-3-
General Information—Hydraulic Systems
Hydraulic Pumps
Hydraulic pumps come in two basic types:
•
Constant displacement - which will continue to put out
its rated flow regardless of pressure, until the relief valve
bypasses the flow.
•
Variable displacement - which will produce only the
flow needed by the implement until the total pump
output is reached. If less than the full pump output is
required, an automatic stroke control mechanism de-
creases the pump output to maintain a constant pres-
sure and flow. The output varies according to demand.
The Closed Center Valve (See Figure 2) is used with variable
displacement pumps. The flow is completely shut off in the
neutral position, causing the pump stroke to adjust to zero flow.
The flow stops, but the pump maintains a static pressure up to the
valve.
There are two basic types of spool valves used in conjunction
with these pumps — Open and Closed Center. In the Open
Center Valve (See Figure 1), the flow goes straight through the
valve when in the neutral position. This type is used for constant
displacement pumps where the flow should never be shut off.
Figure 1
Figure 2
Spool Valves
Open Center
Spool Valve
In Neutral
Position
Closed Center
Spool Valve
In Neutral
Position
Figure 3 shows an internal gear motor (Gerotor) where pres-
sure causes the cavities between the gears to expand on one
side, developing torque. The Gerotor type of hydraulic motor is
used on Hypro pumps for its superior performance character-
istics, including cooler running and higher rpm capabilities.
Hydraulic Motors
Three Systems
Fitting these components together and installing a motor, we
have one of the three types of systems: Open Center, Closed
Center (pressure compensated) and Closed Center Load
Sensing (flow and pressure compensated).
Open Center Systems
In an Open Center System, the hydraulic pump puts out a
constant flow. If the pump puts out more oil than the motor can
use, a portion of the oil must be bypassed around the motor.
When the oil is bypassed around a loop and does no work, the
energy put into it by the pump turns into heat. Therefore, the
amount of oil bypassed should be kept to a minimum. Use the
largest motor possible.
Closed Center (Pressure Compensated) Systems
The Closed Center Pressure Compensated system has a
variable displacement pump which will deliver flow at the
necessary rate to maintain a specified pressure. It is desirable
to equip implements with a motor of a low flow range that will
cause the pump to operate between 1800 and 2100 psi [124
and 145 BAR]. A motor that requires a large volume to obtain
the correct implement speed usually causes the hydraulic
pump in a closed center system to operate at a lower pressure
than desirable. This low pressure results in unnecessary flow
and the generation of heat that lowers the lubricating quality of
the oil and can damage transmission parts. Use the smallest
motor possible.
Closed Center Load Sensing Systems
(Flow and Pressure Compensating)
The Closed Center Flow Compensated System is a variation of
the pressure compensated system, designed primarily for
more efficient operation and the generation of less heat. It
works on the principle of maintaining a constant pressure drop
from the pump to the work port of the selector valve. Any
variation in demand at the motor will cause a change in flow.
The system senses this change in flow due to the change in
pressure drop across the valve, and causes the pump to
compensate by varying the pump flow. No restrictor is used in
the pressure line and no oil is bypassed.
Figure 3
Gerotor Type
Hydraulic Motor
Inlet
Outlet
Inner
Rotor
Rotor
Ring
Low
Pressure
High
Pressure
