Hydraulic Control Valves 341

3/2 valve

Three-port, two-position valve. A directional control valve with three ways for fluid to flow and two positions. A 3/2 valve is used to control a single-acting cylinder actuator.

accumulator

A fluid system component that stores pressurized fluid from the pump as an auxiliary source of fluid power. Accumulators can be controlled to release hydraulic fluid as necessary at the required system pressure.

accumulator circuits

A type of fluid circuit with a secondary source of fluid power. An accumulator circuit may necessitate the use of a pilot-to-close check valve to empty the accumulator.

actuation

The type of energy used to operate and engage a control valve. Actuation methods include manual, mechanical, hydraulic, pneumatic, and electric energy.

actuator

A fluid system component that converts hydraulic energy into mechanical energy. The actuator is at the end of a hydraulic circuit.

American National Standards Institute

ANSI. A private, nonprofit organization that administers and coordinates voluntary standard for products, services, and systems in the United States. The American National Standards Institute establishes symbols for fluid circuit schematics.

ANSI

American National Standards Institute. A private, nonprofit organization that administers and coordinates voluntary standard for products, services, and systems in the United States. ANSI establishes symbols for fluid circuit schematics.

balanced piston

A valving mechanism that remains balanced between system pressure and a spring during normal operation. A balanced piston is located in the main valve of a balanced-piston relief valve.

balanced-piston relief valve

A pilot-operated relief valve consisting of two valves in a common valve body. A balanced-piston relief valve contains a spring and poppet in the pilot portion and a spring-balanced piston in the main valve.

check valve

A normally closed one-way directional control valve that blocks flow in one direction and allows flow in the opposite direction. A check valve opens when hydraulic fluid reaches a certain pressure.

check valve bypass

A flow control valve that routes fluid to a specific circuit or around a specific element. A check valve bypass is often included in pressure control valves and flow control valves to provide an alternate flow path.

control valves

A fluid system device that regulates fluid as it travels from the pump to the actuator. Control valves regulate fluid flow rate, direction, and pressure.

cracking pressure

The set point at which fluid pressure acts on a valving mechanism to open the valve. Cracking pressure opens a valve from its normally closed position.

cylinder actuator

A hydraulic system component that converts hydraulic energy into mechanical energy. A cylinder actuator directs force in a straight line.

DCV

Directional control valve. A fluid system component that determines the path hydraulic fluid takes through a hydraulic circuit. A DCV can start, stop, and alter the direction of fluid flow.

directional control valve

DCV. A fluid system component that determines the path hydraulic fluid takes through a hydraulic circuit. Directional control valves can start, stop, and alter the direction of fluid flow.

directly proportional

A ratio between two values in which one value always changes in the same way as the other. The directly proportional values of pressure drop and flow rate across a nonpressure-compensated FCV mean that as flow rate increases, pressure drop increases proportionally.

direct-operated

A control valve that engages when force acts directly on the main valving mechanism. Direct-operated control valves are actuated through the use of a force, such as hydraulic or electric.

direct-operated reducing valve

A type of normally open pressure control valve that sustains or decreases pressure levels in a hydraulic system. A direct-operated reducing valve has a small passage in the spool that continuously drains fluid to the reservoir.

discs

A valving mechanism inside a valve body that moves to block or allow flow. A disc can rotate or move linearly inside a valve.

double-acting cylinder

A hydraulic linear actuator that applies force in both directions as the piston reciprocates within the cylinder. Double-acting cylinders use fluid pressure to extend and retract the piston.

downstream pressure

Fluid flow away from a given component or at the outlet of a component in a hydraulic system. Downstream pressure in a hydraulic system may be monitored by a pressure reducing valve.

drain line

A line connected to a valve or other component that leads to the reservoir in a hydraulic system. A drain line may be external or internal, depending on the pressure settings of the device.

electrical actuation

The act of opening or closing a valving mechanism with an electrical device such as a solenoid. Electrical actuation methods have good response sensitivity.

FCVs

Flow control valves. A fluid system component that controls the fluid flow rate in a hydraulic circuit. FCVs are used to control the speed of cylinder actuators.

finite positioning

Characterized by being either fully open or fully closed. Finite positioning control valves are typically closed until actuated.

fixed orifice

A nonadjustable opening in a flow control valve through which fluid flows. A fixed orifice in a flow control valve generates a predictable pressure drop.

flow control valve

FCV. A fluid system component that controls the fluid flow rate in a hydraulic circuit. Flow control valves are used to control the speed of cylinder actuators.

flow rate

The total volume of hydraulic fluid that moves through a hydraulic system or individual component in a given amount of time. Flow rate is measured in gallons per minute (gpm) or liters per minute (lpm).

fluid symbols

A standard symbol that represents a fluid system component in a schematic diagram. Fluid symbols are standardized by the American National Standards Institute and the International Organization for Standardization.

four-port, three-position valve

4/3 valve. A directional control valve with four ways through which fluid can flow and three positions. A four-port, three-position valve has a neutral position that blocks flow through the valve.

