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BELTS

BELT DRIVE

Belt Drive 

Definition of a belt 

Belt is in the form of a loop. It connects mechanically two shafts for transmitting power smoothly. A belt drive consists of shafts,pulleys and a belt.

MATERIALS OF BELTS

(I) Leather

(ii) Rubber

(iii) Fiber/cotton

(iv) Balata

Standard Thicknesses of belts

5, 6.5, 8, 10 and 12 mm are the standard thicknesses.

Standard widths of flat belts

25, 32, 40, 50, 63, 71, 80, 90, 100, 112, 125, 140, 160, 180, 200, 224, 250, 280, 315, 400, 450, 500, 560 and 600 mm

TYPES OF FLAT BELTS

  1. Based on Orientation
    1. Horizontal
    2. Vertical
    3. Quarter –turn
    4. Right-angled
    5. Crossed
    6. Reversed drive
  2. Based on Load or duty or power to be transmitted
Sr. No. DUTY or LOAD Power Peripheral velocity
1. Light load 7.5 kW 12 m/s
2. Medium Load 7.5 to 15 kW >12
3. Heavy load > 15 kW >24 m/s

 Important terminology for belts

(i) Speed ratio

Neglecting the slip and thickness of the belt.

Speed ratio= diameter of bigger pulley/diameter of smaller pulley

Hence Speed ratio = N1/N2

Where N1 is the motor or engine speed OR higher speed

N2 is the machine speed or lower speed

(ii) Center distance

(iii) Peripheral velocity

 LAW OF BELTING

The center line of belt as it approaches the pulley must coincide with the central plane of that pulley. Otherwise belt will fly away from the pulley.

TYPES OF PULLEYS USED WITH BELTS

There are two types of pulleys used with belts.

(I) Flat pulleys——–Used with flat belts.

(ii) Grooved pulleys—–Used with V-belts. Grooved pulleys are called sheaves.

Belt Drive and its Types

A belt drive consists of two shafts, two pulleys and a belt. One of these shaft is a motor shaft on which electric motor is mounted. On the other shaft the machine is mounted to which power is transmitted by the belt drive.   Belt drive is used to transmit power from the motor to the machine shaft. It can be from the engine to the machine shaft. This can also be from turbine to generator.

Normally speed of the motor is high because high speed motors are more efficient. Therefore motor shaft is the driving shaft and the machine shaft is the driven shaft. Therefore, in most of the cases the machine has Lesser RPM than that of the motor. Therefore pulley on motor will be smaller pulley and the pulley on the machine will be a larger pulley. Belts do not transmit with 100 % efficiency because of SLIP and stretching of the belt.There are two types of belt drive, namely flat belt drive and V-belt drive.

 Open Belt Drive
 Both drive and driven shafts run in the same direction in case of an open belt drive. For smooth power transmission, belt on one side is more tight than the other side. In a horizontal drive, tightened side is always kept in the lower side of two pulleys because the sag of the upper side slightly increases the angle of contact of the belt on the two pulleys. More angle of contact means more power transmission.

Cross Belt Drive

In case of cross belt drive, both drive and driven shafts run in the opposite directions. But the more angle of contact (angle of wrap) increases power transmission. Chances of slip of belt are not there.
 Rope Belt Drive
Here a rope is used. But these not very common in industries.

 Advantages of a belt drive

  1. Belt drive  is a simple drive.
  2. It is cheap.
  3. No lubrication required.
  4. Have high efficiency.
  5. Requires less maintenance.
  6. Some misalignment is adjustable without loss in efficiency.
  7. Durable and has long life.
  8. Can be used for parallel and non parallel shafts.
  9. Easy to reduce vibrations and noise.

 Disadvantages of a belt drive

  1. Velocity ratio is not truly constant because of slip and stretching.
  2. Heating occurs due to friction. It disturbs its perfect working at higher temperatures.
  3. There is a speed limit of 35 m/s.
  4. Requires center distance adjustment due to stretching.

