Physical properties of Hydraulic Oils

1) Mass Density

• It is define as the mass of the liquid per unit volume. It is denoted by ‘ρ‘. 
• Its unit in SI system is kg/m3 . Mass density of water is 1000 kg/m3.

2) Weight Density

• Weight density (specific weight) is define as the weight of the liquid per unit volume. It is denoted by  ‘w’. Its unit in SI system is N/m3 . Weight density of water is 9810 kg/m3.

3) Specific Gravity

• It is define as the ratio of the density of that liquid to the density of water. It has no unit. Specific gravity of water is 1 and mercury is 13.6 .

4) Compressibility

• It is the ability to get compressed.  Gases are more compressible and liquids are less or incompressible.  It is reciprocal of the bulk modulus. Bulk modulus(K) is defined as the ratio of the change in pressure to volumetric strain.

5) Viscosity

• It is defined as the resistance offered by liquid to flow.  Viscosity is a property of the fluid which opposes the relative motion between the two surfaces of the fluid in a fluid that is moving at different velocities.

5.1 Dynamic viscosity (absolute viscosity)

•      It is defined as the shear stress per unit rate of shear strain.  It is denoted by ‘m’. Its unit in SI system is N×s/m² .

5.2 Kinematic Viscosity

•      It defines the ratio of the dynamic viscosity to mass density. It is denoted by ‘g’.  Its unit in SI system is m²/s.

6) Viscosity Index

• It is number given to hydraulic oil corresponding to change in viscosity with respect to change in temperature. Greater the viscosity index, the lesser change with respect to change in temperature.  It should be as high as possible.

7) Flashpoint

•    It is defined as the temperature at which oil gives off the momentary flash. The flashpoint should be as high as possible to prevent fire hazards.

8) Fire point

•    It is defined as the temperature at which oil catches continuous flame. The fire point should be as high as possible to prevent fire hazards.

9) Oxidation stability

•    Due to oxidation, the desirable properties of hydraulic oil will be severally affected. Hence oil should have a high degree of oxidation stability.

10) Pour point

•    It is defined as the temperature at which oil begins to solidify. Pour point should be low as possible for good hydraulic system.

11) Emulsibility

•    It is defined as the ability of the oil to mix with water to form an emulsion. Oil should not have emusibility.

12) Demulsibility

   It is defined as the property of the oil due to which oil and water get separated out when the mixture kept stable for some time.