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There are times when two or more cylinders need to stroke in a planned sequence. With two or more cylinders controlled by a single directional valve, the cylinder with the lowest resistance always strokes first. If the actuator with the least resistance is first in the sequence, the circuit runs smoothly without other valving.
When the cylinder that must move first has the highest resistance, a single directional control will not work. A separate directional valve for each cylinder is one way to sequence such a circuit. Energizing one solenoid extends the first cylinder. When the first cylinder contacts a limit switch, it energizes a second solenoid, causing the next cylinder to stroke. With this type of sequencing circuit, the first cylinder may lose holding power when the second directional valve shifts. It may require other valves to make sure the first cylinder generates and maintains the force required both before and during the second cylinder’s stroke.
Another way to force fluid to take the path of greatest resistance is to use a pressure-control valve called a sequence valve.
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 often has a bypass check valve for reverse flow.
A sequence valve is a pressure-operated, normally closed, poppet or spool valve that opens at an adjustable set pressure. Some designs use a spring acting directly on the spool or poppet, others are pilot-operated. A sequence valve always has an external drain port to keep from trapping leakage oil. Trapped fluid modifies set pressure at best or keeps the valve from opening at worst. For reverse flow capabilities, use the integral bypass check valve shown in the symbol.
Sequence valves may be internally pilot-operated as in Figure 20-1.This is the standard arrangement for the pilot source. Fluid at the inlet port of the valve cannot pass to the secondary circuit or outlet port, until reaching set pressure. Upon reaching set pressure, the valve opens enough to let excess pump flow pass on to the second operation.
The primary circuit never drops below the sequence valve setting as long as the primary pressure is equal to or greater than the sequence pressure setting. Pressure at the outlet port of the sequence valve is that required to overcome resistance in the secondary circuit when it is not above relief valve or pressure compensator setting.
Figure 20-2 pictures the symbol for an externally piloted sequence valve. In some circuits the pilot signal to open the valve is from a source other than the line feeding it. An external pilot-operated sequence valve opens and allows flow when a remote operation reaches a certain pressure.
Sequence valves produce heat in a hydraulic system. With a pressure setting of 800 psi and resistance in the secondary circuit of 150 psi, there is a 650-psi pressure drop across the valve. This pressure drop results in heat, because its energy does not do useful work. Most sequence circuits require a heat exchanger, especially when they cycle rapidly.
Many older machines use sequence circuits because at the time they were designed there was a lack of understanding of electrical controls. Sequence circuits are unreliable and difficult to set up and maintain. Some older circuits have one directional valve and up to six sequence valves. With this many adjustments to make, it is hard to keep the cycle operating consistently.
Another potential problem with a sequence valve circuit is that actuator position cannot be assured. When a sequence valve shifts, the only sure thing is that pressure has reached a certain level. Pressure build up could be from a damaged or stalled cylinder or a kinked line. When it is necessary to positively locate an actuator, always use a limit switch or limit valve. When it is only necessary to know that pressure has built, a sequence valve in the line keeps fluid from the next action until the limit switch is contacted and pressure increases.
Figure 20-3 shows the symbol for a kick-down sequence valve. Its operation is different from a normal sequence valve. After a kick-down sequence valve reaches set pressure, flow passes through unrestricted. Pressure may have to reach 900 psi before flow passes through the valve, but when it starts passing, a kick-down sequence opens fully. A pressure drop of more than 50 psi across a kick-down sequence valve keeps it full open. (Note that a kick-down sequence valve causes less heat generation, but does not hold pressure on the primary circuit.)
Figures 20-14 through 20-17 show a circuit using a kick-down sequence valve operating two cylinders. A pilot-operated check valve, added to the inlet of the first cylinder, maintains pressure on the first cylinder while the second cylinder strokes at low pressure.
Another use for a kick-down sequence is unloading a pump after the circuit reaches maximum pressure. A kick-down sequence valve keeps unloading the pump until pressure drop across it falls below 50 psi. (See further explanation in conjunction with Figures 20-23.)
When using flow controls with sequence circuits, meter-in flow control is the only workable option. Chapter 10 covering flow controls, explains the reasons for this.
Figures 20-4 through 20-11 provide schematic drawings for a two-cylinder sequence circuit. One 4-way directional control valve controls both cylinders. The sequence is: cylinder1 extend, cylinder 2 extend, cylinder 2 retract, and cylinder 1 retract. Cylinder 2 will not extend until pressure at cylinder 1 reaches 600 psi.
A good feature of a sequence circuit: if cylinder 1 is a clamp, it does not matter how thick the part is. Cylinder 2 will not extend until cylinder 1 securely clamps any thickness part. On the other hand, if the clamp cylinder locks up for any reason before contacting the part, pressure will build and allow cylinder 2 to cycle. Any sequence circuit may fail to operate correctly at any time because of outside influences.