the gland in order to prevent the O-ring from slipping out of the groove if it isn't retained by other means (e.g. dovetail).

At 5% stretch however, the cross-sectional diameter of the O-ring is reduced by about 2.5% (and at 5.9% squeeze, almost 4%) so the gland depth may need to be reduced accordingly in order to achieve the design squeeze on the O-ring. Per ORD5700, for face seal glands in sizes 425 through 475, that design squeeze is 21% to 29% with no extrusion gap.

For face seals subject to external pressure, the gland is usually sized with its inside diameter equal to the mean diameter of the O-ring, with tolerance of +1% of that mean ID, but not more than 0.060". The reason for this is that by ensuring that the O-ring is situated on the correct side of the groove for the anticipated pressure, it won't experience premature failure as a result or being shifted across the groove on every pressurization cycle. For a 2-471, that mean diameter would be 22.230", and for a 2-472, 23.215", which would correspond to 1.25% and 1.20% installation stretch respectively, but these are recommended design minimums.

My guess is that the 23.25" gland ID just provides enough stretch to keep the O-ring from slipping out, and performance in that embodiment was good enough. Provided the cross-sectional diameter reduction and associated gland depth changes are accommodated, the only downside to greater installation stretch is premature aging of the O-ring. (See ORD5700, 3.6).

Sean

-------- Original Message --------
On Jun. 9, 2021, 13:19, Jon Wallace via Personal_Submersibles wrote:

> Can someone with a K-350 measure the ID of the hatch o-ring channel and report back on it? The K350 plans specify a 2-471 O-ring which has an ID of 21.995 inches but I'm measuring an ID of 23.25 on the K600 and I'm thinking the K350 also has an ID of 23.25. That would seem like a bit of a "stretch" installing the 2-471.
>
> There's a 2-472 O-ring that has an ID of 22.995 inches which would require less stretching, but...???
>
> Will post on FB as well.
>
> Jon
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