Lab Manual | constructional details, working principles and operation of the Automotive Transmission Systems

Aim

To study and prepare report on the constructional details, working principles and operation of the following Automotive Transmission Systems:

a) Sliding Mesh

b) Constant Mesh

c) Synchromesh – four speed range

Sliding Mesh type Gear Box

This is the simplest type of gear box (Fig 3). The power is transferred from the engine to the clutch shaft and then to the clutch gear which is always in mesh with a gear on the layshaft. All the gears on the lay shaft are fixed and as such they are all the time rotating when the engine is running and the clutch is engaged. Three direct and one reverse speeds are attained by suitably moving the gear on the main shaft by means of selector mechanism.

Constant Mesh Gear Box

In this type of gear box, all the gears are in constant mesh with the corresponding gears on the layshaft. The gears on the main shaft which is splined, are free. The dog clutches are provided which slide on the main shaft. The gears on the layshaft are, however, fixed.

When the left dog clutch is slided to the left by means of the selector mechanism, its teeth of the gear are engaged with the clutch gear. The same dog clutch, however, when slide to right makes contact with the second gear and second gear ratio is obtained. Similarly movement of the right dog clutch to the left results in low gear and towards right in reverse gear.

Synchromesh Gear Box

This type of gear box is similar to the constant mesh type. In this all the gears on the mainshaft are in constant mesh with the corresponding gears on the layshaft. The gears on the layshaft are fixed to it while those on the mainshaft are free to rotate on the same shaft. Its working is also similar to the constant mesh type, but in the former there is one definite improvement over the latter. This is the provision of synchromesh device which avoids the necessity of double declutching. The parts which ultimately are required to be engaged, are first brought into frictional contact which equalizes their speed, after which these may be engaged smoothly.

Fig shows the construction and working of a synchromesh gear box. In most of the cars, however, the synchromesh devices are not fitted to all the gears. They are only on the high speed gears but on the low speed and reverse gears, ordinary dog clutches are only provided. This is done to reduce the cost.

In other words, the gear wheels which are to be positively connected are first brought into frictional contact and when the friction has equalized their speeds, the positive connections are made. Synchromesh devices can be applied to the sliding mesh gear box but they are universally used with constant mesh gear boxes used in different motor vehicles. The main features of this gear box are:

a) The output gears are free to rotate on bushes on the output shaft. They are internally located by splined thrust bearings. Single or double helical gears remain in constant mesh with the lay shaft gears.

b) The output gears are locked to their shaft by the dog clutch

c) Change of the synchronizing hub takes place when its speed equalizes or synchronizes by theirs cones.

In case this gear is not used, it is left to the skill of the driver to bring one or more gears for meshing. By the skillful use of this clutch and accelerator pedal, he can bring the gears approximately the same speed. Even for the unskilled driver it is not difficult to affect quite satisfactory the change of gears. He can do it simply by using the clutch pedal gear lever with the help of successful commercial gear synchronizers.

Reference Books:

1 Automotive Mechanics – Crouse/Anglin

2 Automobile Engineering by Dr Kirpal Singh

3 Automobile Engineering by Pitman

Viva Questions

1 Why gear box is used in a vehicle

2 What is a synchromesh device?

3 Explain clearly the necessity of a transmission in a vehicle

4 Discuss the advantages of a constant mesh gear box over the sliding mesh type gear box

5 Describe the working of a synchromesh gear box with the help of a neat sketch

6 What is transfer box ? where it is used ?

7 What are the merits and demerits of synchromesh gear box compared to a sliding mesh on constant mesh type

8 Explain the common troubles encountered in gear boxes and suggest suitable remedies

9 What is a synchronizer

10 Why is synchromesh device usually not employed for the reverse gear

11 What may be the possible reason when the gear slips out of engagement

12 What should be the cause when the vehicle is running with excessive noise in the gear box