What Is A Gearbox? (Transmission) – Types, Parts, Function & Diagram

What Is A Gearbox? (Transmission)

What Is A Gearbox

The gearbox is a mechanical device used to increase the output torque or to change the speed (RPM) of a motor. The shaft of the motor is connected to one end of the gearbox and through the internal configuration of gears of a gearbox, provides a given output torque and speed determined by the gear ratio.


High torque is required to start the vehicle from rest, accelerating, hill climbing, pulling a load and facing other resistances. But the IC engine operates over a limited effective speed range which produces a comparatively low torque.  In such a situation, the engine is responsible for the stall and the vehicle rests if the speed falls below the limit.

The torque developed by the engine is increasing within limits with the increase of engine speed and reaches a maximum value at some predominant speed. If the engine directly connects to the driving axle, the engine speed may reduce.

Due to the variable nature of the vehicle resistance resulting in load and gradient changes, it require that the engine power should be available over a wide range of road speeds. Hence, for this reason, the engine speed maintain by using a reduction gear resulting in the road wheels rotating at a proper speed suited to the operating conditions of the vehicle.

Therefore, a single torque multiplication in the rear axle must be interposed and a variable multiplication factor in the gearbox is provided for this purpose.

The Necessity Of Gearbox

To maintain engine speed on all conditions of load and vehicle speed, the gearbox uses a system to maintain engine speed, while sacrificing the same road speed. To enable the engine to run faster on-road wheels as well as to multiply the torque, a gearbox is required.

Parts Of Gearbox

What is Gearbox

1. Clutch Shaft / Driving Shaft / Input Shaft

A clutch shaft is a shaft that takes power from the engine to supply another shaft. The clutch shaft or driving shaft is connected through the clutch and when the clutch is engaged, the driving shaft also rotates. Only one gear is fixed on the clutch shaft and this engine rotates with the same speed as the crankshaft. In addition, the driving shaft and main shaft are in the same line.

2. Counter Shaft / Layshaft

The counter shaft is a shaft that connects directly to the clutch shaft. It has gear which connects it to the clutch shaft as well as the main shaft. It can be run at engine speed or below engine speed according to gear ratio.

3. Main Shaft / Output Shaft

The main shaft or output shaft that rotates at different speeds and also provides the necessary torque to the vehicle. The output shaft is a splined shaft, so that the gear or synchronizer can be moved to engage or disengage.

4. Bearings

The bearings are required to support the rotating part and reduce friction. The gear box has both a counter and main shaft which is supported by the bearing.

5. Gears

Gears are used to transmitting the power from one shaft to another shaft. The amount of torque transmitted through the gears depends on the number of teeth and the size of the gears. Higher the gear ratio, higher the torque / acceleration and lower the speed. All gears except those on the main shaft are fixed to their respective shafts; They can slide in any of the directions along the shaft.

6. Gear Selector Fork

Gear selectors are simple devices that use a lever that selects gears to engage in disengage mechanisms. The motion of the lever slides the engaging part on the shaft. It depends on the type of gearbox whether the lever slides the gear or synchronizer that are already forged along the main shaft.

Types Of Gearbox

1. Manual Transmission

(I) Sliding Mesh Gearbox

What is Gearbox
(I) Sliding Mesh Gearbox

It is the simplest type of gearbox. In this gearbox, spur gears are used. The Figure shows the construction of a sliding mesh type transmission having three forward and one reverse speeds. There are three gears (1, 6 and 5) attached on the main shaft and four gears (2, 3,4 and 7) on the layshaft.

The two gears on the main shaft (6 and 5) can be slided by a shafting yoke and mesh with the gears (3 and 4) on a layshaft. Therefore, it is called a sliding mesh gearbox. A separate idler gear (8) is mounted on the idler shaft.

