The term „Hydraulics“ derives from the ancient greek words „hydro“ (water) and „aulos“ (pipe). In general, hydraulics deal with the flow of liquids and particularly, concerning the area of construction engines, with the transmission of power and energy by the usage of liquids.

History
Historically the Englishman Joseph Bramah (picture) is counted as the founder of hydraulic technology. In 1795, he developed an engine driven by water pressure, which increased the power input by more than 2000 times.

„The year 1905 is marked as the beginning of oil-driven hydraulics, when Williams and Janney used mineral oil as a transmitting medium for a hydrostatic gear-drive, driven by axial pistons, for the first time. The first servo-steering was developed by Harry Vickers (around 1025), the first balanced pressure-valve was developed by him in1936“ (taken and translated from Wikipedia.de)
Therewith the foundations for the hydraulic technology of today were created.

Variants:
We differentiate between three variants of hydraulic power-transmission:
1. the Hydrodynamic Drive
2. the Viscous Clutch
3. the Hydrostatic Drive

Hydraulic systems in construction engines
In construction engines, primarily the hydrostatic drive is used for hydraulic transmission of power. Hydraulic Drives transform the mechanical output of an operating engine into hydraulic power by the utilization of a pump. The are the optimal solution for construction engines in so far, that they allow for an infinite alteration of drive-given speed.

In hydrostatic drives, the pump- and motor-parts are quite similar. The basis is in most cases an axial piston-pump. Axial to the pump‘s drive shaft the pump‘s casing is arranged. On the inside there is a ring of circular piston holes, in which pistons are inserted following the same arrangement. Axial Piston-Motors are utilized in industry (f.e. heavy engineering) as well as in mobile construction machinery. Most of the time, different types of hydraulic motors can be observed. They may have a constant or variable swallowing capacity.

To create a linear motion, hydraulic cylinders are utilized.
In contrast, circular motion is realized by means of Hydraulic Motors and Hydromotors respectively.

These hydraulic systems always rely on a circulation of liquid (Mineral oil, hydraulic oil, ester, glycoles or special mixtures of water). As the density of the hydraulic liquids is very low, high pressures can be transmitted in a very even and exact way.

See the basic principle of a simple hydraulic system in a short animation:

The primar advantage of these hydraulic systems lies in their seperate casting method. Thus the hydraulics can be optimally adjusted to the respective engine by freely repositionable lines of pipes and hoses. This is also achieved by the huge capacity of the comparatively small components.

Furthermore these systems offer more advantages, like (taken from wikipedia.de):

• infinite speed-regulation of the output in a wide range, easy redirection of motion.
• easy generation of very high power levels and torques
• quick and safe protection from overload by means of a pressure regulation valve.
• realization of parallel translatoric and rotatoric output-elements (Hydraulic Cylinders or Hydromotors) with a primary component (pump) in one combined system, thus the effect of a differential is achieved without further effort.
• high durability, as the fluid serves as cooling medium and is self-lubricating.
• easy regulation concepts for an optimum exploitation of the drive motor‘s capacities under highly variable demands for the power of the contruction engine.
• high setting reliablity
• easy control of engine strain by pressure measuring devices.
• start from standstill under full throttle
• hydraulic liquid protects from corrosion (except for water)
  

The disadvantages of hydraulic systems are, by experience:

• High demands on the filtering of the hydraulic liquid
• Losses by oil leakages
• High losses of flow within the hydraulic liquids, which are transformed into heat energy and thus heaten up the engine. (Loss of energy – this happens mainly with older pumps and motors)

Because of their specific advantages and disadvantages, hydraulic drives are often used in mobile working machinery, such as construction engines and agricultural engines. Here the lifting and lowering of loads is mainly achieved by linear movable hydraulic cylinders.

More, typical examples of usage are in:

• Bidirectional excavators
• Agricultural tractors, to lift, power or operate attachment devices
• Excavators: Hydraulic drive for all working devices, including carriage and turning device
• Mobile cranes: Hydraulic drives are used in the telescopic arms, lifting device, winch, turning device, support, steering gear and partially in the driving gear
• Forestry machinery: Hydrostatic driving and operating gear

Further information about hydraulics:

Swallowing Capacity
The term „Swallowing Capacity“ is used in fluid-technology for hydraulic motors for the amount of hydraulic liquid that is consumed by the hydraulic motor per turn.
With controllable hydraulic motors, the swallowing capacity is variable.
The power output (P) of a hydraulic motor ist directly proportional towards its swallowing capacity (V), the rotational speed (n) and the pressure gradient Δp. The product of the applied volumetric current (Q) and the swallowing capacity results in the rotational speed. The volumetric current is the difference between the pressure of the incoming hydraulic liquid (mostly pump pressure) and the pressure of the outgoing hydraulic liquid (mostly tank pressure).

Gear Pump
The Gear Pump is a device for the ranging of liquids and for the power-transmitting operating of hydraulic motors. It is a sub-type of the positive displacement pump.
A Gear Pump smootly ranges the liquid medium (apart from hydrostatically conditional pulsation) and can withstand pressure amounts of approximately 300 bar. The pressure mounts, as in every hydraulic system, by the ranging of a medium against a load. If the load increases, the pressure rises as well.
It is an operating device for hydraulic power-transforming equipment in construction machinery, agricultural tractors and in handicraft and industry, especially in vehicle-construction.