Crane

A crane which performs winding-up and winding-down of a target object, the crane comprising:

• a winch drum configured to rotate for the winding-up and the winding-down of the target object;
• a plurality of electric motors configured to be operated by supply of electric power to output respective driving torques for rotating the winch drum;
• a load deriving unit configured to derive a value of a load of the target object, the load being a load applied to the winch drum;
• an overload safety device configured to monitor the value of the load derived by the load deriving unit and stop an operation of each of the plurality of electric motors when the value of the load exceeds a set hoisting ability value of the crane to stop the rotation of the winch drum;
• and
• an identifying unit configured to identify a failed electric motor among the plurality of electric motors, wherein
• the overload safety device includes:
• a storing unit configured to store the set hoisting ability value;
• a maximum-load calculating unit configured to calculate a maximum load which the winch drum is capable of winding up with the driving torques output from the remaining electric motors other than the failed electric motor identified by the identifying unit;
• an updating unit configured to update the set hoisting ability value stored in the storing unit to a value equal to the maximum load calculated by the maximum-load calculating unit;
• and
• a control unit configured to perform control of the plurality of electric motors, and
• the control unit performs control for stopping the operation of the failed electric motor identified by the identifying unit, causing the remaining electric motors to operate when the value of the load derived by the load deriving unit is equal to or smaller than a latest set hoisting ability value stored in the storing unit, and stopping the operation of the plurality of electric motors when the value of the load derived by the load deriving unit exceeds the latest set hoisting ability value.