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Car transmission system
We have learnt that, the output of an IC engine is delivered, through the crankshaft in the form of rotation.
We need additional provision named Transmission system, to make the vehicle to move forward.
Transmission system transfers the output of an IC engine, to the drive wheels.
The transmission system also reduces the engine RPM, to a lower drive-wheel speed, while increasing torque in the process.
Transmissions can be found on pedal bicycles, fixed machines, and wherever rotational speed and torque needs to be adapted.
The transmission system consists of, following important components and assemblies.
1. Clutch assembly
2. Gear box assembly
3. Propeller shaft
We will explore into each one of them in detail, to have a better learning about transmission system.
1. Clutch Assembly
What is clutch?
Clutch is a mechanism which enables, the rotary motion of a shaft to be transmitted, when desired.
Types of Clutches
Friction Clutch
1.Single plate clutch
2.Multiplate clutch
3.Cone clutch
Single plate clutches are one of the most commonly used type, in modern light vehicles.
It consists of a clutch plate, friction plate, pressure plate, flywheel, bearings, clutch spring and nut-bolts arrangement.
Principle of Friction Clutches
The Principle of a friction clutch can be explained by means of the sketch given.
1.Let shaft A and disc C be revolving at some speed, say N r.p.m.
2.Shaft B and the disc D are stationary, initially when the clutch is not engaged.
3.Now apply some axial force W to the disc D, so that, it comes in contact with disc C.
4.Once the contact is made by the frictional force between C and D, consequently the disc D will also start revolving.
The Speed of D depends upon frictional force present, which is proportional to the force W applied.
If W is increased gradually, the speed of D will be increased, till it becomes equal to the speed of C.
Then, the clutch is said to be fully engaged and full Torque is transmitted.
Torque (T) = π*W*R
W = Force
R = Radius of contact area
Main Parts of a Clutch
The main parts of clutch are divided into three groups:
1. Driving members:
The driving members consist of a flywheel mounted on the engine crankshaft. The flywheel is bolted to a cover which carries a pressure plate and pressure springs
2. Driven members:
The driven member consists of the clutch plate. It is free to slide lengthwise on the splines of the clutch shaft (primary shaft). It carries friction materials on both of its surfaces.
When the clutch plate is gripped between the flywheel and the pressure plate, it rotates the clutch shaft through the splines.
3. Operating members: The operating members consist of a foot pedal, linkage, release levers and the springs.
Functions of a clutch
To disconnect the engine power from the gear box, as required under following circumstances.
a.To start the engine and to warm it up.
b.To facilitate to engage 1st and 2nd gear, to start the vehicle from rest.
c.To facilitate to change the gear as required.
d.Disconnecting drive from the engine, to stop the vehicle after application of brakes.
e.To allow the engine to take up load, gradually without shock or jerk.
Requirements of a clutch
1. Torque transmission: The clutch should be able to transmit maximum torque of the engine.
2. Gradual engagement: The clutch should engage gradually to avoid sudden jerks.
3. Heat dissipation: The clutch should be able to dissipate large amount of heat, which is generated due to friction.
4. Dynamic balancing: The clutch should be dynamically balanced. This is particularly required in the case of high speed engine clutches.
5. Vibration damping: The clutch should have suitable mechanism to damp vibrations to eliminate noise produced during the power transmission.
6. Size: The clutch size should be as small as possible so that it will occupy minimum space.
7. Free pedal play: The clutch should have free pedal play, in order to reduce effective load, on the thrust bearing to minimize wear.
8. Easy in operation: The clutch should be easy to operate requiring as little exertion as possible on the part of the driver.
9. Lightness: The driven member of the clutch should be made as light as possible, so that it will not continue to rotate after the clutch has been disengaged.
Manual clutch working Animation
Basic clutch operation
Gear Box Assembly
Function of the Gear box:
1. The main purpose of the gear box is to provide a method, to vary the torque ratio, between the engine and the road wheels as needed.
2. It also provides a neutral position, so that the engine and the road wheels are disconnected, even with the clutch in the engaged position.
3. A medium to reverse the car by selecting the reverse gear.
What is the purpose of Gear ratio?
We need different gear ratios in the gear box to enable the vehicle move in various speeds.
At the time of starting the vehicle, maximum torque is needed, so a low gear ratio has to be selected, for the movement of the vehicle.
As the engine speed increases, the amount of torque is reduced in the fly wheel, and we need to select higher gear ratio.
Need of a Gear Box
An automobile has to run on roads of various qualities and types, and at varying speeds.
Various types of resistances encountered by the auto-vehicle during its operation are:-
1. Rolling resistance which mainly depends upon
a.Material of the road surface : cemented, asphalted etc.
b.Quality of the road surface : smooth or rough, dry or wet etc.
c.Material and design of tyre-treads
d.Inflation (pressure) in the tyre : over-inflated, under-inflated, or correctly inflated
2. Gradient resistance which mainly depends upon
a.Slope of the road : level, sleepy, down-a-hill, up-the-hill etc.
b.Mass of the vehicle : light, medium or heavy
3. Air resistance which mainly depends upon
Speed of the vehicle : slow, medium or fast
a.Speed and direction of the wind : in-favour, against the vehicle’s motion or crosswinds
b.Body profile of the vehicle : streamlined, bluff body or other type
4. Resistance due to driving needs which mainly depends upon
a.Need of idling, or constant speed
b.Acceleration and its rate
c.Driving habits
d.Frictional resistance
Sum of all these resistances is known as Total Resistance.
