Patent ID: 7748504

Claim:
An automatic transmission comprising: a casing; a transmission mechanism housed within said casing; a brake for braking a rotary element within the transmission mechanism, said brake having multiple friction plates and a hydraulic servo to engage said brake by axially moving a piston member against said multiple friction plates responsive to an oil pressure supplied to an operating oil chamber of said hydraulic servo; and wherein the hydraulic servo comprises: a cylinder formed in an interior surface of said casing: said piston member slidably mounted within said cylinder and forming said operating oil chamber in cooperation with said cylinder; a return plate fixed to said casing; and a plurality of return springs disposed compressed between said piston member and said return plate, for biasing said piston member in a direction for disengaging said brake: wherein said return plate comprises: a plurality of fixing protrusions, for positioning and fixing axial ends of said return springs on said return plate and disposed unevenly spaced in a circle adjacent an outer circumference of the return plate; and a plurality of connecting portions through which said return plate is fixed to said casing, said connection portions being disposed unevenly spaced in a circle adjacent an outer circumference of the return plate; wherein said piston member comprises: an inner circumferential surface configured with a plurality of round grooves which partially surround outer surfaces of said return springs thereby grasping said return springs and preventing said piston member from rotating, said plurality of round grooves being located at positions corresponding to positions of said return springs and unevenly spaced; a cylindrically shaped skirt axially aligned to press against said multiple friction plates of said brake; and a drain hole for discharge of oil from a lower portion of an interior of said skirt.