Hydraulic Schematics and Basic Circuit Design 342

3/2 valve

Three-port, two-position valve. A directional control valve with three fluid passages through which fluid can flow and two positions. A 3/2 valve can control a single-acting cylinder in a circuit.

above fluid level

A conductor in a hydraulic system that ends above the fluid level in the reservoir. An above fluid level connection is represented schematically by a working line symbol that extends to the middle of a reservoir symbol.

accumulator

A fluid system component that stores pressurized hydraulic fluid from the pump and releases it on demand. Accumulators are represented schematically by capsules divided by a horizontal line.

actuation

The type of energy used to operate and engage a control valve. The actuation method for a control valve is indicated by a symbol attached to the valve envelope.

actuator

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

American National Standards Institute

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

ball

A spherical obstruction used to stop and start hydraulic flow in a control valve. A ball symbol is used in a check valve schematic symbol.

below fluid level

A conductor in a hydraulic system that ends below the fluid level in the reservoir. A below fluid level connection is represented schematically by a working line symbol that extends to the bottom of a reservoir symbol.

bidirectional

A hydraulic motor or rotary actuator that can rotate clockwise and counterclockwise. A bidirectional actuator or motor symbol has two inward-pointing energy triangles.

bleed-off circuit

A speed control circuit in which a flow control valve on a return line diverts excess flow from the pump to the reservoir. A bleed-off circuit regulates the amount of fluid entering the actuator to control extension speed.

bypass line

A conductor in a secondary fluid circuit that circulates fluid to clean and cool it for the primary circuit. A bypass line often contains a heat exchanger.

cam-operated

A method of mechanical valve actuation. Cam-operated directional control valves are able to stop or start at precise cylinder piston locations.

cap end

A closure on one end of a cylinder that covers the bore area and protects interior elements from damage. The cap end of a cylinder is opposite the rod end.

case drain

A line on a hydraulic pump or hydraulic motor that drains excess oil leaking internally. A case drain symbol is a drain line leading to a reservoir.

check valve

A directional control valve that blocks flow in one direction and allows flow in the opposite direction. Check valves are represented in a schematic by a ball and seat symbol on a working line.

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 with a flow control valve to provide an alternate flow path while the flow control valve is restricting flow.

computer-aided design

CAD. A computer software program that aids in the automated design and drawing of a part, product, process, or building. Computer-aided design programs allow designers to access standard part and component drawings to evaluate how they can be used in a particular system.

conductors

A passive fluid system component that conveys fluid through the system. Conductors include pipes, tubes, and hoses.

control valves

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

cross

A fitting that joins together four conductors in a fluid system. A cross fitting is represented on a schematic by a "+" with a dot in the center.

cylinder

A linear actuator that houses a piston attached to a movable piston rod. Cylinders exert force in a linear direction.

DCV

Directional control valve. An active fluid system component that determines the path hydraulic fluid takes through a hydraulic system. DCVs are shown in their unactuated state in a schematic diagram.

directional control valve

DCV. An active fluid system component that determines the path hydraulic fluid takes through a hydraulic system. Directional control valves are shown in their unactuated state in a schematic diagram.

double-acting cylinder

A linear actuator that exerts force in both directions as the piston rod extends and retracts. Double-acting cylinders are represented schematically by rectangles with short lines outside the rectangle at both ends to represent the ports.

drain line

A conductor that connects to a component to return fluid to the reservoir in a hydraulic system. A drain line is represented in a hydraulic schematic by short dashes.

electric motors

A machine that converts electricity into mechanical energy or motion. An electric motor is a type of prime mover for a hydraulic system.

electrical actuation

The act of opening or closing a valving mechanism with an electrical device, such as a solenoid. Electrical actuation may be indicated in a control valve symbol by a rectangle with a diagonal line.

energy triangle

A solid triangle used in a hydraulic schematic diagram that indicates whether a device consumes or produces hydraulic energy. An energy triangle shows the direction of fluid flow in a device.

envelope

A blank square used as the foundation for directional control valve and pressure control valve symbols. An envelope includes symbols that provide information about the valve's functionalities.

