Hydraulic (Greek adjective substantivized υδραυλική [ τέχνη ] hydrauliké [ téchne ] " hydraulic [ technology ] " from Ancient Greek ὕδωρ Hydor "the water " and αὐλός Aulós " the tube " ) is the study of the flow behavior of fluids. Is understood in the art including the use of liquid for signal, power and energy transmission.

Historical Development

As the founder of technical hydraulics applies the Englishman Joseph Bramah. In 1795 he developed a water -powered pressure hydro -mechanical machine that worked according to the hydrostatic law of Blaise Pascal and the force introduced 2034fach enlarged. In 1851 William G. Armstrong the Gewichtsakkumulator, a memory, which allows high flow rates could be produced. The London Hydraulic Power Company in 1882, a central water supply for several hydraulic plants in operation.

1905 is considered the beginning of the hydraulic oil, as Harvey D. Williams and Reynold Janney first mineral oil. Transmission medium for a hydrostatic transmission in axial piston swash plate, which offers up to 40 bar plead with a pressure Hele - Shaw developed in 1910 the first practical radial piston engine. 1929 received the engineers Hans Thoma and Henry Kosel a patent on an axial piston bent axis design. The first power steering developed Harry Vickers ( 1925 ), the first pilot operated pressure relief valve he developed in 1936. Jean Mercier built in 1950 on a larger scale, the first hydropneumatic accumulator. For the development of servo - hydraulic works were by John F. Blackburn, Shih- Ying Lee and Jesse Lowen Shearer of importance that have appeared in the U.S. in 1958.


Hydraulics is a technique Gearbox - alternative to mechanical, electrical and pneumatic transmissions, ie it is used to power, energy or Kraft-/Momentenübertragung of the machine ( pump) for the combustion engine ( piston or hydraulic motor ), where the performance parameters are adapted to the demands of the engine. In hydraulics, the power is transmitted through the hydraulic fluid, usually a special mineral oil, but increasingly also with environmentally friendly fluids, such as water or specific esters or glycols. The transmitted power results from the factors pressure and fluid flow. A distinction must be:

  • Hydrodynamic drives work with a pump and a driving turbine. The speed and torque conversion is done by the kinetic energy of the liquid.
  • Viscous couplings transmit power by viscous friction between rotating disks.
  • Hydrostatic drives convert the primary side, the mechanical power of the engine (electric motor, diesel ) by a pump ( driven machine) into hydraulic power to. This power is again converted into loads ( the engine ) to mechanical power and that in the hydraulic cylinders in a linear motion or hydraulic motors into a rotational movement. Hydrostatic drives are often the energetically optimal Gearbox when a continuous adjustment of the output-side speed is required.

By the introduction of fluid under pressure in the cylinder the piston therein and the piston rods are set in a linear motion that is used for operations, and for driving the machine. And rotating actuators may be implemented by fluid pressure, such as with the hydraulic motor.

Hydraulic systems are similar in principle to the pneumatic actuators, used in the compressed air for power and signal transmission, but have deviating properties. So in the oil hydraulic circuit of a fluid is always needed (round reverse), while in the pneumatic exhaust - is released into the environment - usually through a silencer. Only when the water hydraulics is sometimes omitted circuits. Compared to the pneumatics hydraulics has the advantage that much higher forces can be transmitted and very uniform and precise movements are possible because the compression of the hydraulic fluid is so low that it hardly acts prejudicial in technical applications.


The wide distribution in many industries owes the hydraulic following benefits:

  • The separate design concept, namely the flexible connection between the input and output and an optimal constructive adaptation to space requirements. The connection between the motor and pump used tube and hose lines, which are largely free of installation. For mechanical drives, however, it is necessary to establish a direct connection between the engine and transmission and further differential across eg a propeller shaft, flexible disk, flexible disk or a chain. Therefore, the position of the transmission by the position of the motor there is largely fixed.
  • Modern splash-free clean-break couplers allow safe and clean separation of consumer and pump. The dirt and air entry into the hydraulic system is reduced to an absolute minimum.
  • Stepless speed position of the power take-off within very wide limits, a simple reversal of the direction of movement
  • Produce linear output movements with simple technical devices at very high efficiencies
  • Easy generation of very large forces and torques
  • Safe and fast-acting overload protection through pressure relief valve
  • High power density, ie comparatively small components for great achievements especially in comparison to electric motors
  • Realization of parallel operating translational or rotational drive components ( hydraulic cylinders or hydraulic motors) with a primary ( pump) in a common system, while the effect of a differential results in no additional effort.
  • High durability, since the fluid is self-lubricating and can be used as a cooling medium
  • Simple control strategies for optimum utilization of the drive motor with widely varying performance requirements of the machine.
  • High positioning accuracy
  • Uniform motions due to the low compressibility of the hydraulic fluid
  • Standardization through the use of standardized components, connection dimensions, installation spaces, etc.
  • Simple display of the burden of pressure gauges
  • Low inductance of the hydraulic motors and cylinders
  • Starting from standstill at full load
  • Quick (but with the same working pressure slower than pneumatics), sensitive, uniform and infinitely adjustable cylinder and engine speeds.
  • Corrosion protection by hydraulic liquid ( other than water )
  • No electric stray field of the power take-offs


  • A disadvantage of hydraulic actuators is the elasticity of the fluid, which under pressure for compression. This gives rise may pressure or movement oscillations. This problem but affects only for drives with high demands on the uniformity of the velocity at highly variable loads, such as feed drives in machine tools. This has to be countered by means of flexible couplings, which increases costs.
  • High demands on the filtration of hydraulic fluid
  • Switching noise of the valves
  • Risk of leakage, oil leakage
  • ( Increase viscosity and effort at decreasing temperatures ) Temperature dependence of hydraulic oils,
  • Flow losses, which are converted into heat and heat the plant. (Energy loss )
  • Tendency to vibration caused by pressure surges and associated noise.