(II) Constant Mesh Gearbox

What is Gearbox
(II) Constant Mesh Gearbox

Figure shows the construction of a constant mesh type gearbox having three forward and one reverse speeds. In this type of gearbox, all gears are constantly in mesh and dog clutches are used for engaging and disengaging the gears. The dog clutches (D) and D2) are mounted on the main shaft. One (D2) is connected between clutch gear and reverse gear whereas the other (D)) is placed between low speed gear and reverse gear. The splines are provided on the main shaft for the linear movement of dogs. Dog clutch can slide on the shaft and rotate along with it. All gears are rigidly fixed on the counter shaft.

All main shaft and layshaft gears, and idler gears are engaged by dog clutch to obtain opposite and slow speed. Only reverse gears are spur gear type and all others are helical gears.

As compared with the sliding mesh type, the constant mesh type gearbox meshes more readily with the gears having less danger of damaging during meshing because the gear diameters are smaller with few numbers of teeth. So, this type has more defects when compared to a synchromesh type. The necessity of double clutching is needed so that it is not used to any large extent.

(III) Synchromesh Gearbox

What is Gearbox
(III) Synchromesh Gearbox

Synchromesh gearbox uses synchronizer instead of sliding dog clutches to affect the ratio change. The synchromesh gearbox is similar to the constant-mesh gearbox, but the synchromesh gearbox is provided with a synchronizer, the device by which two gears to be engaged are first brought into frictional contact which equalizes their speed, afterward they are engaged smoothly.

To engage, when the gear lever is moved the synchronizer cone meets with a similar cone on the pinion. Due to friction, the rotating pinion is made to rotate at the same speed as the synchromesh unit. To give a positive drive further, movement of the gear lever enables the coupling to override several spring load balls and the coupling engages with the dogs on the side of the pinion.

Since both pinions and synchromesh units are moving at the same speed, this engagement is done without noise or damage to the dogs. A slight delay is necessary before engaging the dog teeth so that the cones have a chance to bring the synchronizer and pinion to the same speed.

2. Epicyclic Gearbox

What is Gearbox
2. Epicyclic Gearbox

An epicyclic gear train (also known as planetary gear) consists of two gears so that the center of one gear rotates around the center of the other. A carrier connects the centers of two gears and rotates to carry one gear, called planet gear or planet pinion, around the other, called sun gear or sun wheel. The rays of the planet and the sun form traps so that their pitch circles are rolled without slip. A point on the pitch circle of the planetary gear traces an epicyclic curve. In this simplified case, the sun gear is fixed and there is planetary gear rolled around the sun gear.

An epicyclic gear train can be assembled so the planetary gear is rolled onto a fixed, external gear ring or inside the pitch circle of the ring gear, sometimes called the annular gear. In this case, the curve detected by a point on the planet pitch circle is a hypocycloid.

The combination of epicyclic gear trains with a planet engaging both a sun gear and a ring gear is called planetary gear train. In this case, the ring gear is usually fixed and the sun gear is operated.

3. Automatic Transmission

What is Gearbox
3. Automatic Transmission

Various speeds are obtained automatically in gearboxes known as automatic gearboxes. Generally, the driver selects the car condition such as neutral, forward or reverse. The gear selection, timing, and engagement of gear for the required gear speed select automatically when the accelerator press or depress.. The Automatic gearbox does not require a gear change lever and a clutch pedal. Since, both clutch and transmission are a combined unit which works automatically. The automatic gearbox operates in two ways namely.1. Hydramatic transmission, and2. Torque converter transmission

Nowadays, automatic transmissions are popular with various names prescribe by the manufacturers. They may differ in construction slightly. Somebody employs only fluid coupling with the planetary transmission. But others may include a torque converter with fluid coupling and planetary transmission as per their requirements.

(I) Hydramatic Transmission

What is Gearbox
(I) Hydramatic Transmission

In the case of a dramatic transmission gearbox, the planetary gear sets connect in such a way that power may transmit through them. A centrifugal governor in the transmission chooses the proper gear according to the speed and throttle position.

The gear shifting from one gear to another gear is done through hydraulically operated pistons by actuating springs. These springs control the brake bands on the planetary gear sets and clutches within the planetary unit. The various shifts achieves by the throttle and centrifugal governor.

