Here is a list of machine devices used in thermal engineering.
1. Individual and Group Drive System:
The arrangement used to transfer the power from a prime mover to the machine is known as ‘driven system’. Thus this system connects the output of the prime mover to the input of machine.
i. Individual Drive:
When output from one prime mover is used to drive one and only one machine, the arrangement is known as individual drive e.g. electric motor driving a compressor or a pump.
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ii. Group Drive:
When the output from one prime mover is used to drive number of machines, the arrangement is known as group drive, e.g., a car engine is used to drive the car through a gear box. Simultaneously it also operates the cooling water pump of radiator, cooling fan, compressor of air conditioner etc.
This type of arrangement is generally used in machine shops. The prime mover is first connected to the line shaft with belt and pulley arrangement. The line shaft is supported by bearings and runs over all the machines which are to be driven. On the line shaft, for each machine two pulleys are mounted.
Out of those, one is known as loose pulley. It can run freely over the shaft while the other is known as fast pulley which is keyed to the line shaft and rotates along with it. Thus using this arrangement the machine can be started or stopped just by shifting the belt on ‘fast’ or ‘loose’ pulley respectively.
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Advantages and Disadvantages of Group and Individual Drives:
Advantages:
1. Only one prime mover generating sufficient power to drive all machines. Hence the initial cost is less.
2. Number of machines, types of machines can be increased without arranging for additional prime mover.
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3. Prime mover can be located out of the machine shop, giving pollution free, noiseless working place.
4. Power losses in prime movers, friction are less giving high overall efficiency.
5. Suitable where number of machines are required to operate simultaneously and the total load remaining more or less constant.
6. Less floor space is required for same number of machines.
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Disadvantages:
1. The prime mover must be kept working even when one of the machines is to be operated.
2. Failure of prime mover entirely paralyses all the operations.
3. Not suitable when number of machines is very small or where the total load is much fluctuating.
2. Belt Drive:
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This arrangement is used to transmit the power from one shaft to another when distance between them is large.
The main components of this drive are discussed as follows:
1. Belt:
These are made of material like canvas, leather, rubber etc. These materials give high friction coefficient between them and pulleys. Sometimes belts are reinforced with steel wires to improve the strength. They are manufactured with different cross sections and accordingly are known as Flat Belt, (with rectangular c/s) and ‘V’ belt (trapezoidal c/s). The low initial cost, no maintenance are major advantages, while non-positive transmission and low transmission efficiency (due to slipping of belt) are major disadvantages of belt drives.
2. Pulley:
It is a circular disc with a flat surface or a groove. It is mounted on shaft with help of a key. Thus it rotates with the shaft. The belt is made to pass over it. Due to the friction between belt and pulley surface, the power is transmitted.
Comparison of Flat and V-belt:
Flat Belt:
1. Cross section is rectangular.
2. Made of canvas or leather.
3. Can be used for distant shafts (distance up to 5 m).
4. Can be used for non-parallel shafts.
5. Open as well as crossed belting is possible.
6. Have a joint.
7. High slippage over pulleys hence low efficiency.
8. Can be repaired.
9. No standardisation of length.
V-Belt:
1. Cross section is trapezoidal.
2. Made of molded rubber, reinforced with steel wires, covered with canvas sheath.
3. Are used for relatively closer shaft (upto 1 m).
4. Normally used for parallel shaft.
5. Only open belting is possible.
6. These are endless.
7. Due to wedging action, low slippage, hence high efficiency.
8. No repair possible.
9. These are available in only standard lengths.
Advantages of V-belt over Flat Belt:
Following are the few advantages of V-belt over flat belts:
1. Transmission efficiency of V-belt is more than flat belts as it is also in contact from sides.
2. V-belts are comparatively silent during operation.
3. In case of V-belts the contact area is more when compared with flat belts as they are at contact from sides.
4. V-belts are endless and they generally used in the power transmission in the industry to run pumps, compressors.
5. The life of V-belts is more when compared with flat belts.
Application of Flat Belt and V-belts:
1. Flat belts are generally used in floor mills and Lathe machines to transmit power.
2. V-belts are used to transmit power in industries i.e., to run compressors, pumps, blowers etc.
3. In automobiles V-belt is used to drive the fan to blow air over the radiator etc.
3. Ropes:
Rope is a bundle of steel wires twisted in a special way. These are used to transmit power when the distance between shafts is very large (8 – 10 m). In this drive also, the rope is made to pass over pulleys. Pulleys used in rope drive have a circumferential groove in order to prevent ‘running out’ of rope. Ropes are used in cranes, elevators, etc.
