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Fluid is that deforms continuously on the application of shear stress, no matter how much small is it. Fluid comprises both gases and liquid. The technique of using liquid for power transmission is called as hydraulics while which uses gases for power transmission is called Pneumatics.In most hydraulics system mineral oils will be used while in most pneumatic system atmospheric air will be used.
Principle behind fluid power
The basic principle which governs any fluid system is the pascal law. The law applies to fluid power transmission. The other laws which will be used in the analysis, design and manufacturing of any fluid system are Boyle's law, Charles's law and Bernoulli theorem.
Property of fluid
For reliable and proper working of hydraulic system the fluid used should have the following certain qualities. Some of them are
- Viscosity The fluid used should have its viscosity in the specified range. Higher viscosity will lead to higher friction and wear while lower viscosity will lead to leakage problems
- Viscosity Index This determines the change of viscosity with respect to temperature. Oil with High VI is preferable
- Oxidation resistance The continuous use of oil will promote oxidation. So the oil should be able to resist oxidation
- Pour and Cloud point These determines the lowest range of temperature below which the fluid will not transmit power
- Flash and fire point These determines the highest range of temperature above which fluid will catch fire and will cease to transmit power.
- Lubrication The fluid should be able to lubricate the moving parts in the system.
- Heat generation and transmission The amount of heat generated should be less. The fluid should have capacity to transmit the heat generated
- Corrosion resistance The fluid should not corrode the parts of the system.
Some of the most commonly used fluid for power transmission are water,emulsions,Water glycol, Synthetics, air and compressed gases 
Parts of fluid system
The most of fluid system will employ some or all of the following parts
- Pump or Compressor: This gives the motion to the fluid which is used to perform useful work
- Control Valves: These are used to regulate and control the flow of the fluid
- Tubes, pipes and Hoses: These carry the fluid from the pump/compressor to the actuator
- Actuator: The actual work is performed by the actuators.
- Discharge Tank: The liquid after performing work will be discharged into a tank in case of liquids /gases or will let into the atmosphere in case of air
- Seals: These prevent the leakage of fluid into the atmosphere or surroundings
- Filter, Lubricator and Regulator Circuit (FRL UNIT):These circuit are used to remove dirt present in the fluid. They will also lubricate and regulate the flow of fluid
- Accumulator: These are used to produce head which can be converted into useful work
Cylinders are primary type of actuators used in fluid circuit. The cylinder used in fluid circuit will consist the following parts
- Cylinder barrel
- Piston rod
- Cylinder Head
Whenever fluid is supplied to the cylinder the piston will move. This movement of piston is called as stroke. The piston can move in forward or reverse direction. The cylinders can be classified as double acting or single acting. In single acting cylinder work will be performed only in the forward stroke. The fluid will be supplied to the cylinder only during the forward stroke. The return stroke is achieved by the spring mounted in the cylinder. In double acting cylinder there will be two ports for the supply of fluid. Work can be obtained both at forward and reverse strokes. Directional control valves required for the easy operation of double acting cylinder.
Advantages of using Fluid Power
The transmission of power through fluid has following advantages compared to the other modes
- Power transmission requires fewer mechanical parts. The problem of breakage of parts like gears, cams, belts and chains is not involved.
- Lesser noise and less vibration can be ensured from the system by proper installation of fluid system
- Forces can be conveyed up and down with less loss in efficiency
- System overloading can be prevented by use of automatic relief valves
- Economical compared to other modes
Use of fluid power
The hydraulic press, invented by Englishman John Brahmah, was one of the first workable pieces of machinery developed that used hydraulics in its operation. Today, hydraulic power is used to operate many different tools and mechanisms. some of the devices which uses hydraulic power are
- Hydraulic jack
- Hydraulic doorstops
- Hydraulic brakes
- Power steering
- Various construction vehicles and equipment
Some devices which uses Pneumatics are
Difference between Hydraulics and Pneumatics
 Precise control of velocity is achievable in hydraulics while due to compressible nature of gas, Pneumatics cannot have precise control over velocity. In Hydraulics, self-regulation action of oil can be used while in pneumatics oil mist has to be used for lubrication. The pressure range for hydraulics is 500-5000 psi while the pressure range for pneumatics is below 100 psi The air after performing work can be sent into atmosphere, whereas oil has to be discharged into a tank.