How Does a Hydraulic System Work? Industrial Fluid Power Principles

How Does a Hydraulic System Work? Industrial Fluid Power Principles

A hydraulic system uses pressurized fluid to transmit power and control mechanical movement. From industrial presses and injection molding machines to lifting equipment and mobile machinery, hydraulic systems are used when high force, controlled motion, and compact power transmission are required.

But how does a hydraulic system actually work? The basic principle is straightforward: a hydraulic pump moves fluid through a circuit, valves control where the fluid goes and how it behaves, and hydraulic actuators convert the fluid's energy into linear or rotary motion.

Understanding the difference between pressure and flow is also essential. The pump primarily supplies fluid flow, while system pressure develops as that flow encounters resistance. This relationship determines how much force an actuator can produce and how quickly it can move.

The Basic Principle: Pascal's Law and Hydraulic Fluid

The operating principle of a hydraulic system is based on Pascal's Law. In a confined fluid, pressure applied to the fluid is transmitted throughout the fluid in all directions.

Hydraulic oil is also relatively incompressible compared with air. This makes it suitable for transmitting force through a hydraulic circuit with limited volume change.

In practical applications, this principle allows hydraulic systems to:

• Multiply force: Pressure acting over a larger piston area can produce a greater output force.

• Control movement: Valves and flow controls can regulate the direction and speed of an actuator.

• Transmit high power: Hydraulic components can deliver substantial force and torque within a relatively compact system.

• Handle variable loads: Pressure-control components can help the system respond to changing load conditions.

For a hydraulic cylinder, the basic relationship is:

Force = Pressure × Effective Area

This relationship helps explain why both working pressure and cylinder area matter when sizing a hydraulic system.

Hydraulic Pressure vs. Flow: What Is the Difference?

One of the most important concepts in understanding how a hydraulic system works is the difference between pressure and flow.

A hydraulic pump primarily supplies fluid flow. Pressure develops when the moving fluid encounters resistance from a load, restriction, valve, actuator, or another part of the circuit.

• Flow rate affects actuator speed. More fluid supplied to a cylinder per unit of time generally allows the cylinder to move faster.

• Pressure affects available force or torque. Higher working pressure can allow an actuator to generate greater force, provided the component is rated for that pressure.

• Restrictions can change operating conditions. A restriction can reduce flow through part of the circuit while causing pressure to rise upstream.

• Component ratings matter. Pumps, valves, hoses, cylinders, motors, and fittings must be selected for the required pressure and flow conditions.

For example, a hydraulic cylinder may need sufficient flow to reach the required operating speed and sufficient pressure to overcome the load. Increasing pressure alone does not automatically make the cylinder move faster.

This is why hydraulic system design normally considers pressure and flow together rather than treating them as interchangeable specifications.

For additional technical background, Bosch Rexroth's Basic Knowledge of Hydraulics training material covers hydraulic system design, pumps, motors, cylinders, and pressure and flow control valves.

The Main Components of a Hydraulic System

A functional industrial hydraulic circuit consists of several interconnected components. Each part has a different role in generating, controlling, transmitting, and returning hydraulic energy.

1. Hydraulic Power Unit and Pump

The hydraulic pump converts mechanical energy from an electric motor, engine, or other prime mover into hydraulic energy by moving fluid through the circuit.

Common pump types include gear pumps, vane pumps, and piston pumps. The appropriate pump depends on factors such as required flow, operating pressure, efficiency, displacement, duty cycle, and application.

It is useful to remember that the pump does not simply create a fixed pressure. It supplies flow, while resistance within the hydraulic circuit determines the pressure required to move that flow through the system.

Naboer's Products section includes Parker hydraulic pumps and valves for industrial applications.

2. Hydraulic Reservoir

The reservoir stores the hydraulic fluid and provides a controlled location for fluid to return before it is pumped through the circuit again.

A properly designed reservoir can also help:

• Allow entrained air to separate from the fluid.

• Help dissipate heat generated during operation.

• Provide a reserve of fluid for changes in actuator volume.

• Allow some contaminants and particles to settle before the fluid re-enters the circuit.

3. Hydraulic Control Valves

Hydraulic valves control the direction, pressure, and flow of fluid. Their function depends on the type of valve and its position within the circuit.

• Directional control valves: Direct hydraulic fluid to different ports or actuator chambers.

• Pressure control valves: Limit, reduce, or regulate pressure in selected parts of the circuit.

• Flow control valves: Regulate the flow rate supplied to an actuator and therefore influence its operating speed.

For example, the Parker D1FB series is a proportional directional control valve family designed for hydraulic flow and directional control. Naboer lists Parker D1FB models such as the D1FBE01FC0NF00 for industrial applications.

Parker's own hydraulic valve resources also provide catalogs and technical information covering directional controls, pressure controls, manifolds, and related hydraulic valve components.

