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Transmission in a ...

January 23, 2011 00:00

Transmission is a vital link in the power train of any modern vehicle. The purpose of transmission or transaxle is to transfer power from the engine to the wheels through the use of gears of various sizes to give the engine a mechanical advantage over the driving wheels. Today, we take a look at the car’s transmission. How does it work?

During normal operating conditions, power from the engine is transferred through the engaged clutch to the input shaft of the transmission. Gears in the transmission housing alter the torque and speed of this power input before passing it on to other components in the drive train. Without the mechanical advantage the gearing provides, an engine can generate only limited torque at low speeds. Without sufficient torque, moving a vehicle from standstill would be impossible. Thus, a proper transmission is necessary to ensure a car’s proper working.

Transmission and Transaxle

Vehicles are propelled in one of three ways:

1. Rear wheel drive

2. Front Wheel drive

3. All four wheel drive

Rear wheel drive

Vehicles propelled by rear wheels use a transmission system. The transmission gearing is located within an aluminium or iron casing called the transmission case assembly. The transmission case assembly is attached to the rear of engine, which is normally located in the front of the vehicle. A drive shaft links the engine’s output shaft to transmission with the differential (read later on) and drive axle located in a separate housing at rear of the vehicle. The differential splits the driveline power and redirects it to the rear drive axles, which then passes it on to the wheels. For many years, rear wheel drive was the conventional method of propelling vehicles

Front wheel drive

Front wheel drives are propelled through the front wheels. Unlike the transmission used in rear wheel drive and four wheel drive, here the mechanism is called Transaxle. A transaxle combines the transmission gearing, differential and drive axle connections into a single case aluminium housing located in front of vehicle.

Major advantage of this type of drive is the good traction on slippery roads due to the weight of the drive train components being directly over the driving axle of the vehicle.

Four wheel drive

Front wheel drives typically use a transmission and transfer case. The transfer case mounts on the side or back of transmission. A chain or gear drive inside the transfer case takes power from the transmission and transfers it to two separate drive shafts. One drive shaft connects to the differential on front drive axle. The other drive shaft connects to differential on rear drive axle.

Working of Transmission

A transmission has three specific shafts: the input shaft, counter shaft, and main shaft. The so-called clutch gear is an integral part of the transmission’s input shaft and always rotates with input shaft. The input shaft itself is connected to the engine and takes power in a 1 on 1 proportion. It is by pressing the clutch pedal in our car, that we disconnect the input shaft from the engine so that we can change gears on the main shaft.

The counter shaft are actually several gears machined out of a single piece of steel. The counter shaft may also be called counter gear or cluster gear.

The main gears on main shaft (also known as the output shaft) transfer rotation from counter gears to the main shaft / output shaft.

The main gears are also called speed gears. They are mounted on the output shaft using roller bearings. Speed gears freewheel around the output shaft until they are locked to it by their shift synchronizer unit. It is this locking which transfers power to the output shaft.

The flow of power and motion is as follows.

Power flows from the transmission input shaft to the clutch gear. The clutch gear meshes with the large counter gear of the counter gear cluster. This cluster gear is now rotating. Since cluster gear is meshed with speed gears on main shaft, the speed gears are now also turning.

There can be no power output until one of the speed gears is locked to the main shaft. This is done by shifting the fork through the gear lever which moves the synchronizers to engage the selected speed gear to the main shaft. Power travels along the counter gear until it reaches this selected speed gear. It then passes through this gear back to main shaft and out of the transmission to the drive line. Indeed, when you use the clutch of the car and shift gears, you are locking different speed gears on the counter shaft to the corresponding gear on the main shaft. Here, the reason for pressing the clutch pedal is to ensure that there is no power transferred to the input shaft, so that we can change gears easily.

The main shaft connects to the differential. The differential divides power to the relevant wheels on the left and right side of the vehicle. This power is different across wheels – for example, when you are taking a turn, the outer wheels needs to turn more than the inner wheel and hence needs more power.

Types of transmission mechanisms

There are mainly two types of transmissions, Manual and Automatic transmission. In this article we have dealt with manual transmission, where the driver needs to operate the transmission system himself.

Most manual transmissions and transaxles are constant mesh, fully synchronized units.

Constant mesh means that whether or not one of the gears is locked to the output shaft, it is in mesh with the counter gear. All gears rotate in the transmission as long as the clutch is engaged.

Fully synchronized means that the unit brings the rotating shafts and gears to the same speed before shift occurs. This promotes smooth shifting. In a vehicle equipped with a four speed manual shift transmission, all four forward gears are synchronized. Reverse gear may or may not be synchronized, depending on transmission/transaxle.

Classification of manual transmissions

Transmissions can be divided into groupings based on the number of forward speed gears they have. In the past three and four speed were common. However with growth in technology and necessity for fuel economy, additional gear systems such as five speed and six speed gears have been introduced. In addition, overdrives have been introduced. Function of the overdrive is to reduce engine speed at given vehicle speed, which increases top speed, improves fuel economy, and lowers engine noise level.

