Software Radios for Wireless

Described herein is a hydraulic servo suitable for friction coupling elements in automatic transmissions. The hydraulic servo of the invention is compact in construction and easy to assemble, including an annular drum rotatably or fixedly mounted in an automatic transmission case, a piston provided in a predetermined position of the drum, a return spring provided on the piston, and a connecting member for connecting the return spring and the piston, characterized in that the piston is provided with a guide sleeve at the tip end of one end face thereof for guiding one end of said connecting member.