Imaginary pivot lift crane

摘要

A lift crane having a separated transporter-supported front ring segment and a transporter-supported rear works including a counterweight. The front ring segment is connected by a frame element to the rear works and supports a carrier which pivotally mounts a forwardly-inclined boom and a rearwardly-inclined mast. Movement of the carrier across the front ring segment is permitted by two equal-length links pivotally connected to opposite sides of the carrier and the respective opposite sides of a horizontal bracket mounted on the frame element. The center of rotation of the carrier as it traverses the front ring segment is substantially coincident with the imaginary vertex of a truncated isosceles triangle formed by the carrier and lines running axially along the links. When the carrier is centered on the front ring segment, the center of rotation is also substantially coincident with a vertical axis running through the rearward-most point of the mast. Since the rearwardmost point of the mast is substantially coincident with the center of rotation, this point of the mast remains substantially stationary during the movement of the carrier across the front ring segment. A fixed length pendant connecting the rearwardmost point of the mast to the rear works also remains substantially stationary and in a vertical plane during the movement of the carrier across the ring segment, thus minimizing the amount of side loads and torsional forces induced in the mast due to such carrier movement.