Rn and their progeny concentrations in residences in Hong Kong

A self-energizing disk brake having a pair of thrust plates positioned within a stationary housing and between brake disks connected to a shaft to be decelerated. An actuating means to rotate the plates relative to each other to cause spherical balls trapped in complementary indentations in the facing surfaces of the plates to push the plates apart against the bias of a spring means into frictional engagement with the brake disks. Centering cams and lugs on the plates are capable of selective engagement with guide surfaces and projections formed on the housing. The cams and lugs are provided with yieldable vibration dampening elements having a low coefficient of friction to minimize the axially directed forces resisting separation of the plates.