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What is in this article?:
Fluid Power eBook — Fluid Power Circuits Explained
Foreward
Chapter 1: Hydraulic Accumulators
Chapter 2: Air Logic Circuits
Chapter 3: Air-Oil Circuits
Chapter 4: Slip-In Cartridges
Chapter 5: Counterbalance Valve Circuits
Chapter 6: Fluid Power Cylinders
Chapter 8: Directional Control Valves
Chapter 9: Filtration
Chapter 10: Flow Control Circuits
Chapter 11: Flow Divider Circuits
Chapter 12: Fluid Motor Circuits
Chapter 13: Pressure Intensifier Circuits
Chapter 14: Proportional Control Valve Circuits
Chapter 15: Pumps
Chapter 16: Reducing Valves
Chapter 17: Regeneration Circuits
Chapter 18: Pressure-relief Valves
Chapter 19: Rotary Actuator
Chapter 20: Sequence Valve Circuits
Chapter 21: Servovalve Circuits
Chapter 22: Synchronizing Circuits
Chapter 23: Sample Actual Circuits
Chapter 16: Reducing Valves
When one branch of a fluid power circuit must operate at a lower
pressure, use a reducing valve to provide it. Reducing valves control
their outlet or downstream pressure only.
Air line regulators, Figure 16-1, reduce pressure for a pneumatic circuit. Because air in
the supply line to a machine is at maximum pressure, energy can be saved by reducing
pressure whenever possible. With a compressor setting between 115 and125 psi and a
machine requirement of 70 psi, approximately 40% of the input energy would be lost
without a properly adjusted regulator. The air-driven machine will work at the higher
pressure, but it consumes more compressor horsepower than necessary.
Figure 16-1. Self-relieving type air line
regulator.
Another use for air line regulators is on retraction strokes of air
cylinders. Reducing pressure on a cylinder’s retraction stroke saves air
and thus consumes less compressor horsepower.
In multiple actuator circuits, it is often impossible to size all actuators to operate at
maximum system pressure. For example: when a cylinder needs 5000 lb of force and one
standard bore produces only 4712 lb at maximum pressure, the designer must go to the
next larger standard bore. However, the next larger bore produces 7363 lb of force, which
can cause machine or part damage. Instead, install a pressure-reducing valve in the
branch circuit with the over-sized cylinder, as in Figure 16-2, to lower that branch’s
pressure to generate the required cylinder force.
Figure 16-2. Pressure-reducing valve with
bypass check valve.
A standard reducing valve is normally open. When downstream pressure
goes higher than its setting, the valve closes, blocking flow. If pressure
downstream tries to increase — say due to resistance from an opposing
cylinder — a reducing valve also blocks reverse flow. Escalating pressure
in the downstream line continues until something bursts or gets
mechanically damaged.
Figure 16-3 shows the symbol for a reducing-relieving valve. A reducing-relieving valve
sets maximum outlet pressure, then relieves fluid to tank when outlet pressure tries to go
higher. The overpressure could be due to outside forces or possibly high temperature in
some environments. A reducing-relieving valve has an integral relief valve with a full-
flow line to tank. When pressure in the downstream circuit rises 3 to 5% above reduced
pressure, trapped fluid relieves to tank. Adjusting the reduced pressure automatically sets
the maximum relief pressure.
Figure 16-3. Pressure-reducing-relieving valve
with bypass check valve.
Hydraulic reducing valves always have a drain line open to tank for
control oil flow. Drain oil flows when reducing valve outlet is lower than
its inlet. This generates a small amount of heat in the system. Blocking
the drain line forces the valve wide open and lets outlet pressure rise to
system pressure.
Multiple pressures in one circuit
Figure 16-4 has a schematic diagram for two cylinders that need
different pressures. One option a novice designer might use is to add a
second relief valve. However, second relief valve B reduces pressure in
the whole circuit. System pressure cannot go above 400 psi — making
high-pressure relief valve A useless.
Figure 16-4. Using relief valves for two pressures.
In Figure 16-5, reducing valve C replaces relief valve B. Now each
cylinder operates at a different pressure. Note that there is no bypass
check valve on reducing valve C. When the system does require reverse flow
through the reducing valve, the bypass check valve can be omitted. However, for a circuit
with reverse flow always use a bypass check.
