CHAPTER 4
Creation and Control of Fluid Flow
Fluid Power Circuits and
Controls, John S.Cundiff, 2001
INTRODUCTION
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Primary flow control device in any circuit is
the pump. It converts mechanical power to
fluid power.
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Fixed displacement pump delivers a fixed
volume of fluid per revolution called the
displacement (in3/rev).
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INTRODUCTION (contd…)
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Pump is driven at given rpm. Flow is measured with
a flow meter. Pump displacement is defined by
3
in / min
rev/min
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3
= in /rev
Variable flow pumps: displacement can be changed
as the pump is operating.
Leakage through the clearance between moving and
stationary parts increases with pressure, while
output flow decreases, refer to flow vs. pressure
curves.
INTRODUCTION (contd…)
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Pump volume efficiency is
evp = Qa / Qt
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Qa = actual output (in / min)
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Qt = theoretical output (in / min)
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Theoretical flow is
Qt = NVpth
N = pump speed (rpm)
Vpth = pump displacement
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INTRODUCTION (contd…)
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Overall pump efficiency is
eop = Output power (hyd)/ Input power (mech)
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Output power is
Pout = PQ / 231
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P = pressure (psi)
Q = flow (in3 / min)
Pump volume efficiency and overall efficiency both
decrease as pressure increases. The rate of
decrease depends on pump design.
Fixed Displacement Pumps
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Vane and piston pumps are available as fixed or
variable displacement pumps.
Gear pumps are available with fixed displacement
and are less expensive.
Disadvantages of gear pumps:
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Leakage in some gear pumps is high.
Leakage increases as pump wears over time.
Operating cost is higher
Pressure-balanced gear-pumps have volumetric
efficiencies that exceed or rival those of piston
pumps.
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Fixed Displacement Pumps (cont..)
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One gear is driven with
input shaft and it drives
the second gear.
Fluid is captured by the
teeth as they pass the
inlet
Oil travels around the
housing and exits at the
outlet.
Design is simple and
inexpensive
Fixed Displacement Pumps (cont..)
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The gerotor has an inner
drive gear and an outer
driven gear.
Inner gear has one tooth
less than outer gear. This
feature creates chambers
of decreasing volume
resulting in “plumbing
action”.
A port plate ensures that
fluid enters the chambers
when it is largest and exits
when smallest.
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Fixed Displacement Pump Circuits
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Circuits with fixed displacement pumps are called
constant-flow circuits.
Key Concept – Each revolution of a fixed
displacement pump delivers a certain volume of
fluid to the circuit. This fluid ultimately returns to
the reservoir, either as a return flow, or a leakage
flow.
Fixed Displacement Pump Circuits
(cont.)
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Simple circuit with fixed displacement.
Fluid passes through the open center
DCV back to reservoir.
Pressure required to overcome
pressure drop in the lines and through
DCV develops.
When an operator shifts the DCV, flow
is diverted to the cylinder. Now the
pressure equals all drops.
A key disadvantage of a constant flow
circuit is that pressure must build from
very low level to the level required to
accelerate the load.
Cycle time is important !
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Fixed Displacement Pump Circuits
(cont.)
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When load is such that operating pressure is
close to the cracking pressure of the relief valve,
the pump flow can be divided into three flows.
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–
–
Flow through relief valve back to the reservoir.
Flow to the load.
Leakage flow through DCV back to the reservoir
(smallest of the three flows)
Variable Displacement Pump Circuits
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Variable displacement pump
circuits are called demand
flow circuits.
Circuit has no relief valve.
Although not shown, relief
valve should always be used
to ensure that pressure can
never reach unsafe levels.
A dangerous pressure spike
can be produced by rapid
deceleration of a large load.
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Variable Displacement Pump Circuits
(cont..)
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Vane pump have a series of
spring loaded vanes that slide
back and forth in slots.
The chamber formed between
adjacent vanes and cam ring
decreases in size as the rotor
turns. Fluid flows into chamber
when it is maximum size and
exits when it is minimum size.
