362 Machine Drawing
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d:\N-Design\Des19-1.pm5
19.2.6 Automobile Gear Box
19.2.6.1 Introduction
The top speed of a car depends upon the maximum power of its engine, and this is developed near
the engine’s maximum power. A typical car engine may run at 4000 rpm for a top speed of 110
kmh. But road wheels of average size turn at only about 1000 rpm to cover 100 km in an hour. So,
they cannot be connected directly to the engine. There must be a system which allows the road
wheels to make one revolution for every four of the engine. This is done by a reduction gear in the
final drive (differential).
The relation between the rotational speed of the engine and the wheels is the axle ratio; 4:1
being common. As long as the car is driven at a steady speed on the level, this gearing is sufficient
but when the car meets a hill, its speed will drop and the engine will falter and stall. A slow-
running engine cannot provide enough torque for climbing hills or starting from rest. Selecting a
lower gear enables the engine to run faster in relation to the road wheels and also multiplies the
torque.
19.2.6.2 Gear Ratios
The lowest gear in the gear box must multiply the engine torque sufficiently to start the fully
laden car moving up a steep hill. A small car needs a lower gear ratio of 3.5:1. Other typical gear
ratios in a small car with a four-speed gear box are 2:1 in second, 1.4:1 in third and 1:1 in top
gear. All these are multiplied by the axle ratio, so that, if the axle ratio is 4:1, the corresponding
ratios between the engine speed and the road wheel speed are 14:1, 8:1, 5.6:1 and 4:1.
19.2.6.3 Transmission
The gear box shown in Fig. 19.6 is a constant-mesh type in which all the gear wheels cannot be
fixed to their shafts. There has to be a system which permits all the gear wheels except those
required for a particular ratio, to run freely. Usually, all the gear wheels on one shaft are fixed to
it and the wheels on the other shaft can revolve freely around their shaft until a ratio is selected.
Then, one of the free-running wheels is locked to the shaft, and that pair of wheels can transmit
power.
Transmission gears are made of high quality steel, carefully heat-treated to produce smooth,
hard surface gear teeth with a softer but very tough interior. They are usually drop-forged. The
teeth on transmission gears are of two principal types: spur and helical. The helical gear is
superior in that it turns more quietly and is stronger because more tooth area is in contact.
The locking of the gear wheels to a shaft is done by collars, which are splined to the shaft.
This method of fixing, allows the collar to revolve with the shaft and also slide along, to lock onto
the gear wheel on either side, or remain between them, allowing both to spin freely.
Around each collar, is a groove engaged by a two-pronged fork which is fixed to a sliding rod
mounted in the gear box housing. One, two or three of these selector rods are linked to the gear
lever. Moving the gear lever causes selector rod to slide to or fro. As it slides, the collar gripped by
the selector fork is slid along the shaft to engage with, or move away from, a gear.
19.2.6.4 Synchromesh for Smooth Gear Changes
In the simplest type of constant mesh gear box shown, the gears may be engaged simply by
shifting the gear lever from one position to the next as fast as possible. To do the job more quietly
and smoothly, the pair of dogs had to be allowed to reach the same speed, so that they would slide
together without clashing.
Drivers today are relieved from the need for double de-clutching for change of speeds by a
synchronising device built into the sliding collars in the gear box. This synchromesh device is
usually fitted to all forward gears. Synchromesh works like a friction clutch. It has a collar which
