Hydraulic machines are machines which use fluid power to do work. Hydraulic lifts and hydraulic platforms are common examples.

In these type of machines, high pressure hydraulic fluid is transmitted throughout the machine to various hydraulic motors and hydraulic cylinders. The fluid is controlled directly or automatically by control valves and distributed through hoses and tubes.

The popularity of hydraulic machinery is due to the very large amount of power that can be transferred through small tubes and flexible hoses, and the high power density and wide array of actuators that can make use of this power.

 Hydraulic power

The science of Fluid pressure provides some of the theory of hydraulics.

A force acting on a small area can create a much larger force by acting on a larger area by virtue of hydrostatic pressure.
A large amount of energy can be carried by a small flow of highly pressurized fluid.
Hydraulic machinery offers a very large amount of power and force with relatively small components. A typical hydraulic cylinder with a 75 mm (3 inch) bore, for example, can supply 89 000 N (20,000 lbf). The power transmission in a hydraulic system is easily controlled with valves.

Some parts of a hydraulic system will operate at about 2000 kPa (300 psi) (pilot controls, vehicle brakes). The main hydraulic actuators (for example, cylinders or fluid motors) will typically operate in the range of 7000 - 42000 kPa (1000 - 6000 psi). With advances in materials and design, there is a trend toward higher pressure, with some systems operating to 100 000 kPa (15,000 psi) and some exotic systems with titanium hardware operating at over 350 000 kPa (50,000 psi).