HYDRAULIC SYSTEMI AINTENANCE
Section I. DESCRIPTION AND DATA
Hydraulic system maintenance procedures not covered in this chapter can be found in TM10-3930-660-20.
16-2. PRINCIPLES OF OPERATION.
System Description. The hydraulic system of the forklift supplies a controlled flow of filtered hydraulic oil for
operation of the boom, carriage, forks, and frame tilt functions. Directions used in the system descriptions are as the
reader is sitting in the operators seat.
Principle components of the hydraulic system and how the components are connected are shown on pages 16-2 and 16-3.
A two section gear pump draws hydraulic oil from the hydraulic reservoir through the suction strainers whenever the
engine is operating. The small section (1.48 cu. in.) of the gear pump supplies oil for the brake system, frame tilt function ,
and charge pressure oil for the axial-piston pump.
From the small section of the gear pump, oil flows past the relief valve. If system pressure exceeds 1750 psi (12066 kPa),
oil pressure unseats the relief valve and allows excess oil to flow through the hydraulic filter back to the reservoir.
Past the relief valve, oil flows through the brake valve, through the frame tilt valve and finally to the axial-piston pump inlet
port where the oil is used as charge pressure oil. The remaining oil flows back to the reservoir.
The large section (3.94 cu. in.) of the gear pump supplies oil to the priority valve for the steering system and to the main
valve for boom hoist and extend functions. From the main valve, oil flows back through the hydraulic filter and is returned
to the reservoir.
Oil flows from the large section of the gear pump to the priority valve. The steering system is first priority, receiving a
portion of the total oil flow first. When the steering system is satisfied, oil pressure increases and moves the spool to
return the total oil flow to the main valve. A load sensing line from the steering valve to the spring end of the priority spool
monitors the pressure in the steering valve, to insure proper oil flow and pressure is provided to the steering valve. When
the steering cylinder bottoms, pressure in the steering valve increases. The pilot pressure is relieved through the 2500 psi
(17236 kPa) relief valve in the priority valve and all the remaining pump flow is directed to the main valve.