Wet Friction Applications
Mechanical seals, brakes, and clutches are susceptible to form of a surface damage known as hot spotting. They often emerge as macroscopic dark spots or patches on the surface and are considered to be a form of failure. The shape and distribution of these spots are quite different depending on the material properties and operating conditions. Hot spots are thought to occur as a result of intensive localized frictional heating. Concentrated temperatures bring about a relatively large local thermal expansion and consequently a large contacting pressure that further aggravates the frictional heating. Hence, the cycle continues until failure occurs. This entire process is often referred to as thermoelastic instability (TEI) and there are theoretical analyses bearing the same name that deal with this subject. Hot spots, if not prevented, can cause an unacceptable performance or durability in friction systems. In face seals, hot spots may be a prelude to heat checking (multiple radial cracks) and eventual leakage. Automotive disk brakes sometimes exhibit a low frequency vibration and noise caused by uneven torque resulting from TEI.

A wet clutch is composed of a set of concentric discs commonly referred to as separator and friction plates with the automotive transmission fluid (ATF) serving as the lubricating agent as well as a coolant. During the first stage of the engagement process, pressure is applied hydraulically by means of a piston and hydrodynamic pressure is developed in the ATF. At this stage, a relatively thick film of fluid separates the surfaces and thus the behavior of the clutch is governed by the theory of hydrodynamic lubrication. During the engagement process, the film thickness is reduced to the extent that the surface asperities come into contact. As a result, the contact pressure at the asperity level begins to support a major portion of the imposed load, thus significantly influencing the behavior of the wet clutch. The film thickness undergoes a further reduction as the porous friction material is compressed and deforms elastically. The surfaces are subsequently pressed together and locked when their relative speed drops to zero.

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