Change in chamber size
provides the pumping action.
Variable Displacement Pump Circuits
(cont..)
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The cam ring is held in
position with a threaded rod
turned with hand wheel. The
ring slides to the left when
the wheel is turned.
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Vane pump can be
converted to pressure
compensated pump by
replacing the hand-wheel
adjustment with a spring.
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Variable Displacement Pump Circuits
(cont..)
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A small cylinder called the
compensator, is placed on the
opposite side.
Outlet pressure acting on the
compensator piston creates a
hydraulic force that opposes
spring force. When outlet
pressure rises to certain point,
hydraulic force becomes
greater than spring force and
cam ring shifts to the left.
As pressure continues to rise,
ring shifts more to the left until it
is centered on the axis of
rotation.
Pump displacement is almost
zero.
Variable Displacement Pump Circuits
(cont..)
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Maximum pressure is limited by the compensator spring
in the pressure-compensated pump.
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Pressure compensated valve can maintain deadhead
pressure with very little energy input. Hydraulic power
output is proportional to pressure times flow. If flow is
zero, hydraulic power output is zero.
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Advantage of demand flow circuit over constant flow
circuit:
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Pressure is available from the instant the DCV is shifted, it does
not build from zero.
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Variable Displacement Pump Circuits
(cont..)
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Flow vs. Pressure
characteristics for pressurecompensated variable
displacement vane pump
When pressure reaches
2900 psi (cutoff pressure),
the cam ring begins to shift
and pump flow decreases.
Rate of decrease (slope of
the curve) is set by the
spring constant of the
compensator spring.
Variable Displacement Pump Circuits
(cont..)
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Exploded view of pressure compensated variable
displacement pump.
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Variable Displacement Pump Circuits
(cont..)
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There are two piston pump designs:
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Axial Piston Pump.
–
Radial Piston Pump.
Variable Displacement Pump Circuits
(cont..)
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Axial piston pump has a
series of cylinders mounted
parallel to the axis of rotation.
Arrangement is similar to
shell chambers in a revolver.
Each piston installed in the
cylinder has a spherical end
that mounts in a shoe.
The shoe is held against a
swashplate by a spring in the
cylinder block.
Swashplate remains
stationary as the cylinder
block rotates with the input
shaft.
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Variable Displacement Pump Circuits
(cont..)
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When the swashplate is at an angle to the shaft,
it moves the piston back and forth in the
cylinders as the cylinder block rotates.
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This movement provides the “pumping action”.
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It is helpful to follow the motion of one piston as
the cylinder block makes one revolution.
Variable Displacement Pump Circuits
(cont..)
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Schematic illustrating
the motion of one
piston during a single
rotation of the cylinder
block.
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Axis of rotation is in the
plane of the paper.
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Variable Displacement Pump Circuits
(cont..)
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Implementation of actual axial
piston pump design
Three key components:
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–
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Cylinder block spring.
Holds block in the position so
the piston shoes are always in
contact with swashplate. This
spring rotates with the cylinder
block.
Yoke spring assembly.
This spring holds the
swashplate against the
actuator piston .
Actuator Piston.
When fluid flows into the
cylinder the actuator piston
extends and reduces the angle
of the swashplate.
Variable Displacement Pump Circuits
(cont..)
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Axial piston pump can be
configured as a pressurecompensated pump.
Outlet pressure (high
pressure), Ps, is incident on
the end of the compensator
valve spool.
Ps multiplied by the area of
the spool gives a hydraulic
force, Fh, which is opposed
by the spring force Fs,
produced by compensator
valve spring.
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Variable Displacement Pump Circuits
(cont..)
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When Ps increases to the point where Fh equals Fs, the spool shifts downwards and
the fluid flows to the actuator piston.
The pressure at the actuator piston is Pc = Ps - ΔP , where Δ P is the pressure drop
across the orifice formed when the compensator valve cracks open.
As Ps increases, the compensator valve opens more, ΔP decreases, and Pc
approaches Ps .