Understanding of Torque and Speed
Basically, the faster the crankshaft spins with the same amount of force, an engine will make more power.
A car with more HP than torque will always be quicker, since this gives a car acceleration and speed.
Torque is a force multiplied by distance, and the easiest way to understand it is by using a wrench. As an individual provides force to the wrench, it travels a distance and provides torque to tighten a bolt.
Sliding Mesh Type GearBox
It is the simplest type of gearbox.
The arrangement of gears is in a neutral position.
The clutch gear is fixed to the clutch shaft. It remains always connected to the drive gear of the counter-shaft.
Gear is in Neutral Position
In neutral position, the engine power will not be transmitted to the axle.
Now the clutch gear is transmitting the power, to gear on the counter-shaft only.
The main shaft does not get any power in the neutral position.
In neutral position, just the clutch shaft gear is engaged to the counter-shaft gear.
All the other gears are free, and therefore the main output shaft is not rotating.
First Gear position
By operating the gear shift lever, the larger gear on the main shaft, is moved to mesh with the first gear of the countershaft.
In this, the main shaft and the clutch shaft both rotate in the same direction.
Second Gear Position
By operating the gear shift lever, the larger gear of the main shaft is disengaged from the first gear of the countershaft.
Then the smaller gear of the main shaft, meshes with the second gear of the countershaft.
Third Gear Position
By operating the crankshaft lever, the second gears of the main shaft and countershaft are disengaged.
Then the third and top gear of the main shaft, is forced axially against the clutch shaft gear.
Fourth Gear Position
By operating the crankshaft lever, the third gear of the main shaft and countershaft are disengaged.
Then the main shaft is forced axially, against the clutch shaft gear.
Reverse Gear Position
By operating the crankshaft lever, the larger gear of the main shaft meshes with the reverse idler gear.
The idler gear is inserted between the countershaft gear, and the main shaft bigger gear. Then the main shaft turns in the direction opposite to the clutch shaft.
This changes the rotation of the wheels from forward to reverse so that the vehicle moves backward.
Propeller Shaft
The propeller shaft is a component assembly, used for transmitting mechanical power, torque, and rotation, from the main shaft of the gear box to the rear wheels.
These shafts are also known as drive-shaft, driving shaft, tail shaft.
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 gear box and the differential, changes due to uneven road condition.
Propeller shaft is used between components, that cannot be connected directly, because of distance or position.
Another purpose is to allow relative motion and flexibility between the rotating components.
As the torque is carried by the driveshaft, it is subjected to torsion or shear stress. So they should be strong enough to bear the stress, while not heavier, as this will increase their inertia.
Driveshafts are used in, with different configurations for front-wheel drive, four-wheel drive, and front-engine rear-wheel drive vehicles.
Parts of a Propeller Joint
1. U-joint: A universal joint (U-joint) is a mechanical joint used to connect rotating shafts. Nowadays, the driveshaft and universal joints are mostly seen on rear-wheel drive and four-wheel drive vehicles.
2. Tube: A tube is a part of a drive shaft, often used in front engine and rear drive automobiles.
3. Centre Bearing: Centre bearing is used to connect the two sections of the drive shaft. These bearings are meant to keep both parts of the driveshaft solid to reduce harmonic vibrations when the vehicle is accelerating.
4. Midship Shaft: Midship shafts are the basic components of a coupling shaft and are part of a drive shaft that is attached to the frame on a center bearing.
5. End Yoke: An end yoke is used for accuracy and durability. The benefit of using an end yoke is to help reduce noise and vibration to keep your driveline running easily.
6. Slip Yoke and Tube Yoke: A slip yoke is attached to the driveshaft itself using a universal joint. The slip yoke is fitted to transfer power by sliding in and out of the transfer case. The tube yoke is also necessary to allow the U-joints to rotate well with the drive shaft.
7. Flanges: Flanges are used for automotive purposes to connect the driveshaft to the transmission, transfer case, and differential.
Requirements of a propeller shaft
In order to achieve efficient functions, the propeller shaft requires the following:
1. High Torsional Strength: They need to be made of a solid or hollow spherical cross-section in order to obtain high torsional strength during working.
2. Toughened and Hardened: Hard and rigid materials are required to make these. Usually they are made of excellent quality steel and are induction hardened.
3. Efficiently Combined: They need to be connected very firmly when they are in operation. Therefore, they are usually welded with the help of a submerged carbon dioxide welding process.
4. Dynamically Balanced: Since the rotation factor can be important at high speeds, the propeller shaft is tested on an electronic balancing machine.
5. Low Thrust Load: Since the resonance is bad for the shaft’s life. To avoid this phenomenon, they transmit the excessive dynamic force to the end support of the shaft.
Universal Joints
A universal joint is used where two shafts are connected at an angle to transmit torque.
The transmission main shaft, the propeller shaft and the differential pinion shaft are not in one line, and hence the universal joints connect them.
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 trunnion, are assembled into bearings in the ends of the two 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.
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