Hydraulic Pump; Hydraulic Cooler - Holiday Navigator 2003 Owner's Manual

Chassis Information
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Hydraulic Pump

Hydraulic Cooler

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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 the
pressurized fluid to the power steering gear and the switching valve of the
engine cooling system. Each half of the pump is equipped with an internal
by-pass pressure relief spring. If the hydraulic pressure should exceed the
specified pressure limit, the internal by-pass relief valve will be forced open to
keep the hydraulic fluid at operating pressure. The hydraulic pressure generally
is not rated in psi but is rated in the term Bar. One bar is equivalent to
approximately 14.5 psi. Hydraulic system pressures with a system at no load
may be as low as eight bars on the output side of the pump. This is due to the
hydraulic fluid flow of the pump. When a load is placed on the hydraulic
pump, such as turning the steering wheel, 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 would be the power steering gear or 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.
The hydraulic cooler is an important part in the hydraulic system. This
helps keep the hydraulic fluid from overheating. When a load is placed on the
hydraulic system, heat is created in the fluid. The hydraulic pump builds
pressure that creates heat in the fluid. The restriction from the loads applied
also creates heat. This heat must be dissipated to keep 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 or arctic climates.
As with any oil, lower temperatures 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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