turn causes shafts 72 and 70 to rotate and thereby to lower cable 54 and as it is unwound from pulley 52 a power source generally indicated by reference numeral 100 is also mounted on frame 30 and comprises a hydraulic housing 102 which includes a piston 104 which is raised vertically upon actuation of the power source a bell crank lever 106 is pivoted at 108 to the framework 30 and has a follower 110 mounted on one end of the lever 106 for engagement with piston 104 the opposite end of lever 106 is bifurcated and has mounted thereon followers 112 pivotally mounted to the inside surfaces of the bifurcated portions a space 114 is formed between the followers 112 adaptor 60 has a reduced portion 116 which forms upper and lower shoulders 118 and 120 respectively a slotted key 122 has upper and lower surfaces 124 and 126 respectively which bear against shoulders 118 and 120 respectively the space 114 between followers 112 is wider than the adaptor and allows the followers to straddle the adaptor and bear against the upper surface 124 of key 122 actuation of power means 100 will cause piston 104 to move upwardly and partially rotate arm 106 around pivot 108 this will force followers 112 downwardly against the upper surface 124 of key 122 with a force which is sufficient to overcome the frictional driving force between.