(II) Torque Converter Transmission

What is Gearbox
(II) Torque Converter Transmission

A torque converter is a type of fluid coupling that transmits rotational power from a prime mover, such as an internal combustion engine, to a rotating driven load. In a vehicle with an automatic transmission, the torque converter connects to the power source to the load. It is usually located between the engine’s flexplate and transmission. The manual transmission will have an equal space mechanical clutch.

The main feature of a torque converter is its ability to increase torque when the output rotational speed is so low that it allows the fluid from the winding vanes of the turbine to separate from the stator while it close against its one-way clutch, thus providing the equivalent of a reduction gear.This is a feature beyond simple fluid coupling, which can match rotational speed, but does not multiply torque, thus reducing power.

Torque converter transmission system employs fluid coupling, torque converter and epicyclic gear arrangement. If all different devices are combines into one unit, they will do their duties jointly without any interruptions.

Purpose Of Gearbox

  1. It helps the engine to disconnect from driving wheels.
  2. It helps the running engine connect to the driving wheel smoothly and without shock.
  3. It provides the leverage between engine and driving wheels to be varied.
  4. This helps in reducing the engine speed in the ratio of 4 : 1 in case of passenger cars and in a greater ratio in case of heavy vehicles like trucks and lorries.
  5. It helps the driving wheels to drive at different speeds.
  6. It gives the relative movement between engine and driving wheels due to flexing of the road spring

Function Of Gearbox

  1. Torque ratio between the engine and wheels should be varied for fast acceleration and for climbing gradients.
  2. It provides means of reversal of vehicle motion.
  3. Transmission can disconnect from the engine by the neutral position of the gearbox.

Gear Ratio

Gear ratios are gear reduction steps in the gearbox. A gear reduction multiplies the engine torque by gear ratio amount. Torque requirement at the wheel depends on operating conditions.

For example :

Moving a vehicle from a standstill requires much more torque than the peak torque of the engine. Therefore the torque multiplies by the first gear ratio.

Once starting the vehicle and moving using first gear, it requires less torque at the wheels to keep it moving. Hence it requires no multiplication or very less multiplication.

If the vehicle suddenly encounters a gradient , it will require more torque on the wheels to keep the vehicle moving. Hence an intermediate ratio requires.

What Is A Clutch And It’s Working?

Where Is A Single Plate Clutch?

What Is A Multi Plate Clutch?

What Is A Cone Clutch?

What Is ACentrifugal Clutch?

Advantages And Disadvantages Of Gearbox

Advantages Of Manual Transmission

  • The vehicle is more attractive to the driver.
  • The driver has complete control over the gears and when to shift the gears.
  • The cost of a manual vehicle is lower than that of an automatic vehicle.
  • Transmission cost is less for repair.
  • It provides better mileage.

Disadvantages Of Manual Transmission

  • Manual transmission can be annoying in heavy traffic.
  • There may be some problems in learning the new driver.
  • Precise control over hills is necessary to avoid stalling or rolling back.
  • Hands and feet can hurt when using gears and clutch.

Advantages Of Automatic Transmission

  • It is easy to drive in stop-and-go traffic.
  • This transmission is fast and smooth.
  • Current automated vehicles offer the same mileage as manual transmission.
  • The automatic transmission is very comfortable for the driver when driving.

Disadvantages Of Automatic Transmission

  • Buying an automatic transmission vehicle is more expensive than a manual transmission vehicle.
  • There are more moving parts in the automatic transmission, which increases the cost of repair.
  • The gear change takes a little time and the gear shifting detects, and at times a slight shock is also failing.
  • You cannot make the automatic gear more or less at your own will, suddenly there is a problem in overtaking the car.


Q. What is Gearbox?

The gearbox is a mechanical device used to increase the output torque or to change the speed (RPM) of a motor. The shaft of the motor is connected to one end of the gearbox and through the internal configuration of gears of a gearbox, provides a given output torque and speed determined by the gear ratio.

Q. What Is a Gear Ratio?

Gear ratios are gear reduction steps in the gearbox. A gear reduction multiplies the engine torque by gear ratio amount. Torque requirement at the wheel depends on operating conditions.

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