Figure 16-5. Using a reducing valve for two pressures.
With the reducing valve installed in the line that feeds the directional
control valve, pressure at both ends of the cylinder is reduced. Also,
when the pump is at pressure, reducing valve drain line flow is constant.
Drain flow amounts to 20 to 70 in.3 minimum, and produces heat. With
several reducing valves in a system, drain line flow might require a
larger pump and a heat exchanger.
Figures 16-6 and 16-7 show the preferred location for a reducing valve. In Figure 16-6,
the circuit is at rest. There is no drain flow with the reducing valve in the line between the
directional valve and the actuator. This arrangement eliminates oil heating and provides
extra flow to other actuators. When both ends of the cylinder need pressure reduction
and/or different pressures, use the arrangement in Figure 16-7. The components cost
more up front but the energy it saves often pays for the extra reducing valve.
Figure 16-6. Using a reducing valve for two pressures (circuit at rest
with pump running).
Figure 16-7. Using two reducing valves for two pressures (circuit at rest
with pump running).
A reducing valve is normally open from inlet to outlet, but closes when
reaching the outlet pressure setting. When an actuator at reduced
pressure reverses suddenly, the reducing valve does not have time to
open. Oil forced out of the cylinder that tries to go back through the
reducing valve keeps pressure on the outlet, holding it shut. A small
pilot-drain flow in this blocked reverse flow condition allows very slow
reverse cylinder movement. A reducing valve with a bypass check may
try to stay closed but will not block flow, so the cylinder reverses easily.
Two-pressure circuit with a pressure-reducing valve
Always connect the drain line of a pressure-reducing valve to a free-flow
tank line. Backpressure in the drain line adds to the spring setting, thus
raising the set pressure. A constant backpressure can be offset by a
lower spring setting, avoiding a problem. With intermittent and/or
fluctuating backpressure, the reduced outlet pressure changes when the
backpressure changes.
Some circuits require a reduced pressure to position a part, then full
pressure to do the work. A reducing valve easily gives two pressures by
opening or blocking the drain line. Figures 16-8 through 16-11 show a
simple way to get two pressures using a reducing valve and a normally
open 2-way directional valve.
Figure 16-8 shows a normally open 2-way directional control valve piped in the drain
line. There is no leakage from the drain port in the at-rest condition.
Figure 16-8. Using a reducing valve for dual pressure (circuit at rest with
pump running.). (contacting work at low pressure).
Figure 16-9 shows the directional valve on CYL2 shifted to advance the
cylinder to the work at low pressure. During this part of the cycle the reducing valve
stays open.
Figure 16-9. Using a reducing valve for dual pressure
(cylinder 2extending at low pressure).(pressing work at high pressure).
Figure 16-10 shows the cylinder contacting the work with pressure at the reducing valve
setting. The low pressure continues as long as required. During this time the operator can
check part alignment or other details. If a problem is detected, the operator simply
reverses the cylinder to realign any out of place or problem components.
Figure 16-10. Using a reducing valve for dual pressure
(cylinder 2contacting work at low pressure).
After determining all is well, the operator energizes the solenoid on the 2-way directional
valve as shown in Figure 16-11. This blocks drain flow from the reducing valve.
Blocking drain flow at the reducing valve causes it to open fully. Backpressure in the
blocked drain line, plus the internal valve spring, pushes and holds the spool open. When
the reducing valve opens, full system pressure goes to the cylinder to generate high force.
This action poses no problem to the reducing valve. This circuit is a reliable way to get
two pressures for an actuator.
Figure 16-11. Using a reducing valve for dual pressure
(cylinder 2pressing work at high pressure).
Remotely operating a pressure-reducing valve
Pilot-operated reducing valves have a remote pressure-control port. Connecting this port
to other pressure valves allows pressure to be changed from a remote location. For
example, Figure 16-12 shows a reducing valve with a directional valve and two remote
relief valves connected to the remote-control port. With the directional control valve in its
center position, set the pressure with the knob on the reducing valve. This setting is
always the highest reduced pressure for the circuit.
Figure 16-12. Using remote pilot port for three different system
pressures (no solenoids energized).
Energizing solenoid A1 of the directional valve, as in Figure 16-13,
connects the remote pilot port to the remote relief valve SET 350 psi.
