Page 126 - Automotive Engineering

Transmissions and driveline CHAPTER 5.1

The detail of this process is covered in the paper re- 5th gear:
ferred to above.
Fifth gear on the LT77 is an ‘overdrive’ gear. This means
that the output shaft of the gearbox rotates faster than
5.1.3.4 Gear ratios – how they the input. The numbers work out as:
are achieved 37 teeth on layshaft/19 teeth on the mainshaft gear
Ratio – 1.947, i.e. mainshaft rotates at 1.947 times
Section 5.1.2 discussed why different ratios were needed quicker than the layshaft
and how they were selected. Given the overall design Combining the ratios gives 0.666 1.947 ¼ 1.297,
package of a transmission, the next task is to design the i.e. mainshaft/output shaft rotates at 1.297 times the
gear pairs within the casing to achieve the required ratios input speed
between the input and output shaft. The inverse of this is again normally quoted, so you
As an illustration of how the ratios in a gearbox are would see 5th gear quoted as being 1/1.297, which is
achieved, included below are the gear tooth numbers 0.771
used in a version of the Land Rover LT77 manual gear-
box. This was used in the Land Rover, Range Rover 5.1.3.5 The clutch
Classic and a number of other vehicles in both two- and
four-wheel-drive versions.
In vehicles there is a requirement for a device to provide
The constant gears: a coupling from the engine crankshaft to the trans-
mission. This allows the engine to be started and run
22 teeth on the input shaft/33 on the layshaft ‘con- without the vehicle moving and the vehicle to be started
stant’ gear (driven gear) from rest under control at various rates of acceleration. In
Ratio – 0.666, i.e. layshaft rotates at 0.666 of the manual gearboxes, the drive from the engine also has to
speed of the input shaft (slower) be disconnected during gear changes.
3rd gear: Clutches are associated with manual gearboxes and are
normally operated by the driver. Recent developments in
29 teeth on layshaft/27 teeth on the mainshaft gear both the passenger car and commercial truck market have
Ratio – 1.074, i.e. mainshaft rotates at 1.074 of the meant that an increasing number of vehicles are auto-
speed of the layshaft (quicker) mating the function of the clutch. The vast majority,
Combining the ratios gives 0.666 1.074 ¼ 0.715, however, remains controlled mechanically by the driver.
i.e. mainshaft/output shaft rotates at 0.715 of the The spring pressure clamps the pressure plate onto
input speed (slower) the driven plate and the flywheel; with the assembly like
The inverse of this is normally quoted to cause a bit this, the drive is passed from the engine to transmission
of confusion! So you would see 3rd gear quoted as (see Fig. 5.1-13). When the driver depresses the clutch
being 1/0.715, which is 1.397 pedal, the movement is passed to the release bearing by
either hydraulics or cable and the release bearing then
2nd gear:
pushes or pulls the diaphragm spring (depending on
19 teeth on layshaft/27 teeth on the mainshaft gear whether the clutch is a ‘push’ or ‘pull’ design), see
Ratio – 0.704, (slower) Fig. 5.1-14. The outer part of the release bearing is held
Combining the ratios gives 0.666 0.704 ¼ 0.469, (by the release lever) so it does not rotate, and the inner
i.e. mainshaft/output shaft rotates at 0.469 of the
input speed (slower) Pressure plate – rotates with
As with 3rd, by convention this is quoted as 2.132 – Engine flywheel and clutch cover and
clamps driven plate to flywheel
(1/0.469) flywheel
Conical diaphragm spring –
1st gear: provides clamp force
14 teeth on layshaft/31 teeth on the mainshaft gear
Gearbox input shaft
Ratio – 0.452, i.e. mainshaft rotates at 0.452 of the
speed of the layshaft (slower) Clutch driven plate –
Combining the ratios gives 0.666 0.452 ¼ 0.301, splined onto input shaft
i.e. mainshaft/output shaft rotates at 0.301 of the Friction surfaces
Clutch cover rotates
input speed (slower) with flywheel
As with 2nd and 3rd by convention this is quoted as
3.322 – (1/0.301) Fig. 5.1-13 Diagram of a conventional single-plate clutch.

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