Power assisted on/off valve

1. 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).

2. The power-assisted on/off valve according to claim 1 , wherein said operational member comprises a piston body that is slidably fitted in said housing, wherein said piston body moves in a valve-closing direction by a spring pressure force so as to closely approach the valve stem, and said piston body moves in a valve-opening direction by air pressure so as to move away from the stem valve.

3. The power-assisted on/off valve according to claim 1 , wherein the motion-transmission surface on said pressing member comprises a surface that is orthogonal to the axis of the pressing member, wherein the motion-transmission surface on said housing is a conical surface.

4. The power-assisted on/off valve according to claim 3 , wherein the motion-transmission surface on said pressing member comprises base surfaces of three radial grooves that are formed in radial directions in said pressing member, wherein said radial-direction movable bodies are fitted in the radial grooves.

5. The power-assisted on/off valve according to claim 3 , wherein said motion-transmission surface on said housing comprises a hard ring body that is a separate member from said housing.

6. The power-assisted on/off valve according to claim 1 , wherein said radial-direction movable bodies are steel balls.