A mechanical servo unit comprises a primary element (9) having a wedge portion (10,11), a single pair of pressure rollers (1,2) and members (7,8) each having a pair of substantially even counter surfaces (2, 5 and 4, 6) formed thereon, one of the members (8) abutting a secondary element (19) and all being arranged within a cylindrical housing (12) the pairs of the counter surfaces (3, 4 and 5, 6) defining a narrowing aperture (15,16) therebetween and pressure rollers (1,2) being supported directly on the counter surfaces (3, 4, 5, 6) wherein action or movement of primary element (9) toward secondary element (19) is transmitted via wedge portion (10, 11), rollers (1, 2) and counter surfaces (3, 4, 5, 6) to secondary element (19) for transmission of amplified action.