Fluid actuated shift bar housing assembly

A fluid operated shift bar housing assembly for a change gear transmission, said assembly comprising:

• a shift bar housing having an axially extending shift bar bore therein;
• a shift bar slidably received in said shift bar bore, said shift bar having an axially centered position, a first axially displaced position wherein said shift bar is axially displaced from said centered position in a first axial direction and a second axially displaced position wherein said shift bar is axially displaced from said centered position in a second axial direction;
• a shift fork associated with said shift bar and axially moved thereby, said shift fork effective to cause engagement of a first transmission gear in the first axially displaced position of said shift bar and effective to cause engagement of a second transmission gear in the second axially displaced position of said shift bar, said shift fork effective to disengage said first and second transmission gears in the centered position of said shift bar;
• a first constantly pressurized fluid chamber defined by said housing, a first centering piston slidably and sealingly received in said first constantly pressurized fluid chamber, said first centering piston applying a first axially directed force on said shift bar when said shift bar is axially displaced from said centered position in said first axial direction tending to bias said shift bar toward said centered position;
• a second constantly pressurized fluid chamber defined by said housing, a second centering piston slidably and sealingly received in said second constantly pressurized fluid chamber, said second centering piston applying a second axially directed force on said shift bar when said shift bar is axially displaced from said centered position in said the second axial direction tending to bias said shift bar toward said axially centered position;
• a first selectively pressurized fluid chamber defined by said housing, a first shifting piston slidably and sealingly received in said first selectively pressurized fluid chamber, pressurization of said first selectively pressurized fluid chamber effective to cause said first shifting piston to axially move said shift bar from said centered position to said first axially displaced position with an axial force greater than and opposite from said first axially directed force;
• a second selectively pressurized fluid chamber defined by said housing, a second shifting piston slidably and sealingly received in said second selectively pressurized fluid chamber, pressurization of said second selectively pressurized fluid chamber effective to cause said second shifting piston to axially move said shift bar from said centered position to said second axially displaced position with an axial force greater than and opposite to said second axial force;
• valve means connectable to a source of pressurized fluid for constantly pressurizing said first and second constantly pressurized fluid chambers and for selectively pressurizing and exhausting said first and second selectively pressurized fluid chambers;
• and
• a detent mechanism effective to apply a resilient detent force to said shift bar when said shift bar is in the first and the second axially displaced positions thereof, said detent mechanism applying a resilient detent force tending to resist axial movement of said shift bar, said resilient detent force having an axial component in both the first and second axial directions having a smaller magnitude than both said first and second axial forces.