Two Marks Q&A

 two marks q & a : Fluid Power Priniciples and Hydraulic Pumps - Hydraulics and Pneumatics

TWO MARKS Q & A

1. What is the function of a hydraulic pump ?

A hydraulic pump is a device which converts mechanical energy into hydraulic energy.

2. How can you classify pumps ?

1. Non-positive displacement pumps, and

2. Positive displacement pumps.

 (a) Rotary pumps (such as gear pumps, vane pumps, and screw pumps)

(b) Reciprocating pumps (such as axial and radial piston pumps)

3. Where are non-positive displacement pumps employed? Why?

The non-positive displacement pumps are suitable for low-pressure, high-volume flow applications. Since these pumps cannot withstand high pressures, they are used only for transporting fluid from one place to another.

4. What do you mean by non-positive displacement pump? State its implications.

In non-positive displacement pumps, the fluid flow is achieved using the inertia of the fluid in motion. Since these pumps cannot withstand high pressures, they are used only for transporting from one place to another. For the same reason, they are not preferred in the fluid power industries.

5. Why are centrifugal (or non-positive displacement) pumps not preferred for fluid power applications?

Because the centrifugal pumps cannot withstand high pressures. Also they are suitable for low-pressure high-volume flow applications.

6. What are positive displacement pumps? Why are they called so?

• The positive displacement pumps discharge a fixed quantity of fluid into the hydraulic system per revolution of pump shaft rotation.

• Since the flow of fluid is guaranteed on every revolution of the shaft, this type of pump is named as 'positive' displacement pump.

7. Name any four positive displacement pumps.

1. External and internal gear pumps.

2. Lobe and screw pumps.

3. Vane pumps.

4. Piston pumps.

8. Why are positive displacement pumps universally used in fluid power industries ?

Positive displacement pumps are primarily used where pressure development is the prime requirement. This type of pumps is capable of delivering high pressure fluid, so it is universally used in fluid power systems.

9. Name any four types of gear pumps used in hydraulic systems.

1. External gear pumps,

2. Internal gear pumps,

3. Lobe pumps, and

4. Gerotor pumps.

10. Why all the gear pumps are fixed displacement pumps ? (or) Why gear pumps cannot be used as variable displacement pumps?

In gear pumps, the volumetric displacement depends on the design parameters (i.e., diameter and width of gear teeth) of gears and they cannot be varied. So the displacement cannot be varied in gear pumps.

11. What type of gears are generally used in gear pumps ?

The gears in the gear pumps can be of spur gear, helical gear or herringbone gear.

12. List the merits and demerits of gear pump.

Advantages

(i) These pumps are self-priming.

(ii) They give constant delivery for a set rotor speed, uniform discharge with negligible pulsations, and do not require check valves.

(iii) If necessary, these pumps can pump in either direction (by changing the direction of the gear rotation).

Disadvantages

(i) The liquid to be pumped must be comparatively clean.

|(ii) The pump cannot be operated against a closed discharge without damage. Hence relief valves are required.

(iii) Variable-speed drives are required to provide changes in pumping rate.

13. How can you specify a pump ?

A pump can be specified by its capacity, discharge pressure, and speed of rotation.

14. Which pump external gear, internal gear, screw, vane and piston-generates the least noise? Why?

Screw pump generates the least noise, because of rolling action of the screw spindles. Also in the screw pump, the fluid does not rotate but moves linearly.

15. Why do screw pumps generate less noise while running?

Screw pump generates the least noise, because of rolling action of the screw spindles. Also in the screw pump, the fluid does not rotate but moves linearly.

16. Name two designs of vane pumps.

1. Unbalanced vane pumps, and

2. Balanced vane pumps.

17. Differentiate between fixed and variable displacement pumps.

In fixed displacement pumps, the displacement of the pump cannot be varied by any means. But in variable displacement pumps, the displacement of the pump can be varied by some suitable arrangements.

18. When do you call a vane pump as a balanced or unbalance one?

A vane pump can be referred as unbalanced pump if all the pumping action takes place on one side of the rotor. But the balanced vane pump has two intake and two outlet ports which are diametrically opposite to each other.

19. What is a balanced vane pump?

The balanced vane pump has two intake and two outlet ports which are diametrically opposite to each other. The two high-pressure zones balance the forces on the rotor shaft and hence a complete hydraulic balance is achieved.

