Hydraulic servo for friction coupling devices of automatic transmissions

Claim 1

A hydraulic servo for an automatic transmission including a casing, said hydraulic servo comprising: an annular drum of two-piece construction and having a first piece which includes a first cylinder splined to the casing, a second cylinder, and a side wall connecting said first and second cylinders, and a second piece which includes a third cylinder formed separately from said first piece and connected to said side wall at a suitable position between said first and second cylinders, said first and third cylinders defining a first cylindrical annular space therebetween and said second and third cylinders defining a second cylindrical annular space therebetween, said second annular space being radially inward of said first annular space; a piston fitted within said first annular space, between said first and third cylinders, said piston having inner and outer cylindrical surfaces and sealing means for forming seals between said piston and said first and third cylinders on the interior of said first annular space, thereby defining the working area of said piston as being located entirely within said first annular space; and a return biasing means provided within said second annular space between said third and second cylinders; and including: a connecting member having one end thereof securely fixed to said piston and joining said piston at a point axially aligned with said third cylinder, and a second end located within said second annular space; retainer securely fixed to said second cylinder; and return springs located within said second annular space and disposed between said second end of said connecting member and said retainer.