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Increase in Pc increases the hydraulic force produced by the actuating piston, and it
rotates the yoke until it is perpendicular to the shaft and the pump displacement is
zero.
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The pump will hold this pressure and deliver no flow until something is done to lower
the pump outlet pressure.
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Pressure- compensated axial piston pump can be used in a circuit without relief valve.
Variable Displacement Pump Circuits
(cont..)
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Radial Piston Pump:Principles
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Cylinders are positioned radially
around the axis of rotation. As
the shaft rotates, the connecting
rods push the pistons back and
forth in the cylinders to develop
the pumping action.
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Commonly used for pneumatic
systems in the fluid power
industry.
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Variable Displacement Pump Circuits
(cont..)
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Improvement in Efficiency with Load Sensing
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Load Sensing was developed to improve the efficiency of a circuit. It requires
a variable displacement pump.
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An example:
–
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An application requires max. 20 GPM at max. 2500 psi. Select a pump to supply
20 GPM at 2500 psi. However, application requires mostly less than 20 GPM at
2500 psi. Percentage of total operating time the system operates under reduced
load, and the way the system responds to this condition, is a key issue. The
specific reduced load situation considered is the activation of a cylinder requiring
metered flow rate of 9 GPM at 1300 psi.
Consider the operation of an open-centered and close-centered circuit that
meets the functional objective and then consider a closed-center circuit with
load sensing.
Variable Displacement Pump Circuits
(cont..)
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Open-Center Circuit with fixed
displacement pump delivers 20
GPM continuously. The relief
valve is set on 2500 psi. When the
DCV is manually shifted, the pump
builds pressure to 1300 psi, and
the load begins to move.
The operator partly closes the
DCV, thus creating a restriction,
until the restricted flow gives the
desired load speed.
The flow is dumped at some
pressure less than 2500 psi, the
full open position of the relief
valve.
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Variable Displacement Pump Circuits
(cont..)
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To produce the graph at bottom of Fig 4.17, assume 11
GPM is dumped at 2500 psi.
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The power loss for the open circuit is
Ploss oc = 2500(11) = 16 hp
1714
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Alternate calculation:
Max. Power loss=Total Pump power – power utilized
Power loss = 2500(20) – 1300(9) = 22.34 hp
1714
1714
Variable Displacement Pump Circuits
(cont..)
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Closed center circuit has a
variable displacement pump.
Compensator valve spring is a
2500 psi spring. The pump builds
2500 psi before the compensator
valve moves to open a pathway
for fluid to flow to the yoke
actuating piston.
The piston extends and forces
the swashplate to the zero
displacement position. The pump
then maintains the 2500 psi and
supplies only enough flow to
replace leakage.
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Variable Displacement Pump Circuits
(cont..)
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The operator cracks open the DCV to start the load.
Pressure drops slightly and the compensator valve moves slightly to
partly close the pathway to the yoke actuating piston.
Piston retracts slightly and the swashplate tilts slightly so that the
pump is now delivering fluid to the DCV and thus the load.
This sequence of events continues until the operator has opened
valve to the position where 9 GPM is flowing to the load.
Pressure is less than 2500 psi, depending on the characteristics of
the compressor valve spring.
Load pressure is 1300 psi, thus pressure drop across the DCV is
2500 – 1300 = 1200 psi.
Pump is delivering 9 GPM, thus the power loss is
Ploss cc = 1200(9) = 6.3 hp
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Variable Displacement Pump Circuits
(cont..)
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Closed Center Circuit with Load
Sensing is achieved with a special
compensator mounted on a
variable displacement pressure
compensated pump.
Assume special compensator is
set to destroke the pump at 200
psi. As the DCV is shifted to
extend the cylinder, a pilot line
senses the 1300 psi required to
move the load. Work port pressure
is added to the “destroke”
pressure, and the pump delivers
the required 9 GPM at 1300 + 200
= 1500 psi.
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Variable Displacement Pump Circuits
(cont..)
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Actual power loss in this case is
Ploss ls = 200(9) = 1 hp
1714
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The three-way, two-position DCV is shifted with pilot pressure. The pump is destroked
when this valve is in the position shown.