4. Hydraulic Actuators

Actuators convert hydraulic energy back into mechanical movement.

• Hydraulic cylinders produce linear motion for pushing, pulling, lifting, clamping, pressing, or positioning.

• Hydraulic motors produce rotary motion for driving wheels, shafts, conveyors, winches, and other mechanical loads.

The actuator must be matched to the required force or torque, operating speed, stroke or displacement, pressure, and installation conditions.

5. Filtration and Cooling

Hydraulic fluid cleanliness has a direct effect on component reliability. Contamination can damage valves, pumps, seals, and other precision components.

Depending on the circuit, filtration may be provided on the suction, pressure, or return side. Cooling equipment or heat exchangers may also be required when the system generates more heat than the reservoir and surrounding environment can dissipate.

6. Hydraulic Fluid, Hoses, Pipes, and Fittings

The fluid and hydraulic connections form the transmission path between the major components.

Hydraulic oil provides several functions beyond power transmission. It also lubricates moving surfaces, helps carry heat away from components, and protects internal parts against corrosion and wear.

Hoses, pipes, fittings, seals, and connection interfaces must also be selected according to the required pressure, flow, temperature, fluid compatibility, and installation conditions.

How Does a Hydraulic System Work Step by Step?

A typical hydraulic circuit can be understood by following the fluid through one complete operating cycle.

1. Fluid Is Drawn From the Reservoir

The prime mover turns the hydraulic pump. The pump draws hydraulic fluid from the reservoir through the inlet side of the circuit.

2. The Pump Supplies Flow

The pump moves the fluid into the pressure side of the hydraulic circuit. At this point, the pump is supplying flow. The pressure that develops depends on the resistance presented by the downstream components and load.

3. Valves Control the Flow

A directional control valve determines where the fluid is sent. Depending on the circuit, a solenoid, mechanical control, hydraulic pilot signal, or electronic controller may shift the valve.

Pressure-control and flow-control valves can also regulate the operating conditions of the circuit.

4. The Actuator Performs the Work

When pressurized fluid enters one side of a hydraulic cylinder, it acts on the piston area and generates linear force.

Hydraulic cylinder force = System pressure × Effective piston area

In a hydraulic motor, the fluid instead produces rotary motion and torque.

5. Fluid Returns Through the Circuit

As the actuator moves, fluid from the opposite side of the cylinder or motor outlet returns through the circuit. Depending on the system design, the returning fluid may pass through a control valve, return filter, cooler, or other conditioning components before reaching the reservoir.

The cycle then repeats as the control valve changes the direction or operating condition of the actuator.

Hydraulic System Flow Path

A simplified open-circuit hydraulic system can be represented as:

Reservoir → Pump → Control Valves → Actuator → Return Filter/Cooler → Reservoir

Each section performs a different function:

• Reservoir: Stores and conditions the hydraulic fluid.

• Pump: Supplies fluid flow.

• Valves: Control direction, pressure, and flow.

• Actuator: Converts hydraulic energy into mechanical movement.

• Filter/Cooler: Helps control contamination and temperature.

This basic flow path is common in many industrial hydraulic systems, although actual circuits can include accumulators, multiple pumps, proportional valves, counterbalance valves, pressure-compensated controls, and other components.

Open-Circuit vs. Closed-Circuit Hydraulic Systems

Hydraulic systems can use different circuit architectures depending on the application. Two important categories are open-circuit and closed-circuit systems.

Note: Open-circuit and closed-circuit hydraulic systems should not be confused with open-center and closed-center valve circuits. These terms describe different aspects of hydraulic system design.

Why Are Hydraulic Systems Used in Industrial Applications?

Hydraulic systems remain common in industrial equipment because they can provide high force and controlled movement without requiring extremely large mechanical transmission components.

High Force in a Compact System

Hydraulic cylinders can generate substantial linear force from relatively compact components. This makes hydraulic power useful for pressing, lifting, clamping, forming, and other high-load operations.

Controlled Movement

Flow-control valves, proportional valves, variable-displacement pumps, and electronic controls can be combined to regulate actuator speed and movement.

Overload and Pressure Protection

Pressure relief and other pressure-control components can limit excessive system pressure when the load or operating conditions change.

Flexible Power Distribution

A hydraulic power unit can supply multiple actuators through hoses, pipes, valves, and manifolds. This allows power to be distributed across different sections of a machine without placing the main motor next to every actuator.

Common Industrial Applications of Hydraulic Systems

Hydraulic systems are used across many types of industrial equipment, particularly where high force, controlled motion, or compact power transmission is required.

• Metal forming presses: Hydraulic cylinders provide the force required for pressing and forming operations.

• Injection molding machines: Hydraulic circuits can control clamping, injection, and other machine movements.

