Automatic transmission for vehicle

An automatic transmission for a vehicle comprising:

• an input shaft connected to an output shaft of an engine;
• a first planetary gear unit including a helical sun gear, helical planet pinions supported for rotation on a planet carrier, and a helical ring gear, the sun gear or the ring gear being connected to the input shaft, the ring gear or the sun gear being connected through a one-way clutch to a fixed member such that the ring gear or the sun gear can be restrained from rotation relative to the fixed member, the planet carrier being connected to a first output shaft;
• a first clutch interposed between the input shaft and the first output shaft and capable of being engaged by the action of flyweights operated by centrifugal force and of being disengaged by thrust exerted on the helical gears of the first planetary gear unit;
• a second planetary gear unit including a sun gear, planet pinions supported for rotation on a planet carrier, and a ring gear, the planet carrier being connected to the first output shaft, the sun gear or the ring gear being connected to a second output shaft, the ring gear or the sun gear being connected through a one-way clutch to the fixed member such that the ring gear or the sun gear can be restrained from rotation relative to the fixed member;
• a second clutch interposed between the first and the second output shaft, and being controlled by a control means so as to engage the first and the second output shaft or disengage the first and the second output shaft from each other;
• a third planetary gear unit including a helical sun gear, helical planet pinions supported for rotation on a planet carrier, and a helical ring gear, the sun gear or the ring gear being connected to the second output shaft, the ring gear or the sun gear being connected through a one-way clutch to the fixed member such that the ring gear or the sun gear can be restrained from rotation relative to the fixed member;
• and
• a third clutch interposed between the second and a third output shaft, and capable of being engaged by the action of flyweights operated by centrifugal force and of being disengaged by thrust exerted on the helical gears of the third planetary gear unit,
• and the engagement and disengagement of the second clutch with the first and second output shaft controlled by the control means provides smooth and shockless upshifting and downshifting of the automatic transmission, wherein the control means engages the second clutch while the vehicle is stopped.