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is the use of fluids under pressure to generate, control, and transmit power. Fluid power is subdivided into hydraulics
Hydraulics is a topic in applied science and engineering dealing with the mechanical properties of liquids. Fluid mechanics provides the theoretical foundation for hydraulics, which focuses on the engineering uses of fluid properties. In fluid power, hydraulics is used for the generation, control,...
using a liquid such as mineral oil
A mineral oil is any of various colorless, odorless, light mixtures of alkanes in the C15 to C40 range from a non-vegetable source, particularly a distillate of petroleum....
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
, and pneumatics
Pneumatics is a branch of technology, which deals with the study and application of use of pressurized gas to effect mechanical motion.Pneumatic systems are extensively used in industry, where factories are commonly plumbed with compressed air or compressed inert gases...
using a gas such as air or other gases. Compressed-air and water-pressure systems were once used to transmit power from a central source to industrial users over extended geographic areas; fluid power systems today are usually within a single building or mobile machine.
A fluid power system has a pump driven by a prime mover (such as an electric motor or IC engine) that converts mechanical energy into fluid energy. This fluid flow is used to actuate a devices such as:
- A Hydraulic cylinder
A Hydraulic cylinder is a mechanical actuator that is used to give a unidirectional force through a unidirectional stroke. It has many applications, notably in engineering vehicles.- Operation :...
or Pneumatic cylinder
Pneumatic cylinders are mechanical devices which utilize the power of compressed gas to produce a force in a reciprocating linear motion....
, provides force in a linear fashion
- A Hydraulic motor
A hydraulic motor is a mechanical actuator that converts hydraulic pressure and flow into torque and angular displacement . The hydraulic motor is the rotary counterpart of the hydraulic cylinder....
or Pneumatic motor
A pneumatic motor or compressed air engine is a type of motor which does mechanical work by expanding compressed air. Pneumatic motors generally convert the compressed air to mechanical work through either linear or rotary motion...
, provides continuous rotational motion or torque
Torque, moment or moment of force , is the tendency of a force to rotate an object about an axis, fulcrum, or pivot. Just as a force is a push or a pull, a torque can be thought of as a twist....
- A Rotary actuator provides rotational motion of less than 360 degrees.
The choice of liquid or gas as the fluid power medium is governed by the application requirements:
- Cost: Pneumatics are considerably more expensive to build and operate. For one, air is used as the compressed medium, so no reservoir is needed to store fluid, nor is there any need to provide means to drain or recover fluid. With increasing working pressures, pneumatics require larger parts than hydraulics.
- Precision: Unlike liquids, gases change volume significantly when pressurized making it difficult to achieve precision.
- Safety: Compressed gases tend to expand at high velocities when decompressed, thus pneumatics are typically limited in utilities with a working pressure up to around 100 psi (7 bar).