a pair of connecting pins 120 and 122 welded or otherwise secured to the upper edges of the pair of pawls the connecting pins 120 122 are disposed on opposite sides of the leaf chain 80 a torsion spring 124 is wound around the pawl support shaft 110 and terminates at one end 126 in a leg that bears against the inside of the gusset 64 of the body 60 the other end of the spring 124 terminates in a leg 128 extending into the space between the leaf chain 80 and the forward connecting pin 122 against which it bears the pair of cam pawls 114 are thus biased to pivot around the shaft 110 in a clockwise direction as viewed in fig2 each of the pawls 114 has a cam edge 130 whose radius relative to the axis of the shaft 110 increases downwardly while the cam edge 130 could take the form of a friction brake shoe preferably it is defined by the crests of a space apart series of hardened teeth 132 each of which is formed with asymmetrical flanks adapted for unidirectional biting engagement with the inside surface of one of the flanges 118 of the guide column 18 when the brake is actuated more particularly the radius of the cam edge 130 is such that when it occupies a retracted position as shown in fig2 the radius at the upper end of a pawl.