Safety-guard for a crane

Claim 1

In a crane having a base, a boom pivotally mounted on the base so that the lift angle thereof may be varied, and at least one hydraulic cylinder unit connected to the base and the boom for varying the lift angle of the boom, the hydraulic cylinder unit comprising a cylinder, a piston slidably inserted in the cylinder in a fluid-tight manner, and piston rod connected to the piston at one end thereof and projected outwardly from the cylinder and having a hollow end, a safety guard comprising: a. support means secured to the piston rod within said hollow end thereof for supporting stress-detecting means, b. stress-detecting means for detecting stress in the axial direction of the cylinder unit by the moment applied to the boom by the weight of the boom and a load carried by the boom, the stress-detecting means including at least one strain-detecting gauge fixedly mounted within said piston rod on said support means, the stress-detecting means providing an electrical output signal corresponding to the stress in response to strain of said support means detected by the gauge, c. safety-limit signal generating means for generating an electrical signal which corresponds to a predetermined maximum allowable moment applied to the boom, and d. safety means for receiving the electrical output signal from the stress-detecting means and the electrical signal from the safety-limit signal generating means and for determining a relationship therebetween.