System and method for controlling the elevation of a boom hoist device

Claim 1

A multi-purpose lifting vehicle having a boom hoist device which can be selectively used in a crane mode of operation or in a front end loader mode of operation, hydraulic cylinder means operably connected to the boom hoist device, and a hydraulic system for delivering hydraulic fluid to the hydraulic cylinder means, said hydraulic system comprising a sump serving as a source of hydraulic fluid; a mode selector valve having first, second, and third operating positions, pump means communicating with said sump for delivering hydraulic fluid from the sump to said selector valve, means defining a first flow path for returning hydraulic fluid to the sump from the selector valve, means defining a second flow path, including a one-way flow valve, for delivering hydraulic fluid from the selector valve to the raising end of said cylinder means and a first normally closed valve, which when placed in an open position, provides a first by-pass path about said one-way flow valve, means defining a third flow-path providing fluid communication between the selector valve and the lowering end of said cylinder means, and means defining a fourth flow-path including a second normally closed valve, which when placed in an open position, provides a second by-pass path about said one-way flow valve, said fourth flow-path providing fluid communication between said raising and lowering ends of said cylinder means through said selector valve, whereby when the control valve is in its first operating position it provides communication between, on the other hand, said pump means and said second flow-path, and, on the other hand, said third flow-path and said first flow-path to deliver fluid to the raising end of the cylinder means and to permit fluid to leave the lowering end of the cylinder means to raise the boom hoist device, and the one-way flow valve prevents reverse flow of fluid from the raising end of the cylinder means, whereas when the control valve is in its second operating position and said first normally closed valve is open communication is provided between, on the one hand, the raising end of the cylinder means and said first flow-path and, on the other hand, said pump means and the third flow-path to deliver fluid to the lowering end of the cylinder means and permit fluid to leave the raising end of the cylinder means to lower the boom hoist device and when said selector valve is in its third operating position and said second normally closed valve is open fluid communication is provided through said selector valve and said second normally closed valve between said raising and lowering ends of said cylinder means.