Two Marks questions and answers

TWO MARKS QUESTIONS AND ANSWERS

1. What are the primary functions of a fluid in any fluid power system?

1. To transmit power.

2. To lubricate moving parts.

3. To seal gaps and clearances between mating components.

4. To dissipate heat generated by internal friction.

5. To prevent rust and corrosion.

2. Differentiate between a liquid and a gas.

3. What is density of a fluid? What are the three different ways of expressing it?

• The density of a fluid is that quantity of matter contained in unit volume of the substance.

• The three ways of expressing the density are: 

1. Mass density,

2. Weight density, and 

3. Specific gravity (or relative density).

4. Define viscosity and bulk modulus of a hydraulic fluid. 

• Viscosity may be defined as the shearing force required to move two plane surfaces relative to one another with a film of fluid between them.

• Bulk modulus is the measure of compressibility of a fluid. It is the reciprocal of compressibility.

5. What is meant by ‘viscosity' of a fluid ? Also state Newton's law of viscosity.

• Viscosity is the measure of the ability of a fluid to flow. It is the measure of the fluid's internal resistance to shear or flow at a definite temperature and pressure.

• Newton's law of viscosity: It states that the shear stress on a fluid element layer is directly proportional to the rate of shear strain.

6. Define the terms absolute viscosity and kinematic viscosity.

• Absolute viscosity, also known as coefficient of dynamic viscosity, is defined as the shear stress required to produce unit rate of shear strain.

• Kinematic viscosity is defined as the ratio of dynamic viscosity to mass density.

7. What are the undesirable effects when an oil having very high viscosity is used?

When the oil is too thick (i.e., oil having very high viscosity), the following undesirable effects will result in the system :

1. Increase in load and hence increased power loss.

2. High operating temperatures because there will be internal friction.

3. Excessive wear of parts.

4. Reduced internal leakage.

8. What are the undesirable effects when an oil viscosity is too low?

When an oil viscosity is too low; then the following undesirable effects will result in the system :

1. Less internal friction.

2. Smaller pressure losses in pipes and valves.

3. Increase in control action and component response.

4. Increased internal and external leakage.

5. Cannot lubricate properly.

9. State the effect of temperature and pressure on viscosity of liquids.

The viscosity of the liquids decreases with increase in temperatures whereas the viscosity increases with increasing pressure.

10. State the effect of temperature and pressure on viscosity of gases.

The viscosity of gases increase with increase in temperature. Over the normal range of pressures, the viscosity of gases is found to be independent of pressure.

11. “The viscosity of liquid decreases but that of gases increases with increase in temperature”—Why?

• Since the shear stress in the liquids (due to the intermolecular cohesion) decreases with increase of temperature, therefore the viscosity of liquid decreases with increase in temperature.

• If the temperature of a gas increases, the molecular interchange will increase. Therefore the viscosity of a gas increases with increase in temperature.

12. What is viscosity index ? What do you infer from V.I. number of an oil ?

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

• The rate of change in viscosity with changes in temperature is relatively less with

high V.I. oils than the low V.I. oils.

13. Why should a hydraulic fluid have high bulk modulus ?

The higher the bulk modulus, the less elastic or more stiffer the fluid. High bulk modulus values are desirable since they result in more stable and less elastic systems.

14. Define cloud point of an oil. Also give its significance.

• Definition: Cloud point is defined as the temperature at which wax or other dissolved solids beings to crystallise and become noticeable when liquids are chilled under specified conditions.

• Significance: The cloud point is of interest for use at low temperatures.

15. Define pour point of an oil. Also mention its significance.

• Definition: The pour point of an oil is the temperature below which the oil will not flow freely.

• Significance: This property is very important for hydraulic systems required to operate in extremely cold weather.

16. Define the terms 'flash' and 'fire' points, relative to hydraulic fluids.

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

• The fire point is the minimum temperature at which sufficient vapour is continuously generated to 'sustain combustion.

17. Mention the significance of flash and fire points.

Flash and fire points indicate the temperature at which the liquid begins to volatilize. A lower flash or fire point is an indication that the liquid has become contaminated with a more volatile product (such as fuel).

18. What is demulsibility? Write its significance.

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

• Significance: This property is significant because the operation of many hydraulic systems are conducive to the forming of moisture or of stable water-in-oil emulsions.

19. What is oxidation stability?

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

20. What is neutralization number of hydraulic fluid ? Write its significance.

• The neutralization number is a measure of acidity or alkalinity of a hydraulic fluid. 

• High acidity causes the oxidation rate in an oil to increase rapidly.

21. List out any four desirable properties of a good hydraulic fluid.

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.

22. List the general types of hydraulics fluids.

1. Petroleum base hydraulic oils, and

2. Non-petroleum base hydraulic oils.

(a) Water,

(b) Emulsions,

(c) Glycols,

(d) Water glycols,

(e) Phosphate esters,

(f) Silicate esters,

23. Name any four hydraulic fluids that are commonly used.

1. Petroleum oils.

2. Water-in-oil and oil-in-water emulsions.

3. Glycols.

4. Phosphate esters.

24. Name any three fire resistant fluids.

1. Water-in-oil emulsions.

2. Water glycols.

3. Phosphate esters.

25. Why is water not commonly used as a hydraulic fluid?

Because the water exhibits the following disadvantages:

1. Water does not possess lubricating property.

2. Water evaporates soon and hence it can be used only over a limited range of temperature.

3. It is high corrosive in nature.

REVIEW QUESTIONS

1. What are the functions of a fluid in any fluid power systems?

 [Ans. Refer Section 2.1.1]

2. List and explain the various functions of a hydraulic fluid in a hydraulic system.

3. Explain any eight characteristics of fluid used in hydraulic system.

 [Ans.  Refer Section 2.4]

4. Write notes on neutralization number.

 [Ans. Refer Section 2.4.20] 

5. List all properties that a good hydraulic fluid should possess. (or) What are the desirable properties of hydraulic fluids?

 [Ans. Refer Section 2.5]

6. List and explain various hydraulic fluids.

 [Ans.  Refer Section 2.6]

7. Explain the advantages and limitations of using petroleum based hydraulic oils.

 [Ans.  Refer Sections 2.6.1.2 & 2.6.1.3]

8. Write briefly about the following hydraulic fluids.

(1) Petroleum-base oils

(2) Water emulsions

(3) Water glycols 

[Ans. Refer Sections 2.6.1, 2.6.2.2 & 2.6.2.3]

9. Discuss the factors to be considered in the selection of hydraulic fluids. (or) How to select oil for the industrial application?

 [Ans.  Refer Section 2.8]