Chassis Information --- Section 10
The Executive 2005
503
Hydraulic Pump
The hydraulic pump creates pressure by meshing sets of gears together inside a close tolerance
housing. A filtered supply of hydraulic fluid from the hydraulic reservoir enters the intake side of the
pump. The meshing gear assembly "squeezes" the oil through the pump to the output side delivering
pressurized fluid to the switching valve mounted on the hydraulic fan motor. The ISM engine uses a
two-part hydraulic pump assembly. One side of the pump drives the hydraulic fan motors; the other
side is used for the power steering. The ISX and Series 60 engines uses a transmission-mounted
hydraulic pump for the fan motors. The power steering pump is mounted behind the compressor on the
ISX, and directly to the accessory drive on the Series 60. Each pump type is equipped with internal by-
pass pressure relief valves. If hydraulic pressure should exceed specification, the internal by-pass valve
will force open to lower fluid pressure and maintain a specified rating. Hydraulic pressure is generally
not rated in psi but in the term Bar. One bar is equivalent to approximately 14.5 psi. Hydraulic pres-
sures with a system at no load may be as low as eight bars on the output side of the pump. When a load
is placed on the hydraulic pump, such as fan speed ramping up, hydraulic fluid flow slows from
hydraulic fluid restriction and pressure increases.
This may be understood as a faucet with a garden hose attached. Crimping the hose with the faucet
on will create pressure from the restriction. This principle applies to the hydraulic system. The
hydraulic pump is the supply; the load is the hydraulic fan motors. Hydraulic system pressure at full
load can exceed 130 bar or 2000 psi. Hydraulic system pressure falls dramatically after the load. The
return line pressure may be as low as six to eight bars. The fluid enters the hydraulic cooler where the
heat is dissipated.
Hydraulic Pump (Series 60 & ISX):
The hydraulic pump is driven by a separate gear assembly mounted to the transmission called a
PTO (power take-off). The intermediate shaft and coupling between the PTO and the hydraulic pump
are lubricated by transmission fluid. No maintenance is required.
Hydraulic Cooler
The hydraulic cooler operates to prevent hydraulic fluid from overheating. When a load is placed on
the hydraulic system, heat is created in the fluid. Heat must be dissipated to prevent the hydraulic
fluid from overheating and breaking down. After cooling, the fluid is filtered before returning to the
reservoir.
Care must be used when starting an engine in very cold climates. As with any oil, lower tempera-
tures thicken the oil. Hydraulic system pressure increases due to the viscosity of the fluid. Although the
hydraulic pump is equipped with pressure relief valves, the thick oil on the return line can exceed the
operating pressure of the hydraulic cooler.
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