4. Chain Drive:
A chain is built up of rigid links, which are hinged together in order to provide the necessary ping action around the driving and driven wheels. These wheels have projecting teeth, which fit into suitable recesses in the links of the chain and thus enable a positive drive to be obtained. They are known as chain sprockets and bear a superficial resemblance to spur gears.
The pitch of the chain is the distance between a hinge centre of one link and the corresponding hinge centre of adjacent link.
The pitch circle diameter of the chain sprocket is the diameter of the circle on which the hinge centres lie, when the chain is wrapped round the sprocket.
Types of Chain:
There are two types of chains in common use for transmitting power, namely-
(a) The roller chain and
(b) The inverted tooth or salient chain.
(a) The Roller Chain:
The construction of this type of chain is shown in Fig. 42.7.
The inner plates A are held together by steel bushes B, through which pass the pins C riveted to the outer links D. The roller R surrounds each bush B and the teeth of the sprockets bear on the rollers. The rollers turn freely on the bushes and the bushes turn freely on the pins. All the contact surfaces are hardened so as to resist wear and are lubricated so as to reduce friction.
Figure 42.7 (a) shows a simple roller chain, consisting of one strand only, but duplex and triplex chains, consisting of two or three strands, may be build as shown in Fig. 42.7(b), each pin passing right through the bushes in the two or three strands.
(b) The Inverted Tooth or Salient Chain:
The construction of this type of chain is shown in Fig. 42.9. It is built up from a series of flat plates, each of which has two projections or teeth. The outer faces of the teeth are ground to give an included angle of 60° and they bear against the working faces of the sprocket teeth. The inner faces of the link teeth take no part in the drive and are so shaped as to clear the sprocket teeth.
The required width of chain is built up from a number of these plates, arranged alternately and connected together by hardened steel pins which pass through hardened steel bushes inserted in the ends of the links. The pins are riveted over the outside plates. The chains may be prevented from sliding axially across the face of the sprocket teeth by outside guide plates without teeth.
The number of teeth on the smaller sprocket of a chain drive should preferably be not less than 17 and the speed ratio should not exceed 6 or 7 to 1.
5. Gear Drives:
Following types of gears are widely used in practice.
i. Spur Gear:
In this type, the teeth are cut on cylindrical surface of hub. The teeth in this gear are parallel to the axis of the shaft. These are used to transmit power between two parallel shafts.
The manufacturing of these gears is relatively simple. These are used where the gear ratio (reduction in speed) is upto 5.
Application:
In small gear boxes.
ii. Helical Gear:
The only difference in spur gear and helical gear is, in helical gear the teeth are cut making certain angle with axis of shaft. Gradual loading of tooth is characteristic of the gear, which results in smooth, noiseless operation. These gears have high power transmitting capacity and are used upto gear ratio of 10.
iii. Bevel Gear:
In this type, the teeth are cut on conical surface. These are used to transmit power between two shafts having intersecting axes.
Application:
Differential of 4 wheeler automobile.
iv. Worm and Worm Wheel:
These are used to transmit the power between to non-intersecting shafts. This gearing gives very high gear ratio (up to 100).
v. Rack and Pinion:
The toothed flat strip is known as rack and small circular toothed wheel is known as pinion. This arrangement is used to convert rotary motion into linear motion.
6. Clutches:
Clutch is a device used to connect or disconnect two coaxial shafts. Clutches have very wide application in automobiles.
