34 and vice versa and preferably that a full battery of spades 23 must be activated in a single and quick maneuver this is accomplished by means of regular wire rope as normally employed in various types of lifting hoists etc high tensile steel wire rope is wound on drums and passed over sheaves at operating radii in order of five to seven times its diameter this is made feasible from the build up of wires into strands and strands wound into rope if pre formed to conform with radius of curvature as defined by the six anchor rings 18 and slipped into place as shown in fig4 a wire rope 21 can be rotated around its longitudinal axis with only slight resistance and so due to construction thus with spades 23 securely fastened to wire rope 21 six spades are simultaneously put into engaged or disengaged position it has been found however that for satisfactory operation spades 23 should press firmly against under tread face 13 while engaged and likewise should press firmly against side wall 17 while disengaged such pre load of spade 23 is accomplished by pre stressing wire rope 21 before installation this is done by simply pre forming it to take a radius of curvature smaller than that.