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AUTOMOBILE RUNNING SYSTEM AND POWER TRAIN 8. TRANSMISSION GEARBOX

In the last lesson we studied the clutch. The clutch, as we saw, normally remains in the engaged position and transmits engine power to the next component in the transmission system, the transmission gearbox. The gearbox then has the function to transform this power according to requirement i.e. either give more torque at low speed or decrease the torque and increase the speed at the output. These days we have gearboxes having gear ratio lower than 1 i.e. the rpm at the output of the gearbox is greater than the input i.e. the rpm of the engine. Such is called the overdrive and helps the car to move at very high speed. Gearbox also serves to drive the car in reverse.

8.0

Objectives After going through this lesson, you will be able to:

i. Describe a gear and recognize its various types. ii. Differentiate between the various types of gearboxes and give the advantage of the synchromesh gearbox. iii. Describe the working of the synchronizer. iv. Describe the working of the gear shift mechanism.

8.1

Introduction

The gearbox is placed in the transmission system after the clutch. It enables the vehicle to be driven at different speeds and at the same time providing differing torque values. When we start the car from rest, we need greater torque to make the car start to move. At this time we use the first gear. Next we change into the second, then third gear and so on. Also, in the city a vehicle is driven at a slower speed than on a highway. All this is achieved through the gearbox. The gearbox is also used to reverse the car. In the reverse gear the engine continues to rotate in the same direction but the car moves backward.

Self-check Questions 1. Give the functions of the gearbox 2. What is overdrive? 3. When is greatest torque required?


4. In which gear is the car moved at high speed? 5. Car is moved in reverse due to the engine rotating in reverse direction. State whether True/False.

8.2

Gears

It is important to understand first what a gear is. A gear is a cylindrical disc on which teeth, of a special profile, are cut on the circumference. The gearbox works on the principle that when two rotating gears are in mesh, the smaller gear rotates at a higher speed than the larger gear. The larger gear, on the other hand, exerts more leverage or torque. There are different types of gears that are used in a vehicle. Some of the commonly used types are: spur gear, helical gear, herringbone gear, bevel gear, worm and wheel, and rack and pinion.

8.3

Types of Gearboxes

The different types of gearboxes are the sliding mesh gearbox which is now obsolete and the constant mesh gearbox which is used in two-wheelers. The gearbox used in the Maruti 800 car is a synchromesh gearbox.

Fig. 8.1: Sliding mesh gearbox.

Fig. 8.2: Constant mesh gearbox.

Fig. 8.3: Synchromesh gearbox used in Maruti 800 car.


8.4

Synchronizer

Synchronizing means equalization of the speeds. A synchronizing device, or synchronizer, matches the speeds of the dog clutch and a gear on the main shaft so that their engagement is smooth. Dogs are teeth cut on the face of the gear. In a synchromesh gearbox, all the driver gears on one shaft and the driven gears on the output shaft are always in mesh and gears on the output shaft rotate on bushes, independent of the shaft. Depending upon the speed required, we select the appropriate gear and mesh the dog teeth of the synchronizer sleeve with those on the face of the gear. This then locks the gear with the output shaft dog clutch and drives the output shaft at the speed of that particular gear. This is because the synchronizer sleeve is mounted on the main shaft on splines and power flows through the dog clutch to the output shaft. Fig. 8.3: Synchronizing device.

8.5

Gearshift Mechanism

The figure shows the gear selector lever and gear selector mechanism, with the help of which gears are shifted. In order to shift gear, the clutch pedal is pressed and the gear selector lever shifted to the desired high or low gear and then the clutch pedal is released.


Fig. 8.4: Gear change mechanism (a) Engaged. (b) Disengaged.

Self-check Questions 6. 7. 8. 9. 10. 11. 12.

What is a gear? Out of the 2 gears which provides greater torque and which greater speed? What happens to speed when the torque increase? ______________ gearbox is used in two-wheelers ______________ gearbox is used in the Maruti 800 car Synchronizer is used to _____________________________ In a constant mesh and synchromesh gearbox all gears are always in mesh, so how do we get different gear ratios?

8.6

Assignments

8.6.1 Class Assignment 1. Discuss different uses of gears. 8.6.2 Home Assignment 1. See the gears in a sugarcane juice extracting machine and note the size of gears. Compute the gear ratio.

8.7

Summing Up

We have different torque and speed requirements while driving a car. Engine power remaining constant, the gearbox provides this variation. It also helps move the car in reverse direction even with the engine crankshaft rotating in the same direction.


There are various types of gearboxes. The sliding mesh gearbox is now obsolete. Constant mesh gearbox is used in 2-wheelers while the constant mesh gearbox is used in 4-wheelers. Synchromesh gearbox works best because it synchronizes (make same), speeds of the gear to be meshed and the dog, which is splined to the output shaft, before engagement. In order to shift gears we first have to disengage the power coming from the engine to prevent clashing. This work is done by the clutch.

8.8

Possible Answers to Self-check Questions

1. 2. 3. 4. 5. 6.

Provide torque or speed as desired and move the car in reverse direction gear ratio where the driving gear turns at a speed slower than the driven gear when moving the car from rest top gear False gear is a cylindrical disc on which teeth, of a special profile, are cut on the circumference 7. larger gear, smaller gear 8. speed decreases 9. constant mesh gearbox 10. synchromesh gearbox 11. synchronize (make same) speeds of 2 members before meshing them 12. gears rotate freely on bearings on the output shaft and the dseired gear is locked to the output shaft by dog clutch

8.9

Terminal Questions

1. What is a gear? List 5 uses of gears that you know or have seen. 2. Name the different types of gearboxes and their main difference. Also state where each is used. Which is considered best? What is its advantage? 3. Describe the construction and working of synchronizer. What does it do? 4. Explain the procedure of gear shifting.

8.10

References and Suggested Further Reading

1. AA BOOK OF THE CAR. Drive Publications Ltd, London. 2. Maruti 800 Service Manual 3. http://www.howstuffworks.com

8.11

Glossary

Bevel gears

Gears whose axes intersect at 90O.


Constant mesh gearbox

Gearbox in which gears are free to move on bearings on the main shaft and are locked to the shaft through the dogs when a shift is completed.

Dog clutch

Gear having teeth on the face.

Gear

Toothed wheel with teeth of special design.

Gear ratio

Ratio of speed of driver to speed of driven gear.

Helical gear

Gear having teeth which are inclines as a helix.

Herringbone gear

Two helical gears joined together as a pair.

Overdrive

Gear ratio where the driving gear turns at a speed slower than the driven gear.

Rack and pinion

Rack is a straight shaft with teeth with which a small gear called pinion is meshed to convert rotary motion to linear.

RPM

Revolutions per minute.

Sliding mesh gearbox

Gearbox in which gears are shifted to mesh or de-mesh.

Splines

Teeth used to mate 2 parts.

Spur gear

Gear having straight teeth.

Synchromesh gearbox

Gearbox in which gears are free to move on bearings on the main shaft; synchronizer hub and gear are locked to the shaft through the synchronizer sleeve when a shift is completed.

Synchronizer

A system of hub, ring, and sleeve which synchronizes (makes same) the speeds of 2 gears before meshing them.

Torque

Rotary force.

Transmission gearbox

A set of gears which transform engine power to either greater torque or speed.


Worm and wheel

Shaft having screw threads and which meshes with a wheel to provide large gear ratio.


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