four-port, two-position valve

4/2 valve. A directional control valve with four ways for fluid to flow and two positions. A four-port, two-position valve is commonly used to control a double-acting cylinder actuator.

hydraulic actuation

The act of opening or closing a valving mechanism with pressurized oil or water. Hydraulic actuation converts fluid pressure into linear or rotary motion.

hydraulic circuits

A series of connected components that move pressurized hydraulic fluid through conductors to produce mechanical force. A hydraulic circuit is often part of a larger hydraulic system.

hydraulic motor

A device that converts hydraulic energy into mechanical energy. A hydraulic motor is a rotary actuator that can rotate continuously.

hydraulic pump

A mechanical power source that generates fluid flow. A hydraulic pump receives energy from a prime mover, such as a motor or engine.

infinite positioning

Characterized by the ability to be fully open, fully closed, or any position in between. Infinite positioning valves use a variable orifice to control fluid flow.

in-line check valve

A directional control valve with an inlet and outlet port located at opposite ends of the valve body. An in-line check valve permits forward flow and blocks reverse flow.

International Organization for Standardization

ISO. An international organization that establishes documented standards, rules, and guidelines to ensure that products, processes, and services are fit for their purpose. International Organization for Standardization symbols may be used for fluid circuit schematics.

ISO

International Organization for Standardization. An international organization that establishes documented standards, rules, and guidelines to ensure that products, processes, and services are fit for their purpose. ISO symbols may be used for fluid circuit schematics.

loads

The force or pressure placed on a hydraulic system or specific component. The load exerted on a system must be compatible with the hydraulic pump and other system components.

manual actuation

The act of opening or closing a valving mechanism by hand using a device attached to the valve body. Manual actuation does not require a mechanical power source.

needle valve

A flow control valve containing a conical obstruction that may extend or retract within the valve body to change the size of the orifice. A needle valve has an orifice that is either fixed or adjustable.

needle valve

A flow control valve with an orifice that is either fixed or adjustable. A needle valve consists of a conical obstruction that may extend or retract within the valve body to change the size of the orifice.

neutral position

A valve position in which all ports are blocked. Neutral position prevents flow through the valve.

nonpressure-compensated

A type of flow control valve used to regulate flow rate when actuator loads are stable. Nonpressure-compensated valves are unable to adjust flow rates in response to pressure changes.

normally closed

A valve position in which the valving mechanism is unactuated and covering a port. A normally closed valve opens when its cracking pressure is reached.

normally open

A valve position in which the valving element is unactuated and not covering a port. A normally open valve closes when it actuates.

overpressure

A condition that occurs when the pressure in a hydraulic system exceeds recommended levels. Overpressure can lead to equipment damage and/or personal injury.

override

A means of bypassing the normal function of a device. Overrides exist for various operating circumstances that can occur during operation.

PCVs

Pressure control valves. A fluid system component that regulates fluid pressure in a hydraulic system. PCVs limit or reduce fluid pressure.

pilot port

An opening on a valve body through which fluid travels when actuating the valving mechanism of a pilot-operated valve. A pilot port is connected to the main valve body.

pilot pressure

The auxiliary fluid pressure used to actuate or control a component. The pilot pressure needed to actuate a control valve is lower than the valve's regular cracking pressure.

pilot-operated

A control valve that engages when force acts on a connected pilot port to move the main valving mechanism. Pilot-operated control valves are often used to override a regular valve function.

pilot-operated reducing valve

A type of normally open pressure control valve that operates in two stages. A pilot-operated reducing valve has a pilot portion that directs fluid to the main valve.

pilot-operated relief valve

PORV. A relief valve consisting of two valves in a common valve body. A pilot-operated relief valve contains a spring and poppet in the pilot portion and a spring-balanced piston in the main valve.

pilot-to-close check valve

A check valve that blocks flow in both directions when pilot pressure applied to the pilot port closes the valve. A pilot-to-close check valve opens to allow forward flow when pilot pressure is removed.

pilot-to-open check valve

A check valve that opens to allow unrestricted reverse flow when pilot pressure is applied to the pilot port. A pilot-to-open check valve can be used to hold a cylinder actuator in place.

pneumatic actuation

The act of opening or closing a valving mechanism with compressed air. Pneumatic actuation converts air pressure into linear or rotary motion.

poppets

A conical valving mechanism that opens and closes in response to variations in pressure. A poppet is typically held in place by a spring.

ports

A passage in a component to which a conductor may be connected. Ports, or ways, in a control valve allow fluid to enter and exit the valve.

