Frictional engagement device for an automatic transmission

A frictional engagement device in an automatic transmission having a first member and a second member which may be engaged and disengaged by use of oil pressure, said frictional engagement device comprising:

• a casing connected to said first member and having at least one first oil passage;
• a first piston means disposed in said casing and having at least one second oil passage, said first piston means defining a first oil pressure chamber together with said casing, said second oil passage being connected to said first oil pressure chamber, said first piston means being axially movable relative to said casing by a predetermined distance by supplying pressurized oil into said first oil pressure chamber, and one of said first piston means and said casing being rotationally movable relative to the other by a predetermined angle;
• a second piston means disposed in said first piston means, said second piston means defining a second oil pressure chamber together with said first piston means, said second piston means being axially movable relative to said first piston means by supplying pressurized oil into said second oil pressure chamber;
• first group of frictional engagement elements being axially spaced and axially slidably connected to said first piston means;
• and
• second group of frictional engagement elements being axially spaced and axially slidably connected to said second member, said second groups of frictional engagement elements facing said first groups of frictional engagement elements respectively,
• wherein said second piston means pushes one of said first group of frictional engagement elements and said second group of frictional engagement elements when said second piston means is axially moved by supplying pressurized oil into at least one of said first oil pressure chamber and said second oil pressure chamber,
• wherein said first oil passage in said casing is connected to said second oil passage in said first piston means and pressurized oil supplied in said first oil pressure chamber can be discharged when one of said casing and said first piston means rotationally moves relative to the other after said first piston means is axially moved.