A friction coupling (3") for locking a shaft (1) relative to a hub (2) comprises an inner sleeve (4), which is arranged for frictional engagement with the shaft (1), an outer sleeve (5), which is arranged for frictional engagement with the hub (4), and means (6; 6a) for actuating the friction coupling by radial deformation of at least one of the inner sleeve (4) and the outer sleeve (5). The friction coupling further comprises a compensating gap (7), which is arranged between said actuating means and the other of the inner sleeve (4) and the outer sleeve (5), and adapted to counteract radial deformation of the other of the inner sleeve (4) and the outer sleeve (5).