Generation of hot spots in a wet multidisk clutch during short-term engagement

Hot spots observed in clutches and brakes result from unstable thermoelastic behavior of the frictional system. The phenomenon has been a subject of numerous experimental and theoretical studies, most of which considered steady-state operating conditions. However, most of the clutches and brakes used in automotive and heavy-duty equipment operate in transient mode, at rapidly varying sliding speed. In the paper, generation of hot spots during short-term engagement with varying speed is investigated. Both theoretical analysis based on finite element model of thermoelastic contact problem and experimental stand tests are presented. The study shows that severe hot spots can be produced during short-term clutch operation with high initial sliding speed and that small geometric imperfections of the disks are able to effectively trigger that process. Tests performed with two different friction materials and with different geometric parameters of the disks indicate how elastic properties and geometry influence hot spotting. It is demonstrated, in particular, that the problem can be effectively mitigated even if an instantaneous sliding at high speed (exceeding the critical speed for thermoelastic instability) cannot be avoided.