Hydraulics

Hydraulics is the technology of force and energy transmission using incompressible fluids in closed systems. Based on Pascal’s law, it enables the amplification of forces and the precise control of both linear and rotary movements. In industry, mobile machinery, and plant engineering, hydraulics is one of the key drive technologies.

Fundamentals and operating principle of hydraulics

Hydraulics utilizes the physical property of fluids to distribute pressure evenly in all directions. A hydraulic system converts mechanical energy—such as that from an electric motor or internal combustion engine—into hydraulic energy, transmits it via a fluid, and converts it back into mechanical work at the load. The two key variables here are pressure, which determines the force transmission, and flow rate, which controls the speed of movement.

Pascal’s Law as the Physical Basis

Pascal’s law forms the foundation of all hydraulics. It states that a force exerted on a stationary, incompressible fluid generates a pressure that spreads uniformly in all directions and acts perpendicularly on all surrounding surfaces. This principle enables force multiplication: A small force acting on a small piston area generates the same pressure as a large force on a large area. Thus, a small input force can generate a multiple output force, which gives hydraulic presses and jacks their enormous power.

Force and Pressure Transmission in Practice

In industrial hydraulic systems, operating pressures typically range between 200 and 350 bar. High-pressure systems can reach pressures of up to 400 bar, and in special applications even up to 700 bar. The transmitted force depends directly on the pressure and the piston area. The larger the effective area of a hydraulic cylinder, the greater the force at the same system pressure. This relationship makes hydraulic systems particularly compact: Compared to mechanical or electric drives, they achieve a significantly higher power density for the same size.

Main components of a hydraulic system

A hydraulic system consists of several functional groups that together ensure the flow of energy from the drive to the consumer.

Hydraulic pumps as energy generators

The hydraulic pump converts mechanical drive energy into hydraulic energy by delivering hydraulic fluid and pressurizing it. The most common types are gear pumps, vane pumps, and piston pumps. Gear pumps are characterized by their robustness and simple design, but operate at a constant flow rate. Variable-displacement axial piston pumps adjust their displacement volume to meet demand and achieve volumetric efficiencies exceeding 95 percent. The choice of pump depends on the required pressure, flow rate, and controllability of the application.

Hydraulic cylinders and hydraulic motors as consumers

Hydraulic cylinders convert fluid pressure into linear motion. They are available in single- and double-acting versions, with or without end-of-stroke cushioning. Hydraulic motors, on the other hand, generate rotary motion from fluid pressure. Here, too, designers distinguish between fixed-displacement and variable-displacement motors, depending on whether the displacement can be adjusted during operation. Variable-displacement motors allow the speed and torque to be varied while maintaining a constant flow rate.

Hydraulic Valves for Control and Regulation

In hydraulics, valves control the direction, pressure, and flow rate of the fluid. Directional control valves determine the direction of movement of cylinders and motors. Pressure valves, such as pressure relief valves and pressure control valves, protect the system from overload and maintain the pressure at a setpoint. Flow control valves, including throttle valves and proportional valves, regulate the flow rate and thus the speed of the actuators. Modern electrohydraulic servo valves enable highly dynamic and precise control.

Auxiliary components

In addition to the main components, the system includes a hydraulic tank, filters, a cooler, a pressure accumulator, and measuring instruments. The tank serves as a fluid reservoir and heat exchanger. Filters keep out particles that could damage sensitive valves and pumps. Accumulators buffer pressure fluctuations and provide additional flow rates for short periods.

Open and closed hydraulic circuits

Hydraulic systems differ fundamentally in the way the fluid is circulated.

Open circuit

In an open circuit, the pump draws hydraulic fluid from a tank, delivers it to the consumer, and returns it to the tank after it has flowed through the consumer. The open circuit is simple in design, easy to maintain, and suitable for applications with moderate performance requirements. The disadvantage lies in the larger volume of the tank and the susceptibility to air ingress, especially under varying flow conditions.

Closed-loop system

In a closed circuit, the fluid flows directly from the consumer back to the pump without passing through the tank. A small make-up pump compensates for leaks. Closed circuits are more compact, respond faster, and allow for more precise control. They are used in applications that require high power density and dynamic load changes, such as in the travel drives of tracked vehicles or winches. However, the design complexity and maintenance costs are higher than with an open circuit.

