Fluid pressure booster for braking device

1. A fluid pressure booster for a braking device, comprising:

a control piston on which an input brake operation acts in an advancing direction, and a reaction force based on a fluid pressure generated by a booster fluid pressure generation chamber for actuating a master piston of a master cylinder acts in a retracting direction;

a pressure increasing valve which includes a cylindrical valve seat member which is provided, at a front end thereof, with a valve seat facing a valve chamber communicating with a fluid pressure generation source, a valve body that is urged by a spring toward a side where the valve body is seated on the valve seat and is housed in the valve chamber, and a pressing rod that is coaxially inserted in the valve seat member and pushes the valve body with the front end so that the valve body is separated from the valve seat, and which opens according to the advance of the control piston, and is provided between the valve chamber and the booster fluid pressure generation chamber; and

a pressure reducing valve provided between a release chamber communicating with a reservoir and the booster fluid pressure generation chamber, the pressure reducing valve closes when the control piston advances and opens when the control piston retracts,

wherein a plurality of contraction portions are formed in series in a flow direction from the valve chamber to the booster fluid pressure generation chamber between an outer periphery of a front portion of the pressing rod and an inner periphery of a front portion of the valve seat member in a flow passage through which a working fluid flows such that a distance between the outer periphery and the inner periphery is reduced at each contraction portion.

2. The fluid pressure booster for a brake device according to claim 1 , wherein the front portion of the pressing rod integrally includes:

a pressing shaft portion that movably penetrates through a valve hole defined in a central portion of the valve seat and which is provided at the front end of the valve seat member and is capable of abutting against the valve body with a front end;

a circular truncated cone portion that is coaxially continuous with a rear end of the pressing shaft portion and has, on an outer periphery, a taper surface having an outside diameter which gets larger in a direction extending rearward; and

a straight portion having an outer peripheral surface with the same outside diameter as a large diameter end of the taper surface; and

the contraction portion is formed between the inner periphery of the valve seat member and the outer periphery of the straight portion at least with advance of the pressing rod.

3. The fluid pressure booster for a braking device according to claim 1 ,

wherein the valve seat member is provided with a small diameter hole portion, and at least one large diameter hole portion that is coaxially disposed rearward of the small diameter hole portion and is formed to have a diameter larger than that of the small diameter hole portion, and

the contraction portions are formed, respectively, between the outer periphery of the pressing rod, and inner peripheries of the small diameter hole portion and the large diameter hole portion.

4. The fluid pressure booster for a braking device according to claim 2 ,

wherein the valve seat member is provided with a small diameter hole portion, and at least one large diameter hole portion that is coaxially disposed rearward of the small diameter hole portion and is formed to have a diameter larger than that of the small diameter hole portion, and

the contraction portions are formed, respectively, between the outer periphery of the pressing rod, and inner peripheries of the small diameter hole portion and the large diameter hole portion.

5. The fluid pressure booster for a braking device according to claim 1 ,

wherein a velocity of a working fluid flowing through the pressure increasing valve is suppressed in the first contraction portion.

6. A fluid pressure booster for a braking device, comprising:

a control piston on which an input brake operation acts in an advancing direction, and a reaction force based on a fluid pressure generated by a booster fluid pressure generation chamber for actuating a master piston of a master cylinder acts in a retracting direction;

a pressure increasing valve which includes a cylindrical valve seat member which is provided, at a front end thereof, with a valve seat facing a valve chamber communicating with a fluid pressure generation source, a valve body that is urged by a spring toward a side where the valve body is seated on the valve seat and is housed in the valve chamber, and a pressing rod that is coaxially inserted in the valve seat member and pushes the valve body with the front end so that the valve body is separated from the valve seat, and which opens according to the advance of the control piston, and is provided between the valve chamber and the booster fluid pressure generation chamber; and

a pressure reducing valve provided between a release chamber communicating with a reservoir and the booster fluid pressure generation chamber, the pressure reducing valve closes when the control piston advances and opens when the control piston retracts,

wherein at least one first contraction portion is formed between an outer periphery of the pressing rod and an inner periphery of the valve seat member such that a distance between the outer periphery and the inner periphery is reduced at the first contraction portion in a flow direction from the valve chamber to the booster fluid pressure generation chamber; and

wherein the pressing rod integrally includes:

a pressing shaft portion that movably penetrates through a valve hole defined in a central portion of the valve seat and which is provided at the front end of the valve seat member and is capable of abutting against the valve body with a front end;

a circular truncated cone portion that is coaxially continuous with a rear end of the pressing shaft portion and has, on an outer periphery, a taper surface having an outside diameter which gets larger in a direction extending rearward; and

a straight portion having an outer peripheral surface with the same outside diameter as a large diameter end of the taper surface; and

the contraction portion is formed between the inner periphery of the valve seat member and the outer periphery of the straight portion at least with advance of the pressing rod wherein the pressing rod further integrally includes:

a second circular truncated cone portion that is coaxially continuous with a rear end of the straight portion and has, on an outer periphery a second taper surface having an outside diameter which gets larger in a direction extending rearward; and

a second straight portion having an outer peripheral surface with the same outside diameter as a large diameter end of the second taper surface,

wherein a second of the contraction portions is formed between an inner periphery of a large diameter hole portion of the valve seat member and the outer periphery of the second straight portion; and

wherein a working fluid flows through the contraction portions.