Power transmission by belt drive is one of the most common and universally used methods of transmission system when two shafts are parallel (up to 10 m) to each other as shown in Fig. 9.1. A belt drive consists of two parallel shafts and a pulley is mounted on each shaft.
An endless belt runs over the surface of the pulley. There may be slippage between them and hence it cannot be called as a positive drive. When the belt runs over the pulley, there is always a friction which acts in between the pulley surface and the belt surface in the opposite direction of motion. The belt transmits power by friction only. The belt drive system can be used for long center-to-center distance of the shaft. For effective transmission, friction between the pulley surface and the belt surface should be as high as possible.
As it is well known, in most of the systems friction is not a desirable phenomenon and should be as minimum as possible.
1. Flat Belt Drive:
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A belt is a thin band made of leather, synthetic rubber, canvas, or thread embedded in rubber or balata. These belts are made flat and rectangular in cross section. The belts are made endless by joining the two ends of the belt by pins or stitching as shown in Fig. 9.2.
The system may be applicable for individual drive or group drive. Individual belt drive can be used when each machine will have its own electric motor. In case of group drive, a high-capacity motor drives an overhead shaft called main shaft or live shaft and the main shaft drives another shaft called counter shaft which drives another machine shaft.
The rotational power from driving pulley to the driven pulley is transmitted due to friction between the belt surface and the pulley surface. The belt will have two sides, one side will be in tension called tension side where as the other side will be in lesser tension called slack side as shown in Fig. 9.3.
The tension side (T1) and the slack side (T2) of the belt depend on the direction of rotation of the driving pulley.
Advantages and disadvantages of flat belts:
Advantages:
i. Simple method, universally used arrangement, operation is smooth if the belt is of proper size.
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ii. Low maintenance cost and long life.
iii. Flexibility is more.
iv. The level of shock is less.
v. Suitable for two parallel shafts.
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vi. Suitable for long distances between two center-to-center shafts.
Disadvantages:
i. Endless belt is made by joining the two ends by pins. The belt tends to get damaged near the joints which reduces its life. This may require periodic replacement of the belt.
ii. The system is not suitable for short-distance shaft.
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iii. Efficiency is found less due to slippage and creep, if the size of the belt is not proper.
iv. The system is not a positive drive.
Flat belt arrangement may be of two types:
(a) Open flat belt drive and
(b) Cross flat belt drive.
(a) Open Flat Belt Drive:
Figure 9.3 shows an open flat belt drive arrangement. There are two pulleys mounted on two parallel shafts. A flat belt runs over the pulley straight. The belt keeps running in the same direction. This arrangement is most suitable when the center-to-center distance of the two shafts is large and both sides of the belt are parallel to each other. Pulley A is driver pulley and B is driven pulley and the rotation of both A and B is clockwise. The lower portion of the belt is the tight side with tension T1 and the upper side of the belt is the slack side with tension T2 such that T1 > T2.
(b) Cross Flat Belt Arrangement:
The arrangement of a cross flat belt arrangement is shown in Fig. 9.4. It is used when two shafts are parallel to each other but they are to be rotated in opposite direction. Driving pulley shaft A is rotated in clockwise direction whereas driven pulley shaft B is rotated in anti-clockwise direction. The two-shaft center-to-center distance is L.
In this driving system, there is a junction point where belts are crossing each other and they will have wear and tear due to constant rubbing effect during operation. This effect is continuous. However, the rubbing effects can be avoided by increasing the center-to-center distance equal to 20 times the width of the belt. Such system is found suitable when the system is operated at low velocity.
Belt Slip:
In case, the frictional resistance between the pulley rim surface and the belt surface is less, there occurs is a difference in the relative motion between both the surfaces which is known as belt slip. Belt slip can be calculated as the difference between the linear speed of the pulley rim surface and the belt surface. The usual method is to be measured as percentage.
Belt slip is caused due to the following reasons:
(a) Continuous run of the belt, pulley rim surface becomes very smooth,
(d) Decrease in the coefficient of friction in between the two surfaces.
(c) Increase in the length of belt due to constant operation.
(d) Large difference in the tension of the tight side (T1) and the slack side (T2).
Creep in Belt Drive:
Creep is caused due to the presence of the relative motion of a belt or pulley. It is due to the increased length of belt. During operation, there is a continuous run of the belt or pulley and there is an alternate contraction and stretching of the belt. Thus, there is a loss of power. Due to creep in the belt, less effective power can be transferred and hence, there occurs a decrease in the speed ratio.
Jockey Pulley/Idle Pulley:
A small pulley which is placed on the slack side of the belt and nearer to the driven pulley B is called jockey pulley as shown in Fig. 9.5. Pulley C is the jockey pulley and it is also called idle pulley.
Following are the advantages and disadvantages of an idle pulley:
(a) It increases the tension T2 in the slack side of belt.
(b) It increases the angle of contact.
(c) It reduces slip.
(d) It increases the power transmission effectiveness.
(e) It reduces the belt life due to increase in the slack side tension by the placement of jockey pulley.
Stepped Pulley System:
In case of a stepped pulley system, a single pulley is made in three steps as shown in Fig. 9.6(a). It is made of cast iron. Two such pulleys are mounted on two parallel shafts as shown in Fig. 9.6(b).
In Fig. 9.6(b), A is the driving shaft and B is the driven shaft. The shafts are placed parallel to each other and they are so aligned that the largest pulley of A falls just opposite to the smallest pulley of driven pulley B. The diameter of all steps of A and B are so adjusted that the same belt can be used. The arrangement is useful for changing the velocity ratio by shifting the belt from one step to another. Sometimes, the system can be used with a four-step pulley instead of a three-step pulley.
