COMPARISON OF POWER TRANMISSION DRIVES CLASS NOTES

 

 

COMPARISON OF POWER

TRANMISSION DRIVES CLASS NOTES

 

 All these do power transmission from the

motor to the machine. These all work on

the principal of friction.

Their selection depends on

(i) Velocity ratio

(ii) Peripheral speed

(iii) Center distance

 

Advantages of V-Belt over Flat Belt

  • Friction is more due to groove (wedge). Thus these transmit more power.

  • Reduced angle of contact and hence more power transmission

  • There is no slip in a V-belt since belt runs in a groove.

  •  Compact since distance between two shaft axes is less.

  • Silent drive.

  • It has more speed.

  • Higher velocity ratio or reduction ratio.

  •   Use it in all orientations i.e. horizontal, vertical or inclined installations.

  • Use number of V-belts . Breakdown of one of the belts does not stop the operation immediately.  Remaining belts become overloaded.

DISADVANTAGES

  • Difficulty using  grooved pulleys than the flat pulleys.

  • Difficult to disconnect as compared to flat belts.

  • Not useable for large center distances

  • Smaller life of V-belt  than flat belts.

  • V-belts complicates the belt drive.

ADVANTAGES OF A BELT DRIVE OVER CHAIN AND A GEAR DRIVE

  1. Belt drive is silent.

  2. Belt drive is more smooth.

  3. Installation is easy and convenient.

  4. Reliability is high.

  5. Permits higher peripheral velocity up to 20 m/s.

  6. Highly flexible as it permits the pulleys to run in the same or opposite directions

  7. It is a variable speed drive with the use of stepped pulleys.

  8. Cheap

  9. No lubrication required.

  10. Easy to connect or disconnect with the use of an idler pulley.

  11. Suitable for larger center distance between the two shafts.

  12. Flexible with respect to the center distance.

  13. It is suitable for higher velocity ratio (10:1) as compared to a chain drive (7:1).

  14. Use number of V-belts. If one of the belt breaks, other belts become slightly overloaded and operation of the drive continues.

  15. Noise is less.

DISADVANTAGES

  1. Overall dimensions are large.

  2. With constant use, belt elongates which introduces slip.

  3. Slip causes reduction in power transmission

  4.  Humidity and temperature affect the belt working

  5. Belts have limited power transmission capacity

  6. Dust, dirt, water and oil affect the performance of the belt

  7. Exert more force on shafts thus requiring stronger shafts

  8. Creep in belts affects badly the performance of the belt drive.

  9. Requires high maintenance and inspection but maintenance is relatively cheap.

  10. Life is less. There is no shock and vibration between the driver and driven.

  11. It is less safe.

POLYGONAL EFFECT IN A CHAIN DRIVE

Chain runs on wheels with teeth. These wheel with teeth on which a chain runs are sprockets. Chain forms a polygon instead of a circle when rapped round a smaller sprocket with less number of teeth.  The chain does move along the pitch circle. Then it moves through smaller chordal distance causing vibrations, noise and impact on the sprocket teeth. It is the polygonal effect. Thus the radius and hence the velocity is continuously changing in between the two teeth’s. The magnitude of speed variation is dependent on the number of teeth on the smaller sprocket. Polygonal effect reduces the chain life too. To reduce the polygonal effect, use minimum number of teeth on smaller sprocket as 19. With this, speed variation is only 1.6 % which is small.

Advantages and Disadvantages of a Chain Drive over a Belt Drive

Advantages

(i)  Use chains for both long and short distances.
(ii) There is no slip in a chain drive; hence achieve perfect velocity ratio.
(iii) Metallic chains occupy less space than a belt or rope drive.
(iv) Its transmission efficiency is high (up to 98 percent).
(v) Its operation is highly reliable despite chain and sprocket wear
(vi) Load on the shaft is less.
(vii) Single chain can transmit motion to number of shafts.
(viii) Chain transmits more power than belts.
(ix) Provides high speed ratio of 8 to 10 in one step.
(x) Can operate with bad alignment
(xi) Gives high efficiency despite having hundreds of wear surfaces and poor lubrication
(xii) These become overloaded.
(xiii)  Operate under adverse atmospheric conditions.

Disadvantages

(i) It is costly.
(ii) Needs accurate installation.
(iii) Needs lubrication.
(iv) Have more velocity variations.
(v) Complex in actual operation.
(vi) Chain performance changes with wear

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