THE
AUTOMATIC TRANSMISSION
Fa
GENERAL DESCRIPTION
The automatic transmission incorporates a three-
element fluid torque converter with a maximum torque
conversion ratio
of
2 : 1 coupled
to
a bevel gear train
which provides four forward gears and reverse.
Engine power is transmitted from the crankshaft
converter output gear through
an
idler gear
to
the input
gear which drives the bevel reduction gears
in
the gear
train assembly.
The final drive is transmitted from a drive gear
to
a
conventional-type differential unit (similar
to
that fitted
to
a synchromesh transmission unit), which in turn
transmits engine power through two flange-type coupling
drive shafts employing constant velocity joints
to
the
road wheels.
The complete gear train assembly, including the
reduction gear and differential units, runs parallel to,
and below, the crankshaft and is housed in the trans-
mission casing which serves also as the engine sump.
The system
is
controlled by a selector lever within
a gated quadrant marked with seven positions, and
mounted centrally on the floor
of
the car. The reverse,
neutral, and drive positions are for normal automatic
driving, with the first, second, third,
and
fourth positions
used for manual operation
or
over-ride as required.
This allows the system to be used as a fully automatic
four-speed transmission, from rest
to
maximum speed
with the gears changing automatically according
to
throttle position and load.
If
a lower gear is required
to
obtain greater acceleration,
an
instant full throttle
position,
Le.
'kick-down' on the accelerator, immediately
produces the change.
Complete manual control
of
all four forward gears by
use
of
the selector lever provides rapid changes. However,
it is very important that downward changes are effected
at
the correct road speeds otherwise serious damage may
result
to
the automatic transmission unit. The second,
third, and top gears provide engine braking whether
driving automatic
or
manual; in first gear a free-wheel
condition exists when decelerating. Manual selection to
third
or
second gear gives engine braking and also allows
the driver to stay in a particular lower gear
to
suit road
conditions
or
when descending steep hills.
The hydraulic system
Oil is drawn
fro~
the transmission casing through the
main gauze strainer and pick-up pipe by the main oil
pump which has a high potential output
and
serves both
the engine lubrication and transmission systems with a
common oil supply. The oil passes through drillings in
the cylinder block
and
a pipe to the external full-flow
filter and thence to the valve block.
The valve block assembly controls pressures to the
transmission. Separate valves control the converter
and
engine pressures. The oil passes from the valve block
through a long connecting pipe
to
the converter stator
unit. Three short interconnecting pipes take the necessary
line pressure to each
of
the servos which control brake
band' operation in manual
and
automatic selector
positions.
Fa.2
The power flow through the bevel reductions gear is
coupled
to
the final drive gear pinion by means
of
two
multi-disc clutch assemblies operated hydraulically in
manual
and
the automatic selector positions.
In
the event
of
tow-starting the engine,
an
auxiliary
oil pump
of
low capacity is employed which is responsive
to
vehicle speed only; immediately the engine starts the
main pump automatically takes over.
The governor system
The governor is driven by the auxiliary pump gears
and
is
of
the spring-loaded mechanical type, with its bob-
weights mounted on short links. A rod linkage transfers
the movement
to
the governor valve incorporated
in
the
valve block chest.
A spring-loaded rod connected
to
the carburetter
provides
an
over-ride device. The spring tends
to
be
compressed by accelerator pedal
operation~
and
transfers
this load by levers
to
the governor. The effect is
to
delay
travel
of
the governor, which
in
turn
delays gear shifts
more as the accelerator is depressed.
The torque converter
This
is
fitted onto a taper
on
the rear
of
the crankshaft.
Basically it comprises three elements,
Le.
an
impeller, a
turbine, and a stator,
but
it is only serviced as a unit.
There is a continuous supply
of
oil circulating through
the unit; this assists in dissipating the heat generated,
and the out-flow passes through a low-pressure valve
which maintains a 30 lb.fsq. in.
(2,1 kg.fcm.
l
)
pressure
within the converter to improve efficiency.
The brake bands and servos
Three brake bands are used. One is for reverse
and
the
others provide second
and
third speed reactions. The
clamping load is applied by three hydraulic servos
in
a
common casing.
The multi-disc clutches
For
forward motion a single-piston multi-disc clutch
carries the drive
and
is engaged
at
all times during
forward motion
of
the car. This forward clutch unit
is
fitted on one side
of
the final drive pinion
and
on
the
other side is a top
and
reverse clutch assembly which has
a tandem piston arrangement. This feature is necessary
because the clutch is also engaged for reverse and since
a greater torque capacity
is
required in this case both
pistons are pressurized.
The valve block
This assembly
is
constructed
of
three sections,
Le.
the
lid, valve chest, and pipe chest.
A linkage arrangement locates the selector valve
and
this in turn is controlled by the selector rod, externally
connected by cable
to
the gear selector lever
in
the car.
The function
of
the various valves is as follows:
The
selector
valve,
directs oil from the main supply to
either the governor valve for automatic gear-shifting
or
alternatively
to
the appropriate clutch
or
servo for
manual selection.
MINI. Issue
1.
4908
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mk1-performance-conversions.co.uk
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