Abstract: A safety circuit for a lifting device extensible boom is presented wherein the boom angle of elevation and boom length are each converted into a parallel coded word. The two coded words, one representing boom angle and the other boom length, are processed in a digital logic network which provides inhibit functions that prevent boom extension or boom lowering when either of the two boom actions will place the boom in a position which will damage the boom or topple the machine. A plurality of switches are associated with the boom to generate and apply the boom length code word to a monitoring circuit, which responds thereto to provide a switch malfunction signal when one of the switches is defective, whereby boom extension or lowering are inhibited.

Abstract: A protective circuit for an overload sensor of a lifting boom, such as an aerial platform boom. Excess deflection of the boom under load, due to such factors as weight of the load, boom angle and boom extension, is detected by the actuation of an overload sensor switch engaged by the free end of a beam which extends generally parallel to the boom, the other end of the beam being fixed to the boom. The overload sensor switch is in a protective circuit which causes interruption of current to controls which effect movement of the boom in a manner to increase the boom deflection, and which circuit causes the interrupted circuit to remain interrupted independently of the position of the overload sensor switch. The circuit requires positive resetting once there has been such interruption. The protective circuit includes a drop out relay in series with the overload sensor switch, a latching relay with re-set coil, and control circuit switches controlled by the latching relay.

Abstract: A safety circuit for a lifting device extensible boom is presented wherein the boom angle of elevation and boom length are each converted into a parallel coded word. The two coded words, one representing boom angle and the other boom length, are processed in a digital logic network which provides inhibit functions that prevent boom extension or boom lowering when either of the two boom actions will place the boom in a position which will damage the boom or topple the machine. A plurality of switches are associated with the boom to generate and apply the boom length code word to a monitoring circuit, which responds thereto to provide a switch malfunction signal when one of the switches is defective, whereby boom extension or lowering are inhibited.