Sunday 3 February 2013

Flat and V-Belts







 Flat and V-Belts

INTRODUCTION:

  • Power can be transmitted either by flexible or non—flexible type of machine elements. 
  • In flexible elements, the center  distance can be changed conveniently and in non—flexible type of machine elements, the centre distance cannot be changed. 
  • The flexible type of elements are: belts, chains, and ropes. 
  • The non—flexible type of elements are : gears, clutches, couplings, and power screws. 
  • Flexible elements are used for power transmission over long distances and non flexible type over short distances.
Factors for Selection of Belt Drive:

The following factors must be considered while selecting a belt drive:

1. Speed of driving and driven shaft.

2. Speed reduction ratio.

3. Power to be transmitted. 

4. Centre distance of hafts.

5. Space available.

6. Layout of shaft.


Factors on which Coefficient of Friction Depends:

The coefficient of friction between belt and pulley depends upon the following factors-

1. Belt material.

2. Pulley material.

3. Belt slip. 

4. Belt speed.


Classification of Flat Belt Drives:

Flat belts can be used for horizontal, vertical, quarter—turn, right—angled, crossed and reversed drive. Flat belt drives are classified as:
 

Slip in Flat Belt drive:

Slip is the relative motion between the belt and the pulley rim. The slip is given by, s = total percentage slip on the pulleys


Crowning of pulley rim:

  • A convex curvature (called crowning) is given to the pulley rim to make the belt run in the middle of the pulley width. 
  • The crowning depends upon the diameter and width of the pulley rim.
     
Selection of Flat—Belts from Manufacturers Catalogue:

The following information is required to select the flat—belt from manufacturer’s catalogue:

1. Power to be transmitted

2. Input and output speeds.

3. Centre distance available.

The basic procedure for belt selection from the catalogue of the Dunlop Rubber Co. (India) Ltd. is as follows:

1. Calculate the maximum power transmitted by the belt from the rated power:
 

  • The rated power is based on 1800 angle of arc of contact. For this angle less than 180the belt is required to carry additional tension. 
2.For this purpose a factor called ‘arc of contact’ factor Fd is used to calculate the corrected power to be transmitted be the belt:
 
3. Calculate the corrected power rating for the belt:
 
Where v = belt velocity, rn/s

4. Calculate the product of (width x number of plies):

Width x number i of plies = corrected power/corrected kW rating

5. Select the standard belt width and number of plies


Creep in Belts

  • Elastic creep in belt drive.
  • The tension on the tight side of a belt drive is more than the tension on the slack side. 
  • As a result of this, the belt is stretched more on the tight side as compared to the slack side. 
  • Therefore, the driver pulley receives more length of the belt and delivers less. Hence, the belt creeps (jumps) forward. 
  • The reverse occurs on the driven pulley, which receives less length of the belt and delivers more. 
·         Thus the belt creeps backward. This phenomenon is called creeping of belt;
Creep = (F1 — F2)/(btE)

Where F1, F2 = tensions on the tight and slack sides respectively b, t = width and thickness of belt respectively
E = Young’s modulus of elasticity of belt material.






V-BELTS

 Characteristics of V—Belt Drive The characteristics of V — belt drive are:

1. It requires a grooved pulley.

2. The belt comes into contact with the sides of the pulley groove only.

3. The belts are endless.

4. It gives more positive drive than flat belts.

5. The slip of thebelt is less as compared to flat belts.

6. The wedging action of the belt in the pulley groove offset the decrease in the arc of contact.

7. V- belts are used as a matched set.

8. The pulley groove angle is 40°.

9. They are suitable for speed ratio upto 7 and belt speed upto 25 rn/s.

10. The size of a V — belt is specified by the cross—section followed by inside length.

1.2.2 Equivalent coefficient of friction

Fig. 1.1 shows a V - belt in a grooved pulley. The normal force on the belt face is,


 
What is power rating of a belt?


It is the power transmitting capacity of a belt based on 180° angle of arc of contact and a speed of 1440 rpm of the faster pulley.
 

 Are the angles of groove in a V-belt pulley equal to angle of V-belt? If not, why?


·         No, because the V-belt contracts while transmitting power, therefore the pulley groove angle is made a little smaller than the angle of belt. 
·         If this is not done then the belt will start touching the bottom of the pulley groove in a very short period of operation and no allowance will be available after some wear of the belt.



List at least four materials for flat belt?


1, Leather
2. Fabric
3. Balata
4. Rubber




 the advantages and disadvantages of Flat belt, V-belt, rope and chain?



Advantages:

1. Can be used for short as well as long centre distances.

2. A number of shafts can be driven in the same or opposite directions from a single driving sprocket.

3. More compact drive as compared to belt and rope drives.

4. High efficiency.

5. More positive drive, i.e. less slip.

6. No fire hazards and temperature effects.

7. Can be easily replaced.


Disadvantages

1. Unsuitable for precise motion due to polygonal effect and wear of joints.

2. Require precise alignment of shafts.

3. Require proper maintenance, lubrication and slack adjustment.


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