review and summary

REVIEW AND SUMMARY

• In this chapter, the different types and the various properties of fluids are examined, which is very important in the selection and use of fluids.

• A fluid is a substance which is capable of flowing.

• Fluids can be classified into : 1. Liquids, 2 Gases, and 3. Vapour.

• The various functions of a hydraulic fluid in a hydraulic system are:

1. Primary transmission

2. Lubrication

3. Sealing

4. Cooling (Heat transfer)

5. Prevention of rust and corrosion

6. Contaminants removal

• The definition, formula, units, and the significance of the following physical properties of fluid are explained in this chapter :

1. Mass density,

2. Weight density,

3. Specific gravity,

4. Specific volume,

5. Absolute viscosity, and

6. Kinematic viscosity.

• Viscosity is the measure of the fluid's internal resistance to flow.

• The viscosity of liquid decreases but that of gases increases with increase in temperature.

• Viscosity index (V.I) of a liquid is a number indicating the effect of a change in temperature on viscosity.

• Bulk modulus is a measure of the incompressibility of the fluid. The higher the bulk modulus, the less compressible or stiffer the fluid. Generally it is desirable to have the hydraulic fluid which has the minimum compressibility.

• The pour point of an oil indicates the temperature below which the oil will not flow freely. Any fluid used for hydraulic purposes should have a pour point well below its minimum operating temperature.

• The flash point is the minimum temperature at which sufficient liquid is vapourised under specified conditions to create a mixture that will burn if ignited.

• The fire point is the minimum temperature at which sufficient vapour is continuously generated to sustain combustion. Flash and fire points indicate the temperature at which the liquid begins to volatilize and hence high flash and fire points are desirable.

• The property of a hydraulic fluid to separate rapidly and completely from moisture and resist emulsification is known as demulsibility.

• Oxidation stability is defined as the ability of a liquid to resist reaction with oxygen or oxygen-containing compounds.

• Lubricity is the ability of a liquid to impart low friction under boundary conditions. 

• Compatibility is the ability of the hydraulic fluid to be compatible with the system.

• Required properties of a good hydraulic fluid are :

1. Stable viscosity characteristics,

2. Good lubricity,

3. Compatibility with system materials,

4. Stable physical and chemical properties,

5. Good heat dissipation capability,

6. High bulk modulus and degree of incompressibility,

7. Adequate low temperature properties,

8. Good flammability characteristics,

9. Low volatility,

10. Good demulsibility,

11. Better fire resistance,

12. Lower foaming tendency,

13. Non-toxicity,

14. Good oxidation stability,

15. Better rust-and corrosion-preventive qualities, 

16. Low density and specific gravity,

17. Low coefficient of expansion,

18. Simple and easy handling,

19. Ready availability, and 

20. Inexpensive.

• Types of hydraulic fluids:

1. Petroleum-base (or mineral-base) fluids,

2. Water-base (or fire resistant) fluids, and

(i) Water emulsions, and

(ii) Water glycols.

3. Synthetic fluids.

KEY TERMS ONE SHOULD REMEMBER

Fluids

Liquid 

Gas

Vapour

Density

Mass density

Specific mass

Weight density

Specific weight

Specific gravity

Relative density

Specific volume

Viscosity

Absolute viscosity

Kinematic viscosity

Newton's law of viscosity

Coefficient of dynamic viscosity

Saybolt viscometer

Viscosity index

Cohesion

Adhesion

Surface tension

Capillarity

Vapour pressure 

Cavitation 

Compressibility

Bulk modulus

Cloud point

Pour point

Flash point

Fire point Demulsibility

Oxidation stability

Chemical stability

Thermal stability 

Hydrolytic stability 

Film strength

Oiliness

Lubricity

Compatibility

Volatility

Corrosiveness

Neutralization number

Petroleum oils

Emulsions

Water-in-oil emulsion

Oil-in-water emulsion

Water glycols

Phosphate esters

Fire-resistant fluids