Functioning of different gear engagements in a four speed transmission

Neutral

The input shaft rotates at engine speed whenever clutch is engaged. The clutch gear is mounted on the input shaft and rotates with it. The clutch gear meshes with the counter gear, which rotates around the countershaft.

The counter gear transfers power to the speed gear on the main shaft. However since speed gears one, two, three and four are not locked to the main shaft when transmission is in neutral, they cannot transfer power to the main shaft. The main shaft does not turn, and there is no power output to the driveline.

First gear

The power or torque flows through the input shaft and clutch gear to the counter gear. The counter gear rotates. The first gear on cluster drives the first speed gear on main shaft. A typical first speed gear ratio is 3:1 (three full turns of input shaft to one full turn of output shaft). So, if engine torque entering the transmission is 298NM, it is multiplied three times to 894 NM by the time it is transferred to driveline.

For reverse gear also similar gear ratio is used.

It is to be noted that though torque is multiplied three times, the speed is reduced accordingly. Hence first gear is preferred only for moving the vehicle from stationary condition, generally limited up to 10KMPH

Second gear

When shift from first to second gear is made, the shift fork disengages the first/second synchronizer from the first speed gear and moves until it locks the second gear to main shaft. Power flows from the second gear through the synchronizer to the main shaft (output shaft).

In second gear, the need for vehicle speed and acceleration is greater than the need for maximum torque multiplication. To meet these needs, the second speed gear on the main shaft is designed slightly smaller than the first speed gear. This results in typical gear ratio of 2.2:1 which reflects a drop in torque and an increase in speed.

Third Gear

When the shift from second to third gear is made, the shift fork returns the first/second synchronizer to neutral position. A second shift fork slides the third/fourth synchronizer until it locks the third speed gear to the main shaft. Power flow now goes through the third gear through synchronizer to main shaft and driveline.

Third gear permits a further decrease in torque and increase in speed. The third speed gear is smaller than second speed gear. This results in a typical gear ratio of 1.7:1

Fourth gear

In fourth gear, the third/fourth synchronizer is moved to lock the clutch gear on the input shaft to the main shaft. This means power flow is directly from input shaft to main shaft at gear ratio of 1:1. This results in maximum speed output and no torque multiplication.

Fourth gear has no torque multiplication because it is used at cruising speed to promote maximum fuel economy.

Reverse gear

On reverse gear, it is necessary to reverse the direction of main shaft. This is done by introducing reverse idler gear into the power flow path. The idler gear is located between the countershaft reverse gear and the reverse speed on the main shaft.

Functioning in Transaxle

Like transmissions, some transaxles have five forward speeds. Usually, fourth and fifth gear for smaller cars have overdrive ratios. These high gear ratios compensate for very low final drive gear ratios. Low final drive ratios provide great torque multiplication, which is needed to safely accelerate with small engine.

Final Drive

All vehicles use a differential to provide an additional gear reduction (torque increase) above and beyond what the transmission or transaxle gearing can produce. This is known as final drive gear. In a transmission equipped vehicle, differential gearing is located in rear axle housing. In transaxle however, the final reduction is produced by the final drive gear housed in the transaxle case.

Trouble shooting

Some of the troubles that can arise with usage of transmission are listed below. It is to be noted that most of the troubles with transmissions needs to be inspected by mechanics or people having relevant experience.

  Trouble Possible Cause Remedy
1 Gear clash when shifting from one gear to other Clutch adjustment incorrect, Adjust clutch
Clutch linkage or cable binding Lubricate or repair as necessary
clutch housing mis alignment Needs inspection by mechanic
Lubricant level low Needs inspection by mechanic
Gearshift components or synchroniser blocking rings worn or damaged Needs inspection by mechanic
2 Gear shifting hard Clutch adjustment incorrect, Adjust clutch
Clutch linkage or cable binding Lubricate or repair as necessary
Shift rail binding Needs inspection by mechanic
Internal binding caused by shift forks, selector plates or synchroniser assemblies Needs inspection by mechanic
clutch housing mis-alignment Needs inspection by mechanic
Incorrect lubricant Drain and refill
3 Locking in one gear- ie . cannot be shifted out of gear Shift rail broken or worn, fork bent , etc Needs inspection by mechanic
Broken gear teeth Needs inspection by mechanic
Gearshift lever broken Needs inspection by mechanic

Conclusions

Above are the brief working principles of Manual transmissions. It is aimed at providing information on how a transmission works so that users would be familiar with working of the components and importance of the same. Transmissions are pure physics in motion and though technical in nature, can provide very good insight in how a car actually works.

The above is an attempt to explain a complicated piece of engineering. Do share your questions and insights with us.

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