Accumulator for hydraulically operated friction clutch or brake unit

Claim 1

A hydraulic control for a friction device selectively engageable to change the speed ratio of a transmission comprising a source of fluid pressure, a housing, a hydraulically operated control piston in said housing movable between first and second positions for respectively selectively engaging and releasing said friction device, said control piston cooperating with said housing to form an expansible and contractible control chamber therein for receiving pressurized fluid from said source, return spring means operatively connected to said piston for urging said piston from said first toward said second position, fluid conducting means hydraulically connecting said control chamber to said source, shift valve means hydraulically connected to said fluid conducting means operable to feed fluid from said source to said control chamber to cause said control piston to move toward said first position to effect the engagement of said friction device and operable to exhaust fluid from said control chamber so that said return spring moves said control piston to said second position to release said friction device, a fluid receiving accumulator for modulating the buildup of pressure in said control chamber, said accumulator having an accumulator piston movably mounted therein to provide first and second expansible and contractible chambers, fluid conducting means hydraulically connecting said first accumulator chamber in parallel with said control chamber of said control piston, fluid conducting means operatively connecting a second of said accumulator chambers to said source, a variable rate accumulator spring operatively disposed in a first of said accumulator chambers for continuously contacting said accumulator piston, said spring being a unitary helical spring having first and second coil means, said first coil means having a lead greater than said second coil means so that spring has a high spring rate for initially exerting a large load on said accumulator piston for initially stroking said accumulator piston in one direction in response to the supply of pressure fluid to said first chambers to increase the volume of said first chamber and thereby provide a controlled rate of increase of pressure in said control chamber and having a low spring rate for subsequently exerting a reduced load on said accumulator piston for further stroking said accumulator piston to further increase the volume of said first chamber and thereby reduce the rate of increase of pressure in said control chamber so that said accumulator effects the timed and cushioned engagement of said friction device by said control piston to change the speed ratio of said transmission.