56 Three speed and reverse transaxle automatic transmission mechanical power flow

(Gear train as adopted by some Austin-Rover, VW and Audi 1.6 litre cars)

The operating principle of the mechanical power or torque flow through a transaxle three speed automatic transmission in each gear ratio will now be considered in some depth, see Fig. 5.12.

The planetary gear train consists of two sun gears, two sets of pinion gears (three in each set), two sets of annular (internal) gears and pinion carriers which support the pinion gears on pins. Helical teeth are used throughout.

For all forward gears, power enters the gear train via the forward annular gear and leaves the gear train by the reverse annular gear. In reverse gear, power enters the gear train by the reverse sun gear and leaves the gear train via the reverse annular gear.

First gear compounds both the forward gear set and the reverse gear set to provide the necessary low gear reduction. Second gear only utilizes the forward planetary gear set to produce the intermediate gear reduction. Third gear is achieved by locking the forward planetary gear set so that a straight through drive is obtained. With planetary gear trains the gears are in constant mesh and gear ratio changes are effected by holding, releasing or rotating certain parts of the gear train by means of a one way clutch, two multiplate clutches, one multiplate brake and one band brake.

The operation of the automatic transmission gear train can best be explained by referring to Table 5.2 which shows which components are engaged in each manual valve selection position.

5.6.1 Selector lever (Table 5.2)

The selector lever has a number of positions marked P R N D 2 1 with definite functions as follows:

P — park When selected, there is no drive through the transmission. A mechanical lock actuated by a linkage merely causes a parking pawl to engage in the slots around a ring gear attached to the output shaft (Fig. 5.2). Thus the parking pawl

Table 5.2 Manual valve selection position

Forward Drive and First and Second One way clutch reverse clutch reverse brake gear band clutch

Applied Applied Applied Applied

Applied

Applied Applied

Applied

Applied

Applied

Fig. 5.12 Transaxle three speed automatic transmission layout

locks the output shaft to the transmission casing so that the vehicle cannot roll backwards or forwards. This pawl must not be engaged whilst the vehicle is moving. The engine may be started in this position.

R — reverse When selected, the output shaft from the automatic transmission is made to rotate in the opposite direction to produce a reverse gear drive.

The reverse position must only be selected when the vehicle is stationary. The engine will not start in reverse position.

N — neutral When selected, all clutches and band brake are disengaged so that there is no drive through the transmission. The engine may be started in N — neutral range.

Fig. 5.13(a-d) Three speed and reverse automatic transmission transaxle units

Fig. 5.13 contd

D — drive This position is used for all normal driving conditions, automatically producing 1-2, 2-3 upshifts and 3-2, 2-1 downshifts at suitable road speeds or according to the position of the accelerator pedal. The engine will not start in D — drive range.

2 — First and second This position is selected when it is desired to restrict gear changes automatically from 1-2 upshift and 2-1 downshifts only. The selector must not be positioned in 2 range above 100 km/h (70mph). The engine will not start in this range position.

1 — First gear When this range is selected, the transmission is prevented from shifting into second and third gear. A friction clutch locks out the one way roller clutch so that better control may be obtained when travelling over rough or wet ground or icy roads. Engine braking on overrun is available when descending steep hills.

5.6.2 First gear (D — 1st) (Fig. 5.13(a)) With the manual selector valve in D range, engine torque is transmitted from the converter through the applied forward clutch to the annular gear of the forward planetary gear train. The clockwise rotation of the forward annular gear causes the forward planet gears to rotate clockwise, driving the double (compound) sun gear anticlockwise. The forward planetary carrier is splined to the output shaft. This causes the planet gears to drive the double sun gear instead of rolling 'walking' around the sun gears. This counterclockwise rotation of the sun gears causes the reverse planet gears to rotate clockwise. With the one way clutch holding the reverse planet carrier stationary, the reverse planetary gears turn the reverse annular gear and output shaft clockwise in a reduction ratio of something like 2.71:1.

When first gear is selected in the D range, a very smooth transmission take-up is obtained when the one way clutch locks, but on vehicle overrun the one way clutch is released so that the transmission freewheels.

5.6.3 First gear manual (1 — 1st) (Fig. 5.13(a)) The power flow in first gear manual differs from the D range in that the first and reverse brake are applied to hold the reverse planet carrier stationary. Under these conditions on vehicle overrun, engine braking is provided.

In D range in second gear, the forward clutch and the second gear band brake are applied. The forward clutch then transmits the engine torque from the input shaft to the forward annular gear and the second gear band brake holds the double sun gear stationary. Thus engine torque is delivered to the annular gear of the forward planetary train in a clockwise rotation. Consequently, the planet gears are compelled to revolve on their axes and roll 'walk' around the stationary sun gear in a clockwise direction. As a result the output shaft, which is splined to the forward planet carrier, is made to turn in a clockwise direction at a slower speed relative to the input shaft with a reduction ratio of approximately 1.50:1.

In D range engine torque is transmitted through both forward clutch and drive and reverse clutch. The drive and reverse clutch rotate the sun gear of the forward gear train clockwise and similarly the forward clutch turns the annular gear of the same gear set also clockwise. With both the annular gear and sun gear of the forward gear train revolving in the same direction at the same speed, the planet gear becomes locked in position, causing the forward gear train to revolve as a whole. The output shaft, which is splined to the forward planet carrier, therefore rotates at the same speed as the input shaft, that is as a direct drive ratio 1:1.

With the manual selector valve in the R position, the drive and reverse multiplate brake is applied to transmit clockwise engine torque to the reverse gear set sun gear. With the first and reverse brake applied, the reverse planet gear carrier is held stationary. The planet gears are compelled to revolve on their own axes, thereby turning the reverse annular gear which is splined to the output shaft in an anticlockwise direction in a reduction ratio of about 2.43:1.

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