Clutching device of an automatic transmission

A clutching device of an automatic transmission, the clutching device comprising:

• a clutch drum connected with an input shaft;
• a clutch connected with the clutch drum and also connected with a rotation element of a planetary gear set and configured to connect or disconnect torque delivery between the input shaft and the rotation element;
• and
• a piston unit configured to operate the clutch to connect or disconnect the torque delivery between the input shaft and the rotation element,
• wherein the piston unit comprises:
• a first piston forming a first sealed space with the input shaft and the clutch drum and configured to move toward the clutch to directly press the clutch by a hydraulic pressure supplied to the first sealed space through a first hydraulic line formed in the input shaft;
• a second piston forming a second sealed space with the first piston and the input shaft;
• a spring retainer arranged on the input shaft and forming a third sealed space with the first and second pistons and the input shaft;
• a second hydraulic line formed in the input shaft and configured to supply a hydraulic pressure to the third sealed space;
• a first stopper disposed on the input shaft and arranged between the first piston and the second piston to limit a moving stroke of the second piston;
• a second stopper disposed on the input shaft and configured to abut the spring retainer;
• a first return spring disposed between the first piston and the second piston and configured to provide a first restoring force to the first and second pistons;
• and
• a second return spring disposed between the spring retainer and the second piston and configured to provide a second restoring force to the spring retainer and the second piston.