FCV

Flow control valve. A fluid system component that controls fluid flow rate throughout a hydraulic system. FCV symbols often include curved lines to indicate a flow restriction.

filter

A fluid conditioning device that removes solid contaminants from hydraulic fluid. A filter is represented in a schematic diagram by a diamond with a dashed line in the center.

filters

A fluid conditioning device in a hydraulic system that removes solid contaminants from hydraulic fluid. A filter is represented schematically by a diamond with a dashed line in the center.

fitting

A fluid system component that connects, redirects, extends, or terminates the run of a conductor. Fittings are represented in a schematic by a dot on working lines showing where conductors connect.

fixed-displacement pumps

A hydraulic pump with a fluid displacement that stays the same from cycle to cycle when the pump is operating at a given speed. A fixed-displacement pump symbol consists of a circle with an energy triangle indicating the direction of fluid flow.

flow control valves

FCV. An active fluid system component that regulates fluid flow rate throughout a hydraulic circuit in order to control actuator speed. Flow control valve symbols often include curved lines to indicate a flow restriction.

flow rates

The total volume of 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 conditioning device

A fluid system component that keeps hydraulic fluid clean and maintains fluid temperature. A fluid conditioning device may remove contaminants or heat from fluid.

fluid symbols

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

foot pedals

A fluid power control that activates or deactivates a control valve. A foot pedal is used in some manual actuation methods.

four-port, three-position valves

4/3 valve. A directional control valve with four passages through which fluid can flow and three positions. A four-port, three-position valve controls the extension and retraction of a double-acting cylinder.

gas-charged accumulator

A type of accumulator that uses a bladder or piston to separate the gas chamber from the liquid chamber. A gas-charged accumulator has a blank triangle within the main symbol to represent the compressed gas.

graphic diagram

A diagram that uses standard fluid symbols and lines to represent the components and connections within a hydraulic circuit. Graphic diagrams, commonly referred to as schematics, show the possible fluid flow paths in a hydraulic circuit.

heat exchanger

A fluid conditioning device that cools a system by transferring heat from hydraulic fluid using air or water. A heat exchanger is represented schematically by a diamond with arrows indicating whether it uses liquid or air.

heat exchangers

A fluid conditioning device that cools a system by transferring heat from hydraulic fluid using air or water. A heat exchanger is represented schematically by a diamond with arrows indicating whether it uses liquid or air.

high-low circuit

A type of pump unloading circuit that is powered by a high-pressure, low-flow pump and a low-pressure, high-flow pump. A high-low circuit conserves energy when a cycle requires maximum pressure and very low flow.

hose

A type of flexible fluid conductor. Hoses are represented in a hydraulic schematic by a curved or U-shaped line with dots at both ends.

hydraulic actuation

The act of opening or closing a valving mechanism with pressurized liquid, such as oil or water. Hydraulic actuation is indicated in a control valve symbol by a rectangle with an energy triangle.

hydraulic circuit

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

hydraulic intensifier

A component of a hydraulic system that converts low-pressure fluid power into high-pressure fluid power. Hydraulic intensifier symbols look similar to cylinder actuator symbols but have two cylinders and are located at different places within a schematic.

hydraulic motors

A motor that can rotate continuously to convert hydraulic energy into mechanical power. A hydraulic motor symbol is a circle with one or two inward-pointing energy triangles.

hydraulic motors

An actuator that rotates continuously to convert hydraulic energy into mechanical power. A hydraulic motor symbol is a circle with one or two inward-pointing energy triangles.

hydraulic pump

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

hydraulic systems

A type of power transmission system that uses oil or another liquid under pressure to produce mechanical power. Hydraulic systems contain one or more hydraulic circuits.

in parallel

A circuit configuration in which fluid flows through separate components in a circuit simultaneously because there is more than one path for current to flow. Parallel components may include control valves, pumps, or actuators.

internal combustion engines

A type of prime mover that produces heat energy and mechanical energy internally. Internal combustion engines used for hydraulic systems run on gasoline or diesel.

