| 摘要 |
A linkage assembly of variable length extends between a steerable member and a frame member to define the center position of the steerable member. The linkage assembly includes a resistance assembly for providing a resistance force resisting forces tending to move the steerable member to either side of the center position, and a center holding assembly having a piston that may be locked in position in a cylinder for transmitting steering forces to the resistance assembly. A control valve is operable between a closed position for preventing fluid flow so that the piston is held in its locked position, and an open position for allowing the holding piston to move away from its locked position to permit the length of the linkage to freely change in response to the steering forces. The control valve is operated by an actuator responsive to the amount of the resistance force, such that the piston is free to move in its cylinder when the resistance force exceeds a predetermined amount.
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