BOOK 2 CHAPTER 20: Sequence Valves (2023)

Sequencing valves

There are times when two or more cylinders need to work in a planned sequence. With two or more cylinders that are controlled by a single directional control valve, the cylinder with the least resistance always strikes first. If the actuator with the least resistance is first in the sequence, the circuit will work fine with no other valves.

If the cylinder to be moved first has the most resistance, single direction control will not work. A separate directional control valve for each cylinder is one way to sequence this shift. By energizing a solenoid, the first cylinder extends. When the first cylinder touches a limit switch, it energizes a second solenoid, causing the next cylinder to move. With such a sequential circuit, the first cylinder can lose its holding power when the second directional control valve is switched. Other valving may be required to ensure that the first cylinder generates and maintains the required force before and during the second cylinder's stroke.

Another way to force fluid to follow the path of greatest resistance is to use a pressure control valve called a valve.Folgeventil.

Figure 20-1 shows the schematic symbol for an internally piloted sequence valve. A sequence valve symbol is similar to a relief valve symbol. The main difference is that a sequence valve always has an external drain line and usually a bypass check valve for reverse flow.

A sequence valve is a pressure actuated, normally closed, spring or spool valve that opens at an adjustable set pressure. Some designs use a spring acting directly on the spool or seat, others are pilot operated. A sequence valve always has an external drain connection to prevent oil leakage. Trapped fluid will at best change the set pressure or, at worst, prevent the valve from opening. For reverse flow functions, use the integrated bypass check valve shown in the symbol.

Sequence valves can be internally piloted as shown in Figure 20-1. This is the standard arrangement for the pilot source. The liquid in the inlet port of the valve cannot flow to the secondary circuit or the outlet port until it reaches the set pressure. Upon reaching the set pressure, the valve opens wide enough to allow excess pump flow to the secondary service.

The primary circuit will never fall below the sequence valve setting as long as the primary pressure is equal to or greater than the sequence pressure setting. The pressure at the sequence valve output port is the pressure required to overcome the resistance in the secondary circuit when not exceeding the setting of the unloading valve or pressure compensator.

Figure 20-2 shows the symbol for an external pilot operated sequence valve. In some circuits, the control signal to open the valve comes from a source other than the supply line. An externally piloted sequence valve opens and allows flow when a remote control reaches a specified pressure.

Sequence valves generate heat in a hydraulic system. With a pressure setting of 800 psi and a branch resistance of 150 psi, there is a pressure drop of 650 psi across the valve. This drop in pressure results in heat because its energy is not doing any useful work. Most sequence circuits require a heat exchanger, especially for fast cycles.

Many older machines use sequence circuits because electrical controls were not understood at the time they were designed. Sequential circuits are unreliable and difficult to set up and maintain. Some older circuits have one directional valve and up to six sequence valves. With so many customizations, it's difficult to keep the loop running consistently.

Another potential problem with a sequence valve circuit is that actuator position cannot be assured. If a sequence valve changes, the only certainty is that the pressure has reached a certain level. The pressure buildup may be due to a damaged or stuck cylinder or a kinked line. When it is necessary to safely locate an actuator, always use a limit switch or relief valve. If all that needs to be determined is that the pressure has risen, an in-line sequence valve prevents the fluid from moving to the next action until the limit switch is triggered and the pressure rises.

Figure 20-3 shows the symbol for aKickdown-Folgeventil. Its operation differs from a normal sequence valve. Once a reducing sequence valve reaches the set pressure, flow will flow freely. The pressure may need to reach 900psi before flow begins to flow through the valve, but once it begins to flow a reduction sequence is fully open. A pressure drop in excess of 50psi across a reductant sequence valve will keep it fully open. (Note that a kickdown sequence valve generates less heat but does not maintain pressure in the primary circuit.)

Figures 20-14 through 20-17 show a circuit using a sequential throttle valve operating two cylinders. A pilot operated check valve added to the inlet of the first cylinder maintains pressure in the first cylinder while the second cylinder operates at low pressure.

Another use of a kickdown sequence is to unload a pump after the circuit has reached maximum pressure. A check valve will continue to unload the pump until the pressure drop falls below 50 psi. (See additional explanation along with Figures 20-23.)

When using flow controls with sequence circuits, input flow control is the only viable option. Chapter 10, covering flow controls, explains why.

