Dynamic O-Ring seal Design and Gland Dimensions

Dynamic O-rings

FRICTION - the higher the durometer hardness (above 70 Shore A) the greater the friction. This is due to the fact that the compressive force, at the same squeeze, is greater than with softer compounds.

Compound swell decreases with hardness and may increase the friction. The lower the operating temperature, the harder the seal becomes which can also increase friction. However, thermal contraction of the seal material which reduces effective squeeze may offset any increased friction caused by an increase in hardness.

Breakout friction is the force necessary to start the unit moving. It is dependent upon the length of time between cycles. It also depends on the surface finish of the metal, the rubber hardness, squeeze, and other friction-affecting factors. Breakout friction can be reduced by utilizing softer O-Ring compounds.

Running Friction depends on two factors: (1) The force exerted on the ring's rubbing surface by the compression force of the squeeze. This depends on the hardness of the o-ring, the percent of the squeeze, and the length of the rubbing surface; and (2) the force of the system's pressure against and tending to distort the o-ring into a "D" shape. This depends on friction per square inch of groove area times the area of each particular size 0-ring.

There are many variables which make it difficult to accurately predict the total friction for a particular case. The best way to be sure that dynamic O-Rings do not have excessive friction is by experimentation. Decreasing squeeze, surface finishes, cross-section, and using external lubrication will help.

GROOVE FINISH - Straight-sided grooves are best to prevent extrusion or nibbling, but 5 degree sloping sides are easier to machine and are suitable for pressures up to 1500 psi.

The rubbing surfaces should be 8 to 16 RMS without longitudinal or circumferential scratches. The best surfaces are honed, burnished, or hard chromeplate. Finishes below 5 RMS wipe too clean for good moving seal life.

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