Abstract: A coupling for a flexible shaft for a pump rotor orbiting within a stator. Coupling of the shaft to a driving or driven member is made easily by screwing the end of the shaft by hand into a socket in the member, and by then locking the shaft against rotation in the socket by means of abutting complementary oblique faces at the end of the shaft and at the end of a locking element in a transverse bore intersecting the socket. The screwing in of the shaft into the socket need not be more than fingertight. This permits the use of a shaft of uniform cross section (no flats for spanners) having a smooth surface finish, thereby providing good fatigue life.