Valves are used on hydraulic pumps and air compressors to control fluid direction, pressure, and flow rate. Valves are found in most industrial processes including: water and sewage processing, mining, power generation, oil processing, gas and petroleum, food manufacturing, chemical and plastic manufacturing, and many other fields. Valve mechanisms are classified as a spool, poppet, slide, rotary, and diaphragm.

Spool Mechanism

Used in both pneumatic and hydraulic valves. They slide inside a sleeve, controlling flow between two ports. Characteristics include a short stroke, low friction, requires little actuation force, and is suitable for high pressures.

Poppet Mechanism

A poppet is a component in the valve that covers an internal passage and is held in place by air pressure and a spring. Cartridge valves consist of a poppet, sleeve, and spring that are contained in a machined manifold block. When actuated, a stem pushes the poppet away from the seat to allow air flow. They are used for heavy flows with minimum pressure drop.

Spool (L) vs. Poppet (R) Mechanism

Spool (L) vs. Poppet (R) Mechanism


Pressure Control Valves

Pressure control valves modulate pressure level in fluid-power circuits. There are several types available and they are categorized by function.

Relief Valves

Used to keep the pressure in the lines from going above a level that would damage equipment. A spring holds the relief valve closed until the pressure level of the hydraulic force on the inlet port exceeds the spring pressure. The valve ports the hydraulic fluid back into the reservoir.

Pressure Control Valve

Pressure Control Valve

Reducing Valves

Used to limit pressure levels of branch circuits. When the circuit reaches a predetermined pressure, the reducing valve restricts flow by moving the spool.

Sequence Valves

These valves sense pressures other than maximum. They are normally closed and when the pressure reaches preset levels, they permit flow between inlet and output ports . Sequence valves normally come with a reverse check, which causes the system to sequence in one direction only.

Unloading Valves

Cause a pump to go off-line when a certain pressure is reached, while maintaining pressure. When the pressure falls to a set amount, the valve closes and boosts pressure back to the preset level and then unloads the pump again.

Brake Valve

Used for dynamic braking on a hydraulic motor. It has a relief valve where the back pressure to slow the motor can be set.

Counterbalance Valve

Used in cylinder circuits to put a back pressure on the cylinder and prevent load from uncontrollably falling. It is considered a safety valve in machines like manlifts and presses.

Flow-Control Valves

Fluid flow is controlled by either throttling or diverting it. Throttling limits flow by reducing the size of an orifice and bypassing part of the flow around a circuit, so the actuator only receives the portion needed to perform the task.

Flow Control Valve

Flow Control Valve

  • Meter-in system: inlet flow of an actuator is controlled
  • Meter-out circuit: outlet flow of an actuator is controlled
  • Bleed-off system: fluid is diverted to a reservoir or another part of the circuit

Non-Compensated Flow Controls

Simple valve that meters flow by restricting or throttling. When fluid is restricted, there is a drop of pressure on the downstream that is proportional to the amount of restricted fluid. A common type is a needle valve.

Pressure-Compensated Flow Controls

These valves incorporate a metering orifice to maintain constant flow despite variations in circuit pressure. Pressure drop shifts a balanced spool against a control spring and maintains constant pressure drop.

Directional Control Valves

Direction Control Valve

Direction Control Valve

These valves determine the fluid’s flow path through the circuit. They are classified by ports, position, and way.

  • Ports: The number of plumbing connections to the valve.
  • Position: The number of stops the valve can make during operation.
  • Way: The possible flow paths through a valve.

Proportional and Servo Valves

Proportional Valve

Proportional Valve

Proportional Valves

These are solenoid valves that vary the flow in proportion to the amount of current that flows though the solenoid coil. Fluid flow rate is nonlinear to current flow, but these valves are an inexpensive option for equipment requiring high-speed response at high-flow rates.

Servo Valves

Operation is accomplished from a remote electronic controller. Sensors mounted on hydraulic or pneumatic actuators continuously monitor position, speed, force, and acceleration of the actuator. The actuator valve opens and closes based on these settings. This is called closed-loop feedback.

Flow Capacity

Valve size can be based on the size of pipe connections or on flow capacity. Valve flow capacity can be expressed as a nominal or maximum rating.

Nominal Flow Rating

The amount of flow a valve can handle with relatively low pressure drop. It is typically the hydraulic flow that creates a 50 to 60 psi pressure drop.

Maximum Rating

The maximum flow a hydraulic valve can control without malfunction. If subjected to higher flow, controlling pressure may be insufficient to shift the valve spool mechanism.

Size Ratings

Refers to the physical size of the port.

Flow Coefficient (Cv)

As a valve restricts the flow of a fluid, there is a proportional drop in pressure on the outlet.

PTDA-Handbook-CoverContent on this page was created using excerpts from the Power Transmission Handbook (5th Edition), which is written and sold by the Power Transmission Distributor’s Association (PTDA). The Power Transmission Handbook is just under 400 pages and is a valuable resource for anyone involved or interested in the power transmission industry.

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