TRANSMISSION SYSTEM
Clutch Assembly
A clutch is a mechanical device which engages and disengages power transmission especially from driving shaft to driven shaft. In the simplest application,clutches connect and disconnect two rotating shafts (drive shafts or line shafts).
Types of clutch
Different types of clutches are as follows:
1. Friction Clutch
Single plate clutch
Multiplate clutch
• Wet
• Dry
Cone clutch
• External
• Internal
2. Centrifugal Clutch
3. Semi-centrifugal Clutch
4. Conical Spring Clutch
4.Diaphragm Clutch
- Tapered finger type
- Crown spring type.
5. Positive Clutch – Dog and spline Clutch
6. Hydraulic Clutch
7. Electro-magnetic Clutch
8. Vacuum Clutch
9. Over running Clutch or free-wheel unit.
Principle of Friction Clutches
The Principle of a friction clutch may be explained by means of above figure.
Let shaft A and disc C be revolving at some speed, say N r.p.m. Shaft B and the disc D keyed to it are stationary, initially when the clutch is not engaged. Now apply some axial force W to the disc D, so that, it comes in contact with disc C. As soon as the contact is made the force of friction between C and D will come into play and consequently the disc D will also start revolving. The Speed of D depends upon friction force present, which in turn, is proportional to the force W
applied. If W is increased gradually, the speed of D will be increased correspondingly, till the stage comes when, the speed of D becomes equal to the speed of C. Then, the clutch is said to be fully engaged.
Let W = Axial load applied
T = Torque transmitted
R = Effective mean radius of friction surface.
Then, T = μWR
Thus, we see that the torque transmitted by a friction clutch depends upon 3 factors i.e., μ W and
R. This means that increasing any or all of the above factors would increase the amount of
torque which a clutch can transmit. However, there are upper limits in each of these cases.
Gear Box (Transmission) Assembly
Need of a Gear Box
An auto-vehicle has to run on roads of various qualities and types, and at varying speeds. It alsoencounters with vivid nature of tyre-road adhesion, on level and sloped roads. Having differentbody profiles, various driving needs such as idling and quick acceleration etc., and widelyvarying driving habits of drivers, the vehicles have to overcome several resistances also.Moreover, they also meet the variations in tractive effort at various speeds. Various resistancesencountered by the auto-vehicle during its operation are1. Rolling resistance which mainly depends upon
- Material of the road surface : cemented, asphalted etc.
- Quality of the road surface : smooth or rough, dry or wet etc.
- Material and design of tyres tread
- Inflation (pressure) in the tyre : over-inflated, under-inflated, or correctly inflated
2. Gradient resistance which mainly depends upon
- Slope of the road : level, sleepy, down-a-hill, up-the-hill etc.
- Mass of the vehicle : light, medium or heavy
3. Air resistance which mainly depends upon
- Speed of the vehicle : slow, medium or fast
- Speed and direction of the wind : in-favour, against the vehicle’s motion or crosswinds
- Body profile of the vehicle : streamlined, bluff body or otherwise
4. Resistance due to driving requirements which mainly depends upon
- Need of idling, or constant speed
- Acceleration and its rate
- Frictional resistance.
Sum of all these resistances is known as Total Resistance.
Types of Transmission (Gearbox)
Several kinds of transmissions are employed on auto-vehicles. These can be classified as follows:1. Manual Transmission
- The manual transmission i.e., the selective type gearboxes are the conventional means.
- They are widely used in cars, bases and trucks. It is also known as standard or
- conventional type Gear box.
- Sliding mesh gear box
- Constant mesh gearbox
- Synchromesh gearbox without overdrive
- Synchromesh gearbox with overdrive.
2. Semi-automatic transmission
- Semi-automatic transmission incorporates an intermediate arrangement which is
- combination of manual and automatic systems.
- Electric controlled with a fluid drive.
- Electric controlled with overdrive.
- Fluid-torque drive.