• Construction equipment: Excavators, loaders, and other machines use hydraulic cylinders and motors for lifting, digging, and movement.

• Mining equipment: Hydraulic systems are used for heavy-duty actuation and equipment control.

• Material handling equipment: Hydraulic power is used for lifting, positioning, and clamping loads.

• Industrial machinery: Hydraulic circuits can operate presses, machine tools, production equipment, and automated handling systems.

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What Can Cause a Hydraulic System to Lose Performance?

When a hydraulic machine becomes slow, weak, noisy, or inconsistent, the problem is not necessarily the pump alone.

Common causes can include:

• Internal leakage through worn seals or valve components.

• Contaminated hydraulic fluid.

• Incorrect or insufficient fluid level.

• Air entering the hydraulic circuit.

• Restricted filters or suction lines.

• Incorrect pressure or flow settings.

• Excessive fluid temperature.

• Incorrectly sized or malfunctioning control valves.

For this reason, troubleshooting should consider the complete hydraulic circuit rather than replacing the pump or valve without checking the operating conditions.

Frequently Asked Questions

Does a hydraulic pump create pressure or flow?

A hydraulic pump primarily supplies fluid flow. Pressure develops when the flow encounters resistance within the hydraulic circuit, such as a load, restriction, actuator, or control valve.

How does hydraulic pressure create force?

Pressure acting on an actuator area creates force. For a hydraulic cylinder, the basic relationship is Force = Pressure × Effective Area. A larger piston area can therefore produce greater force at the same pressure.

How do you control the speed of a hydraulic cylinder?

Cylinder speed is mainly related to the flow rate entering the cylinder. Flow-control valves or variable-displacement pumps can be used to regulate the amount of fluid supplied to the actuator.

Why is hydraulic oil used instead of water?

Modern hydraulic fluids provide lubrication, corrosion protection, viscosity control, and thermal characteristics that are suitable for hydraulic pumps, valves, cylinders, and other components. Water-based hydraulic fluids are also available for certain specialized applications.

What causes a hydraulic system to lose power?

Possible causes include internal leakage, insufficient flow, pressure loss, contaminated fluid, air in the circuit, restricted filters, incorrect valve settings, or problems with the pump or actuator. Troubleshooting should consider the entire circuit.

What is the difference between an open-circuit and closed-circuit hydraulic system?

In a typical open-circuit system, return fluid flows back to the reservoir before being pumped again. In a closed-circuit arrangement, fluid can circulate more directly between the pump and actuator, depending on the circuit design. Closed-circuit systems are common in applications such as hydrostatic drives.

Choosing Hydraulic Components for a System

Correct component selection requires more than matching a product category. Engineers and maintenance teams normally need to consider the required pressure, flow, actuator force or torque, operating temperature, fluid type, connection size, mounting arrangement, control method, and duty cycle.

When replacing an existing component, the original part number and configuration should also be checked carefully. A valve or pump from the same product family may have different ports, mounting arrangements, pressure ranges, displacement, or control options.

For example, Naboer's hydraulic system guide can serve as a starting point for understanding the complete circuit, while product-level evaluation should be based on the exact model and technical requirements.

For pressure-reducing valve applications, see Naboer's Parker PRDM Valve Replacement Guide, which explains why the complete model designation, mounting interface, pressure range, and adjustment configuration should be checked before selecting a replacement.

References and Technical Resources

The following manufacturer and industry resources provide additional technical information on hydraulic systems, components, pressure, flow, and hydraulic valve operation.

• Bosch Rexroth – Basic Knowledge of Hydraulics – Covers hydraulic system design, fluids, pumps, motors, cylinders, and pressure and flow control valves.

• Bosch Rexroth – Hydraulics for Beginners – Introduces hydraulic system structure, component functions, pump operation, hydraulic motors, cylinders, and the relationship between velocity, flow, and piston force.

• Parker Hannifin – Hydraulic Valve Systems Central – Provides access to Parker hydraulic valve catalogs, technical resources, service guides, and application information.

• Parker Hannifin – Hydraulic Valve Technical Information – Includes technical information related to hydraulic fluids, valves, system startup, pressure settings, and hydraulic circuit components.

Conclusion

A hydraulic system works by using fluid flow to transmit power and hydraulic pressure to generate force or torque. The pump supplies flow, valves control the fluid, actuators convert hydraulic energy into mechanical movement, and the reservoir, filtration, cooling, and connection components keep the circuit operating within its required conditions.

The most important distinction is that flow is closely related to actuator speed, while pressure is closely related to available force or torque. Understanding this relationship makes it easier to evaluate hydraulic circuits, select components, and troubleshoot performance problems.

For industrial hydraulic applications, component selection should always be based on the complete operating requirements rather than a product family name alone. Pressure, flow, mounting, connections, control configuration, fluid compatibility, and application conditions should all be checked before a component is specified or replaced.