SYSTEM AND METHOD FOR CONTROLLING THE ELEVATION OF A BOOM HOIST DEVICE

摘要

SYSTEM AND METHOD FOR CONTROLLING THE ELEVATION OF A BOOM HOIST DEVICE A multi-purpose lifting vehicle includes a hydraulic cylinder for controlling the elevation of a boom hoist device operably connected thereto. The device is alternately used in crane or front end loader modes of operation. The vehicle includes a hydraulic system for controlling the operation of the cylinder comprising a multi-position mode selector valve for selecting the specific mode of operation of the boom hoist device for raising, lowering and enabling the boom hoist device to float. The control system further includes a holding valve for locking the hydraulic fluid within the raising end of the cylinder for maintaining the elevation of the boom hoist device. A first bypass circuit is provided about the holding valve for enabling fluid to escape from the raising end of the cylinder while fluid is simultaneously flowing to the lowering end thereof. A two-position valve having a closed position for preventing flow of fluid through the valve when the selector valve is in either its raising or lowering positions and an open position for establishing a bypass path about the holding valve when the selector valve is in its float position for enabling fluid to flow to the raising end from the lowering end of the cylinder or to the lowering end from the raising end thereof in accordance with changes of contour of the terrain over which the lifting vehicle is advancing.