Hydrostatics and Hydrodynamics

Hydraulics is divided into two physical operating principles: hydrostatics and hydrodynamics. Hydrostatics uses the pressure of a stationary fluid to transmit force. All conventional hydraulic systems with pumps, cylinders, and valves operate on this principle. Hydrodynamics, on the other hand, utilizes the kinetic energy of a flowing fluid, as found in flow couplings and torque converters. Hydrostatics dominates industrial hydraulics because it is characterized by high forces combined with compact dimensions and precise controllability.

Hydraulic fluids and their properties

The hydraulic fluid is the medium that transmits pressure and energy within the system. Mineral oil-based HLP hydraulic oils in accordance with DIN 51524 are the most widely used. They offer good lubrication, corrosion protection, and resistance to aging. In environmentally sensitive areas, rapidly biodegradable fluids such as HEES (synthetic esters) or HEPR (polyalphaolefins) are used. For applications with an increased fire hazard, such as in foundries or mining, operators use flame-retardant fluids from the HFC (water-glycol) or HFD (synthetic fluids without water content) groups. The viscosity of the fluid must be suitable for the system’s temperature and pressure range, as it affects friction losses, lubricity, and response characteristics.

Applications of Hydraulics

Hydraulic systems are used in nearly all areas of technology where high forces are required in a compact design.

Mobile hydraulics

Mobile hydraulics encompasses all applications on mobile machinery: excavators, wheel loaders, cranes, tractors, combine harvesters, and forklifts. Open circuits with gear or axial piston pumps dominate here. The requirements range from simple lifting functions to complex load-sensing and control systems.

Industrial and Stationary Hydraulics

In the stationary industry, hydraulic power units supply presses, injection molding machines, machine tools, and test benches. Closed circuits and variable-displacement pumps are more common here, as precision and energy efficiency are paramount. Operating pressures of 250 to 350 bar are standard in this sector.

Shipbuilding, Aviation, and Renewable Energy

In shipbuilding, hydraulics control rudders, winches, and flaps. In aviation, it operates landing gear, flaps, and brakes, where low weight and absolute reliability are essential. Wind turbines use hydraulic units for rotor blade adjustment and braking. In hydroelectric power plants, hydraulic cylinders also control turbine guide vanes and safety devices.

Standards and Regulations

The safety requirements for hydraulic systems are governed by DIN ISO 4413. This standard specifies how systems must be designed, constructed, and operated to protect people and machinery. DIN 51524 defines the requirements for hydraulic fluids, while ISO 11158 specifies the classification of industrial hydraulic oils. DIN EN 16840 applies to environmentally friendly fluids. Compliance with these standards is mandatory during the design and commissioning of hydraulic systems and is regularly audited.

Trends and Developments in Hydraulics

The hydraulics industry is constantly evolving, driven by efficiency requirements, environmental regulations, and digital technologies.

Electrohydraulics and decentralized drives

Electrohydraulics combines electric drives with hydraulic actuators. Instead of a central pump that supplies the entire flow rate, decentralized electrohydraulic drives use small pump units directly at the point of use. This reduces line losses, simplifies installation, and lowers energy consumption, as only the pressure and flow rate actually required are generated.

Energy Efficiency and Sustainability

Energy efficiency is increasingly becoming a focus. Variable-displacement pumps with zero-stroke function, accumulators for energy recovery, and demand-based control concepts significantly reduce the energy requirements of hydraulic systems. The use of environmentally friendly fluids reduces the risk of leaks and complies with stricter environmental regulations.

Digitalization and intelligent systems

Sensors measure pressure, temperature, flow rate, and wear status in real time. Digital controllers analyze this data and automatically adjust operating parameters. Condition monitoring and predictive maintenance detect potential failures before they occur and increase system availability. The integration of hydraulic components into Industry 4.0 architectures using standardized communication protocols such as OPC UA is already underway.

Maintenance and Servicing

The reliability of a hydraulic system depends largely on the cleanliness of the hydraulic fluid and the condition of the wear parts. Particle contamination is the most common cause of failures. Regular filter changes, oil analyses, and adherence to recommended oil change intervals are therefore essential. Seals, bearings, and sliding surfaces on pistons and control rings are subject to natural wear and should be inspected during scheduled maintenance. Systematic maintenance extends the service life of components and prevents unplanned downtime.