Patent ID: 8662469

Claim:
A power-assisted on/off valve comprising: a valve stem which opens and closes a flow passage; an operational member which is positioned on an axis of said valve stem and is relatively movable with respect to said valve stem; and a power-assisted mechanism which is interposed between said valve stem and said operational member, reduces the speed and power-assists the movement of said operational member, and transfers said movement of said operational member to said stem valve, wherein said power-assisted mechanism comprises: a transmission rod which is positioned on an axis of said operational member, said transmission rod provided with a reduced-diameter portion that reduces the diameter thereof in a direction toward said valve stem; a pressing member which is interconnected with said valve stem; a pair of mutually opposed motion-transmission surfaces that are formed on the pressing member and on a housing that movably supports said pressing member, wherein a space between said pair of motion-transmission surfaces increasingly narrows toward the outer periphery thereof; and a plurality of radial-direction movable bodies that are interposed between said pair of motion-transmission surfaces and the reduced-diameter portion of the transmission rod and are in contact therewith, wherein said valve stem includes an on/off valve body that approaches and retreats from an annular valve seat to close and open the flow passage; wherein said reduced-diameter portion of said transmission rod is formed so that a substantially constant valve-closing force can be achieved from a valve fully-closed state, at which the flow passage is closed by said on/off valve body seating onto the valve seat, to a valve-seat wear limit, at which a closed-valve state can be maintained even in a case where said valve seat has worn; wherein said pair of mutually opposed motion-transmission surfaces, the reduced-diameter portion of the transmission rod, and the plurality of radial-direction movable bodies are determined so as to satisfy: 2<D/d<50 (wherein D designates an amount of movement of the operational member in the axial direction thereof; and d designates an amount of movement of the valve stem in the axial direction thereof); and the shape of the reduced-diameter portion of the transmission rod is determined so as to satisfy: Fs×Sp=Fo×So (wherein Fs designates a spring reaction force applied to said transmission rod; Sp designates a movement amount of said transmission rod; Fo designates a substantially constant output of said pressing member; and So designates a movement amount of said pressing member, for each stroke position of the transmission rod between a valve fully-closed state and a valve-seat wear limit).