20. How can the unbalanced vane pump be used as a variable displacement pump?

In unbalance vane pump, the variable displacement can be provided by mechanically varying the eccentricity.

21. What are the advantages of balanced vane pumps over the unbalanced vane pumps? 

1. The balanced vane pumps eliminate the bearing side loads and thus permit higher operating pressures.

2. The balanced vane pumps have much improved service lives over simpler unbalanced vane pumps.

22. Give the symbol of variable vane pump.

23. Sketch the graphical symbol of variable displacement reversible pump.

24. What is a pressure compensated vane pump and how does it work?

Variable displacement can be provided if the housing can be moved with respect to the rotor. This movement changes the eccentricity and hence the displacement. Usually a hand wheel or pressure compensator can be used to move the cam ring to change the eccentricity. 

25. Differentiate pressure compensated and non-pressure compensated pumps.

In pressure compensated pumps, the pump flow can be made to zero. Such a pump has its own protection against excessive pressure build up. Also there is no power waste and reduced fluid heading. Whereas non-pressure compensated pumps are hydraulically unbalanced and cause undesirable side load on the bearings of the pump.

26. What are piston pumps ? Name the two basic types of piston pumps.

• In piston pumps, the pumping action is affected by a piston that moves in a reciprocating cycle through a cylinder.

• Types: 

1. Axial piston pumps, and

2. Radial piston pumps.

27. How can you vary the displacement in an axial piston pump?

The variable displacement in an axial piston pump can be achieved by altering the angle of the swash plate (or offset angle). Because in axial pumps, this swing angle determines the piston stroke and hence the pump displacement.

28. Differentiate between axial and radial piston pumps.

In axial piston pumps, the pistons move in-line or parallel to the shaft centre-line. But in radial piston pumps, the pistons move perpendicularly to the shaft centre-line.

29. Define volumetric efficiency of a pump. Also write its significance.

• Volumetric efficiency is the ratio between the actual flow rate produced by the pump and the theoretical flow rate that the pump should produce.

• Significance: The volumetric efficiency indicates the amount of leakage within the pump.

30. Define mechanical efficiency of a pump. What does it indicate ?

• Mechanical efficiency is the ratio between the theoretical power required to operate the pump and the actual power delivered to the pump.

• It indicates the amount of energy losses that occur due to reasons other than leakages. 

31. What is overall efficiency of a pump?

• Overall efficiency is the ratio between the actual power delivered by pump and the actual power delivered to pump.

⸫ ηо = ηvol × ηmech

• It indicates the amount of energy losses by all means.

32. A positive displacement pump has an overall efficiency of 87% and volumetric efficiency of 93%, what is its mechanical efficiency?

Solution:

33. Draw the characteristic curve showing the relationship between discharge pressure and rate of delivery of an external gear pump.

34. What do you mean by 'slip, in pumps ?

Slip is the leakage that occurs between the discharge and suction sides of a pump through the pump clearances.

35. What is pump cavitation ? How can you avoid/eliminate it ?

• The formation, growth, and collapse of vapour filled cavities or bubbles in a flowing

• liquid due to local fall in fluid pressure is called pump cavitation.

• Pump cavitation can be avoided by ensuring that the suction pressure is always greater than that required by the pump.

36. What is pump ripple?

Small variations of fluid flow that takes place during pumping due to some design problems are called pump ripple.

37. A hydraulic pump has a flow rate of 75 lpm and is rated for a maximum pressure of 20,000 kPa. What is the maximum power output of this pump?

Given Data: 

Q = 75 lpm = 75 × 10-3 m3/m = 75 × 10-3/60 = 1.25 × 10-3 m3/s;

P = 20,000 kPa = 20,000 ×103 N/m2

Solution: Hydraulic power = P × Q = (20,000 ×103) (1.25 × 10−3) = 25 kW Ans.

38. Name any four desirable characteristics of hydraulic pumps. 

(i) Pumps should provide safe and maximum system working pressure.

(ii) They should have high volumetric and overall efficiency.

(iii) They should be compact and also have higher power-to-weight ratio.

(iv) They should possess the variable displacement control.

39. List some of the factors one should consider while selecting a pump for a specific application.

(i) Safe and maximum operating pressure.

(ii) Maximum flow rate/delivery requirements. 

(iii) Pump drive speed.

(iv) Type of control.