Hydraulic Lift Circuit

Figure 6-21 shows the lift portion of the hydraulic system. The circuit has two cylinders: a single-acting lift cylinder and a double-acting tilt cylinder. The lift cylinder moves the lifting fork up and down. The tilt cylinder tilts the mast back and forth to support or dump the load.

A two-section, multiple-unit directional valve controls the cylinder’s operation. The first valve has a double-acting D-spool to operate the tilt cylinder, hydraulically, in either direction. The outer envelopes show the typical four flow paths for reversing the cylinder. The second valve has a single-acting T-spool to operate the lift cylinder. This cylinder is returned by gravity; the bypass unloads the pump.

The pump is driven by the lift truck’s engine and supplies the circuit from the large volume end. The enclosure around the two pump symbols indicates that both pumping units are contained in a single assembly. The same applies to the two directional valves and the relief valve that are enclosed. They are in a single assembly.

Figure 6-21 shows the circuit in neutral; the valves are centered. If the figure were to show the operating mode, the outer envelopes on the valve symbols would be shifted over to align with the ports at the center envelopes. The arrows in the envelopes would then show the flow paths from the pressure inlet to the cylinders and/or the return flow to tank.

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