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. The International Organization for Standardization establishes fluid symbols for hydraulic schematic diagrams.

linear actuators

An actuator that reciprocates to convert fluid energy into mechanical power at the end of a hydraulic circuit. Linear actuators include double- and single-acting cylinders.

load

The force or pressure placed on a hydraulic system or specific component. The load exerted on a cylinder actuator results in cylinder extension.

manual actuation

The act of opening or closing a valving mechanism by hand. Manual actuation methods are represented by various symbols for manual devices, such as levers or pedals, attached to a control valve envelope.

meter-in circuit

A speed control circuit that uses a flow control valve at the actuator inlet. A meter-in circuit controls actuator extension speed.

meter-out circuit

A speed control circuit that uses a flow control valve at the actuator outlet. A meter-out circuit controls actuator retraction speed.

needle valve

A flow control valve with a fixed or adjustable orifice that restricts flow. A needle valve symbol has two inwardly curved lines to indicate a restricted passageway.

normally closed

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

passing lines

Two conductors in a fluid system that cross each other without connecting. Passing lines are indicated in a hydraulic schematic by two crossed lines that may have a small curve at the crossing point.

PCV

Pressure control valve. An active fluid system component that regulates fluid pressure in a hydraulic system. PCVs limit or reduce fluid pressure.

pilot line

An auxiliary conductor on a valve that transmits fluid pressure to a pilot port to actuate a valve. Pilot lines are represented in a hydraulic schematic by long dashes.

pilot port

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

pilot pressure

Auxiliary fluid pressure used to actuate or control a component. Pilot pressure actuation methods are indicated by a rectangle with an energy triangle.

pilot-operated valve

A fluid system control valve that actuates when fluid pressure acts on a pilot port to move the main valving mechanism. Pilot-operated control valve symbols contain dashed pilot lines and may be labeled in a hydraulic schematic.

piston rod

The movable part of a cylinder actuator that is attached to the piston. A piston rod is represented in a cylinder symbol as a straight line through the middle of the rectangle.

plug

A fitting that terminates a conductor run in a fluid system. A plug is represented on a schematic by a conductor line with an "X" at the end.

port

A passageway on a valve or cylinder through which fluid can flow. A port is identified as a short line extending off the main symbol of a component, such as a cylinder or valve.

positions

A physical setting on a control valve that can shift to provide different flow paths. Positions for a directional control valve are indicated on a schematic by the number of adjacent squares.

power source

A device that generates the energy needed to power a tool or system. The power source in a hydraulic system is the hydraulic pump.

pressure

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

pressure compensated

A device that automatically controls or adjusts a component in response to pressure changes. A pressure compensated symbol is an arrow attached to the side of a fluid symbol.

pressure control valve

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

pressure lines

A conductor that carries working fluid from the reservoir to the actuator in a hydraulic system. Pressure lines are indicated in a hydraulic schematic by solid lines.

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 FCV symbols include an arrow in line with the working line.

pressurized reservoir

A reservoir that is sealed from the atmosphere and receives pressure produced by the hydraulic system. A pressurized reservoir is represented in a hydraulic schematic by a capsule.

prime mover

The component in a hydraulic system that provides mechanical energy to power the hydraulic pump. Prime movers for hydraulic systems include electric motors and internal combustion engines.

pump unloading circuit

A hydraulic circuit design that conserves energy and pressure. A pump unloading circuit consists of two pumps with different pressures and flow rates.

pushbuttons

A manual control that activates or deactivates a control valve. Pushbuttons are used in some manual actuation methods.

quick disconnect

A specialized fitting used for conductors or components that are connected or disconnected frequently. Quick-disconnect fittings are shown as connected or disconnected in a schematic.

reciprocating

Repeated back-and-forth movement. Reciprocating movements occur linearly.

relief valve

A pressure control valve that drains pressurized fluid from the pump back to the reservoir once a hydraulic system reaches a predetermined pressure. Relief valve symbols include drain lines and reservoir symbols.

reservoir

The main storage container for fluid in a hydraulic system. A reservoir is represented schematically by an open rectangle or a capsule, depending on the specific type.

resistance

The opposition to a force, such as the weight of a load. Fluid takes the path of least resistance.

