19.2: Understanding the Operation of a Direct-Acting Sequence Valve and Its Schematic Symbol

In many hydraulic applications —especially in automated machine tools —tasks must occur in a specific order. One typical example is a clamping operation that must be completed before any secondary action, such as drilling, grinding, or pressing, begins. This is where the sequence valve plays a critical role.

A sequence valve ensures that one actuator completes its motion, and pressure builds before another actuator is allowed to operate. In a clamping application, for instance, the valve prevents flow to the second actuator (like a hydraulic motor or a drill) until the clamp cylinder fully extends and holds its position. Only then does the valve open, allowing the next stage to begin. This sequential control is automatic and entirely pressure-driven.

How the Valve Works

There are two main types of sequence valves: direct-acting and pilot-operated. Both serve the same purpose, but the direct-acting type is simpler in construction and is often the first one students learn about. It provides a great foundation for understanding how sequencing is achieved in hydraulic circuits.

Figure \(\PageIndex{1}\): Schematic representation of a sequence valve.

A direct-acting sequence valve is made up of a valve body, an internal spool, a compression spring, and an adjustable screw or knob to fine-tune the spring tension. The valve has two main flow ports: an inlet (also called the primary port) and an outlet (secondary port), along with a third, smaller drain port connected at the top side of the valve. The inlet port is supplied with oil from the hydraulic system, and the outlet port connects to the next actuator in the sequence.

The spring chamber —where the spool is seated —is isolated from pressure by the smaller drain port, which routes any internal leakage to the tank. This helps ensure that the spring force remains consistent and is not affected by trapped pressure. There’s also an internal passage that routes the inlet pressure to the end of the spool opposite the spring.

In its normal state, the valve is closed. The spring holds the spool in a position that blocks the path from the inlet to the outlet. No oil can pass through to the second actuator. However, once system pressure builds to a level high enough to overcome the spring tension, the spool shifts. This opens the path from inlet to outlet, allowing oil to flow through to the next stage of the circuit.

The pressure at which the valve opens is determined by the spring’s preload, which is adjustable and is called the set crack pressure. This means you can precisely control at what pressure the next actuator is activated.

Reading the Schematic Symbol

The schematic symbol for a sequence valve resembles that of a relief valve, but with a few key differences. In both symbols, you’ll see a normally closed valve with a spring pushing against a movable component (typically an arrow). However, the sequence valve symbol also includes a pilot line that senses pressure from the inlet side and an external drain line coming from the spring chamber.

Figure \(\PageIndex{2}\): The schematic symbol for a sequence valve. 

This symbol tells us a few important things: the valve stays closed under normal conditions, opens in response to upstream pressure, and relieves internal leakage through the drain. Understanding the symbol is essential for interpreting and designing hydraulic circuits that involve sequencing.

Visualizing the Sequence in Action

Figure \(\PageIndex{3}\): Schematic representation of valve sequence action.

Consider a circuit where a clamp cylinder and a hydraulic motor are both connected to a directional control valve (DCV), with a sequence valve placed in the line to the motor. When the DCV is shifted to begin operation, oil flows first to the clamp cylinder. Because the sequence valve is normally closed, no oil reaches the motor initially.

The clamp cylinder extends easily, requiring less pressure than the sequence valve’s setting. As the cylinder reaches full extension and begins to resist further motion—by clamping onto a workpiece, for example—pressure in the system rises. Once it surpasses the valve’s setting (say, 500 psi), the spool inside the sequence valve moves, opening a flow path to the motor. Now the motor begins to operate.

This setup ensures that the clamp cylinder always completes its function before the motor starts. It’s worth noting that the pressure at the motor (downstream of the sequence valve) depends only on the load the motor sees. It is not influenced by the upstream pressure that caused the valve to open. That means the valve regulates the timing of when flow begins, but it does not control the downstream pressure once flow has started.

In the example circuit, the grinder motor requires less pressure than the valve’s opening setting. The sequence valve delays flow until the clamping step is complete, and after that, the motor operates based on its own load conditions.