The coupling comprises two coaxial discs (1, 11; 26, 28; 36,38; 46, 48) a resilient ring (8; 29; 39; 49) of frictional material positioned between the discs, means (14) for retaining the ring substantially coaxially with the discs, and means (7; 27; 37; 47) for biasing the discs into frictional contact with the ring. The clutch includes an additional means for engagement and disengagement of the discs. If the clutch is a slipping clutch it includes setting means (6) for the adjustment of the biasing force exerted by the biasing means (7). The brake has the same element as the clutch but one of the discs is stationary.