| The surface speed of a reciprocating
shaft can affect the function of a seal. Hydroplaning
and frictional heat may occur with excessive speed, while
stick-slip, discussed previously in the friction section,
is most often associated with slow speed.
Hydroplaning occurs when hydrodynamic forces lift the
sealing lip off of the dynamic surface, allowing fluid
to bypass the seal. The lip geometry, as well as the
overall force on the lip, will influence its ability
to resist hydroplaning. Most hydraulic seals are rated
for speeds up to 20 inches/second (0.5 m/second), but
this may be too fast for certain lip geometries or when
the seal has a lightly loaded design. The Seal
Theory-Lip Geometry section of our web-site shows
which lip geometries are subject to hydroplaning. Straight
cut and beveled lip geometries are the most effective
at resisting hydroplaning so long as sufficient lip
loading is present to overcome the hydrodynamic forces.
High surface speeds can create excessive
frictional heat. This can create seal problems when
the
dynamic surface is continuously moving. The under-lip
temperature of the seal will become much hotter than
the system fluid temperature, especially when the seal
is under pressure. If the heat being generated cannot
be dissipated, the seal will experience compression
set, wear, extrusion and/or increased chemical attack.
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