There is no pilot pressure on the left side. The pump pressure applied on the right side
shifts the DCV to the position shown.
There is no hydraulic pressure to add to the spring force (200 psi) in the compensator.
Thus pump builds only 200 psi pressure.
When the operator shifts the three-position DCV, the pilot line is connected to supply
pressure, which is 1300 psi in this example.
This pilot pressure acts on the left side of the two-position DCV, causing it to shift to
the right.
Supply pressure is added to the spring force in the compensator and the pump builds
1300 + 200 = 1500 psi.
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Comparison of Pump Performance
Characteristics for Three Main Designs
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Data was selected from manufacturers’ literature
for a fixed displacement pump of each design:
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Gerotor (gear)
Vane
Piston
Pumps with similar displacements were selected
from three different manufacturers
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Comparison of Pump Performance
Characteristics for Three Main Designs
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Gerotor Pump
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The Hydreco Model 1919 gerotor pump has a displacement
3
of 4.53 in / rev. The manufacturer gives performance data
up to a max. speed of 3000 rpm. The max. pressure curve
is 2500 psi.
–
When driven at 1200 rpm, the volumetric efficiency drops
from 92% at 500 psi to 78% at 2500 psi. (Fig 4.21a)
–
At 1800 rpm, the volumetric efficiency drops from 93% at
500 psi to 84% at 2500 psi. (Fig 4.21b)
Comparison of Pump Performance
Characteristics for Three Main Designs
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Comparison of Pump Performance
Characteristics for Three Main Designs
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The reason for higher volumetric efficiency at
the higher speed can be explained as follows.
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The leakage flow is given by
Q1 = Qt - Qa
Where Q1 = leakage flow (GPM)
Qt = theoretical flow (GPM)
Qa = actual flow
Comparison of Pump Performance
Characteristics for Three Main Designs
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Theoretical flow at 1200 rpm is given by
Qt = 1200(4.53) = 23.53 GPM
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Volumetric efficiency at 1500 psi is evp = 0.85, so
Qa = Qt evp = 23.53(0.85)
= 20.0 GPM
Substitution into previous equation gives,
Q1 = 23.53 – 20 = 3.53 GPM
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Comparison of Pump Performance
Characteristics for Three Main Designs
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Leakage flow is primarily a function of pressure, with
rotational speed being a much less significant factor.
Suppose the leakage flow is same for performance test
run at 1800 rpm, 1500 psi.
Q1 = 1800(4.53) = 35.3 GPM
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= (Qt - Q1) = 31.77
Volumetric Efficiency is
evp = Qa = 31.77 x 100 = 90 %
Qt 35.3
Comparison of Pump Performance
Characteristics for Three Main Designs
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Measured volumetric efficiency was
evp = 88.7%,
slightly less than efficiency calculated by
assuming constant leakage flow. There is some
increase in leakage at higher speeds due to
increased turbulence of the fluid.
Good design practice : Select a smaller pump
and operate it at higher speed to achieve a
higher volumetric efficiency.
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Comparison of Pump Performance
Characteristics for Three Main Designs
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Volumetric efficiency in the 85% to 90% range is achievable with a
gerotor pump (pressure < 1500 psi).
Volumetric efficiency decreases as operating temperature increases.
Viscosity of the fluid decreases and more leakage occurs through
clearances in the pump.
Overall efficiency accounts for the loss of mechanical energy due to
friction and loss of hydraulic energy due to leakage flow.
Clearances between moving parts are thought of as orifice. Fluid on
one side of the orifice has a high pressure and fluid at the other side
has pressure equal to that in the pump housing.
Overall efficiency for Hydreco Model 1919 gerotor pump is plotted in
Fig 4.22a (1200 rpm) and Fig 4.22b (1800 rpm).
Efficiency is near maximum at 1000 psi. Increase in overall efficiency
at higher rpm is smaller
Comparison of Pump Performance
Characteristics for Three Main Designs
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Comparison of Pump Performance
Characteristics for Three Main Designs
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Key comparisons between volumetric and overall efficiencies for the
gerotor pump.
