Patent ID: 8281915

Claim:
An automatic transmission including a first clutch for selectively connecting a first rotary member and a second rotary member disposed coaxially with said first rotary member, and a second clutch for selectively connecting said first rotary member and a third rotary member disposed coaxially with the first rotary member, said automatic transmission comprising: a first drum in which a first piston of said first clutch is axially slidably received, the first drum including a first inner cylindrical wall and a first outer cylindrical wall that are coaxial with each other, and a first bottom wall connecting corresponding axial end portions of said first inner and outer cylindrical walls; a second drum which is disposed coaxially with said first drum, so as to accommodate said first drum, and in which a second piston of said second clutch is axially slidably received, the second drum including a second inner cylindrical wall and a second outer cylindrical wall that are coaxial with each other, and a second bottom wall connecting corresponding axial end portions of said second inner and outer cylindrical walls; an abutting portion for abutting contact of said first and second drums with each other in an axial direction of the first and second drums; and a fitting portion at which said first drum is fitted in said second drum and which inhibits relative rotation of said first and second drums about a common axis of the first and second drums, the fitting portion being a spline-fitting portion constituted by first spline teeth formed on an inner circumferential surface of said first inner cylindrical wall, and second spline teeth which are formed on an outer circumferential surface of said second inner cylindrical wall, wherein said second drum further includes a second annular protrusion extending from a radially intermediate part of said second bottom wall in the axial direction thereof toward said first bottom wall for abutting contact with said first bottom wall, wherein the first inner cylindrical wall and the second annular protrusion overlap in a radial and axial direction, and wherein said abutting portion consists of said second annular protrusion.