Eaton Electrical PVE19 - Manual (Page 10)

10
External Valve Spool Orifice Size Increased
If the external valve spool orifice size is increased, pump
outlet pressure will decrease, lowering force against the
compensator spool. (See Figure 4.) The compensator spring
causes the spool to move, opening the yoke stroking piston
to case drain. As fluid is metered from the yoke stroking
piston, the yoke spring force strokes the yoke to a higher
flow. The increase in flow through the external valve spool
orifice establishes once again the constant pressure drop
(
n P). With differential pressure ( n P) across the external
valve orifice, the compensator spool nulls. Flow from the
yoke stroking piston stops, and the pump flow rate stabilizes
at a higher value.
Operation of the load sensing compensator is such that as
the load pressure varies, the pump outlet pressure will follow
the variations, holding a constant pressure drop (
n P) across
the external valve spool orifice, and a constant flow through
the external valve and actuator. Pump flow will change only
with changes in external valve spool orifice size.
E. Compensator
(Load Sensing with Pressure
Limiting “CVP”, “CVPC”, “CVPD”)
As expected from the above title, these units are a combina-
tion of the flat cut-off and load sensing compensators.
The load sensing portion functions at pressures below the
flat cut-off compensator setting and provides a constant flow
characteristic. If pressure exceeds the flat cut-off compensa-
tor setting, the yoke will stroke to zero flow at maximum
pressure lowering the horsepower requirements for holding
circuits and protecting the pump. Refer to Figure 5
throughout the following circuit explanation.
Assume an actuator load that is increasing gradually. Also,
assume the pump outlet pressure is lower than the flat
cut-off compensator cracking pressure. As actuator load
pressure increases, the load sensing compensator spool
senses the difference between pump outlet pressure and
actuator load pressure. As long as the difference between
the pump outlet pressure and the actuator load pressure
(
n P) is constant, flow to the load will stay constant. As
pressure rises across the load, leakage will increase in the
pump and load. The load sensing portion of the compensator
adjusts pump outlet flow to compensate for leakage while
providing a constant flow through the valve spool orifice.
The pump outlet pressure continues to increase until the flat
cut-off compensator spool reaches cracking pressure. The
flat cut-off compensator spool then meters flow to the yoke
stroking piston. The yoke stroking piston starts moving the
yoke to reduce flow while holding the outlet pressure at
compensator cracking pressure. This action continues until
the pump is fully compensated (zero flow and maximum
pressure).
Standby Operation Feature
Standby defined: When the external valve spool is shifted to
zero flow, the circuit is placed in standby.
The small fixed orifice located in the compensator body
provides a decompression feature for the load circuit during
standby operation. The decompression feature allows the
pump to stroke to zero flow and minimum pressure (
n P), if
the load is blocked and the external valve spool orifice is
closed. (Refer to Figure 5.)
The circuit functions as follows:
Assume the pump is at zero flow with maximum pressure to
the load. The flat cut-off compensator spool will be in the up
position (compressing the spring) and the load sensing spool
will be in the down position due to actuator load pressure
plus the spring force. If the external valve spool orifice is
closed at this time, fluid under pressure will be trapped in the
load circuit and will hold the load sensing spool in the down
position. This will keep the pump outlet pressure at flat
cut–off compensator cracking pressure (a power loss since
no work is being performed at this time). To prevent this
condition from continuing, the small orifice meters the fluid
trapped in the load back through the flat cut-off compensator
spool to case drain. The actuator load pressure will decrease
gradually causing the load sensing spool to open pressure to
the yoke stroking piston, bypassing the flat cut-off compen-
sator. As the actuator load pressure reduces, the pump
outlet pressure will reduce until minimum pump pressure is
obtained. When the minimum flow/minimum pressure
condition occurs, the pump is considered to be in standby.
During standby, the CVP(C) control reduces the input power
well below that of a standard “C” type compensator. This
provides an increase in system efficiency and reduces the
cost of operation.