Slipsheets are used as an alternative to conventional wooden pallets for carrying and transporting loads of stacked boxed packages. Slipsheets are employed to reduce the cost of transporting the substantial weight and volume of wooden pallets. Slipsheets also cost less and are more readily returned for reuse. While a conventional pallet provides a rigid base and may be lifted by the forks of a conventional forklift vehicle, a slipsheeted load requires a lifting vehicle having a rigid, flat platen with sufficient area to support the entire load.
A slipsheet handler also picks up a slip-sheeted load differently than a conventional forklift truck that picks up a palleted load. Because the slip-sheeted load rests directly on the ground, the platen may not easily be slipped underneath the load without pushing or toppling the load. Therefore, conventional slipsheet handlers such as described in U.S. Pat. No. 3,640,414 to Brudi use a gripping jaw that is extendable to the leading edge of the platen to grip a peripheral edge tab of the slipsheet. The gripping jaw may be forcefully closed and withdrawn to pull the load onto the platen for transporting.
To unload the slipsheeted load from the platen, an extendable and retractable vertical push-plate (to which the gripping jaw is generally attached) pushes the load off the platen.
Conventional slipsheet handling apparatus have several disadvantages. First, the large push-plate necessarily employed limits the visibility of the operator, reducing safety and efficiency. As shown in the prior art, the push-plate is actuated by a hydraulically-driven pantograph having pantograph arms with lengths proportionate to the horizontal travel of the push-plate. The pantograph arms extend to a substantial height when the push-plate is retracted, partially obscuring the operator's view. In addition, because the pantograph arms must attach to the push-plate, the push-plate itself must extend to a substantial height, further impairing visibility.
A second disadvantage of the prior art apparatus is that the push-plate tends to damage the load it is pushing. The lower edge of the push-plate is spaced above the platen, and tends to cut into the sides of boxed loads. The gap between the platen and the push-plate is necessary to receive the slipsheet edge tab for proper gripping jaw operation. When pushing a load from the platen, the lower push-plate edge remains spaced above the platen, where it may cause undesirable creasing or rupture of delicate boxed loads.
A third disadvantage of the prior art apparatus is the substantial cost, weight and power loss associated with the several hydraulic cylinders and required sequencer to control the operation of the cylinders, as well as the overall size of the structural components required.