two marks questions and answers

TWO MARKS QUESTIONS AND ANSWERS

On Directional Control Valves

1. What are the control functions of different valves in hydraulic systems?

1. Directional control valves: To control the direction of flow by which the direction of actuator is controlled.

2. Pressure control valves: To control the pressure of the fluid in the circuit by which the force exerted by actuator is controlled.

3. Flow control valves: To control the flowrate of fluid in the circuit by which the speed of the actuator is controlled.

2. Name the three basic types of control valves based on their function.

1. Directional control valves,

2. Pressure control valves, and

3. Flow control (or volume control) valves.

3. What are the three types of control valves based on their configuration ?

1. Poppet (or seat) valves,

2. Sliding spool valves, and

3. Rotary spool valves.

4. What is the function of a DCV?

The function of a directional control valve (DCV) is to control the direction of flow in a hydraulic circuit.

5. How do you classify directional control valves ?

A. Based on the construction, the DCVs are classified as:

(i) Poppet (or seat) valves,

(ii) Sliding spool valves, and

(iii) Rotary spool valves.

B. Based on the number of ports present, the DCVs are classified as:

(i) Two way valves,

(ii) Three way valves, and

(iii) Four way valves.

6. What are the function of check valves ?

Check valves are used:

(i) to allow free flow in only one direction, and

(ii) to prevent any flow in the other direction.

7. What is the difference between a simple check valve and a pilot check valve ? 

The simple check valve does not permit any reverse flow. The pilot-check valve can allow fluid flow in both directions, but the reversed flow depends upon the pilot actuation. 

8. What is two-way valve? 

Two-way valve contains two ports. Two-way valves are generally used as check and shut-off valves.

9. What does a 4/3 control valve represent ?

A 4/3 control valve has 4 ways and 3 positions.

10. What are the different valve actuation methods used?

1. Manual actuation,

2. Mechanical actuation,

3. Electrical actuation, and

4. Fluid actuation.

11. Give the hydraulic symbol for a 3-position, 4-way, closed-centre solenoid operated direction control valve.

12. Give the symbol for an electro-pneumatic 3 position 4 way DC valve.

13. Draw the graphic symbol for the following DC valves :

(a) Push button operated, four way, two position, spring offset DC valve, and 

(b) Solenoid actuator, spring return 4/3 DC valve.

14. Draw a neat sketch of the graphical symbol for 3 position 4 way solenoid energized pilot operated tandem centre direction control valve.

15. Identify the following elements, represented in Fig.(a) and Fig.(b).

Ans. 

(a) Solenoid actuated, spring return 4/2 DC valve.

(b) Shuttle valve.

16. What is the purpose of a shuttle valve? Draw its graphic symbol.

• Shuttle valves are used when control is required from more than one power source. They are generally used to shift the fluid flow from the second and back up source, when the main source becomes inoperative.

• The graphic symbol of shuttle valve:

17. Draw the ANSI symbols for a pilot-operated check valve.

On Pressure Control Valves

18. What are pressure control valves? Mention any two functions.

• Pressure control valves are the devices used to control the fluid pressure in a system. 

• Functions of pressure control valves are :

(i) To limit the maximum pressure in various circuit components as a safety measure.

(ii) To maintain the desired pressure levels in various parts of the circuits.

19. Name various types of pressure control valves.

1. Pressure limiting (or relief) valves,

2. Pressure reducing valves,

3. Sequence valves,

4. Counter balance valves, and

5. Unloading valves.

20. Differentiate between pressure control valve and pressure relief valve.

Pressure control valve controls the fluid pressure in a system whereas pressure relief valve protects a system from excessive fluid pressure over and above the design pressure limit.

21. Define 'pressure override' in pressure control valves.

The pressure at which the valve that opens is called the cracking pressure. The cracking and closing pressure of the control valve is not same. Moreover in most cases, the valve poppet cracks at a pressure lower than the adjusted pressure but the valve closes at a lower pressure than at which it cracks. This phenomenon is known as pressure override.

22. What is the difference between pilot operated and direct operated pressure relief valve? 

Direct operated pressure relief valves are used where the flow rate and the system pressure are reasonably smaller or there is not much variation in system pressure or flow rate. Whereas for a larger flow rate and higher pressure, pilot operated pressure relief valves are used. The great advantage of pilot valve is that it can be kept spatially separated from the main valve.

23. Distinguish between the pressure reducing valve and pressure relief valve by giving the graphical symbols.

Pressure reducing valve maintains a prescribed reduced pressure at its outlet regardless of the valve inlet pressure. But the pressure relief valve limits the maximum pressure that can be applied to the part of the system to which it is connected.

24. What is twin pressure valve?

Twin pressure valve, like counter balance valve, permits free flow in one direction and restricted flow in the opposite direction.

25. What is the function of a sequence valve? 

The sequence valves are used to control the fluid flow to ensure several operations in a particular order of priority in the system.

26. What is the purpose of a counterbalance valve ?

The counterbalance valve is used to permit free flow in one direction and restricted flow in the opposite direction.

27. What is the function of an unloading valve ?

The unloading valve allows pressure to build up to an adjustable setting, then bypasses the flow as long as a remote source maintains the preset pressure on the pilot port.