Cone Pulley Arrangement:
In this type of pulley arrangement, there are two shafts mounted with long frustum of cone as shown in Fig. 9.7(a) and they are kept parallel to each other, but placed in opposite direction. A complete cone pulley arrangement has been shown in Fig. 9.7(b).
In such arrangement, A is the driving shaft and B is the driven shaft. A flat belt runs over the surface of the frustum of cone in a particular position as per the desired speed ratio. There is a belt shifter “C” in between them. The belt may be shifted to vary the speed ratio in an appropriate manner. The arrangement is very useful and commonly used in wood-turning lathe work.
Compound Belt Drive System:
In compound belt drive arrangement, a particular shaft holds two or more pulleys. Such an arrangement is shown in Figs. 9.8(a) and 9.8(b).
As shown in Figs. 9.8(a) and 9.8(b), for A-B combination, A is the driving pulley and B is the driven pulley. For C-D combination, C is the driving pulley and D is the driven pulley. When it is desired to have maximum reduction in the speed, the compound belt drive assembly is considered to be the most important method. It eliminates the larger driven pulley.
In this combination, pulley B and C are the compound pulleys, i.e., pulley C is keyed on the same shaft on which pulley B is. D is another pulley. A belt runs over A-B and another belt runs over C-D. The speed of pulley B (nb) and pulley C (nc) is same, i.e., nb = nc.
The speed of pulley D (nd) can be calculated as:
where na, nb, nc and da, db, dc are the speed and diameter of pulleys A, B, and C, respectively; t is the belt thickness.
Fast and Loose Pulley:
In a well-organized workshop, several machines are driven by a single main driving shaft (known as line shaft) and quite often one machine is to be stopped or run frequently. In order to stop one machine, the driving main shaft is to be stopped which hampers the work of other machines. However, this problem can be eliminated by introducing the technique as given in Fig. 9.9.
The arrangement facilitates the machine to run or stop as per our need. Figure 9.9 shows that power is transmitted from driving pulley A to fast pulley B by means of belt drive. The fast pulley shaft is connected with machine to be stopped. Adjacent to this, there is a free/loose pulley which is free on the shaft and revolves freely.
If the belt is shifted by belt shifter over to the loose pulley, which is revolving freely, the fast pulley rotation gets stopped, thereby stopping the machine shaft rotation. The belt keeps on running but the fast pulley becomes free and machine stops quickly.
Symbols and Formulae to be used for Flat Belt:
As shown in Fig. 9.10, let us assume A and B as two pulleys.
Then,
da = Diameter of the driving pulley, m
db = Diameter of the driven puller, m
ra = Radius of driving pulley
rb = Radius of driven pulley
na = Speed of driving pulley, rpm
nb = Speed of driven pulley, rpm
m = Mass/length of belt (kg/m)
θa = Angle of contact at pulley A
θb = Angle of contact at pulley B
L = Center distance between the driving and the driven pulley
L0 = Length of belt in open belt drive
Lc = Length of belt in cross-drive
T1 = Tight side tension
T2 = Slack side tension
T = Maximum tension in belt = T1 + T2
T0 = Initial tension in belt = (T1 + T2)/2
Tc = Centrifugal tension = mv2
Tco = Initial tension considering centrifugal tension
= (T1 + T2 + 2Tc )/2
T1 – T2= Net or effective tension in belt
b = Width of belt, m
t = Belt thickness
v = Velocity of belt (m/s)
ωa = Angular velocity of the driving pulley = 2Πna
ωb = Angular velocity of the driven pulley = 2Πnb
P = Power transmitted (kW) = (T1 – T2)v/1000
2. V-Belt Drive:
V-belts are found very suitable for high-power transmission systems. The cross section of a V-belt is made trapezoidal as shown in Fig. 9.11. It is molded from pure rubber/ synthetic rubber with fibrous material such as load-carrying cords of nylon which have got fibrous strength. V-belts are structured like an endless loop of a limited length by the manufacturers depending upon the system under consideration.
A V-belt tightly fitted and run in a V-groove pulley to transmit high torque has been shown in Fig. 9.12. As can be seen, two surfaces of the V-belt are in touch with the V-groove surface, thereby improving the frictional forces between the belt and the pulley. Transmission effectiveness is much improved in this system.
Due to large friction forces, wear and tear of the V-belt increases which causes a reduction of the belt life. The manufacturers are making V-belt in different sizes as per the requirement. When a V-belt is used for power transmission, the pulley is modified by providing a wedge-type groove, so that the V-belt can run in the groove.
Figure 9.13(a) shows a V-belt pulley that transmits power with a single V-belt and Fig. 9.13(b) shows power transmission with three V-belts. In “multi V-belt drive,” even if one belt fails, the other belts can transmit powers.
A V-belt has following features:
(a) V-belts are used for the transmission of large power.
(b) Number of V-belts used on same pulley depends on the power to be transferred.
(c) A V-belt can be used for small center-to-center distance as compared to a flat belt.
(d) Slip is completely absent as compared to the flat belt.
(e) It can be used in any position and any direction; even shaft axis may be inclined.
(f) The installation of V-belt is easy.
(g) The replacement of V-belt is easy.
(h) A V-belt drive is very effective and occupies less space.
Limitations of V Belt:
(a) The life of a V-belt is short due to wear and tear.
(b) It is not so durable.
(c) The manufacturing of V-belt is complicated and requires special technique.
(d) If V-belt gets damaged, replacement is the only alternative which expands the cost.
(e) V-belts can be used within the range of velocity 5-50 m/s.
(f) In case of replacement of belts, if one belt gets damaged, all belts of the same set are required to be replaced.