Study on the Clarification of the Mechanism of Scooter Centrifugal Clutch Squeal

A centrifugal clutch is used in many machines such as scooters, lawn mowers, outboard motor of boats, brush cutters, and so on. It may produce a shrill sound, similar to a brake squeal, when the clutch engages for starting. In this study, we have proved that this shrill sound, in another word, clutch squeal, is caused by the self-excited vibrations of the centrifugal clutch. And we have also clarified that the clutch squeal can be restrained by employing an asymmetrical shape for the clutch housing. The clutch squeal tends to occur when the centrifugal clutch becomes hot due to repetitive starting and stopping, which causes the friction coefficient of the friction material on the clutch shoe to increase. It is presumable that vibration on the clutch housing generated by the self-excited vibrations is the cause of the clutch squeal. In an attempt to clarify the cause of clutch squeal, we first measured the sound pressure of the clutch squeal and the vibration shapes of the clutch housing when the squeal occurred. Then, using the complex eigenvalue analysis, which is one of the finite element methods, we got unstable modes of the centrifugal clutch that could lead to self-excited vibrations. By conducting a comparative verification of each result, we have proved that the cause of clutch squeal is the higher frequency self-excited vibrations in the centrifugal clutch. And using the complex eigenvalue analysis, we gained the perspective that applying an asymmetric shape to the clutch housing is a prospective solution to restrain the generation of unstable modes, and this was verified by testing with an actual scooter.