Properties of Fluid Used in Fluid Power System.

Classification of fluids 

1. Ideal fluid: A fluid which is incompressible and is having no viscosity. it is an imaginary fluid, it does not exist. 
2. Real fluid: A fluid which posses viscosity is known as real fluid 
3. Newtonian fluid: In a fluid, in which shear stress is directly proportional to the rate of shear strain
4. Non- Newtonian fluid: A fluid in which the shear stress is not proportional to the rate of shear strain. 
5. Ideal plastic fluid: A fluid in which shear stress is more than the yield value and shear stress is proportional to the rate of shear strain

Properties of fluid

1. Viscosity: 

The viscosity of the fluid is its resistance to flow. The fluid of hydraulic power system is the compromise. if the viscosity of the fluid is too low then the fluid will leak by the internal seal causing a volumetric loss of the efficiency. if the viscosity of the fluid is too high then the fluid is difficult to push through the hoses, fittings and valves causing a loss of mechanical efficiency.

The dynamic viscosity is the shearing resistance of the fluid and it is measured by placing the fluid between two plates and shearing one plates with respect to other.

2. Chemical stability:

It is an important property of the hydraulic fluid. it is defined as the ability of the fluid to resist oxidation and deterioration for long time periods. Under the severe operating condition, the fluid undergoes unfavourable changes. a chemical reaction takes place with a hydraulic component, these reactions result in the formation of sludge, gums, carbon, etc, which clog the openings and cause valve and piston to stick.

3. Cleanliness: 

It is a very sensitive factor which received very considerable attention, especially when the hydraulic system used in the aerospace industry because it is extremely sensitive to the contamination. fluid cleanliness is of primary importance because the contaminants can cause malfunction, prevent proper valve seating, cause wear in the component.

4. Anti-wear and lubrication characteristics: 

The fluid must be capable of covering the contact surfaces of all the moving parts with a thin continuous lubricating film. the lubricating film may be destroyed as a result of high loading forces, insufficient oil delivery and low viscosity, it leads to wear due to fretting.

5.Compatibility:

In the hydraulic system, different materials are used and all the material has their own properties, so the fluid should be fully compatible with the other material used in the hydraulic system. it should not react chemically with any of these materials nor changes their physical properties.

6. Foaming: 

All the fluid contain a certain amount of dissolved air in it.  the amount of the dissolved air depends on the temperature and pressure. material based oil can contain 10 to 12 % by volume of dissolved air. as the temperature of the fluid increases or the pressure decreases the dissolved air is liberated and it forms the bubble. when the fluid contains the entrained air return to the reservoir, the air rises to the surface and cause foam. this foam can cause a severe problem. a foam-depressant additive is used to break down of the foam.

Read moreHow Fluid Power System Works.