ch 16.doc

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O EMAIL INSHARE 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

Transcript of ch 16.doc

Page 1: ch 16.doc

O EMAIL INSHARE

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.

Page 2: ch 16.doc

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.

Page 3: ch 16.doc

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.

Page 4: ch 16.doc

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.

Page 5: ch 16.doc

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.

Page 6: ch 16.doc

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.

Page 7: ch 16.doc

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

Page 8: ch 16.doc

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.

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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.

Page 10: ch 16.doc

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.

Page 11: ch 16.doc

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.

Page 12: ch 16.doc

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.

Page 13: ch 16.doc

Figure 16-13. Using remote pilot port for three different system

pressures (solenoid A1 energized).

Page 14: ch 16.doc

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.

Page 15: ch 16.doc

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.

Page 16: ch 16.doc

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.

Page 17: ch 16.doc

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).

Page 18: ch 16.doc

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.

Page 19: ch 16.doc

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.

Page 20: ch 16.doc

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.

Page 21: ch 16.doc

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.