PRACTICAL APPLICATIONS OF A FLAT BELT DRIVE

(I) Farming

(ii) Water pumps

(iii) Mining

(iv) Saw mills

(v) Electrical generators

PRACTICAL APPLICATIONS OF A V- BELT DRIVE

(i) Stone crushers

(ii) Machine tools

(iii) Paper industry

(iv) Textile industry

(v) High power mills

(vi) Refrigeration and air conditioning machinery

(vii)  Cars

 



BELT-CHAIN-GEAR DRIVES

BELT-CHAIN-GEAR DRIVES 

 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 is transmitted
  • 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.
  •  Can be used in all orientations i.e. horizontal, vertical or inclined installations.
  • Number of V-belts is used. Breakdown of one of the belts does not stop the operation immediately since remaining belts can take the overload.

DISADVANTAGES

  •  Grooved pulleys are complicated as compared to flat pulleys.
  • Difficult to disconnect as compared to flat belts.
  • These cannot be used for large center distance.
  • Life of V-belt is less than of flat belts.
  • V-belts are complicated than flat belts.

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 (Open Flat belt) or opposite directions (Cross flat belt).
  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. Number of V-belts is used. Even if one of the belt breaks, other belts will be slightly overloaded and operation of the drive will continue.
  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. Belt length is effected by humidity and temperature
  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 called sprockets. Chain forms a polygon instead of a circle when rapped round a smaller sprocket with less number of teeth. Thus the chain does move along the pitch circle but moves through smaller chordal distance causing vibrations, noise and impact on the sprocket teeth. It is called 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) Chain can be used for both long and short distances.
(ii) There is no slip in a chain drive; hence perfect velocity ratio is achieved.
(iii) The chains are made of metal and hence occupy less space in width 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 can be overloaded.
(xiii) Can be operated 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.



FLAT BELT DRIVE

FLAT BELT DRIVE

POWER TRANSMISSION
The power is transmitted from one shaft to the another shaft by a belt, chain and gear drive. The belts and ropes are used where distance between the two shafts is large. The chains are used for intermediate distances. The gears are used for shorter distance between the shafts. Gear drive is a positive drive because there is no slip. Belts(or ropes) transmit power because of friction between the belt (or rope) and the pulley. Due to slip and creep in belts, this drive is not a positive drive. Thus it finds limited applications.

BELT DRIVE
Belt drive consists of
(i) Driver and driven pulleys
(ii) Motor and machine shafts
(iii) Two keys
(iv) One belt
TYPES OF BELT DRIVE
Open Belt Drive
Cross Belt Drive
Compound Belt Drive (3 shafts and two belts, four shafts and three belts)
Quarter turn belt drive

ANALYSIS OF FLAT BELT DRIVE

OPEN FLAT BELT DRIVE
When centrifugal tension is neglected
T1 / T2 = eµθ
T1 Tight side tension in Newton (N)
T2 Slack side tension in Newton (N)
µ = Coefficient of friction between the pulley and belt materials    

θ = Angle of contact=angle of lap on the SMALLER PULLEY
Initial tension = Ti = (T1 + T2)/2
Power transmitted = (T1 –T2) v            Watts (W)
v is the linear velocity of the belt in meters
Hence v =πD N/60
Where N is RPM and D is the diameter of the pulley in meters

WHEN CENTRIFUGAL TENSION IS CONSIDERED
Tc = m v2
Where m is mass of belt PER UNIT LENGTH, kg/m
Initial tension = Ti = ((T1 + T2)/2 + Tc)
Ttight = T1 + Tc
Tslack = T2 + Tc
Ttight / Tslack = eµθ
Power transmitted = (Ttight — Tslack) v         Watt

CONDITION FOR MAXIMUM POWER WITH CENTRIFUGAL TENSION (Tc)
Tmax = 3 m v2

T1 = (2/3) Tmax

T2 = (1/3) Tmax

V = (Tmax/3 m) 0.5

Max Power = (2/3) Tmax (1 — eµθ)( (Tmax/3 m)0.5)            Watts




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