Variable effort steering system

In a bypass type variable effort power steering system including

• a steering assist fluid motor having a first working chamber and a second working chamber,
• a pump,
• a steering control valve connected to said pump and to said first working chamber and to said second working chamber operative to port one of said said first and said second working chambers to said pump while concurrently porting the other of said said first and said second working chambers to a low pressure fluid exhaust, and
• a bypass valve in a bypass duct connected to said first working chamber and to said second working chamber having a closed position wherein said bypass duct is blocked and an open position wherein said bypass duct is open for bypass fluid flow from said one working chamber to said low pressure fluid exhaust,
• the improvement comprising:
• a throttling valve housing including a center chamber having a first opening to said bypass duct and a second opening to said bypass duct spaced from said first opening,
• means in said center chamber defining a first valve seat at said first opening to said bypass duct and a second valve seat at said second opening to said bypass duct,
• a valve element disposed in said center chamber for movement between a centered position generally mid-way between said first and said second valve seats and a seated position engaging one of said first and said second valve seats, and
• spring means between said valve element and said throttling valve housing normally maintaining said valve element in said centered position,
• said valve element in said centered position thereof cooperating with said center chamber in said throttling valve housing in defining a bypass flow path between said first and said second valve seats having a flow area calculated to induce a pressure force on said valve element exceeding the force exerted by said spring means when said bypass fluid flow rate exceeds a transition flow rate characteristic of a substantial pressure difference between said one working chamber and said low pressure fluid exhaust so that said valve element is moved by said pressure force to said seated position wherein said bypass flow path is blocked.