Figures 20-4 through 20-11 provide schematic drawings for a two cylinder sequential shift. A 4-way valve controls both cylinders. The order is: Cylinder1Extend cylinder2Extend cylinder2retraction and cylinder1drive in cylinder2will not extend until the pressure in the cylinder1reach 600psi.

Good feature of a sequential circuit: When the cylinder1It's a staple, the thickness of the piece doesn't matter. cylinder2will not extend to the cylinder1Hold all thick parts securely. On the other hand, if for some reason the clamp cylinder binds before it makes contact with the workpiece, the pressure builds up and allows the clamp cylinder2Pedal. Each sequential circuit can fail at any time due to external influences.

Two-cylinder sequential circuit

Figure 20-4 shows a sequential shift of two cylinders at rest. The schematic drawing shows the valve pressure settings. Gauges are placed to show the working pressure as the sequence progresses.

In Figure 20-5 SolenoidA1is excited andCIL1distributes pressure to the pressure gaugesPG1-2j3indicates the required pressure (100psi) to moveCIL1. Even ifCIL2only requires 25psi to move, sequential valvemikeeps liquid out.CIL1extends until it touches a part.

IfCIL1partially touched, Figure 20-6, the system pressure increases rapidly. When pressure rises above 300psi (as seen on pressure gauges).PG1,2, Of3),CIL2it is still stationary. The pressure keeps increasingCIL1up to 350psi. When the pressure is 350psiCIL1, reduction valve(B)tight and safe. The pressure in the rest of the circuit will continue to increase until it reaches 500psi.

When the pressure reaches 500 psi as shown in Figure 20-7, the sequence valve shuts offmiopens just enough to allow excess pump fluid to drainCIL2. When the pressure is onCIL1Drop for some reason, sequence valvemiclose enough to keep the system pressure at 500 psi or more if possible. measure nowPG1read 500 psi while measuringPG2j3Read and measure 350 psiPG5Read what is needed to moveCIL2. pressure onCIL2changes with load fluctuations.

When the pressure increasesCIL2is less than 500 psi, the pressure drop across the sequence valve(MI)it generates heat. When the pressure increasesCIL2Exceeds 500 psi there is no energy loss and therefore no heat. Because of the reducing valve(B),pressure onCIL1it stays at 350psi no matter how high the system pressure rises.

IfCIL2falls below, as in Figure 20-8, the pressure in the manometersPG1jPG5750 psi is reached and the system pressure relief valve begins to dump fluid into the tank. pressure onCIL1stays at 350 psi because of the reducing valve(B)I won't let you go any higher. reducing valve(B)hinderCIL1crushing the partCIL2does his job.

closing magnetB1at the directional control valve(A), Figure 20-9, begins to return the cylinders to their original position. In this circuitCIL2retires first whileCIL1kept under pressure when the directional control valve switches. Pilot operated check valve(C)Catch the oil at the top of the lidCIL1- Note counterPG3– so as not to relax and drop the piece. The oil works nowCIL2and switching valve(D) . CIL2it retracts first because it only takes 100 psi to move it while the pressure adjustment occurs at the sequence valve(D)it's 300 psi. pressure onCIL2it changes on break-in but never goes above 200-250psi.

IfCIL2fully retracted, the system pressure increases rapidly as shown in Figure 20-10. rod end pressureCIL2eventually rises to 300 psi.CIL1still has about 350psi at the end of the cap due to the pilot operated check valve(C) .(press onCIL1can fall down due to leaking seals or pipes in the circuit described here). With a short cycle time, the pressure drop is minimal. If pressure drop is a problem, connect a small accumulator in the line between the pilot operated check valve(C)and the cylinder. Refer to Leakage Recovery Circuit with Accumulator in Chapter 1, Figures 1-24 through 1-27.

When the pressure reaches 300psi in Figure 20-11,CIL1begins to withdraw. Because of the switching valve(D) ,rod end pressureCIL2stays at 300psi. When oil flows through the sequence valve(D) ,first sends a pilot signal to open the pilot operated check valve(C) .after the valve(C)open,CIL1can withdraw. pressure at the end of the rodCIL1is all that is needed to put the cylinder in place.

Simple sequence circuit with modular tubes

Figures 20-12 through 20-15 show a modular or sandwich sequence valve in a circuit. The use of modular valves and manifolds reduces pipe installation time and reduces the number of potential leak points.