With help of clutch we can bring the vehicle to rest, without stopping, the engine, just by disconnecting the input and output shafts. It also enables change of gear without creating a shock load on engine.
Clutches are classified in two types viz.:
1. Positive Clutch:
This type of clutch when engaged connects two shafts rigidly.
2. Non Positive Clutch:
These normally use frictional force to transmit power. Hence these are also known as friction clutch.
There are three types of friction clutch as:
(a) Single plate clutch
(b) Cone clutch
(c) Centrifugal clutch.
(a) Single Plate Clutch:
In this type, there is only one plate on each shaft. A circular plate is fixed to the end of one shaft with a key. While on other shaft, splines are cut along its length. On these splines another circular disk can slide freely.
The surfaces of one of the plates facing other is lined with friction material. The sliding plate is forced to remain in contact with the fixed plate with spring force. While this plate can be pulled with help of ‘puller’ which is fixed to ‘neck’. Thus when the sliding plate is pulled against spring force, it result in disconnecting the two shafts. Due to low power transmitting capacity it has not much practical importance.
(b) Cone Clutch:
If we want to increase the power transmitting capacity of a clutch, we have to increase the pressure between the contact surfaces or the area of contact surface.
But there is limit for the higher value of pressure which the friction linings can withstand, hence one has to increase the contact area.
This is achieved by making the friction linings on inner or outer conical surfaces of the plates mounted on each shaft.
Similar to the plate clutch, one cone is fixed on one shaft while the other can slide. The operation in same as the plate clutch.
(c) Centrifugal Clutch:
This gets engaged due to the centrifugal force. Thus it operates automatically. In this the shoes are held in the position as shown in Fig. 42.19 with springs, which are fixed to the shaft. When the shaft starts rotating due to centrifugal force the shoes start moving in radially outward direction and at a specific speed they make contact with the dish from inside. This dish is fixed to the output shaft. This type of clutch is widely used in small automobiles (below 50 CC).
7. Brakes:
We all are very much conversant with the term brakes. We apply brakes in bicycles, motor vehicles to retard the motion and to stop the vehicle. So we can define brake as a device or appliance which is used to retard or stop the vehicle.
Brakes are broadly classified into 3-types:
1. Electric brakes: e.g., Eddy current brakes.
2. Hydraulic brakes.
3. Mechanical Brakes
Mechanical brakes are further classified as:
(a) Single block or shoe brakes
(b) Double block or Double shoe brake
(c) Band brake
(d) Band and Block brake
(e) Internal Expanding brake.
(a) Single Block or Shoe Brakes:
Figure 42.20 shows a single block or shoe brake. A is the rotating wheel or drum to which a block or shoe. B is pressed so as to retard its speed. OC is the lever on which force F is applied to retard and stop the wheel.
(b) Double Block or Double Shoe Brake:
Figure 42.21 shows a double block or double shoe brake. A is the rotating wheel or drum to which two blocks B are pressed so as to retard its speed. OC and O ‘c’ are the two levers on which forces F and F’ are applied to retard and stop the wheel.
(c) Band Brake:
Figure 42.22 shows a Band brake. A is the rotating wheel or drum to which a band generally belt/rope/flexible band (B) is pressed so as to retard its speed. OC is the lever on which force F is applied. T1 and T2 are the tensions in rope or belt. Similarly we can have the drum rotation in anti-clockwise direction also.
(d) Band and Block Brake:
Figure 42.23 shows Band and Block brake. As the name implies it consists both belt or flexible band and the wooden blocks as shown. Force F is applied for braking.
(e) Internal Expanding Brake:
Figure 42.24 shows an internal Expanding brake.
It consist of two brake shoe S and S’ which are pivoted at O and O’ respectively. The shoes liner material is generally Ferodo.
When break lever is pressed, it will rotate the cam, which in turn presses or expands the shoes S and S’ against’ the drum for breaking. Spring is provided to bring back the shoes in position when break is released.
Application:
Breaks are used in bicycles, motor vehicles, tram cars, railways etc.