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Background Knowledge：
Common sense knowledge about dry friction of mechanical seal
Generally, due to increase in force and decrease in gap of mechanical seal , friction pairs will start to produce dry friction due to temperature increase or other reasons. In addition, most mechanical seals experience semi-dry friction or dry friction briefly when the machine is turned on or stopped. Because dry friction will cause the sealing ring to be thermally cracked or burned and subsequent malfunction, it is necessary to study the dry friction state of mechanical seal.There is a sealing ring made of self-lubricating materials. Despite this，the limit PV value must be known.
The famous formula derived by Grubeyev of the Soviet Union was used to determine the maximum PV value without thermal cracking.IIn the formula，you have to know thermal shock coefficient of the said material. Below are thermal shock coefficients of some common materials
material | Thermal shock coefficient |
Alumina（99% Al2O3） | 2.8 |
Cemented carbide（7% Co） | 71.7 |
Silicon carbide （atmospheric sintering） | 46.8 |
Formed carbon graphite | 3—7 |
Impregnated carbon graphite (Babbitt alloy) | 35 |
Impregnated carbon graphite (high temperature treatment) | 27 |
graphite | 80—170 |
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