The coefficient of friction of rubber material ranges from 0.001 to 10 which depending on the interface conditions.
In case of shaft seals or O-ring
Hydrodynamics film thickness is about 150 mm. Shearing of this film is the prime cause of dynamic or running friction, so that the dynamic coefficient of friction is a function of lubricant viscosity and sliding velocity. Running friction can be mimimized by optimizing vicosity and velocity effects.
Causing
Seal friction create a heat buildup , often causing premature failure due to excessive heat aging of the rubber material.
Rubber Formula :
nitrile seal formulations often include a hight percentage of graphite as a filler. becouse it increased the thermal conductivity , thus carrying heat away from the rubbing surface.
Warning :
Graphite is not used to reduce friction and it is no more than a bulk filler that hardness is increased only.
Trend
Concurent engine oil temperature have increased with smaller engines operating at higher frequency . This combination of more serve operating conditions and changing seals requirements at oil operationg temperatures. But the additional thermal load created by seal frictions can raise this value to 175 to 200 *C at the seal contact .The temperature in excess of the capabilities of nitrile rubber material. Finite element analysis of heat transfer in rubber products is increasingly needed in the design stage, using a specific heat value of the rubber material to maximize service life.
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