Figure 20-12 shows the system in idle state. A constant pump is used as the drive source, which delivers without pressure via a central tandem valve. This cycle generates some heat, but ensures that theClampThe cylinder never drops below a certain pressure as long as theWorkThe cylinder extends and retracts. He alsoWorkthe cylinder cannot even try to extendCIL1makes a limit switch.

In Figure 20-13 SolenoidA1at the directional control valveDV01is energetic. The flow from the pump goes toClampCylinder and expands it into work. Since only a low pressure is required for this and the entire flow rate of the pump is used, no heat is generated.

If thatClampCylinder makes a limit switch, as shown in Figure 20-14, energizes the solenoidA2at the directional control valveDV02. The modular low flow valveDV02ensures that theClampCylinder sees at least 700 psi beforeWorkThe cylinder is extended. If heWorkThe cylinder only requires 450psi to extend, 300psi of energy loss generates heat. With this follow-upClampThe cylinder pressure must not fall below 700 psiWorkcylinder strokes. directional valveDV02, shifted by a limit switch, ensures that theClampthe cylinder touches the workpiece in front of theWorkcylinder cycles

to withdrawWorkDe-energize the cylinder and magnetA2as in Figure 20-15. This directs oil from the pump to the rod end of theWorkCylinder. HeClampCylinder still has 700 psi to hold the part in place while theWorkThe cylinder returns home.

If thatWorkCylinder is fully retracted, switch off magnetA1and energize the solenoidB1at the directional control valve. HeClampcylinder return,DV01is deactivated and the cycle ends.

Add more directional control valves, e.gDV02and more modular sequence valves ensure sufficient pressure for more work functions. A single bolt-on cartridge sequence valve added to the rod manifold in the pump line between the pinch valve and the power cylinder valves can eliminate multiple sequence valves. The additional cost of special manifolds for this arrangement is a good investment.

Sequence circuit with kickdown sequence valve
Figures 20-16 through 20-19 show a reducing sequence valve in place of the standard sequence valve. For a reducing sequence valve, add the modular pilot operated check valve shown in-line at the end of the clamp cover. A pilot operated check valve blocks pressurized fluid at the cap end of the clamp cylinder when the downshift sequence valve opens.

In Figure 20-17 SolenoidA1at the directional control valveDV01is energetic. The flow from the pump goes toClampCylinder through the pilot operated check valve to extend theClampCylinder. Because this requires low pressure and full flow from the pump, no heat is generated.

After making contact with the workpiece, Figure 20-18, energize the solenoidA2at the directional control valveDV02. Modular low sequence kickdown valveDV02causes a pressure of 700 psi to build up insideClampcylinder beforeWorkThe cylinder is extended. If heWorkThe cylinder requires only 450psi to start up, system pressure drops to 450psi with minimal heating.

IsClampThe cylinder maintains performance as the pre-actuated modular control holds 700 psi of fluid. With a short cycle time, the pressure drop is minimal. If pressure drop is a problem, connect a small in-line accumulator between the manifold and the head-end line. See a leakage compensation circuit using an accumulator in Chapter 1, Figures 1-24 through 1-27.

to withdrawWorkDe-energize the cylinder and magnetA2as in Figure 20-19. This increases flow from the pump to the rod end of theWorkCylinder. Pre-action control still maintains 700 psi or moreClampCylinder holds the part in place while theWorkthe cylinder retracts.

AfterWorkRetract the cylinder completely, de-energize the magnetA1and energize the solenoidB1on the directional control valve, retracting theClampcylinder to its home position. Control pressure connectionBopens the pilot operated check valve, allowing trapped liquid to flow out of the attachment endClampCylinder. AfterClampcylinder return,DV01is de-excited and the cycle ends.

Pump unloading with sequential kickdown valve

Figures 20-20 through 20-23 show a downstream sequence valve that automatically unloads a pump at the end of a cycle. The kickdown sequence valve and a single solenoid directional control valve can replace a tandem or open center 3-position valve exhaust circuit. This circuit simplifies electrical controls because it uses only one electromagnet.

A rig application using a single solenoid, a two position spring return valve and a constant displacement pump can be operated in this manner with little heat generation. Figure 20-20 shows the circuit at rest. The pump unloads through the kickdown sequence valve(A)at around 50psi it is ready to cycle.

closing magnetA1at the directional control valve(B),Figure 20-21 directs the oil to the end of the inner cylinder cover. The cylinder is stretched to the pressure required for its movement. Oil from the rod end of the cylinder flows freely into the tank, greatly reducing the pressure in that line. Pressure drop allows kickdown sequence valve(A)to close (or reset) for the retract cycle.

