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MOTOR TRADE THEORY N3
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Text book: Motor & Diesel Trade Theory by HJ Botha
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MODULE 7: EPICYCLIC GEAR TRAINS & AUTOMATIC
GEARBOXES
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MODULE 7: EPICYCLIC GEAR TRAINS
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• Epicyclic gear formation:
All gears are constantly in mesh with no engaging or disengaging of gears.
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The main parts are:
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o Annulus - solid round with internal gear teeth.
o Planetary gears - kept in place by planet carrier/cage and turn inside planetary gear.
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o Sun gear or drive gear - it turns the planetary gears that will then turn the annulus
o Planet carrier/Cage – All planetary gears revolve around the pins which are mounted on
the carrier. Please see the side and sectional views at page 7 – 1 of module 7.
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Gear trains are used in semi and fully automatic gearboxes.
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MODULE 7: EPICYCLIC GEAR TRAINS continue……
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• Please learn the advantages and disadvantages of epicyclic gears over
conventional gears.
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• Gear Ratios: Selection of different gear ratios is achieved by braking/locking one
element and making the other element to drive. Gear reduction is achieved by
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braking the annulus by means of a brake band and making the sun gear to drive
resulting in output shaft turning at slow speed.
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• Reverse, direct drive, free wheel, overdrive and different gear ratios/speeds can
be learned from page 7 - 2 till 7 -5.
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MODULE 7: AUTOMATIC GEARBOX
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• Automatic gearbox consist of the following main components/parts :
o Torgue converter: situated in front of the automatic gearbox and mounted on the flywheel of the
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vehicle engine.
o Double epicyclic gear set
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o Brake bands – front and rear brake bands
o Cluthes – front and rear clutches
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o The hub of the front clutch connected to small sun gear
o Hydraulic pump/Oil pump
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o The input shaft of the gearbox driven by the torque converter
These parts are clearly explained at page 7 – 5 of module 7.
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MODULE 7: AUTOMATIC GEARBOX main components continue……
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o The main shaft in the middle of the gearbox
o Sun gear – small and big sun gear
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o The small planetary gears
o The annulus
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o The output shaft which form part with the annulus and will drive the propeller shaft of the
vehicle.
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o Governor valve
o Shift valve – connected to accelerator pedal
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o Valve body
These parts are clearly explained at page 7 – 5 of module 7.
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MODULE 7: AUTOMATIC GEARBOX continue……
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• Please practice the schematic drawing of a fully automatic gearbox at page 7 -6 of
module 7. You will now see the position of the front and rear clutches, front and
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rear brake bands, small and large sun gears, small and large planetary gears,
planetary carrier/cage, annulus, input shaft and output shaft. Look at the main
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shaft in the middle of the gearbox and it is free in both front and rear end, is free
to turn inside the bushes.
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• Know the main systems of the automatic gearbox:
o Torque converter
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o Epicyclic planetary gear train
o Hydraulic control units: gear lever, accelerator pedal, governor
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o Oil pump
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MODULE 7: AUTOMATIC GEARBOX ……
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• Please summarise the operation/working of automatic gearbox to achieve neutral, first gear, second
gear, third gear, reverse gear and park position at page 7 - 6 till 7 – 8 of module 7.
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• You must know the advantages and disadvantages of automatic gearbox over the conventional
gearbox(manual gearbox)
• Know all the control systems of an automatic gearbox (page 7 - 9 till 7 – 13):
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o The oil pump
o Manual valve
o Primary oil pressure regulator
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o Secondary oil pressure regulator
o Throttle valve/modulator
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o Shift valve
o Valve body
o Governor valve
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o Compensator valve
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MODULE 7: AUTOMATIC GEARBOX ……
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Self assessment:
• Answer the following questions:
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1. Neatly draw a sketch of an epicyclic gear train label the four main parts – sun gear, planet
carrier, planetary gears and annulus.
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2. State four advantages of epicyclic gear train over conventional gear train
3. What is the function of a one way clutch/free-wheel?
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4. Explain how a reverse gear is obtained in an epicyclic gear system
5. Mention three main systems in an automatic gearbox.
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6. Explain how first gear is obtained in an automatic gearbox
7. List eight components of an automatic gearbox
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8. List and briefly describe five control systems of an automatic gearbox.
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MODULE 7: AUTOMATIC GEARBOX ……
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Self assessment continue…..
9. Draw a schematic view of a fully automatic gearbox and label the 10 main parts.
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10. Draw a full view to show the control systems of an automatic gearbox.
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11. What is the purpose of control systems in an automatic gearbox?
12. Draw sketch of an automatic oil pump and label the main parts.
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Good luck!!!
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