Hydraulic Circuit Design | Interview Question and Answers
Hydraulic Circuit Design | Interview viva ,oral Question and Answers
1. What are the three important things that are controlled in a hydraulic system?
In a hydraulic system, the pressure, direction and volume of flow are controlled. The pressure is controlled to control the force. Flow is controlled to control the velocity. Direction is controlled to obtain actuator motion in both directions.
2. What is the function of relief valve in a hydraulic system?
The function of a relief valve is to limit the system pressure to a specified maximum value by diverting the pump flow back to the tank.
3. What are the advantages of compound relief valve over direct acting relief valve?
(i) Compound relief valves are usually smaller than the simple relief valves for the same flow and pressure settings.
(ii) They also have a wider range of maximum pressure setting.
(iii) They can also be operated remotely by connecting a simple relief valve to the vent port.
4. What is the function of an unloading valve?
The unloading valve is useful to control the amount of flow at any given time in systems having more than one fixed delivery pump.
5. What is the function of a sequence valve?
When the operation of two cylinders is required to be performed in sequence, the sequence valve is used.
6. When is the pressure reducing valve used in a hydraulic system?
Pressure reducing valve is used to maintain reduced pressure in specified locations of hydraulic system.
7. Draw the symbols for a pressure relief valve and pressure reducing valve
8. Name one application of a counterbalance valve.
The counterbalance valve is used to maintain back pressure on a vertical cylinder to prevent it from falling due to gravity.
9. What is a check valve?
The check valve is a one way valve, which allows flow in one direction, on the other direction the flow is not permitted.
10. How does a pilot operated check valve differ from a simple check valve?
Pilot operated check valve is a two way valve. This type of check valve allows free flow in one direction; it allows flow in other direction also, when a pilot pressure is applied. But simple check valve always allows flow in only one direction.
11. When are three way valves used in a hydraulic circuit?
Three way valves are used to operate single acting cylinders and two three way valves are needed to operate a double acting cylinder.
12. What is meant by actuation of valve? List the various actuating devices of direction control valves.
Actuation is a method of moving valve element from one position to another. The various actuating devices are: Manually operated, mechanically operated, Solenoid operated and Pilot operated.
13. What is a solenoid and how does it work?
A solenoid is a electromagnetic coil. When it is energised, it creates a magnetic force that pulls the armature. The armature is connected to the spool of the direction control valve and the valve is actuated.
14. Give the hydraulic symbol for 3 position-4 way closed center solenoid operated direction control valve?
15. What is meant when a flow control valve is said to be ‘pressure compensated’?
The flow through the flow control valve varies as the square root of the pressure drop across it. So any change in pressure at the outlet and inlet changes the flow through the valve. Pressure compensated flow control adjusts automatically to pressure changes and maintains a constant pressure drop from inlet to outlet, thus provides constant flow.
16. What is the need for temperature compensation in flow control valves?
As the viscosity of oil varies with temperature, the oil becomes less viscous when temperature increases. As the less viscous fluid flow more readily through an orifice, the increase in temperature causes increase in flow for a valve setting. So temperature compensation is needed to offset the effect of such temperature variations.
17. Name the three ways of applying flow control valves?
The three ways of applying flow control valves are,
(i) Meter-in (ii) Meter-out (iii) Bleed-off.
18. Differentiate meter-in and meter-out controls.
- In meter-in type, the flow control valve is located in the pressure line leading to the work cylinder. This type is used when load characteristics are constant and positive.
- In meter-out type, the flow control valve is installed in the return side of the cylinder. This type is used in machine tools like drills and mills in which the tool is dragged suddenly.
19. What is a flow divider and name the different types.
Flow divider is a flow control valve and is used to divide the fluid flow from a single source into two or more streams of equal flow.
There are two types of flow dividers:
1. Valve type
2. Motor type
20. What is the purpose of a deceleration valve?
The purpose of the deceleration valve is to gradually reduce the flow rate going out of the cylinder to provide deceleration of the heavy loads.
21. What is a modular valve?
Modular valve is a stack of control valves one on the other to form a complete valve pack. This arrangement forms a compact system, thereby reducing assembly cost and eliminates much of the interconnecting pipe work.
22. What is a cat ridge valve?
Cartridge valve consists of a catridge which can be mounted in a standard recess in a valve block. These valves give necessary control function such as pressure, flow and direction.
23. What is a servo valve and what is its function?
Servo valve is a direction control valve which has infinite variable positioning capability. Thus it can control not only the direction of fluid flow but also the amount. It gives a very accurate control of position and velocity of an actuator.
24. What is the purpose of seals in hydraulic system and how are they classified?
The seals are used to prevent the internal and external leakages of hydraulic system.
Seals may be classified as (i) Positive type (ii) Non-positive type;
Seals may be classified in another way as (i) Static seals (ii) Dynamic seals.
