Speed change control system for an automatic transmission

Claim 1

A speed change control system for an automatic transmission having a planetary gear mechanism, and a brake band engageable with a brake drum for altering the transmission ratio, comprising a body defining a cylindrical chamber, a servo-piston having an axial bore and slidably engaged in said cylindrical chamber, a piston rod secured to said servo-piston and to said brake band, an accumulator piston slidably disposed in said axial bore, a first spring means provided between said servo-piston and an inner wall of said cylindrical chamber of said body for returning said servo-piston in a direction, opposite to a direction of engagement of the brake band with the brake drum, for releasing said brake band, a second spring means operatively connected between said accumulator piston and said servo-piston for biasing said accumulator piston, said servo-piston in cooperation with said cylindrical chamber defining apply side and release side oil chambers respectively on opposite sides of said servo-piston, said accumulator piston on one side thereof in cooperation with said axial bore defining an accumulator oil chamber, said body is formed with ports communicating with said apply side and release side oil chambers, respectively, passage means for communicating said apply side oil chamber with said accumulator oil chamber, and means for engaging said accumulator piston with said servo-piston after the accumulator piston moves a predetermined stroke in said direction of engagement of the brake band against said second spring means, said accumulator piston in cooperation with said first and second spring means, said servo-piston and said passage means constituting means for moving said accumulator piston against said second spring means after said servo-piston moves against said first spring means when oil pressure is applied to said apply side oil chamber via one of said ports.