BELT DRIVE CLASS NOTES FOR MECHANICAL ENGINEERING

 

BELT DRIVE CLASS NOTES

FOR MECHANICAL ENGINEERING

 Belts, chains and gears transmit power

from one shaft to the another shaft.

Use belts and ropes 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
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.
Material 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 power 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 of two types.
(I) Flat pulleys——–Used with flat belts.
(ii) Grooved pulleys—–Used with V-belts. Grooved pulleys are sheaves.
Belt Drive and its Types
A belt drive consists of two shafts, two pulleys and a belt. On one shaft, mount the electric motor. On the other shaft, mount the machine. Belt drive transmits 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.
Motor speed and Pulleys sizes
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. Thus, in most of the cases the machine has Lesser RPM than that of the motor. Hence, pulley on motor will be smaller pulley and the pulley on the machine shaft 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. The flat belt drive is of further two types, Open and cross belt drives.
 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, keep the tight side as the lower side.  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
In this, use a rope instead of a belt. 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. 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
ANALYSIS OF FLAT BELT DRIVE
OPEN FLAT BELT DRIVE
Neglecting centrifugal tension
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
Considering centrifugal tension 
Tc = m v2
Where m is mass of belt PER UNIT LENGTH, kg/m
Initial tension = Ti = ((T1 + T2)/2 + Tc)
Firstly        Ttight = T1 + Tc
Secondly  Tslack = T2 + Tc
Thirdly      Ttight / Tslack = eµθ
Power transmitted = (Ttight — Tslack) v         Watt
CONDITION FOR MAXIMUM POWER WITH CENTRIFUGAL TENSION (Tc)
Firstly Tmax = 3 m v2
Secondly T1 = (2/3) Tmax
Thirdly T2 = (1/3) Tmax
V = (Tmax/3 m) 0.5
Max Power = (2/3) Tmax (1 — eµθ)( (Tmax/3 m)0.5)            Watts
SLIP AND CREEP
Slip is there when belt does not grip pulley fully and perfectly. Thus,  there is not full friction between the pulley and the belt. Thus, there will be slip between the driver pulley and belt. There will be slip between the belt and driven pulley.
SLIP BETWEEN DRIVER PULLEY AND BELT
This slip is the difference between the linear velocities of driver pulley and belt. Express it in terms of driver linear velocity.
Linear velocity of driver pulley = [(πd1N1)/60)
Where
d1 is the diameter of the driver pulley
N1 is the RPM of the driver pulley
Linear velocity of belt = v
Let S1 = % slip between driver pulley and the belt
S1 =% of linear velocity of driver velocity
=[(πd1N1)/60) (S1/100)
∴[(πd1N1)/60)] (S1/100) = [(πd1N1)/60)]-v
∴v = [(πd1N1)/60)] [1 — S1/100]              (i)
SLIP BETWEEN BELT AND DRIVEN PULLEY
This slip is the difference between the linear velocities of belt and the driven pulley. Express it in terms of linear belt velocity.
Let d2 be diameter of the driven pulley
N2 is the RPM of the driven pulley
S2 = % slip between the belt and driven pulley
=in terms of velocity of belt
=v x S2/100
Slip = = v — πd2N2/60
v x S2/100 = v — πd2N2/60
v(1 — S2/100 ) = πd2N2/60
Substituting the value of v from equation (i), we get
[(πd1N1)/60)] [1 — S1/100] (1 — S2/100) = πd2N2/60
Multiply brackets containing terms S1 and S2
[(πd1N1)/60)] [1 — S1/100 — S2/100 + S1S2/10000 )= πd2N2/60
Neglecting S1S2/10000, we get
[(πd1N1)/60)] [1 — S1/100 — S2/100)= πd2N2/60
[(πd1N1)/60)] [1 – (S1+S2)/100)= πd2N2/60
Let Total % slip = S = S1+S2, then
[(πd1N1)/60)] [1 – S/100)= π d2N2/60
DIFFERENCE BETWEEN SLIP AND CREEP IN A BELT DRIVE
SLIP
Slip Is due to incorrect tensions in the belt. Due to slip driver pulley moves and the driven pulley does not move. It is more prominent in a V-belt drive than a flat belt drive. It is due to insufficient friction.
CREEP
Belt material is of low modulus. Change in length on tight and slack side are different. Hence creep occurs. It also occurs because of pulleys not being parallel to each other. It is more prominent in a flat belt drive. To avoid creep, crown the pulleys in the center. It results in no relative motion between the pulley and the belt. Both slip and creep cause relative motion between the pulley and belt. Both reduce power transmission. Reduce these to the minimum.

https://mesubjects.net/wp-admin/post.php?post=14392&action=edit    MCQ POWER TRANS BELTS