Patent ID: 7490728

Claim:
A crane having: a hoist rope winch ( 7 ); a hoist rope ( 5 ) with a proximate end attached to, and partially wound around, the hoist rope winch ( 7 ); a lifting hook ( 6 ) attached to the hoist rope ( 5 ) and disposed to be attached to an object to be lifted or lowered; a hydraulicly operated hoist winch drive motor ( 9 ) coupled to the hoist rope winch ( 7 ) to drive same; and an overload protection ( 18 ) includes: a controllable balance valve ( 20 ) disposed to receive hydraulic pressure from a control pressure line and distribute the hydraulic pressure between first and second hydraulic input ports of the hydraulic hoist winch drive motor ( 9 ) to reel the hoist rope ( 5 ) on and off the hoist rope winch ( 7 ) under a predetermined variable retention force; and a control device ( 10 ) disposed to retain a value of a permitted load limit for each potential crane radius and to determine a current radius of, and load on, the crane and being coupled to overload protection ( 18 ); wherein, when the predetermined permitted load limit is reached for the then current radius of the crane the control device ( 10 ) controls the controllable balance valve ( 20 ) to distribute the hydraulic pressure between the first and second hydraulic input ports of the hydraulic hoist winch drive motor ( 9 ) to permit drawing off the hoist rope ( 5 ) from the hoist rope winch ( 7 ) under a predetermined variable hoist rope retention force.