5 has been fixed here by an attachment screw 26 when the attachment screw 26 has been slightly loosened this catch 25 can be shifted in the longitudinal direction of the slit 23 so that for every conceivable v belt pulley diameter or every conceivable position of the cover 7 on the casing 1 it can be positioned and fixed at the maximum distance required by din 42 703 of 3 mm to the outer perimeter of the v belt pulley 5 as can be seen from fig1 the belt anti drop device 22 does not prevent the cover halves 9 10 from being pulled apart an ingoing belt safety device 27 is provided in the cover half 10 accommodating the ingoing belt section 20 of the v belt 6 this comprises a safety pin 28 a peg or similar which extends into the wedge shaped area between the ingoing belt section 20 of the v belt 6 and the v belt pulley 5 which is not encompassed by the v belt 6 at this point whereby the distance from the safety pin 28 to the perimeter of the v belt pulley 5 on the one hand and to the ingoing belt section 20 on the other hand may not exceed a maximum of 4 mm according to din 42 703.