How manual transmission works

Engine crankshaft rotates at a high speed. The high speed power cannot be directly transmitted to the wheels, as it would start rotating at an uncontrollable speed and the driver won’t have any control over the speed on different driving conditions. Therefore, speed reduction is necessary between engine and the wheels. Manual transmission uses a set of gears that help in speed reduction.

The manual gearbox provides a set of gears with different sizes and different number of teeth for different driving conditions. The wheels will be slowest at the 1^st gear and fastest at the top gear. Manual transmission is the most popular form of transmission.

In this article, we will learn about the working of a 4 speed manual gear box with reverse gear.

Why is a transmission necessary?

A vehicle requires moving at different speeds on different conditions. For example, a vehicle moving on a slope requires more torque and hence the vehicle should be operated at lower gear (1^st gear). The higher the torque, the lower is the power from the engine. Whereas, a vehicle moving on a straight surface with less traffic can be operated at higher speeds by switching to higher gears (4^th gear).

Principle of manual transmission:

Manual transmission works on a simple principle of gear ratios. A basic gearbox consists of an input shaft from the engine, an output shaft or main shaft that delivers power to the differential and a counter shaft that transmits the power from input to output shaft.

The power from the input shaft drives the counter shaft and the counter shaft in turn drives he output shaft of main shaft.

Constant Mesh Gearbox:

This is the most common type of manual gearbox used in a vehicle. It consists of an input shaft, a lay shaft or counter shaft, a main shaft or output shaft and a synchromesh device.

The engine drives the input shaft, which in turn drives the counter shaft. The counter shaft gears and the main shaft gears are in constant mesh with each other all the times. That’s why it is called constant mesh gearbox.

The counter shaft gears drive the main shaft gears. But the main shaft gears rotate freely over the bearings and don’t rotate the main shaft unless one of the main shaft gears is locked with the main shaft using the synchromesh device.

The synchromesh device is splined to the main shaft and can slide from left to right or vice-versa. The synchromesh device is commonly known as dog clutch and is operated by means of a selector rod.

Hub and Sleeve arrangement:

There is a hub which is splined to the main shaft and rotates along with the shaft. The hub has external teeth over which a sleeve with internal teeth can slide as per the gear ratio required. Each gear on the main shaft is provided with a synchronizer cone teeth arrangement which rotates freely over a bearing. If the sleeve meshes with the teeth of the synchronizer cone, it is clear that both gear and main shaft will be locked and will start rotating at the same speed.

Synchronizer Ring:

But during the gearbox operation, both the main shaft and main shaft gears will be rotating at different speeds. Hence, meshing the sleeve with the synchronizer cone is a difficult task and can generate a lot of noise. To overcome the problem, a synchronizer ring is provided between the sleeve and the synchronizer gear to match the speed of the gear with the shaft before being meshed with each other.

The synchronizer ring not just rotates along with the hub, but also slides axially.

Engagement of sleeve and synchronizer cone teeth:

When clutch pedal is pressed, the power flow from the engine to transmission is blocked. The sleeve is slid towards the required gear with the help of a selector rod. The sleeve pushes the synchronizer ring against the synchronizer cone.

Due to high frictional force between the cone and the ring, the speed of the gear is matched with the speed of the shaft. When the speeds match, the sleeve is slid further towards the cone and meshes with its teeth. Hence, both gear and the main shaft are locked and both start rotating at the same speed.

The same principle is followed to shift to other gears.

Different gear ratios:

Neutral gear:

All the gears on the main shaft are in constant mesh with the gears on the counter shaft. The gears on the main shaft rotate freely and none of the gears are synchromeshed with the main shaft. Hence, no drive is transmitted from the input shaft to the output shaft.

1^st gear:

The smallest gear (lowest number of teeth) in the counter shaft is synchromeshed with the largest gear (highest number of teeth) on the main shaft. Thus we can achieve maximum torque and minimum speed. 1^st gear is ideal for a standing start of the engine.

2^nd gear:

In 2^nd gear, the gear in the middle of the counter shaft is synchromeshed with the 2^nd biggest gear on the main shaft. This increases the speed and reduces the torque to a certain level. 2^nd gear ratio is ideal for cars ascending a hill.

3^rd gear:

The biggest gear on the counter shaft is synchromeshed with the smallest gear on the main shaft to increase the speed further and reduce the torque. This gear ratio is ideal for cruising.

4^th gear:

The input shaft gear is directly synchromeshed with the main shaft to provide a direct drive from the input shaft to the main shaft. The vehicle can reach its top speed at top gear.

Reverse gear:

The reverse gear uses an idle gear to be meshed between an input gear on the counter shaft and an output gear on the main shaft. When the driver selects the reverse gear, the idle gear is slid in between the two gears. This reverses the direction of rotation of the main shaft.

There is no synchronizer cone and ring mechanism for reverse gear. Hence, reverse gear can only be used when the transmission operation is stopped completely and none of the shafts are rotating. The gear ratio is kept low for reverse, since a vehicle requires more torque when it is moving from a standstill.

Related Links:

• Sliding Mesh Gearbox