Because of their specific advantages of hydraulic drives are often used in mobile working machines such as construction and agricultural machinery. Here the lifting and lowering of loads (forklifts, excavators, lifts, mobile cranes, etc.) is done mainly by linearly movable hydraulic cylinder

Vehicles are often driven with rotating hydraulic transmissions or liquid transducers, for example, with so-called bent-axis and swash plate machines with which high performance can be transferred. The remarkable thing is that the hydraulic transmission can flexibly adjust the movement of a working inflexible or fixed- speed motor to the operating conditions, such as used in diesel locomotives.

Other typical applications are:

  • Hydraulic wrench and Bolt Tensioners for bolt preload
  • Lift with low lift height, but high payload
  • Motor vehicles and muscle -powered two and three wheelers of various type: brakes ( brake fluid, even with bicycle), automatic transmissions, power steering, chassis control, convertible tops
  • Combustion engine: camshaft timing, valve actuation, actuation of injection units
  • Aircraft: total control of the wing flaps and the landing gear extension and retraction
  • Track brakes in the marshalling yard
  • To drive or to control agricultural tractors to lift Attachments
  • Automotive: vehicles are lifted by means of a lifting platform
  • Excavator: hydraulic drive all attachments including rotary and chassis
  • Mobile Cranes: Hydraulic drive of the telescopic poles, lift and winch, Slewing gear, support, guidance, and some traction drive trucks, eg forklifts: all movements including steering and travel
  • Tractors: power lift package with position controller for the tools; steering hydraulics
  • Forestry equipment: Hydrostatic drive and work drives
  • Commercial Vehicles: tipping hydraulics; Tailgates; Steering aids ( power steering); Clutch and brake operation; hydrostatic drive on the front axle (MAN)
  • Tanks and frigates: Hydrostatic superposition steering, Power Hydraulic Straightening drives for the main gun turrets or for the
  • Machine Tools: main and auxiliary movements on presses, shears and bending; Feed motion of grinding machines, operating functions, such as tool changers, workpiece and tool clamping and locking of axes on all metal cutting machine tools. In this case, the force can be multiplied by the pressure transducer.
  • Metallurgy: drives on electric and walking beam furnaces; continuous casting; Charging and cooling beds
  • Rolling mills: nip regulations ( regulation of the thickness of the rolled material by means of so-called hydraulic adjusting cylinders ); all auxiliary movements for feeding the rolled stock; Straightening and shearing drives;
  • Actuator: electro hydraulic control fluids ( pressure, flow) in process engineering, power plants, pipelines
  • Mining in the cylinder -powered roof, hydrostatic drives in mining machines and boring machines
  • General: hydraulic ram for moving heavy loads, for example in the construction industry for advancing and installation of bridge beams and other heavy prefabricated parts for heavy-duty transport or as a rescue device in THW and fire.
  • Automotive: For deep drawing of hoods, fenders or other body parts.

Due to the hydraulic press can be achieved with low levels of physical force, a large force effect. By manually pump the pump piston ( 2) of a car jack can be lifted on the plunger (3) a ton load.

Function Description: If the pump piston (2 ) is pressed down, the valve (4) and the valve (5 ) opens closes, so hydraulic oil flows into the press cylinder. The plunger ( 3) stands out. Is the pump piston moves upwards, the valve (4) and the valve ( 5) opens close. As a result, from the reservoir (1) continue to flow the hydraulic oil. Acts on the pump piston with an area of ​​0.5 cm ², a force of 100 N (corresponding to an applied mass of about 10 kg), which gives a pressure of

This static pressure also acts in the press cylinder. The plunger has an area of ​​40 cm ² acting on it a force of

Which one can lift about 800 kg. To press the plunger against this burden by half an inch upwards, a volume of 40 cm ³ to be moved. To this end, several pump strokes of together 80 cm are required. Is the hydraulic or pressure energy

This energy is equal to the work that is spent at the pump piston and performed by the plunger:

Symbols and Circuits

  • Schematic diagram and circuit diagram of a simple hydraulic system with a cylinder

A circuit diagram is the diagram of a hydraulic system. The components are represented by standard symbols. These plans are part of the required documentation to each facility, especially important to create and maintain the system. The list of character ( fluid power ) contains a comprehensive list of symbols for hydraulics and pneumatics, as symbols for memory, filters, pumps and compressors, cylinders and valves.

Circuit diagrams can be created individually, company-specific or standards (DIN ISO 1219). You can represent parts such as labor and control circuits, the steps of the workflow, the components of the circuit with its labeling as well as the lines and connections. The spatial arrangement of the components is not included in the rule.

An icon indicates only the function of a component / device, it says nothing about the structural design and installation position of the hydraulic components. Symbols are depicted in one color, and normally they are unconfirmed, current condition or shown in the starting position. For modeling can be used relationships such as electro - hydraulic analogy.