Fluid power is the technology that deals with the generation, control, and transmission of pressurized fluids.

As the term fluid refers either to gases or to liquids, fluid power is also subdivided into the categories of hydraulics and pneumatics. The differences being that with hydraulics the medium used is a liquid (ie mineral oil or water) and for pneumatics it is a gas (ie air or another inert gas).

Practical use

In practice fluid power is used to achieve one of two generic tasks:

Transport fluid
Delivering the fluid from one location to another to achieve a useful purpose.
An example of this in the case of hydraulics is a lube oil system to provide oil to lubricate bearings on heavy equipment.

Transport energy
In this case the fluid flow is used to actuate a device specifically designed to operate from the flow provided. In general these devices can be described as either a cylinder (providing a linear output force/motion) or motor (providing a rotary output).


Application

Hydraulics and pneumatics are similar in many ways, but there are clear reasons for using one over the other.

* Cost: Pneumatics costs considerably less to build and operate. This is partly because usually, air is used as the fluid, so no reservoir is needed to store fluid, nor is there a need to provide means to recover fluid. When higher force or torque is needed, pneumatics needs much larger motors or cylinders than hydraulics. So, when higher forces are needed, usually, hydraulics is used.
* Precision: Fluids vary in controllablity. Unlike liquids, gases change volume significantly when pressurized, making it hard to achieve precise position if needed. So, when precision is needed, usually, hydraulics is used.
* Safety: Gases under high pressure are explosive. So for lower forces, usually, pneumatics is used only into the range of 100 psi (7 bar), due to the hazard of a line breach. Hydraulics is used for higher forces.