Fluid actuated shift bar housing assembly having a centering cylinder therein

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

• a shift bar housing having a plurality of generally parallel axially extending shift bar bores therein;
• a shift bar slidably received in each of said shift bar, bores, each of said shift bars 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 each of said shift bars and axially moved thereby, said shift forks effective to cause engagement of a first transmission gear ratio in the first axially displaced position of said shift bar and effective to cause engagement of a second transmission gear ratio in the second axially displaced position of said shift bar, said shift forks effective to disengage said first and second transmission gear ratios in the axially centered position of said shift bars;
• each of said shift bores having associated therewith a first cylinder defined in said housing, a first shifting piston axially fixed to each of said shift bars and slidably and sealingly received in said first cylinder to define a first and a second selectively pressurized fluid chambers, 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 and pressurization of said second selectively pressurized fluid chamber effective to cause said first shifting piston to axially move said shift bar from said centered position to said second axially displaced position;
• valve means connectable to a source of pressurized fluid for selectively pressurizing and exhausting said first and second selectively pressurized fluid chambers;
• said assembly characterized by:
• each of said shift bores having associated therewith a centering cylinder defined by said housing;
• a first and a second centering piston slidably and sealingly received in said centering cylinder, said first and second centering pistons axially separatable in said centering cylinder by a maximum distance generally equal to the axial movement of said shift bar between the axially centered position and said first axially displaced position and also generally equal to the axial movement of said shift bar between said axially centered position and said second axially displaced position, said first and second centering pistons having opposed faces defining a third selectively pressurized and exhausted fluid chamber therebetween, said first centering piston displaced in said first axial direction from said second centering piston, a first stop means limiting axial movement of said first centering piston relative to said shift bar in said first axial direction, a second stop means limiting axial movement of said second centering piston relative to said shift bar in said second axial direction, said first and second centering pistons, when engaging said first and second stop means, respectively, axially separatable by a distance generally equal to said maximum distance;
• fluid passageways defined in said centering cylinder and said first and second centering pistons to provide constant fluid communication between a selectively pressurized and exhausted fluid passage and said third selectively pressurized chamber;
• a neutral selection valve associated with said valve means for selectively pressurizing and exhausting said selectively pressurized and exhausted fluid passage and
• each of said selectively pressurized and exhausted third chambers in fluid communication with said selectively pressurized and exhausted fluid passage whereby all of said shift bars may be positively urged to the neutral axially central positions thereof by means of a single neutral selection valve.