Comparison of Pump Performance
Characteristics for Three Main Designs
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Vane Pump
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The Vickers Model 25V (T) vane pump has a
displacement (cam ring fixed for maximum
3
displacement) of 4.81 in /rev.
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The specifications:
–
–
Maximum Speed 1800 rpm
Maximum Pressure 2500 psi
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Comparison of Pump Performance
Characteristics for Three Main Designs
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Volumetric efficiency at 1200 rpm decreases from 97.3%
at 500 psi to 84% at 2500 psi (Refer Fig 4.24a).
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The vane pump volumetric efficiency is 5 to 6% higher
than the gerotor pump. The volumetric efficiency is
higher because leakage flow is less.
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Volumetric efficiency at 1800 rpm decreased from 97.8%
at 500 psi to 88% at 2500 psi (Refer Fig 4.24b). With the
gerotor pump, it is higher at higher shaft speed across
the whole pressure range.
Comparison of Pump Performance
Characteristics for Three Main Designs
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Comparison of Pump Performance
Characteristics for Three Main Designs
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Overall efficiency of vane pump is quite low at low
pressures.
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Overall efficiency at both 1200 and 1800 rpm increases
to a maximum of 1000 psi. (Refer Fig 4.25a and 4.25b)
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Decrease in overall efficiency as pressure increases
from 1000 psi to 2500 psi is less for the vane pump than
the gerotor pump.
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The vane pump efficiency curve is “flatter”, indicating
that vane pump performance will change less as load
pressure changes in 1000 to 2500 psi range.
Comparison of Pump Performance
Characteristics for Three Main Designs
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Comparison of Pump Performance
Characteristics for Three Main Designs
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Comparison of Volumetric and Overall Efficiency for a Vane Pump
operated at 1200 and 1800 rpm.
Comparison of Pump Performance
Characteristics for Three Main Designs
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Axial Piston Pump
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The Sauer-Danfoss Series 90-075 variable
displacement axial piston pump has a
3
displacement of 4.57 in /rev.
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The specifications :
–
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Maximum speed : 3950 rpm
Maximum pressure : 7000 psi
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Comparison of Pump Performance
Characteristics for Three Main Designs
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The pump has a higher maximum speed than the gerotor
pump (3000 rpm) or the vane pump (1800 rpm). It is
rated for much higher pressure (7000 psi vs. 2500 psi).
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Volumetric efficiency at 1200 rpm decreases from 99%
at 1000 psi to 89% at 6000 psi (Refer Fig 4.26a)
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Volumetric efficiency at 1800 rpm decreased from 98.8%
at 1000 psi to 90.3% at 6000 psi. (Refer Fig 4.26b)
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Increase in volumetric efficiency at higher speed was
less for the piston pump than the gerotor or the vane
pump.
Comparison of Pump Performance
Characteristics for Three Main Designs
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Comparison of Pump Performance
Characteristics for Three Main Designs
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Overall efficiency of the piston pump is quite
high (>90%) for the pressures < 4000 psi.
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Overall efficiency at 1800 rpm is slightly higher
than 1200 rpm. (Refer Fig 4.27a and Fig 4.27b).
Since leakage flow is predominantly a function
of pressure, not speed.
Comparison of Pump Performance
Characteristics for Three Main Designs
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Comparison of Pump Performance
Characteristics for Three Main Designs
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Comparing power loss (expressed as percent of input power) for the
three pump designs, calculated at 1200 rpm and 1500 psi.
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The trend in loss is such that the vane pump is about midway
between the gerotor and the piston pumps.
Comparison of Pump Performance
Characteristics for Three Main Designs
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Losses due to friction are higher in the vane pump than piston pumps. Both
pumps have relatively high number of moving parts as compared to the
gerotor pump.
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Vane pump has a larger lubricating film area than the piston pump, so the
friction power loss is 11% for the vane pump and only 5% for the piston
pump.
