The overdrive gear unit is an independent planetary gear unit with a gear ratio less than 1.0 (approx. 0.7-0.8). It is combined with a conventional 3-speed planetary gear unit and is equivalent to the 4th speed gear. The overdrive gear unit consists of the planetary gear set, brake (B0), clutch (C0), and one-way clutch (F0). The power is input to the overdrive carrier and output from the overdrive ring gear. Normally, when the vehicle speed goes over 40 km/ h in the “D” range, shifting up to the overdrive gear becomes possible. It is also possible to drive without shifting up to the overdrive gear if that suits the driver.
The illustration on the left is for a 3- speed planetary gear unit with an overdrive gear unit added (A140 series).
“P” or “N” Range
When the shift lever is in “N” or “P”, the forward clutch (C1) and direct clutch (C2) do not operate, so input from the input shaft is not transmitted to the differential drive pinion shaft. In addition, when the shift lever is in “P”, the parking lock pawl engages with the parking gear to which the differential drive pinion shaft is splined, thus preventing the vehicle from moving.
Parking lock mechanism for FR vehicles
When the shift lever of an automatic transmission for a FR vehicle is in the “P” range, the parking lock pawl is engaged with the front or rear planetary ring gear, which is splined with the output shaft, preventing movement of the vehicle. However, on FR- based 4WD vehicles, the movement of the vehicle cannot be prevented if the transfer mechanism is in neutral, even if the automatic transmission is in “P”. For this reason, be sure to set the parking brake when parking.
1. In overdrive
In overdrive, the O/D brake (B0) locks the O/D sun gear, so when the overdrive pinion gears revolve clockwise around the overdrive sun gear while rotating around the pinion shafts. Therefore, the overdrive ring gear rotates clockwise faster than the overdrive carrier. The length of the arrow shows the rotational speed and the width of the arrow shows the torque. The longer the arrow, the greater the rotational speed and the wider the arrow, the greater the torque.
2. Not in overdrive
The overdrive planetary gear set acts as a direct drive mechanism, rotating as a single unit to output the input power (rotating speed and torque) as is. The length of the arrow shows the rotational speed and the width of the arrow shows the torque. The longer the arrow, the greater the rotational speed and the wider the arrow, the greater the torque.