Overload protection system for a crane

A multi-stage system for protecting a crane against overload, comprising:

• (a) a structural frame;
• (b) an elongated boom, said boom having an inboard end pivotally secured with respect to the structural frame about a substantially horizontal axis and an outboard end adapted for guiding a load hoist line reeved thereabout, said boom being pivotally movable throughout a range of movement;
• (c) a boom hoist line interconnected to the outboard end of said boom for pivoting said boom about said horizontal axis to raise and lower the outboard end of the boom;
• (d) a load hoist line, said load hoist line reeved about the outboard end of the boom for supporting a load connected to the load hoist line;
• (e) means for selectively retracting and extending the load hoist line to raise and lower a load supported by the load hoist line;
• (f) means responsive to a load on said boom hoist line for generating a plurality of different control signals, which control signals are adaptively dependent upon the magnitude of the load on the boom hoist line throughout the range of boom movement;
• and
• (g) control means responsive to said signal generating means for adaptively providing multiple levels of corrective action to control both the boom hoist line and the load hoist line, said control means being operative to prevent lowering of the boom in response to a first control signal and operative to vary the magnitude of the load supported on the load hoist line in response to a second control signal.