Three Speed and Reverse Transaxle Automatic Transmission
A transaxle three speed automatic transmission is presented in Fig. 25.41. The planetary gear train uses 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 through the forward annular gear and leaves the gear train by the reverse annular gear. Whereas in reverse gear, power enters the gear train by the reverse sun gear and leaves the gear train through the reverse annular gear. With planetary gear trains the gears are in constant mesh and gear ratios changes are effected by holding, releasing or rotating certain parts of the gear train by means of a one-way clutch, two multi-plate clutches, one multiplate brake and one band brake.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. For better understanding of the operation of the automatic transmission gear train Table 25.2 maybe referred, which represents the components engaged in each manual valve selection position.
Selector Lever.
The selector lever (Table 25.2) has a number of positions marked PRND21 with definite functions as follows:P-Park. In this position, there is no drive through the transmission. A mechanical lock actuated by a linkage causes a parking pawl to engage in the slots around a ring gear attached to the output shaft (Fig. 25.41). Thus the parking pawl locks the output shaft to the transmission casing due to which backward or forward movement of the vehicle is arrested. The engine may be started in this position.
Fig. 25.41. Transaxle three speed automatic transmission layout.
R-Reverse.
In this position, the output shaft of the automatic transmission is made to rotate in the opposite direction to produce a reverse gear drive. The reverse position must only be used when the vehicle is stationary. The engine does not start in reverse position.N-Neutral.
When this position is selected, all clutches and band brake are disengaged, as a result there is no drive through the transmission. The engine may be started in this position.Table 25.2. Manual valve selection position.
| Range | Forward clutch FC | Drive and reverse clutch (D + RJC | First and reverse brake (I + R)B | Second gear band 2GB | One-way clutch owe | Ratio |
| PandN | - | - | - | - | - | - |
| D- 1st 2 - 1st | Applied | - | - | - | Applied | 2.71:1 |
| 1-lst | Applied | Applied | - | 2.71:1 | ||
| D-2nd 2-2nd | Applied | - | - | Applied | - | 1.5:1 |
| D-3rd | Applied | Applied | - | - | - | 1.00:1 |
| R | - | Applied | Applied | - | - | 2.43:1 |
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 does not start in D-drive range.2-First and Second.
This position is chosen to restrict gear changes automatically from 1-2 upshifts and 2-1 downshifts only. The selector must not be positioned in 2 ranges above 100 km/h. The engine does not start in this range position.1-First.
When this position is selected, the transmission is not permitted to shift 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.First Gear (D-lst).
When the manual selector works in D range, engine torque is transmitted from the converter through the applied forward clutch to the annular gear to 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 in the clockwise direction producing a.reduction ratio of around 2.71:1 (Fig. 25.42A).The power flow in first gear manual (1-lst) differs from the D range first gear (D-lst) in that the first and reverse brake is applied to hold the reverse planet carrier stationary. Under these conditions engine braking is provided (Fig. 25.42A) on vehicle overrun.
Second Gear (D-2nd).
When in D range 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 in a clockwise direction. The second gear band brake holds the double sun gear stationary. 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 relative to the input shaft with a reduction ratio of approximately 1.50:1 (Fig. 25.42B).Third Gear (D-3rd).
In this D range engine torque is transmitted through both forward clutch, and drive and reverse clutch. The drive and reverse clutch rotates the sun gear of the forward gear train clockwise. The forward clutch turns the annular gear of the same gear set also clockwise. Since both the annular gear and sun gear of the forward gear train revolve 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, being splined to the forward planet carrier, also rotates at the same speed as the input shaft with the drive ratio of 1:1 (Fig. 25.42C).Reverse Gear (R).
When the manual selector valve is in the R position, the drive and reverse multiplate brake is applied to transmit clockwise engine torque to the reverse gear set sun gear. The reverse planet gear carrier is held stationary. The planet gears are forced to revolve about their won axes, so that the reverse annular gear, which is splined to the output shaft, is also rotated in an anticlockwise direction with a reduction ratio of about 2.43:1 (Fig. 25.42D).
Fig. 25.42. Three speed and reverse automatic transmission transaxle unit.