Abstract: A mechanism for selectively locking a stationary element and a movable element mounted on the stationary element. The mechanism comprises a pawl block mounted within a casing for sliding movement in opposite first and second directions along a first axis substantially transverse to an engagement surface on the stationary element. The pawl block has a substantially "V"-shaped cavity defined by two ramp surfaces which, together with the engagement surface, define a substantially triangular recess. A striated roller member is mounted within the recess for simultaneous circumferential engagement with the engagement surface and at least one of the inclined ramp surfaces when the pawl block is in its locking configuration.