The coupling transmits the drive from the drive member (13) to the driven shaft (1). It is formed by a bush (3) is rigidly connected to the drive member (13), with four equally spaced longitudinal slots. The bush has an outer surface with a cross-section defined by a spiral curve, and fits inside a second bush (8) which has a matching bore. When the inner bush is rotated w.r.t. the outer bush, the inner bush is radially compressed and grips the shaft. Thus the drive is transmitted by friction from the drive member to the shaft.