25. State the difference between positive seals and non-positive seals.
Positive seals prevent even a small amount of leakage. Positive seals are used for external applications such as pipe and flange joints. Nonpositive seals allow small amount of fluid to pass and are used for internal applications.
26. What is the difference between static seals and dynamic seals?
Static seals prevent leakage between non-moving parts and dynamic seals prevent leakage in moving components.
27. List out the most widely used types of seal configurations.
Static seals–Gaskets, O-rings
Dynamic seals– (i) Contact seals
Lip type seals, U rings, cup seals, chevron rings, wipers and scrapers
(ii) Clearance type seals-Piston rings.
28. What is an intensifier and when is it used?
An intensifier is a device which converts low pressure fluid power into high pressure fluid power. Intensifiers are used when a great force is needed for a relatively short distance.
31. Name the three basic types of accumulators used in hydraulic system.
(i) Weight loaded type (ii) Spring loaded type (iii) Gas loaded type
32. State the various functions of accumulator in hydraulic circuit?
Leakage compensation, auxiliary power source, emergency power source, shock suppressor, thermal expansion compensator.
33. What is a hydrostatic drive?
A hydrostatic drive consists of a positive displacement pump, driving a positive displacement hydraulic motor.
34. What is the difference between open-loop and closed-loop hydrostatic Transmission?
In open-loop transmission, all the fluid discharged by the motor returns to the oil reservoir. In closed-loop transmission the fluid flows in a continuous path from the pump outlet port to the hydraulic motor inlet port and then directly back to the inlet of the pump.
35. What are the three basic arrangements in hydrostatic drives?
(i) Variable displacement pump, fixed displacement motor-constant torque transmission
(ii) Fixed displacement pump, variable displacement motor-constant power transmission
(iii) Variable displacement pump, variable displacement motor-variable torque, variable power transmission.
36. What are the two braking systems used in hydrostatic drives?
The two braking systems used in hydrostatic drives are regenerative and dynamic braking.
37. What do you understand by the term power pack?
Power pack consists of a pump, electric motor, reservoir and associated valving assembled to one unit to supply pressurised fluid. They are relatively small in size and provide functions of pressure, direction and flow control within the basic package.
38. What are the main criteria in the design of hydraulic system?
(i) Simplicity (ii) Reliability (iii) Efficiency (iv) Cost effectiveness (v) Maintainability (vi) Safety.
39. List the parameters related to the selection of hydraulic cylinders?
(i) Purpose (ii) Stroke requirement (iii) Thrust (iv) Speed (v) Acceleration and deceleration (vi) Cylinder mountings (vii) Special seal requirement
40. What is the purpose of stop tube in a hydraulic cylinder?
In cases, where the cylinder is rigidly fixed or pivoted at both ends, there is a possibility for excessive loading to occur. The effect of side loading can be reduced by a stop tube inside the cylinder.
41. What is cracking pressure of relief valve and how does it affect the selection of relief valve?
The pressure at which the relief valve opens is called the cracking pressure. The relief valve with a higher cracking pressure is selected because once the relief valve cracks, the flow is lost over the relief valve even when the system has not yet achieved the maximum system pressure.
42. Which type of flow control valve is preferable, if load on the actuator undergo changes?
Pressure compensated flow control valve is selected, if load on the actuator undergo changes.
44. What is the difference between hydraulic tubing and hoses?
Tubes are constructed of cold drawn steel and they are used where high pressures are encountered. Hose is made up of rubber or thermoplastic tube reinforced with steel wire or textile braiding. Hoses are used for medium pressure applications and moving machine members.
45. List the parameters affecting the selection of a pump.
Maximum operating pressure, maximum delivery, pump drive speed, type of fluid, fluid contamination, pump pulsation, pump noise, size and weight of pump, efficiency and cost.
46. What is the purpose of baffle plate in a fluid reservoir?
The baffle plate is provided between the pump’s inlet and return lines for preventing the continuous recirculation of the same fluid into the system. So the foreign particles from the returning fluid are allowed to settle down without any disturbance and the trapped air is allowed to escape.
47. What is an air breather in a fluid reservoir?
As the fluid level in the tank goes up and down during the various operations, a hole is provided at the top, for the inlet and exit of air and is known as air breather.
48. What are the main functions of a fluid reservoir?
The functions are,
(i) To provide a chamber in which any change in volume of the fluid in the hydraulic circuit can be accomodated.
(ii) To provide a radiating surface for allowing the fluid to cool.
(iii) To provide a volume of fluid to allow entrapped air to separate and heavy contaminants to settle.
49. Give the empirical rule for sizing reservoirs.
(i) If there is no volume changes in the system, the minimum reservoir capacity should be twice the pump delivery pressure.
(ii) For mobile applications, the reservoir capacity should be 3-4 times the pump delivery per minute.
(iii) For high pressure system, the reservoir capacity should be 2-15 litres per installed horse power.
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