gearbox presntation

1. Transmission System
The mechanism which converts the engine output
into different speed and power combinations at the
road wheels
There are many different types of transmission.
For driving licence purposes they are only classed
as Manual or Automatic
Pictures produced with kind permission
from Volvo Bus Sweden – copyright applies

2. In the transmission diagram different size gear ratios join
together to enable the road wheels to turn at a wide variety of
speeds within the engine limits.

3. Gear Ratios
In the picture to the right,
suppose the smaller gear has 13
teeth
The second, larger gear has 26
teeth.
The gear ratio is therefore 13/26
or 1/2 (written as a ratio of 1:2)
For every revolution of the
smallest gear the largest gear
has turned much slower –
only ½ a revolution

4. Typical Example
Gear Gearbox
Ratio
Road speed at 1000
revolutions of the engine
First – lots of engine power
but not much speed
2.97:1 7.7 mph
Second 2.07:1 11.0 mph
Third 1.43:1 15.9 mph
Fourth – here the ratios are
1.00:1 22.7 mph
the same
Fifth – more speed but not as
much engine power
0.84:1 27.1 mph
The gear selector is used to change gear ratios
Warning! - Because low gears have more power
but not much speed it is easy to lose traction

5. In easy terms – you can accelerate quickly in a low gear but you
can’t attain a high speed
You can reach a high speed in a high gear but you can’t
accelerate as quickly

7. There are many different types of gearbox and gear
selectors
– In general most buses and many coaches have some form
of automatic gears
– Often, this modern type of transmission senses and
selects the appropriate gear according to road speed and
the load on the engine
– The driver is usually able to override the system by use of
the gear selector or ‘kickdown’

8. ‘Kickdown’
– By depressing the accelerator fully down this provides for
quicker acceleration and:
– It may also result in the vehicle automatically changing
down to a lower gear
– The engine may rev higher than normal before changing
gear

9. Selecting drive
– Application of the footbrake before selecting drive is a very
important safety feature for any automatic transmission
– It ensures foot placement is correct before releasing the
handbrake
– Some transmissions will not allow drive to be selected
unless the footbrake is applied and the handbrake is on

10. Using the handbrake
– Fully applying the handbrake whenever you are stationary
is important on an automatic vehicle
– Automatic vehicles may move forwards or backwards in
gear with the handbrake off – this is called ‘creeping’
– Secure the vehicle and select NEUTRAL when stationary
for a long period to avoid danger and damage. Never leave
the vehicle in gear with the engine running.

11. Manual selection
– This gives the same flexibility as a manual gearbox
– It is especially useful for uphill or downhill gradients
– It also allows you to hold the vehicle in the same gear
– On downhill gradients selecting a low gear will assist
braking, especially if the vehicle has a retarder fitted.

12. Driving safely
– You should know how to change gear
– When to select gears
– Which gear to select
– Some coaches have high and low ratio transmissions
which can total up to 16 different gears!

13. How to change gear
– Consult the vehicle manufacturers handbook
– Seek advice from a qualified trainer
– There are many different types of gearbox that will require
further training

14. How to change gear
• Engage the clutch smoothly when moving away and changing
gear
• Select the appropriate gear
• Avoiding taking your eyes off the road when you change gear
• Ensure your foot does not remain on the clutch pedal when
you are not using it

15. When to change gear
– Steep downhill gradients may require the selection of a
lower gear. This will increase engine speed but reduce the
vehicle speed
– Modern buses and coaches have a braking retarder.
Efficiency may be improved when the engine speed is
increased
– The engine braking effect and retarder will avoid excessive
use and overheating of the service brake

16. Which gear to select
– Select the correct gear(s) to combine safety, performance
and economy
– Avoid maximum acceleration through the gears where
possible. This may compromise safety, comfort and cause
excessive wear of mechanical components
– Reduce speed safely before selecting lower gears

17. Which gear to select
• Change gear in good time before a hazard or junction
• Choose the most appropriate gear for your speed and the
road conditions
• Avoid holding on to the gear lever unnecessarily
• Avoid ‘coasting’ with the clutch pedal depressed or the gear
lever in neutral. It is illegal

21. Understanding how the gears work will help you to
get the most out of them.
You should be aware of the manufacturers advice
for the particular vehicle that you drive
Don’t forget to check your driving licence!
Many drivers are restricted to driving automatic
passenger carrying vehicles only

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