Dynamic seals require additional design factors
including lighter squeeze to help eliminate friction, wear,
and heat buildup.
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
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
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 ring into a "D" shape. This
depends on friction per square inch of groove area times the
area of each particular size 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
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.