return line

A conductor carrying active fluid from the actuator back to the reservoir. Return lines often contain fluid conditioning devices.

return lines

A conductor that carries working fluid from the actuator back to the pump in a hydraulic system. Return lines are indicated in a hydraulic schematic by solid lines.

rod end

The end of a cylinder that contains the piston rod that extends out of the cylinder barrel. The rod end of a cylinder is opposite the cap end.

rotary

The ability to spin or turn around an axis, without a change in linear position. Rotary components are represented schematically by circles.

rotary actuators

An actuator that rotates in fixed degrees to convert fluid energy into mechanical power at the end of the hydraulic circuit. Rotary actuators are represented schematically by semicircles with one or two inward-pointing energy triangles.

schematic

A diagram that uses standard fluid symbols and lines to represent the components and connections within a hydraulic circuit. A schematic, also known as a graphic diagram, shows the possible fluid flow paths in a hydraulic circuit.

seat

The fixed surface within the valve body on which a valving mechanism sits. A seat symbol is a set of angled lines and is used in a check valve symbol.

sequence valve

A normally closed pressure control valve that diverts fluid from a primary branch of the circuit into a secondary branch after a critical pressure is reached. Sequence valves are typically used in circuits containing hydraulic intensifiers and multiple actuators.

single-acting cylinder

A linear actuator that exerts force in one direction as the piston rod extends or retracts. A single-acting cylinder is represented schematically by a rectangle with a port at the cap end or rod end.

solenoid

A coil of wire that generates an electromagnetic force when a current is applied. Solenoid actuation is represented in a control valve symbol by a rectangle with a diagonal line.

spring-loaded accumulator

A type of accumulator that uses a preloaded spring to release pressurized hydraulic fluid. A spring-loaded accumulator schematic symbol has a sawtooth line within the main symbol to represent the spring.

spring-retracted

A single-acting cylinder actuator that retracts when fluid pressure decreases and the spring decompresses. A spring-retracted cylinder symbol has a sawtooth line down the center to represent the spring.

strokes

The action a piston rod takes when it moves in or out of a cylinder. The stroke of a piston rod is its length of travel.

tee

A fitting used to change the direction of a conductor run in a fluid system. A tee fitting is represented on a schematic by a "T" with a dot where the lines meet.

temperature-compensated FCV

A flow control valve with a variable orifice that adjusts automatically in response to fluid temperature and viscosity. Temperature-compensated FCV symbols have a solid line terminating in a dot to indicate a thermometer.

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 can control a single-acting cylinder in a circuit.

unidirectional

A hydraulic motor or rotary actuator that is able to rotate in only one direction. A unidirectional actuator or motor symbol has one inward-pointing energy triangle.

unloading valve

A pressure control valve located near the pump that drains fluid from the pump to the reservoir at low pressure levels. An unloading valve may be used to drain a low-pressure pump.

valving mechanism

The internal component in a control valve that covers a port. The valving mechanism opens and closes to allow or block flow.

variable feed circuit

A speed control circuit with a two-way directional control valve and a flow control valve installed in parallel. A variable feed circuit results in varying speeds at different parts of the circuit.

variable-displacement pumps

A hydraulic pump that can move an adjustable amount of fluid per cycle. Variable-displacement pump symbols consist of a circle with a diagonal arrow inside and an energy triangle indicating the direction of fluid flow.

vented reservoir

A reservoir that uses atmospheric pressure and gravity to push fluid into the pump inlet. A vented reservoir is represented in a schematic by an open rectangle.

viscosity

A hydraulic fluid's resistance to flow. Viscosity decreases as temperatures increase.

weight-loaded accumulator

An accumulator with a dead weight attached to the top of a piston. A weight-loaded accumulator schematic symbol has a small square within the main symbol to indicate the dead weight separating the liquid chamber and gas chamber.

working line

A conductor in a fluid system that carries active fluid to or from the actuator. A working line is represented in a hydraulic schematic by a solid line.

wye

A fitting used to join angled conductors in a fluid circuit. A wye fitting is represented on a schematic by a "Y" with a dot where the lines meet.