Powerhouse cranes generally include a large amount of rope because of the extreme lift heights and the use of a double reeved rope configuration to provide true vertical lifting. The drum needs to be sized to store this rope. Generally, the length and/or the diameter of the drum can be increased to add rope storing capacity. Both solutions result in separate problems. As the length of the drum is increased, the length of the trolley frame that supports the drum must also be increased. A longer trolley frame experiences greater bending moments, and therefore the load members of the trolley frame must be increased in size to compensate. As the diameter of the drum is increased, so is the amount of torque which is required to turn the drum. Typically, a larger gearbox is necessary to provide more torque.
The costs associated with providing enlarged drums, gearboxes and trolley frames add significantly to the overall price of the crane. The components often need to be custom designed for each application, thereby resulting in the manufacturing of only a single crane at a time. Use of mass produced components could significantly reduce the overall cost of these cranes.
Accordingly, the invention provides a crane that can utilize mass produced drums, trolley frames and gearboxes.
The invention includes the use of two lift trains. Each lift train includes a drum that is single reeved together with the drum of the other lift train. In some embodiments, both lift trains include similarly sized drums, gearboxes and motors where the components of each lift train are generally smaller than those typically used on powerhouse cranes. Because the costs associated with the components increase exponentially with the size and torque requirements, the cost of two smaller lift trains is less expensive than the cost of a single custom built lift train.
With two drums to carry more or less the same amount of rope, the length of each drum can be reduced. When the length of the drum is reduced, the length of the trolley frame is also reduced, resulting in the ability to use a mass produced trolley frame.
When the diameter of the drum is reduced, the torque requirements are also reduced, resulting in the ability to use a smaller mass produced gearbox. A ring gear external to the gearbox may be utilized to increase the torque of a smaller gearbox such that very high ratios (e.g., ratio of 600 to 1) can be achieved with a standard three stage helical gearbox.
Although the cost of components utilized on a powerhouse crane can be reduced as discussed above, the components must provide a lifting arrangement that meets all safety requirements including fleet angle requirements. In one embodiment, the invention provides a lifting arrangement that meets all fleet angle requirements by staggering the axial position of each drum in relation to the other drum. The fleet angles in the full up and full down positions can be equalized by this positional shift between the two drums to optimize the fleet angles and thus maximize rope life. A bottom block with two separate sheave nests can also be utilized to optimize the rope fleet angles. Use of two separate sheave nests allows for optimum placement of the sheave nests with respect to the corresponding drum. Additionally, the width of the bottom block can be increased such that the sheave nests can be placed at any location with respect to the drums. Generally, the sheave nests are located near the ends of the bottom block.