PORV

Pilot-operated relief valve. A relief valve consisting of two valves in a common valve body. A PORV contains a spring and poppet in the pilot portion and a spring-balanced piston in the main valve.

position

A setting on a control valve that can shift to block or allow fluid flow. Positions on a directional control valve indicate the distinct flow paths the valve provides.

pressure

The exertion of force upon a fluid or object. Pressure is measured in pounds per square inch (psi) or pascals (Pa).

pressure control valve

PCV. A fluid system component that regulates fluid pressure in a hydraulic circuit. Pressure control valves limit or reduce fluid pressure.

pressure drop

The difference in pressure measurements between any two points of a system or a component. A pressure drop occurs when force acts on a fluid as it flows through a hydraulic system component.

pressure override

The difference in fluid force per unit that exists between two separate points in a fluid system or fluid system component. Pressure override, sometimes referred to as pressure drop, is the difference between a valve's cracking pressure and full-flow pressure.

pressure reducing valve

A normally open pressure control valve that sustains a particular pressure level in a hydraulic system. A pressure reducing valve closes when a preset maximum pressure is reached downstream.

pressure-compensated FCV

A flow control valve with a variable orifice that adjusts automatically in response to pressure drops across a pressure-compensated spool. Pressure-compensated FCVs maintain consistent flow rates despite changes in load.

relief valve

A pressure control valve that drains excess pressurized fluid from the pump back to the reservoir once a hydraulic system reaches a predetermined pressure. Relief valves are typically placed after the hydraulic pump.

relief valves

A pressure control valve that drains excess pressurized fluid from the pump back to the reservoir once a hydraulic system reaches a predetermined pressure. Relief valves are typically placed after the hydraulic pump.

reservoir

The main storage container for fluid in a hydraulic system. A reservoir supplies fluid to the pump.

restriction check valve

A directional control valve that permits forward flow and allows a small amount of reverse flow in the normally closed position. Restriction check valves are similar in design to right-angle check valves but include a restriction plug.

restriction orifice

A basic flow control valve consisting of a disc or block with a fixed bore. A restriction orifice, also known as a restriction plate, is sized according to the intended pressure loss.

restriction plate

A basic flow control valve consisting of a disc or block with a fixed bore. A restriction plate, also known as a restriction orifice, is sized according to the intended pressure loss.

restriction plug

A device within a restriction check valve's valving mechanism that alters the size of the outlet port. A restriction plug allows for a limited amount of flow in the reverse direction in a restriction check valve.

right-angle check valve

A directional control valve with an inlet and outlet port located at right angles to each other. A right-angle check valve allows unrestricted flow in one direction and blocks reverse flow.

schematic diagrams

A diagram that uses symbols to represent fluid system components. A schematic diagram shows the arrangement and functionality of a circuit.

seat

The component of a control valve on which the valving mechanism rests. A seat forms a seal with the valving mechanism.

sequence valve

A normally closed pressure control valve that diverts fluid from a primary branch of the circuit into a secondary branch of the circuit after a critical pressure is reached. Sequence valves are used to control two or more actuators in a hydraulic circuit.

sequence valves

A normally closed pressure control valve that diverts fluid from a primary branch of the circuit into a secondary branch of the circuit after a critical pressure is reached. Sequence valves are used to control two or more actuators in a hydraulic circuit.

single-acting cylinder

A linear actuator that directs force in one direction as the piston extends within the cylinder. Single-acting cylinders use an external force to retract the piston back into the cylinder.

solenoid

A coil of wire that generates an electromagnetic force when a current is applied. When activated, solenoids can be used to change a valve's position.

spool

A cylindrical valving mechanism that slides back and forth in the valve body. A spool is used in a directional control valve to open and close ports.

swing check valve

A directional control valve with an inlet and outlet port located at opposite ends of the valve body. A swing check valve has a hinged valving mechanism that opens when fluid reaches cracking pressure.

tank port

A valve outlet port that drains to the reservoir. A tank port is often labeled as "T" in a schematic.

temperature-compensated FCV

A flow control valve with a variable orifice that adjusts in response to fluid temperature and viscosity. Temperature-compensated FCVs typically include pressure compensators as well.

three-port, two-position valve

3/2 valve. A directional control valve with three ways for fluid to flow and two positions. A three-port, two-position valve is used to control a single-acting cylinder actuator.

upstream pressure

Fluid flow toward a given component or at the inlet of a component. Upstream pressure in a hydraulic system is monitored by relief valves.

valve body

The outer framework of a valve assembly that encloses all internal parts of a control valve. The valve body withstands fluid pressure from connected conductors.

valving mechanism

The internal component in a control valve that covers a port. The valving mechanism opens and closes to regulate fluid flow.

variable orifice

An adjustable opening in a control valve through which fluid flows. Variable orifices are used in flow control valves to regulate fluid speed and quantity.

variable orifice

An adjustable opening in a valve through which fluid flows. Variable orifices are used in flow control valves to control fluid speed and quantity.

velocity

The speed of fluid flow through a hydraulic conductor. Velocity may be measured in gallons per second (gps) or liters per second (l/s).

ways

A passage in a component to which a conductor may be connected. Ways, or ports, in a control valve allow fluid to enter and exit the valve.

work port

A passageway in a control valve that connects to an actuator. A work port can receive pressurized fluid from either direction.