3. Automatic Transmission
Automatic transmission is used mainly on luxury cars, or heavy vehicles which require frequent change of gears in the city and crowded highway driving. In automatic transmission, the different ratios between the engine crankshaft and the wheels are achieved by automatic means. The driver does not need to shift gears, such transmission
- make use of a fluid coupling or torque converter, as well as mechanical, hydraulic or electrical controls.
- Hydromatic drive
- Torque-converter drive
Principle of Gearing
A simple Gearing principle is shown below where two gear are engaged with each other. Gear A is called Driving Gear or Driver and Gear B is called Driven Gear.Suppose,NA = Speed of Gear A T = Number of teeth on Gear A ANB = Speed of Gear B T = Number of teeth on Gear B BTherefore, the speed ratio of Gear A and Gear B
With the following example, the principle of gearing will be very clear to you.
In the example, the DRIVER has 60 teeth and because it is the largest we say that it revolves once. The DRIVEN gear has 30 teeth. Simply divide 60 teeth by 30 teeth to work out the number of revolutions of the driven gear.
Propeller Shaft and Universal Joint
The rotary motion of the transmission main shaft is carried out through the propeller shaft to the differential casing, the rear wheels to rotate. The propeller shaft has to withstand the torsional stresses of the transmitting torque and yet, it must be light and well balanced so that vibrations and whip will not occur at high speeds. For these reasons, it is made of a strong steel tube. Solid propeller shafts are also used. Some applications include bearings at or near the propeller shaft center to support the shaft. On some applications, the propeller shaft is in two sections, supported by a center bearing and coupled together by universal joint.
It is to be noted that the transmission main shaft and the differential pinion shaft are not in one horizontal level. The rear axle housing with differential is attached to the frame by springs, therefore, the distance between the gearbox and the differential, changes due to road irregularities. This also changes the angle of driver. In order that the propeller shaft must take curve of these two changes it is provided with one or more universal joints to permit variations in the angle of drive. Also, it must be provided with a sliding joint that permits the effective length of the propeller shaft to change.
Requirements of a Propeller Shaft
For efficient discharge of its functions, the following requirements are expected in a propeller
shaft.
- High torsional strength (therefore, they are made circular in cross-section, preferably
- hollow circular).
- Toughened and hardened (they are generally induction hardened).
- Efficiently welded (they are generally welded by submerged arc CO process). 2
- Dynamically balanced (they are tested on electronic balancing machine).
- Least stress concentration (the fillet radius is ground on centreless grinding machine).
- Reduced thrust loads under high torque.
- Higher fatigue life (to achieve it, they are made of superior quality steel).
- Continuous operating torque
- Continuous true running angles
- Desired service life
- Power source
Universal Joints
An universal joint is used where two shafts are connected at an angle to transmit torque. In the transmission system of a motor vehicle, the transmission main shaft, the propeller shaft and the differential pinion shaft are not in one line, and hence the connections between them are made by universal joint which is used to connect the transmission main shaft and the propeller shaft. The other universal joint which is used to connect the other end of the propeller shaft and the differential pinion shaft. Thus, connections between the three shafts are flexible and at an angle with each other. The universal joint permits the torque transmission not only at an angle, but also while this angle is changing constantly.
A simple universal joint consists of two Y-shaped yokes, one on the driving shaft and other on the driven shaft, and a cross-piece called the spider. The four arms of spider, known as trunnions, are assembled into bearings in the ends of the two shaft yokes. The driving shaft and the driven shaft are at an angle to each other, the bearings in the yokes permits the yokes to swing around on the trunnions with each revolution.
A simple universal joint does not transmit the motion uniformly when the shafts are operating at an angle, except in constant velocity type universal joint. Because, the pivot pins do not revolve in the same plane, the driven shaft will increase to a maximum and decrease to a minimum, twice in each revolution. Although the degree of variation is small, it may be minimized by the use of two universal joints. The two joints are arranged so that the nonuniform rotation of each joint tends to neutralize that of the other.
Types of Universal Joints
In automobiles, the following types of universal joints are used.
- Cross
- Constant velocity
- Rubber coupling
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