A gearbox is a transmission device that is used between engine’s output shaft and the final drive in order to provide required torque and power to the wheels of the vehicle. It is the second element of the power train in an automobile.
A gearbox is required because different torque and speed is required in a vechile according to different road conditons.
There are mainly two types of gearboxes :-
1) Manual Transmission gearbox.
2) Automatic transmission gearbox.
Lets see about these two types of gearbox in details :-
1) Manual Transmission gearbox :- This is the most used gerbox in the vehicles on the road due to its low cost. This type of gearbox has 5 or 6 speed ratios or gears and a reverse gear. In this type of gearbox driver has to manually change the gears and requires a specific skill to drive a vehicle with manual transmission gearbox. In vehicles with manual gearbox the gears or the speed ratios have to be changed using a gear lever.
Manual transmission gearbox is further divided into three types. Only these three types of manual gearbox are made since the origin of manual gearboxes.
Three types of manual gearboxes are :-
i) Sliding Mesh Gearbox :-
Sliding mesh gearbox is a transmission system which consists of various gears and shafts that are organized together in an organized fashion.
This is the oldest type of gearbox in Manual transmission gearboxes. This is the first transmission system invented by French inventors Louis-Rene and Emile Levasso.
The shifting of gears in this gearbox is obtained by meshing of the gear in the main shaft with gear in the lay shaft by left or right sliding of the gears in the main shaft.
This gearbox basically have four gears first,second, third and reverse. First gear is obtained when smallest gear in lay shaft meshes with biggest gear on the main shaft, second gear is obtained middle sized gear in the main shaft meshes with middle sized gear in the lay shaft, third gear is obtained when smallest gear in the main shaft meshes with biggest gear in the lay shaft and the reverse gear is obtained by using an idle gear between the main shaft and the lay shaft which makes the rotation of main shaft and lay shaft in same direction and the vehicle moves in reverse direction.
ii) Constant Mesh Gearbox:-
This is gearbox which was invented to overcome the limitations of sliding mesh gearbox.In this gearbox all the gears are always in mesh. The gear remains fixed and not slide like the sliding mesh gearbox.
This gearbox has different parts like coounter shaft, main shaft, clutch shaft, gears and dog clutch.
In this gearbox all the gears in the main shaft and the counter shaft are always engaged with each other . Different transmission ratio or speed ratio are obtained by using the dog cluth .
When the dog clutch is engaged with different gears of main shaft different gear ratios are obtained. If the dog clutch is not in contact with any gear of main shaft the gears of main shaft rotates freely and does not rotates the main shaft as they are connected with main shaft using bearings. The main shaft rotates only when one of the dog clutch is engaged with any of the gear of the main shaft. Reverse gear is obtained in this gearbox using the same technique that was in sliding gearbox i.e using the idle gear between main shaft gear and counter shaft gear.
iii)Synchromesh Gearbox :-
Synchromesh gearbox is a modification of the constant mesh gearbox for smooth gear changes. The main difference between the constant mesh gearbox and synchromesh gearbox is that synchronizer is used in place of dog clutch in synchromesh gearbox.
In contsant mesh gearbox lots of noise and vibration is created during the gear changing process. To overcome this synchromesh gearbox was developed which synchronizers are used which has conical grooves cut over its surface that provide frictional contact to the gears which is to be meshed in order to equalize the speed of main shaft, lay shaft and clutch shaft which in turn provides smooth shifting of gears. This gearbox is used in nearly 52% of the vehicle on the road today.
2) Automatic transmission gearbox :-
This is another main type of gearbox other than manual transmission gearbox. A transmission in which different gear ratios are obtained automatically is known as automatic transmission. In this type of transmission driver only selects condition whether to move forward or reverse. Automaic transmission doesnt require any gear or clutch as both are combined unit and works automatically.
Automatic transmission are of two different types :-
i) Epicyclic gear box:-
This gearbox doesn’t use any sliding gears or dog clutch for obtaining different speed ratios instead a brake band is used to obtain different speed ratios. This gearbox has sun gear,planet gears and a ring gear.Planet gers are smaller than the sun gear.Sun gear is present in the middle of this gearbox the same way as the sun is present in centre of between the solar system and the planet gear revolves around it. Planet gears are smaller than the sun gear. In most of the cases there are four planet gears which is present around the sun gear. The ring gear contains teeths in its inner circumference and surrounds the planet gears and keep them intact. The sun gear is connected with all the planet gears and planet gears are connected with ring gear from outside. The sun gear is attached to the clutch shaft thus moves with power of engine. A brake band is used to stop the movement of ring gear. When the sun gear rotates with engine power, the planet gears also rotate in opposite direction and the ring gear attached to the planet gears also rotates. These three types of gears rotates in their position and doesn’t chnage their postion. When the brake band is used to stop the movement of ring gear, the ring gear stop moving and the planet gears still rotates, since the planet gears rotates and the ring gear is still the planet the gears start moving around the sung gear and using ring gear as its path. Planet gears are connected to the driven shaft which rotates as the planet gears rotates around the sun gear. After that if brake band is released and the ring gear is free to rotate, the planet gears become stationary as the ring gear starts rotating and hence the driven shaft become stationary. Epicyclic gearbox conatins a large number of such units to obtain different speed ratios.
ii) Hydraulic Torque converter :-
Hydraulic torque converter is a type of fluid coupling which is used to transfer rotating power from engine of an automobile to transmission.
This gearbox consists of mainly three parts impeller, turbine and stator. These three parts are enclosed in a single case. The impeller is connected to the engine and the turbine is connected to transmission.
The impeller rotates with power from the engine and as the impeller rotates the oil inside the torque converter assembly is directed towards the turbine due to centrifugal force.As it hits the turbine blades, the turbine blades starts rotating. Since the turbine rotates so the transmission system starts rotating and hence the vechile starts moving. The function of the stator is that it direct the returning fluid in the direction of the impeller rotation and that helps in multiplying torque. The stator changes the direction of fluid almost upto 90 degrees.