In the design of hydraulic and pneumatic systems,Circuit diagrams are an indispensable “language” for maintenance and troubleshooting.Master various component symbols and their functions,It can help us quickly understand how the system works.This article will introduce in detail the common component classifications, symbols and core functions in fluid circuit diagrams, providing you with a practical guide for interpreting this “mechanical language”.

Common classification of fluid circuit components

There are many types of fluid circuit components, which can be divided into the following core categories based on their functions. Each type of component plays a unique role in the system:

Universal fluid drawing elements

• Pipeline: A channel that transports fluids in a system.
• Control line: An auxiliary oil circuit used to control the operation of components such as valves.
• Crossing lines: Pipes that cross but are not connected.
• Connecting lines: Pipes that are directly connected.

Power and energy storage components

• Pump: Converts mechanical energy into hydraulic energy, providing fluid power for the system.
• Fuel tank: Stores hydraulic fluid and also dissipates heat and precipitates impurities.
• Motor: The power source that drives the pump.
• Hydraulic motor: Converts hydraulic energy into rotational mechanical energy.
• Filter: Removes contaminants from the fluid and protects system components.
• Accumulator: Stores hydraulic energy and absorbs system shock and pulsation.
• System relief valve: Limits the maximum system pressure to ensure safety.

Actuator

• Hydraulic cylinder: Converts hydraulic energy into linear mechanical motion.
• Pneumatic actuator: Converts air pressure into linear mechanical motion.
• Fluid motor: Converts hydraulic energy into rotary mechanical motion.

Control elements

• Flow control valve: Controls fluid flow.
• Throttle valve: Adjustable flow control valve
• Check valve: Adjustable flow control valve with bypass
• Check valve
• Hydraulically controlled check valve

• Pressure Control Valve: Regulates and controls system pressure.
• Relief valve
• Unloading valve
• Brake valve
• Sequence valve
• Counterbalance valve
• Pressure reducing valve

• Directional control valves: control the direction of fluid flow.
• 2-position 2-way valve
• 2-position 3-way valve
• 2-position 4-way valve
• 3-position 4-way valve (4-way, 3-position, with multiple neutral positions: Open [H-type] / Closed [O-type] / Tandem [U-type] / Float [Y-type])

• Operating Component: The trigger device that controls valve actuation.
• Push button
• Mechanical
• Foot pedal
• Hand lever
• Pneumatic pilot
• Hydraulic pilot
• Solenoid

Detailed explanation of fluid circuit components

Needle Valves

• Function: Used to throttle or shut off fluid flow. Flow rate varies with pressure or viscosity.
• Type: Some needle valves offer pressure or temperature compensation, maintaining stable flow despite varying operating conditions.

Check Valves

• Core Feature: One-way flow components that allow fluid to flow in only one direction. For example, fluid can flow freely from right to left, but flow from left to right is blocked.

Pilot-operated check valves

• Pilot-to-open:
• When there is no pilot pressure, flow is permitted in one direction only and reverse flow is blocked.
• When the pilot line is pressurized, the check valve opens, allowing flow in both directions.

• Pilot-to-close:
• When there is no pilot pressure, flow is permitted in one direction only, and reverse flow is blocked.
• When the pilot line is pressurized, the check valve closes, blocking flow in both directions.

Pressure Gauges

• Purpose: Measure oil pressure at a specific point in a system. Units are usually PSI (pounds per square inch) or bar (bar).
• Conversion: 1 bar = 14.5 PSI = 0.1 MPa.

Flow Control Valves

• Function: Controls fluid flow in one direction, while leaving it unrestricted in the opposite direction.
• Control Methods:
• “Meter-in”: Controls fluid flow into the actuator.
• “Meter-out”: Controls fluid flow out of the actuator.
• Special Types: Some valves feature pressure or temperature compensation to minimize the impact of varying operating conditions on flow.

Shut-off valves

• Purpose: Isolate one section of a fluid system from the rest of the system for maintenance or emergency shutoff.

Air bleed valves

• Function: Automatically remove air bubbles from the pressurized hydraulic system, preventing cavitation from affecting system performance.

Level switches

• Typical applications: Detecting when the oil level in a tank has dropped to the minimum operating level, triggering an alarm or protective action.

Temperature switches

• Typical applications: Detecting whether the oil temperature in the tank has reached the maximum operating temperature to prevent system damage from overheating.

Pressure switches

• Function: They trigger when system pressure rises or falls below a set value. They can be either adjustable or fixed.

Pressure Reducing Valves

• Purpose: Reduces pressure in branch circuits, ensuring that each circuit operates at its required pressure.

Pressure relief valves

• Core function: Limit the maximum pressure in a hydraulic system or a portion of a circuit to prevent system overload.

Counterbalance Valves

• Purpose: Controls overriding loads (such as the gravity load of a lowered object) and supports the load when the actuator is stopped.
• Note: These valves typically have preset values; do not adjust them at will.

Flow Fuses

• Features: Normally open valves that automatically close when the inlet/outlet pressure differential exceeds the designed set point; resettable by reverse flow.
• Application: When connected in series with an actuator (such as a hydraulic cylinder), they can limit the actuator’s maximum operating speed.

Accumulators

• Accumulators are used to store hydraulic energy and absorb shock in hydraulic systems.
• Warning: Before working on any component, be sure to release all hydraulic energy.
• Safety Tips – Accumulator Handling: Before servicing any hydraulic component, be sure to release all stored hydraulic energy in the accumulator to avoid injury from unexpected operation.

Directional Control Valves

• Directional control valves are used to direct fluids into the appropriate piping for a specific operation. These valves are typically electrically controlled.

Hydraulic Pumps

• Hydraulic pumps are used to pump fluid from the power unit to the rest of the hydraulic system. Some pumps come with control options such as a pressure compensator or flow compensator.

Fluid Motors

• Fluid motors are used to convert hydraulic energy into mechanical rotational motion.

Filters

• Function: Removes contaminants from fluids and protects system components.
• Additional Features: Some filters are equipped with a contamination indicator (optional), a one-way bypass valve (optional), and an automatic drain.

Oil Filters

• Function: Removes large solid particles from water or oil. Some models include an optional one-way bypass valve.

Water Modulating Valves

• Purpose: Automatically regulates the oil temperature in the tank by controlling the amount of water flowing through the heat exchanger.

Heat exchangers

• Coolers：remove heat from the circulating oil in a hydraulic system. The most common type is a water-to-oil cooler, but air-to-oil coolers can also be used.

• Heater: Heats the fluid to ensure the system starts normally in a low temperature environment.

Hydraulic Cylinders

• Hydraulic cylinders are used to convert fluid energy into mechanical linear motion.

Quick Disconnects

• Function: Used to quickly disconnect piping and separate equipment components.
• Type:
• With check valve: No fluid leakage during disconnection.
• Open: Fluid may escape during disconnection.

Proportional (Servo) Valves

• Features: These electrically controlled hydraulic valves control hydraulic pressure and/or flow proportionally to an electrical input signal.

Mufflers

• Purpose: Reduces exhaust noise. Commonly used in pneumatic systems.

Air Blows

• Indication: The number of air blows in the symbol can be adjusted based on the actual application.

Mastering the symbols and functions of these hydraulic and pneumatic circuit components is the basis for interpreting system circuit diagrams and troubleshooting.

Contact: Nancy Zhu, Sales manager, JW GROUP.

Email: nancy@jwgroup.cc

Web: https://jwcylinder.com

Mobile/Whatsapp:+86 15902166721

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