1. Technical Field
The present invention relates to a fluid power assist steering gear. In particular, the present invention relates to pressure relief poppet valves in the steering gear and adjustment of such valves.
2. Description of the Prior Art
Fluid power assist steering gears for vehicles are known. One such steering gear is disclosed in U.S. Pat. No. 4,648,307. The steering gear disclosed in U.S. Pat. No. 4,648,307 includes a housing defining a fluid chamber. A piston divides the fluid chamber into first and second chamber portions. The piston is drivingly connected with a steering linkage to effect angular movement of steerable wheels of the vehicle when the piston moves. The piston moves through a work stroke due to fluid pressure acting in one of the chamber portions. A fluid flow control valve, when actuated, directs pressurized fluid into the one chamber portion and simultaneously vents the other chamber portion to effect movement of the piston. An input shaft is moved by the vehicle operator to actuate the control valve.
A vehicle typically has a pair of steering stops which limit the maximum amount of angular movement of the steerable wheels. One steering stop limits the amount of angular movement in one direction and the other steering stop limits the amount of angular movement in the opposite direction. Each of the steering stops is mounted on a part of the vehicle, such as an axle, and engages a part connected with one of steerable wheels upon movement of the steerable wheels the maximum angular amount in one direction.
A pair of pressure relief poppet valves are mounted at axially opposite ends of the piston. Each poppet valve relieves fluid pressure in a pressurized chamber portion just prior to the steerable wheels reaching a respective limit of angular movement in one respective direction and just prior to engagement of the part connected with one of the steerable wheels with a respective steering stop. Relieving pressure in the pressurized chamber portion when a steering stop is engaged prevents damage. Specifically, the power steering pump and the driving connection between the piston and the steering linkage are protected from hydraulic and mechanical loads which may be transmitted if the pressure is not relieved.
Each of the poppet valves includes a valve seat and a valve member which is movable relative to the valve seat. Fluid flow through a poppet valve is blocked when the valve member engages its valve seat. Fluid flow through the poppet valve is permitted when the valve member is moved to a postion not engaging its valve seat. The position of the valve seat relative to the piston is adjustable. The positions of the valve seats are adjusted by moving the piston to cause the valve seats to engage respective walls of the fluid chamber and to move relative to the piston on continued movement of the piston. Adjustment of the valve seats is effected initially by turning the steerable wheels of the vehicle in both directions until the steering stops engage the respective parts connected with the steerable wheels. The seats are thereby adjusted to positions so that a respective valve member engages a wall of the fluid chamber and moves away from its valve seat to vent the fluid pressure in a chamber portion through the poppet valve when the piston is near an end of its maximum work stroke which is just prior to a steering stop being engaged.
If for some reason the maximum amount of angular movement of the steerable wheels is decreased, the steering gear of U.S. Pat. No. 4,648,307 must be removed from the vehicle, disassembled and the poppet valve seats moved to a position so they may be automatically adjusted after reinstallation of the steering gear. For example, if tires of a larger diameter than the original tires are installed on the steerable wheels, the steering stops are reset to decrease the maximum amount of angular movement of the steerable wheels. The length of the maximum work stroke of the piston correspondingly decreases. Thus, the valve members cannot engage a wall of the fluid chamber to unseat and thereby vent the fluid pressure in a chamber portion moving the piston. Therefore, there would be no pressure relief when a steering stop is engaged and the aforementioned damage to the steering gear could occur.