28. What is the use of a hydraulic fuse?

The hydraulic fuse, analogous to an electrical fuse, is used to prevent the system pressure from exceeding beyond the allowable limit in order to protect the system components from damage.

On Flow Control Valves

29. What are flow control valves? Why are they referred as speed-control valves ? (or) Why flow control valve is essential in hydraulic circuits? 

• Flow control valves, also known as volume-control valves, are used to regulate the rate of fluid flow to different parts of a hydraulic system.

• Since control of flow rate is a means by which the speed of hydraulic machine elements is governed, therefore flow control valves are also referred as speed-control valves.

30. List the three ways of applying flow control valves. 

(i) Non-pressure compensated flow control valves, 

(ii) Pressure compensated flow control valves, and

(iii) Pressure and temperature compensated flow control valves.

31. Name few non-pressure compensated flow control valves.

1. Disk-type globe valve,

2. Plug-type globe valve,

3. Ball-type globe valve, and

4. Needle valve.

32. Draw the graphic symbol for a pressure-compensated flow control valve.

33. Draw the ANSI symbol for: (a) Adjustable, temperature and pressure compensated flow control valve, and (b) Counter balance valve.

On Servo Valves

34. What is a servo system?

A servo system is one in which a comparatively large amount of power is controlled by small impulses or command signals and any errors are corrected by feedback signals.

35. List six elements of a basic closed-loop servo system.

1. Command signal,

2. Servo amplifier,

3. Servo valve,

4. Actuator,

5. Load, and

6. Feedback transducer.

36. What are servo valves ? Write their purpose in fluid power systems.

• Servo valves are nothing but DC valves having infinitely variable positioning capability.

• The servo valves are used to control not only the direction of fluid flow, but also the amount of flow.

37. How can you classify servo valves? 

1. Mechanical type servo valve, and

2. Electrohydraulic servo valve,

(i) Single-stage servo valve,

(ii) Two-stage servo valve,

(iii) Jet pipe servo valve, and

(iv) Flapper jet servo valve.

38. Define the terms ‘lap' and 'null' with respect to servo valves.

• Lap is the length relationship between the metering lands to the spool and the port openings in the sleeve or body.

• Null is the relational condition between the spool and valve port where the valve supplies no control flow at zero load pressure drop.

39. Mention any four applications of hydromechanical servo valves.

Hydromechanical servo. valves are employed in steering devices (such as power steering system of automobiles), test and training devices, copying devices (such as machine tools), and heavy-duty mobile equipment.

40. How does an electrohydraulic servo valve differ from the hydromechanical servo valve? 

In electrohydraulic servo valves, electrical signal is used to control the hydraulic output. But in mechanical servo valves, mechanical linkages are used for the same purpose.

41. Write few applications of electrohydraulic servo valves.

The electrohydraulic servo valves are widely employed in more sophisticated control systems such as on tape-controlled machine tools, high-speed printing presses, press brakes, etc.

42. What is solenoid valve and how does it work?

• A solenoid valve is an electromechanically operated valve.

• The valve can use a two-port design to regulate a flow or use a three or more port design to switch flows between ports.

On Proportional Control Valves

43. What are proportional control valves ?

Similar to servo valves, proportional valves are also infinite variable position valves. They also use a variable force direct current solenoid to control the output from the main spool.

44. How does a servo valve differ from proportional valve?

Servo valve spools differ from proportional valve spools because they have no overlap in center condition. Spool overlap makes proportional valves (and the activators they control) respond slowly. With no overlap or underlap, any servovalve spool movement gives immediate flow and actuator response. However, this type is difficult to manufacture, which makes the valve expensive.

45. List the types of proportional control valve.

1. Proportional pressure relief valve,

2. Proportional pressure reducing valve, and

3. Proportional direction control valve.

On Accessories

46. Name any six ancillary equipment commonly used in the hydraulic system.

1. Fluid reservoir,

2. Filters and strainers,

3. Heat exchangers,

4. Pressure and temperature switches, 

5. Accumulators, and

6. Intensifiers.

47. What is the purpose of a fluid reservoir ? (or) What are the functions of reservoir? 

Fluid reservoirs are basically used to provide a storage facility for the hydraulic fluid used by the system. In addition, the reservoirs also serve to separate entrained air, remove contaminants, and dissipate heat from the fluid.

48. What is the purpose of a heat exchanger in hydraulic systems?

Heat exchangers, also known as hydraulic coolers, are devices used to dissipate the heat generated in a hydraulic system.

49. What is the use of a temperature switch?

A temperature switch is an instrument that automatically senses a change in temperature and opens or closes an electrical switching element when a predetermined temperature point is reached.

50. Differentiate pressure switch and temperature switch.

• Pressure switch is used to sense a change in pressure automatically, and opens or closes an electrical switch when a predetermined pressure is reached.

• Temperature switch is used to sense a change in temperature automatically, and opens or closes an electrical switch when a predetermined temperature is reached.

51. What is the use of a shock absorber in hydraulic systems ?

A shock absorber is a device that brings a moving load to a gentle rest through the use of metered hydraulic fluid.