How A Gear box (Transmission) works? What’s Gear Ratio?

Gear Box Working Principle:

A Gear Box is an assembly consisting of various gears, synchronizing sleeves and gear-shifting mechanism fitted inside a metal housing. The metal housing usually made of aluminium/iron casting accommodates all the gears in it. Gearbox is a part of the ‘transmission’ system as the gears play an important role in transmitting the engine power to the wheels.

What is Transmission?

All the components of the drive-train that help to transmit the engine power to the wheels are a part of the ‘Transmission’ system. Of which, the gearbox is an integral part. These components include the clutch, gearbox, couplings, propeller shaft, axle shafts, and differential. In general, the term 'Transmission' usually refers to the car’s gear box. Some car designs integrate the gear box and differential assembly into a single unit called ‘Transmission’ or 'Trans-Axle'.

What is the Gear Ratio?

The gear ratio is the ratio between the input and output gears. The driving gear and driven gears in a gear box define the gear ratios. The input gears get the drive from the engine and they rotate the output gears which, in turn, drive the wheels. The ratio of the number of rotations of output gear to the number of rotations of input gear is called Gear Ratio.

The Gear Ratio can also be obtained by the following formula:

Gear Ratio = No. of teeth of output gear / no. of teeth of the input gear

For e.g., If no. of gears on input (driving) gear = 30, no. of gears on output (driven) gear = 105

Then the Gear ratio = 105/30 = 3.5:1 because to rotate the output (driven) gear by 1 rotation, you need to rotate the input (driving) gear by 3.5 rotations.

A typical chart of Gear ratios in a gear box of an MUV:

Following is a chart of gear ratio found in an MUV's gear box.

Gear	Ratio
1st gear	3.78:1
2nd gear	2.20:1
3rd gear	1.42:1
4th gear	1:1
5th gear (Overdrive)	0.83:1

The gear ratios vary from vehicle to vehicle. In trucks, the gear ratios are typically higher than cars as they have to carry the heavy load.

How does a Gear Box work?

A gear box contains gears of different sizes.  This is mainly because of the varying demands of the vehicle in terms of the torque required at the wheels depending upon the road, terrain & load. For e.g., if a vehicle is climbing a slope, it needs higher torque as compared to while cruising on a straight road.

In a gear box, the first gear is biggest in size and provides maximum torque output while producing minimum speed. Hence, it is used when climbing slopes.  All the gears between 1^st and last gear vary in size; in a decreasing ratio. Thus, it provides a varying combination in terms of pulling ability and speed. So, the vehicle could be driven smoothly without any drop in its acceleration. The gear box basically improves the vehicle's driveability in all conditions.

What is an Overdrive?

In contrast, the last gear or the top gear, at times an Overdrive, is smallest in size. However, it provides minimum traction but maximum speed. A gearbox with an Overdrive means its output is higher than its input that connects to the engine. In other words, the overdrive gear rotates faster than the engine speed. Thus, it provides higher speed and better efficiency as the engine runs at a lower rpm in relation to vehicle speed.

In some advanced designs, there are more than one 'Overdrive' gears; usually two. Thus, Dual Overdrive (aka 'Double Top') provides even higher speed and better efficiency in a vehicle.

Gearbox Operation:

Generally, there are two sets of gears in a conventional gear box – the input & the output. The input gears are fixed on the countershaft making it a single unit. It drives the individual gears on the main-shaft which rotate freely on the bearings. Thus, the gearbox passes the drive to the wheels depending upon the gear which engages on the main-shaft. When you push the shifter-sleeve towards the desired gear, that gear locks onto the main-shaft and rotates it. Thus, the main shaft rotates at the speed of the engaged gear and provides the output as per the engaged gear’s ratio.

Gearbox: Speed vs Traction

You need both the speed as well as the traction while driving an automobile. The gears in the gearbox help you choose either of them depending on the driving conditions. The lower gear i.e. 2nd and 1st gear will give you greatest traction while the higher gears i.e. 5th and 6th (if available) will give you the highest speed. The number of gears in the gearbox provides the perfect combination of traction and speed. Thus, it helps the driver/rider choose the most appropriate combination to enhance efficiency at all times. Hence, choosing the right gear according to road and load conditions is very important. With short gearing, you get better acceleration or pick-up while with tall gearing you achieve higher top speed.

Types of Gear Box:

In general, an automotive gearbox is mainly classified into four categories:

1. 1. Manual – With up to 6 forward gears in a car and up to 13 forward gears in a truck
   2. Fully Automatic Transmission - up to 9 speed
   3. Continuously Variable Transmission - CVT
   4. Automated Manual Transmission (AMT) – up to 5 speed.
   5. Dual Clutch Transmission

According to the shifting mechanism, the manufacturers further classify the automotive gear into three more categories:

1. 1. Sliding mesh – typically used in two-wheeler/bikes
   2. Constant mesh - typically used in old generation trucks
   3. Synchromesh - used in the newer generation  cars & trucks

According to the types of shifting mechanism location:

1. 1. Column Shift – Gear lever mounted on the steering column, operated by hand.
   2. Floor Shift - Gear lever mounted on the floor, operated by hand
   3. Peddle Shifters - Gear-shifters fitted on the steering wheel, operated by fingers.

For more information on the newer generation gear boxes, click here:

http://www.zf.com/corporate/en_de/products/innovations/8hp_automatic_transmissions/8hp_automatic_transmission.html

Watch how a gearbox works here:

Read more: How torque helps to improve efficiency in a vehicle?>>