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Multiple Pump Circuits
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Multiple Pump Circuits.
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Multiple pump design places two or more pumps in the
same housing and drives them with a single input shaft.
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Sometimes only one inlet is provided, but each pump has
is own outlet.
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Advantage – Isolation of circuits.
Multiple Pump Circuits
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The pump supplies flow to a hydraulic
motor and cylinder.
When solenoid activated, two position
DCV is shifted, flow is directed to the
motor.
If the manually activated three-position
DCV is shifted, flow is diverted to the
cylinder.
Pressure in the motor and cylinder
decides how the flow will divide.
If motor pressure is higher, flow will go
to the cylinder
By this the motor will stop until the
three position DC is recentered or
cylinder reaches its full extension and
the pressure builds to the pressure
required by the motor circuit.
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Multiple Pump Circuits
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It is undesirable to interrupt the
motor operation to activate the
cylinder.
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An efficient solution to this is to
use a multiple pump, the same
electric motor and the same
hydraulic components.
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An exception is that the threeposition DCV now has a open
center rather than a closed
center.
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Each pump has a relief valve to
protect the circuit.
Multiple Pump Circuits
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Gear pumps are available with more than two pumps in
the same housing, commonly referred to as a tandem
pump.
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Many manufacturers of all three pump designs supply
models with a pad for mounting a second pump on the
primary one.
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Different pump designs can be mixed
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Ex : Multiple gear pump can be mounted on the auxiliary mount
of a piston pump.
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Pump Mounts
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Pump Mounts are gear boxes that are designed
to power hydraulic pumps.
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They have provision for mounting one to four
pumps on the output side.
Pump Mounts
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Basics of Directional Valves
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The valves in the next figures control the direction of
the oil flow and are called directional control valves
(DCVs).
Each of the valves shown is a four-port device (4
connections to the hydraulic circuit).
Ports P and T provide pressure and return, while A &
B are for connection to the actuator or circuit to be
controlled by the valve.
Directional Control Valve
Goering, 2003, Off-Road Vehicle Engineering Principles
32
4 Way – 3 Position Valve
Center Conditions
Goering, 2003, Off-Road Vehicle Engineering Principles
• Closed Center: has no flow when valve centered
•Open Center: relieves pressure to tank when centered
•Float Center: relieves pressure from load and pump
Valve Basics, con’t
These DCV’s are three-position valves
with the valve spool shown in the
centered position.
z The valve spool can be slid to the right
to align the left-most box with the
external ports, or to the left to align with
the right-most box with external ports.
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33
4 Way – 3 Position Valve
Lab-Volt, Hydraulic Trainer Manual
4 Way – 3 Position Valve
Lab-Volt, Hydraulic Trainer Manual
34
Flow Control Valves
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Flow control valves.
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Used in constant-flow (fixed displacement pump) circuits to control actuator
speed.
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Simplest type – needle valve also called non-pressure-compensated flow
control valve.
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Turning manual adjustment on needle valve causes needle to move down into
the orifice, thus reducing orifice area.
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Pressure drop across valve (ΔPfc ) is increased by continuously restricting the
orifice until enough pressure is produced.
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Extra turn will further reduce the orifice, increase the ΔPfc , increase the
pressure relief valve, dump more fluid to the reservoir.
Flow Control Valves
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Flow across the valve represents an energy loss.
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Needle valve is inexpensive, but the operating cost is high because
of energy loss.
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Pressure-compensated flow control valve has the provision for
changing the ΔPfc as the load pressure changes.
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Total pressure at the relief valve is
Pr = ΔPfc +ΔPL (maintained constant)
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As ΔPL increases , ΔPfc decreases, and vice versa.
Constant Pr means constant load on pump and constant flow on
relief valve.
35
Flow Control Valves
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Schematic of a pressurecompensated flow control valve.
A force balance on the spool of the
valve shown.
A spring is rated at 100 psi, i.e it
produces force equivalent to a 100
psi pressure.
