Importance

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Abstract

A mobile lift crane includes a carbody having moveable ground engaging members; a rotating bed rotatably connected to the carbody such that the rotating bed can swing with respect to the ground engaging members; a boom pivotally mounted on a front portion of the rotating bed; a mast mounted at its first end on the rotating bed; a backhitch connected between the mast and a rear portion of the rotating bed; a moveable counterweight unit; at least one hydraulic cylinder; and at least one arm pivotally connected at a first end to the rotating bed and at a second end to the hydraulic cylinder. The arm and hydraulic cylinder are connected between the rotating bed and the counterweight unit such that extension and retraction of the hydraulic cylinder changes the position of the counterweight unit compared to the rotating bed. In one method of operation, the counterweight is positioned forward of a point directly below the top of the mast when no load is on a load hoist line suspended from the boom; and positioned reward of the top of the mast when the hoist line supports a load; and the moveable counterweight is never supported by the ground during crane pick, move and set operations other than indirectly by the ground engaging members on the carbody. In another method the crane is used to perform a pick, move and set operation with a load wherein the move able counterweight is moved toward and away from the front portion of the rotating bed by extending and retracting the hydraulic cylinder during the pick, move and set operation to help counterbalance the load, but the counterweight is never supported by the ground other than indirectly by the ground engaging members on the carbody.

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First Claim