Planetary Gearbox

The planetary gearbox, including the overdrive unit (4th gear), consists of three planetary gear trains, three clutches, three freewheels and tour brakes.

Components, 1st-4th Gears:

Four forward speeds and reverse are obtained by engaging the elements of the planetary trains in different combinations.

This is achieved by engaging and disengaging the hydraulically operated clutches and brakes.

The method of obtaining the different speeds is described in the following section "Power flow through gearbox".

Power Flow Through Gearbox
Since 4th gear (overdrive) remains in the direct-drive mode in all gears other than 4th itself, the power flow through the overdrive unit is described only under "4th gear" below.

However, the flow is shown in each of the other cases to illustrate the flow through the gearbox as a whole.

Power Transmition In Different Gears:

The following components are used to transmit power in the different gears:

B Brake
C Clutch
F Freewheel
P Planetary train
I Input shaft
II Intermediate shaft
III Sun wheel shaft
IV Output shaft

1st Gear, Selector In Position:

Both planetary trains (P1 and P2) transmit driving power, dividing the torque between them.

Front clutch C1 is engaged and transmits power from the input shaft, through the intermediate shaft, to the rear ring gear.

The rear ring gear forces the rear planets to rotate about their own axes, causing the planet carrier (which is rigidly attached to the output shaft) to rotate clockwise.

The rear planets simultaneously drive the sun wheel shaft counterclockwise at a speed determined by the front planetary train.

The front planet carrier is prevented from rotating counterclockwise by freewheel F2, causing the sun wheel to drive the front ring gear through its planets.

The front ring gear which (like the rear planet carrier) is rigidly attached to the output shaft, rotates clockwise.

Thus, power is transmitted in common by both front and rear trains at a ratio of 2.81:1 in the AW72L, and 2.45:1 in the AW70 and AW71.

Selector In Position 1
The difference between this and position D is that brake B3 is also applied, preventing the front planet carrier from rotating. The action of the brake means that torque is also transmitted from the output to the input shaft. In other words, engine braking is available in position 1.

2nd Gear, Selector In Position D:

Only rear planetary train P2 transmits power, the front train rotating unloaded.

Front clutch C1 is engaged and transmits power from the input shaft, through the intermediate shaft, to the rear ring gear.

The resultant tendency of the rear planets to drive the sun wheel shaft counterclockwise is prevented by freewheel F1. (The outer hub of the freewheel is locked by brake B2.)

The rear ring gear instead forces the planets to orbit the sun wheel, causing the planet carrier also to rotate at a reduced speed.

Since the rear planet carrier and output shaft are rigidly attached to each other, the output shaft will rotate at a reduced speed compared with the input shaft. The speed ratios are 1.49:1 in the AW72L and 1.45:1 in the AW70/AW71.

Selector In Position 2
The difference compared with D is that brake B1 is also applied to permit engine braking in 2nd. In this case, torque may also be transmitted from the output to the input shaft.

3rd Gear:

Both the front and rear clutches, C1 and C2, are engaged. The complete planetary assembly rotates as a unit with no difference in reduction ratio or torque (Ratio 1:1).

NOTE: Brake B2 is also applied in 3rd to facilitate downshifting from 3rd to 2nd. (If brake B2 - which is used in 2nd - is already applied, downshifting from 3rd to 2nd will be faster). As already described, brake B2 does not brake the sun wheel shaft, but only the outer hub of freewheel F1, enabling the sun wheel to rotate freely in 3rd. (In 2nd, on the other hand, the sun wheel is locked since it would otherwise rotate in the opposite direction.)

4th Gear (3rd + overdrive):

Fourth gear (overdrive), which is engaged automatically from 3rd, lowers the reduction ratio from 1:1 to 0.73:1 in the AW72L and from 1:1 to 0.69:1 in the AW70/AW71.

Brake B0 is applied on engagement, locking the sun wheel in the overdrive unit.

The overdrive unit ring gear then rotates at a higher speed than the engine. This speed is transmitted to clutch C1 in the gearbox, from whence the power is transmitted further to the output shaft as in 3rd gear (see above).

Overdrive may also be disengaged completely, permitting the gearbox to be used in 1st, 2nd and 3rd gears only. Disengagement is performed by means of a switch on the gear selector lever, a function described under "Control system, manual downshift valve 4-3".

Reverse:

In reverse gear, power is transmitted only by front planetary train P1.

Clutches C0 and C2 are locked, connecting the input shaft to the sun gear shaft. Brake B3 is applied, locking the front planet carrier.

As a result, the sun wheel shaft drives the front planets counterclockwise. These, in turn, drive the front ring gear (which is rigidly connected to the output shaft) counterclockwise.

Neutral And Parking Positions
Clutch C0 only is engaged "ready" for starting (direct-drive mode).

All brakes are released, except in the parking position, in which brake B3 is applied while the engine is running.