Force balance on the spool is
PcAc – PcAr – 100 Ac ) = 0
2
Ac = area of spool cap (in )
2
Ar = area of spool rod end (in )
Pc = pressure in cavity between
the two spool ends.
Flow Control Valves
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Pressure Pc must equal 100 psi for spool to be in
force balance.
Spool finds the position that maintains 100 psi in the
center cavity.
If inlet pressure is 500 psi, pressure drop across the
orifice in Fig 4.31a is
= 400 psi.
The pressure drop between the center cavity and the
outlet valve is 100 psi.
This pressure drop sets the flow through the orifice
created by the position of the handwheel adjustment.
36
Flow Control Valves
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Refer Fig 4.31b
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Complete schematic for the
flow control valve is shown.
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The spring cavity opens to
the downstream pressure.
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The downstream pressure
adds to the spring pressure
to give the total pressure in
the center cavity.
Flow Control Valves
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Orifice equation (from chapter 2)
Q = k ΔP
If ΔP is constant, then Q will be constant.
Using the pressures shown in Fig 4.31b,
Cavity pressure = Downstream pressure - Spring pressure
= 200 + 100 = 300 psi
The spool finds a position where the orifice ΔP is
ΔPo = 500 – 300 = 200 psi
37
Flow Control Valves
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If the downstream pressure goes as high as 400 psi,
Pc = 400 +100 = 500 psi
ΔPo = 500 – 500 = 0 psi
Then
Denoting the downstream pressure as PL and the
pressure drop across handwheel orifice as ΔPhw , total
pressure at the inlet is
Pfc = ΔPo + ΔPhw + PL
Three cases from this equation are as follows.
Flow Control Valves
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Case 1 PL = 0
Pfc = 400 + 100 = 500 psi
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Case 2
PL = 200
Pfc = 200 + 100 + 200 = 500 psi
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Case 2
PL = 400
Pfc = 0 + 100 + 400 = 500 psi
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For all the three cases , the pressure at inlet valve is
held constant. Pump and relief valve “see” the same
pressure, thus flow is held constant.
38
Flow Control Valves
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Flow dividers valves split the flow from a
single pump to supply two circuits that
operate at different pressures.
Orifices can give splits anywhere from 50:50
to a 90:10 split.
Flow Divider Valves
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The flow divider valve shown can be used with a fixeddisplacement pump.
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The flow divider valve, or priority valve, is also pressure
compensated .
Goering, 2003, Off-Road Vehicle Engineering Principles
39
Flow Control Valves
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Cross section of flow divider is
shown, where flow enters port 1
and exits port 6.
Key component is the spool
(component 2)
Spool has passage drilled down
the center.
Fluid enters spool at 3 and
splits to flow along passage 4 in
both directions.
Orifices at both ends of
passage 4 are identified as
component 5
If they are same size, flow
divider is designed for 50:50
split.
Flow Control Valves
z
When flow through both orifices is same, pressure drop is same at both
ends, and spool is in force balance.
z
If pressure at left port is lower than at right port, fluid entering 1 takes the
path of least resistance and flows to left port.
z
Higher flow at left orifice produces higher pressure drop at the orifice.
z
Greater pressure on the upstream side of left orifice creates a force
imbalance on spool and shifts it to the left.
z
Spool moves closer to end plate and partially blocks the orifice. End of the
spool moves towards the end plate.
z
Spool moves until it finds the position where flow is equal in both directions.
40
Circuits using Flow Control Valves
z
z
Flow control valve to meter into the actuator (Fig 4.34)
A flow control valve with a built-in check valve is used for the
meter-in and meter-out circuits so that the cylinder can be
retracted at full speed.
Circuits using Flow Control Valves
z
Flow control valve to meter flow out of the actuator (Fig 4.35)
41
Circuits using Flow Control Valves
z
z
No check valve is needed for the bleed-off circuit, because a
return flow path through the DCV is provided.
Flow will take the path of least resistance, therefore, it will go
through the DCV rather than through the flow control valve.
END OF CHAPTER 4
THANK YOU
42