Pressure in the system now drops to and holds at 350 psi. Energizing
solenoid B1 of the directional valve, as in Figure 16-14, connects the
remote pilot port to the remote relief valve SET 700 psi. Pressure in the
system now rises to 700 psi and holds at that level.
Figure 16-13. Using remote pilot port for three different system
pressures (solenoid A1 energized).
Figure 16-14. Using remote pilot port for three different system
pressures (solenoid B1energized).
Figure 16-15 shows the reducing valve’s remote port connected to an infinitely variable
electrically modulated relief valve. An electronically controlled relief valve changes the
reduced pressure infinitely with a remote electrical controller.
Figure 16-15. Using remote pilot port with a servo press controller for infinitely variable pressure.
Pressure-reducing-relieving valves
When it is possible for an external force to increase pressure in a
reduced-pressure circuit, use a reducing-relieving valve. Most modular
valves now have the reducing-relieving function. When in doubt, specify
reducing-relieving valves where they are required.
Figure 16-16 shows a large-bore cylinder opposing a smaller-bore
cylinder. With a standard reducing valve, oil in the cap end of the 2-in.
bore cylinder (CYL1) is blocked after reaching reduced pressure. With a
6-in. bore CYL2 opposing CYL1, pressure could increase to 9000 psi in its cap end.
Pressures this high could cause machine damage and be unsafe.
Figure 16-16. Using a reducing valve in circuit with unmatched
opposing cylinders (both extending and locked up).
Figure 16-17 shows a reducing-relieving module installed. Now, pressure
in the end of cylinder CYL1 only increases to 430 psi. At 430 psi, the relief function
takes over and the cylinder retracts.
Figure 16-17. Using a reducing valve in circuit with unmatched
opposing cylinders (larger cylinder driving smaller back).
A cylinder in a high-temperature location may have a similar problem. (Normally,
hydraulic systems are not installed in areas with excessive heat, but it is a possibility.)
With the cylinder extended at reduced pressure, as in Figure 16-18, heat could cause
pressure at a conventional reducing valve outlet to increase and cause failure. Figure 16-
19 shows how a reducing-relieving valve allows any heat-expanded oil to relieve to tank.
Figure 16-18. Using a reducing valve with cylinder in high-temperature
area (cylinder stalled).
Figure 16-19. Using a reducing-relieving valve with cylinder in high-
temperature area (cylinder stalled).
With slow heat build-up in a location with conventional ambient
temperatures, oil expansion that raises pressure is slow enough to pass
through the normal drain function.
All pilot-operated reducing valves have a drain line that bypasses control
oil. There is always a small amount of oil passing through it. When drain
flow is sufficient to handle backpressure from outside forces or heat, a
reducing-relieving valve may be unnecessary. If in doubt, specify a
reducing-relieving valve for safety’s sake.
Modular pressure-reducing-relieving valves
When buying modular reducing valves or reducing-relieving valves,
different options help reduce heat in a circuit while still maintaining good
control.
Figures 16-20 and 16-21 show a reducing-relieving valve in the pump port line. The
valve has an internal pilot that maintains reduced pressure at the outlet port. This means
there is heat-generating flow from the drain line whenever the pump is running.
Figure 16-20. Reducing valve circuit using a reducing-relieving valve on
port P — with both cylinders contacting a load.
Figure 16-21. Reducing valve circuit using a reducing-relieving valve on
port P — with both cylinders retracted.
Remotely piloting the reducing-relieving valve from port A, as in Figure
16-22, reduces pressure only on the extension stroke of the cylinder.
While the cylinder retracts and holds, as in Figure 16-23, the reducing
valve drain is not bypassing oil. (Some manufacturers put the reducing
valves directly in the A or B ports and use bypass checks for reverse free flow.)
Figure 16-22. Reducing valve circuit with reducing-relieving valve on
port A — with both cylinders contacting a load.
Figure 16-23. Reducing valve circuit with reducing-relieving valve on
port A — with both cylinders retracted.
In either case, drain flow only takes place during a small portion of the
cycle. This may sound unnecessary, but some circuits have multiple
reducing valves. Excess drain flow can cause heating and fluid waste.
(Pilot flow cannot be used to operate other actuators.)
Extra time spent on circuit design pays off in energy-efficient systems
that perform better in the work place.