The cylinder extends until it hits the part. The pressure builds up until the punch penetrates the part. A limit switch then deactivates the electromagnet.A1at the directional control valve(B) .directional valve(B)the spring returns to normal and the cylinder stroke is reversed and retracted.

The cylinder will retract with the pressure required for its movement, Figure 20-22. Kickdown sequence valve(A)it remains closed because its setting is greater than the pressure retracting the cylinder. The cylinder retracts to the end of its stroke or to the stop.

When the retract cylinder stops, Figure 20-23, pressure builds up at the end of the rod. When the pressure reaches the kickdown sequence valve setting(A) ,The valve opens and discharges the pump into the tank at about 50 psi. The circuit returned to the conditions of Figs. 20-20.

A kickdown sequence valve is a unique component that can simplify the electrical control of single and twin cylinder systems. At the same time, energy loss and heat generation are minimal.

CAUTIOUS:When using any pressure relief valve, the only certainty is that it has reached the set pressure.

Pressure-compensated pump with self-regulating relief valve

Some designers use a relief valve with a pressure-balanced pump to reduce pressure spikes when the pump is rapidly transitioning from full flow to no flow. Figure 1-16 in Chapter 1 shows and explains a circuit that uses a constant pressure pump and a relief valve. Figures 1-17 through 1-19 show another circuit that uses a battery to protect the pump. The accumulator circuit virtually eliminates pressure spikes; In addition, it offers faster actuator response at the beginning of a cycle. However, there are pressure compensated pump circuits that require over pressure protection that an accumulator alone cannot provide. The schematic diagrams in Figures 20-20 through 20-24 show a circuit with a cylinder opposed by a force in excess of its pressure rating. When an external force begins to push on the cylinder, the pressure at the end of the cap increases. Without a relief valve in the circuit, pressure can easily exceed the ratings of valves, lines, pumps, and cylinders. This is because a pressure-compensated pump will compensate for the lack of flow at the set pressure, but will not allow backflow to relieve pressure above the set value.

safety valve(A),Installed anywhere in the discharge line, it protects the system when set at 150 to 200 psi above the pump trim. The inlet of a pressure-compensated pump should never have a higher pressure than the compensator setting. Adding a check valve(B)at the pump outlet ensures that the pressure at the pump never exceeds the trim tab setting. However, the relief valve(D)can cause problems as indicated in Chapter 1, page ACC7. Figures 20-25 through 20-27 show a less problematic overpressure protection circuit.

Figure 20-25 shows a low pressure sequence valve with internal pilot and external drain.(A)connected to the pump outlet. (Valve(A)has a low pressure spring rate between 50 and 250 psi.) The output of the sequence valve(A)Go straight to the tank. isolation check valve(B)in the pump outlet line before the sequence valve keeps backflow and overpressure away from the pump. pilot line(C)from the pump outlet before the check valve(B)goes to the external drain port of the sequence valve(A).

With the circuit inactive and the pump running, the system pressure is the pump trim setting. The pump pressure at the inner pilot valve attempts to open the sequence valve(A),but at the same time keep it closed by the external drain hole. With a sequence valve spring setting of 65 psi, the tank will not open until pressure is past the check valve(B)it goes at least 65 psi higher than the trim tab setting on the pump as shown in Figure 20-26.

The main reason this overpressure relief circuit is better than a circuit with a standard relief valve is that adjusting the compensator on the pump not only changes the system pressure, it also automatically increases the relief pressure. The pump never discharges to tank and the circuit always discharges when the pressure rises above the sequence valve spring setting.

With this overpressure circuit, there is no protection against pressure surges when the pump scales have to work quickly. Using the accumulator shown in the schematic drawings saves the pump when it needs to compensate quickly. The accumulator also makes the circuit more responsive early in the cycle.

This drain valve relief acts anywhere in the circuit to protect each line from over-pressurization. Figure 20-27 shows a sequence valve T-connected to the head end line with excessive external force. If an external force attempts to retract the cylinder, the cylinder is free to move if the pressure at the canopy door rises just above the trim tab setting. At all other times, the sequence valve remains closed since its inlet never sees a pressure greater than the compensator setting.