Service Manual

HYDRAULIC COMPONENTS (cont'd)
A Charge Pump is a small constant
displacement pump which is driven by the input
shaft. It provides a base circuit pressure and
flow which can be used for the following
functions:
& Transferring oil from the reservoir to the
& Pushing oil through a system filter
4 Supplying oil to remote lift cylinders
hydrostatic pump
Valve - Function
Valves are used to provide control in a hydraulic
system. They can be used to control:
& Pressure
& Direction Of Flow
& Volume Of FIOW
We will first review these three functions of
valves and then review some of the common
types of valves you may encounter.
Pressure Control valves may be used to
provide an upper pressure limit or to maintain a
minimum pressure in a circuit.
For an example, let's
look at a lift system. High
pressure is generated when objects heavier
than the system design are encountered or
when a cylinder reaches the end of its stroke.
The pressure control (relief) valve opens when
the upper pressure limit of the system is
reached.
With the valve open, a pressure escape path
routes the oil back into the low pressure side of
the circuit, protecting the system components
from damage.
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3.4297.407
Productivity Series