Axial Gap & Excessive Tolerances

Axial Gap & Excessive Tolerances

In an axial application, if the hardware is not able to achieve face to face contact with zero clearance gap, there are two potential impacts to the O-ring or seal. The first risk is extrusion. The second is inadequate compression.

Or perhaps the hardware has face to face clearance, but the tolerance on the groove and sealing surface is fairly large. Like a clearance gap, the impact will be to the designed squeeze, also called compression.

Risk of extrusion is a function of the amount of clearance, the fluid pressure, and the seal’s ability to resist extruding. Its possible the O-ring material can be selected for greater extrusion resistance, however if the pressure is high enough, or the gap large enough, any O-ring or other seal profile will extrude. Possibly a thermoplastic or rigid seal will be needed in this event.

The second risk created with a clearance gap or excessive tolerances is the increased variation in squeeze. Application and test data has consistently shown that for a solid O-ring there is an optimum amount of designed squeeze. Typically this in the 25% range, and with tolerances, the rough range of 20 to 30% compression. If the gap opens up a few thousandths of an inch, this can increase the designed squeeze range to 18 to 32% or even more, 16 to 34%. High variation in squeeze can lead to premature compression set, high compressive load forces, and possible rupture of the seal.

Some alternative seal options which compensate for axial gaps and tolerances are:

• Diamond & Precision Cut Seals (Axial)
• Hollow Profiles (Axial)
• Lip Seals (Axial)
• Transfer Port Seals (Axial)