GB1400402A - Safety devices for jib cranes of variable working radius - Google Patents

Abstract

1400402 Crane safety devices STRAWSON HYDRAULICS (CONSULTANTS) Ltd and WHARNCLIFFE FABRICATORS & ENGS Ltd 4 July 1972 [7 April 1971 6 Oct 1971] 8993/71 and 46480/71 Heading B8B A safety device for a jib crane (16), Fig. 1 (not shown), of variable working radius utilizes a plurality of printed circuit boards 122, used selectively, to relate the load to the maximum allowable safe working load at a particular work- ing radius and under particular working conditions. A mercury switch (44), Fig. 2 (not shown), used in conjuction with a servomotor (48) produces an electrical signal corresponding to the elevation of the jib, while a spring-loaded, jib-length, cable drum (52), Fig. 3 (not shown), ultilizes an electrical proportioning device to produce an electrical signal corresponding to the jib length. These two electrical signals are compounded to produce a further electrical signal corresponding to the working radius of the crane. The printed circuit board 122 comprises a non-conductive lamina 124 with conductors 126, 128 extending in parallel along its upper and lower edges. A metallic line 130 is also presented just proud of the lamina 124 and represents the safe load/radius curve (for a particular working condition) supplied by the crane manufacturer. A resistance element is associated with the circuit board 122 and comprises an insulation rod 132 with several thousand turns of resistance wire 134 and bonded to a supporting strip of insulation material 136. In use a voltage is applied across the conductors 126, 128 and the resistance element is caused to move, in contact with the conductors 126, 128 and the wire 130, across the board 122 in response to the changes in working radius of the crane. The device acts as a potential divider and its output signal is dependent on the curve of the line 130 and can be compared with the actual load on the crane as measured by an electronic load cell (60), Fig. 4 (not shown), acting against the load rope (32). The result of comparing the signals may be used to activate a visual alarm, followed by an audible warning, when the safe working load is approached or exceeded, when a recorder may be brought into action. Instead of resistive devices the safety device may utilize inductive or electrostatic devices, Figs. 8 to 10 (not shown).