Two Marks q & a

TWO MARKS Q & A

1. What is the function of a hydraulic actuator?

A hydraulic actuator is used for converting hydraulic energy into mechanical energy. 

2. How can you classify the hydraulic actuators?

1. Linear actuators (also called 'hydraulic cylinders'), and

2. Rotary actuators (also called 'hydraulic motors').

3. What is the function of a hydraulic cylinder ?

The hydraulic cylinder is used to convert fluid power into linear mechanical force and motion.

4. Mention few applications of hydraulic cylinders.

The hydraulic cylinders are basically used for performing work such as pushing, pulling, tilting, and pressing in a variety of engineering applications such as in material handling equipment, machine tools, construction equipment, and automobiles.

5. Name different types of hydraulic cylinders.

1. Single-acting cylinders,

2. Double-acting cylinders,

3. Telescoping cylinders,

4. Tandem cylinders, and

5. Through rod cylinders.

6. By what means, single-acting cylinders are retracted ?

The single-acting cylinders are retracted using gravity or by the inclusion of compression spring at the rod end of the cylinders.

7. Differentiate between a single-acting and a double-acting hydraulic cylinder.

Single-acting hydraulic cylinders can deliver a force in only one direction. But in double-acting cylinders, liquid pressure can be applied to either side of the piston, thereby providing a hydraulic force in both directions.

8. What is the purpose of seals in hydraulic system ?

The seals are used to prevent the internal and external leakages of hydraulic system.

9. What are the three types of seals used in a double-acting cylinder ? Also write their functions.

1. Piston seals: They are provided in between piston and barrel to avoid any leakage. 

2. Wipe or Scraper seal: It is fitted on the outside of the piston rod bearing to prevent grit, water or other matter being drawn into the bearing.

3. O rings: O rings, also called internal sealing rings, are fitted behind the bearing to prevent high pressure fluid leaking out along the rod.

10. Why are double-acting cylinders known as differential cylinders?

Since the piston rod is attached at one side only, the cylinder exerts greater force when extending [F = P × Apiston] than when retraction [F = P × (Apiston - Arod)]. This results in different pressure levels on either side of the piston and that's why double-acting cylinders are also called as differential cylinders.

11. What is meant by cylinder cushioning? (or) Why cushioning mechanism is needed for hydraulic cylinders?

When the pressurised fluid is allowed to enter inside the cylinder, the piston accelerates and travels in the cylinder barrel. If the piston is allowed to travel at the same speed till the end of the stroke, it will hit the end cap with a great impact. To avoid this impact, the piston needs to decelerate at the end of the travel. The arrangement made at the end caps to achieve the same is called 'cylinder cushion'.

12. What is a telescopic cylinder ? When is it normally used ?

• A telescopic cylinder consists of a series of rams (pistons) nested in a telescopic assembly which provide a long extension.

• Telescopic cylinders are useful for application requiring a long stroke but with only limited space available for the unextended ram.

13. Sketch the graphical symbol of telescoping cylinder.

14. What are the advantages of telescope cylinders? (or) Mention the significance of a telescopic cylinder with an industrial example.

• Telescopic cylinders are used where there is a space constraint.

• Such cylinders are useful for applications requiring a long stroke but with only limited space available for the unextended ram.

• Some of the applications of telescopic cylinders include hydraulic cranes, high-lift fork trucks, dump trucks, dipper wagons, etc.

15. What are tandem cylinders? When are they normally used?

• A tandem cylinder is one in which two or more piston rod combinations are assembled as a rigid unit with all pistons mounted on single rod.

• The cyЯnders are used to obtain a low-force, high-speed action followed by a high- force, low-speed action.

16. Differentiate double-rod and tandem cylinder.

• A tandem cylinder is one in which two or more piston and rod combinations are assembled as a rigid unit with all pistons mounted on single rod.

• A double-rod cylinder, also known as through rod cylinder, has single piston with piston rods extending from both ends of the cylinder.

17. What is meant by rodless cylinder? Where it is preferred?

• As the name implies a rodless cylinder has no piston rod, being constructed with a slide table assembly mounted directly above the piston. The table and piston are connected either directly, using a slot cut along the top of the cylinder body, or indirectly, through magnets located on both the piston and table.

• Rodless cylinders were originally developed to overcome the space limitations of conventional actuators, providing relatively compact, space saving devices, capable of moving or positioning components or tools in assembly and automation systems.'

18. Name any four different types of hydraulic cylinder mountings,

1. Side foot mount,

2. Centerline mount,

3. End foot mount,

4. Side-flush mount,

5. Rear flange mount,

6. Front flange mount,

7. Clevis mount,

8. Basic mount, etc.

19. What is the function of a hydraulic motor (or rotary actuator)?

The function of a hydraulic motor (or rotary actuator) is to convert hydraulic energy into rotary mechanical energy.

20. Name the basic types of rotary actuators. Which will provide the maximum degree of rotation?

• Types: 

1. Continuous rotary actuator, and

(a) Gear motor, (b) Vane motors, and (c) Piston motors.

2. Limited-rotation hydraulic motors.

(a) Vane type, and (b) Piston type.

• Continuous rotary actuators will provide the maximum degree of rotation.

21. How do you rate/specify ahydraulic motor ?

The hydraulic motors are usually specified in terms of the torque developing capacity or differential pressure.

22. Which hydraulic motor is generally the most efficient ? Why ?

The piston motors are the most efficient motors. Because they are positive displacement motors.

23. Mention few applications where piston motors are employed. 

Piston motors are suitable for low-speed, high torque traction applications such as earth- moving machinery, agricultural tractors, railway locomotive and other industrial applications. 

24. What do you mean by a limited-rotation hydraulic motor? (or) What is semi-rotary actuator? 

The limited-rotation motors provide rotary output motion over a finite angle. Usually rotation of the shaft of these motors is 90°, 180°, or 270°.

25. Mention few applications of limited-rotation hydraulic motors.

The important applications of limited-rotation hydraulic motors include conveyor sorting, valve turning, air bending operations, flip over between workstations, positioning for welding, lifting, rotating, and dumping.

26. What are the main advantages of gear motors?

The gear motors are simple in design, and very cheap in cost.

27. Draw the hydraulic symbols for fixed and variable displacement bidirectional motors.

28. Define volumetric efficiency of a hydraulic motor.