Steer wheel control system with dual pistons connected to a common shaft

An apparatus for holding at a selected center position at least one steerable member moveable to either side of the selected center position in response to rotational reciprocation of a steering shaft by steering forces, said apparatus comprising resistance means for providing a holding force resisting steering forces tending to move said steerable member to either side of the selected center position, trim means operable from a location remote from said resistance means for changing the selected center position of said steerable member, and accumulator means for holding a supply of fluid and means for pressurizing the fluid held in said accumulator means;

• wherein said resistance means comprises:
• a centering shaft and means for connecting said centering shaft to the steering shaft for rotational reciprocation therewith, a rotational position of said centering shaft defining a centering position corresponding to said selected center position,
• a first centering piston cooperating with a first centering cylinder to define a first centering chamber,
• a second centering piston cooperating with a second centering cylinder to define a second centering chamber, each of said first and second centering pistons being arranged to reciprocate independently of the other between a rest position and an active position for compressing a fluid within its corresponding chamber,
• actuator means comprising first push means for causing compressive movement of said first centering piston, second push means for causing compressive movement of said second centering piston, and arcuate means connected to said centering shaft for arcuate movement around an axis to either side of a neutral position corresponding to said centering position in response to rotational reciprocation of said centering shaft, said arcuate means being arranged to engage each of said first and second push means independently of the other so that rotation of said centering shaft away from said centering position to one side causes compressive movement of said first centering piston without moving said second centering piston and rotation of said centering shaft away from said centering position to the other side causes compressive movement of said second centering piston without moving said first centering piston;
• and,
• centering fluid means for providing a flow of centering fluid between said accumulator means and each of said first and second centering chambers so that fluid pressure against said centering pistons provides said holding force by opposing movement of said steerable member toward either side of said selected center position and provides a return force continuously biasing said steerable member toward said selected center position during movement of said steerable member to either side of said selected center position, the rest position of each of said centering pistons corresponding to the neutral position of said arcuate means, and said fluid pressure causing each of said centering pistons to bias said arcuate means toward said neutral position during said compressive movement;
• and wherein said trim means comprises:
• a first trim piston cooperating with a first trim cylinder to define a first trim chamber and having a first stop element for engaging said first push means;
• a second trim piston cooperating with a second trim cylinder to define a second trim chamber and having a second stop element for engaging said second push means,
• and trim fluid means for selectively providing a flow of trim fluid between said accumulator means and each of said trim chambers such that each of said trim pistons is movable relative to its trim cylinder to vary said center position by changing the neutral position of said arcuate means, said trim fluid means comprising trim valve means operable between a closed position for preventing said trim fluid flow so that said trim pistons are held in a locked position, and an open position for allowing said trim fluid flow so that said trim pistons are free to move away from said locked position in said trim cylinders, said movement of the trim pistons causing trim fluid flow to one of said trim chambers and trim fluid flow from the other of said trim chambers to permit trimming movement of said steerable member to another center position by changing the centering position of said centering shaft in response to said steering forces.