1. A pneumatic booster comprising: a shell body; a power piston provided in said shell body to divide an interior of said shell body into a constant-pressure chamber and a variable-pressure chamber; a valve body connected to said power piston, said valve body having a tubular portion extending through and projecting rearward from said shell body; a valve mechanism provided in said valve body, said valve mechanism being actuated in association with movement of a plunger connected to an input rod; and a negative-pressure passage provided in said valve body, said negative-pressure passage communicating at one end thereof with said constant-pressure chamber, the other end of said negative-pressure passage opening in an inner surface of said tubular portion as a negative-pressure passage opening; wherein said valve mechanism is actuated in response to advance of said input rod to introduce atmospheric air into said variable-pressure chamber through an opening of said tubular portion, thereby producing a pressure difference between said constant-pressure chamber and said variable-pressure chamber, and said power piston is propelled by the pressure difference; wherein said negative-pressure passage opening opens toward a proximal end side of said input rod; said valve mechanism having: an annular atmospheric valve seat formed on a rear end of said plunger; a negative-pressure valve seat formed on a peripheral edge of said negative-pressure passage opening; a poppet valve member provided to liftably rest on said atmospheric valve seat and said negative-pressure valve seat; and a poppet spring that presses said poppet valve member in a direction for resting on said atmospheric valve seat and said negative-pressure valve seat; wherein said poppet valve member opens or closes said negative-pressure passage opening by lifting from or resting on said negative-pressure valve seat; and wherein a poppet back space defined by said tubular portion and said poppet valve member in an area at a back of said poppet valve member communicates with said variable-pressure chamber.

2. A pneumatic booster according to claim 1, wherein said poppet valve member has: an annular poppet proximal end portion secured to the tubular portion of said valve body; an approximately tubular bellows portion connected to said poppet proximal end portion; and an annular valving portion connected to a distal end of said bellows portion, said valving portion liftably resting on said atmospheric valve seat and said negative-pressure valve seat; wherein the distal end of said bellows portion is connected to an inner peripheral portion of said valving portion.

3. A pneumatic booster according to claim 1, wherein said poppet valve member has: an annular poppet proximal end portion secured to the tubular portion of said valve body through an annular retainer provided inside said tubular portion; an approximately tubular bellows portion connected to said poppet proximal end portion; and an annular valving portion connected to a distal end of said bellows portion, said valving portion liftably resting on said atmospheric valve seat and said negative-pressure valve seat; wherein said poppet spring is provided between said valving portion and said retainer.

4. A pneumatic booster according to claim 3, wherein said poppet spring is provided in said poppet back space.

5. A pneumatic booster according to claim 3 or 4. wherein said poppet proximal end portion is fitted to an inner periphery of said retainer and has an outer diameter smaller than an inner diameter of said poppet spring.

6. A pneumatic booster according to any one of claims 3 to 5, wherein the distal end of said bellows portion is connected to an inner peripheral portion of said valving portion.

PNEUMATIC BOOSTER
The present invention relates to a pneumatic booster for use in a brake system of a vehicle.
Fig. 4 of the accompanying drawings illustrates one example of conventional pneumatic boosters [see Japanese Patent Application Unexamined Publication (KOKAI) No. 2004-1632], The pneumatic booster 1 shown in Fig. 4 has a shell body 2 and a power piston 5 that divides the interior of the shell body 2 into a constant-pressure chamber 3 and a variable-pressure chamber 4. The pneumatic booster 1 further has a valve mechanism 9 that is actuated in association with the movement of a plunger 8 disposed in a tubular portion 39 of a valve body 6 connected to the power piston 5. The plunger 8 is connected to an input rod 7. The tubular portion 39 projects rearward from the shell body 2.
The valve mechanism 9 includes an atmospheric valve 10 and a vacuum valve 11 and has a poppet valve member 12 that constitutes a part of each of the atmospheric and vacuum valves 10 and 11. The atmospheric valve 10 selectively allows or blocks communication between the variable-pressure chamber 4 and the atmosphere. The vacuum valve 11 selectively allows or blocks communication between the constant-pressure chamber 3 and the variable-pressure chamber 4. The poppet valve member 12 has an annular poppet proximal end portion 12a secured to the valve body 6, an approximately tubular bellows portion 12b

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contiguous with the poppet proximal end portion 12a, and an annular valving portion 12c that is connected to the distal end of the bellows portion 12b and that liftably rests on an atmospheric valve seat 20 and a negative-pressure valve seat 23 (described later). The distal end of the bellows portion 12b is connected to an outer peripheral portion of the valving portion 12c.
Of the interior space (tubular space) 37 in the tubular portion 39, a region 37a rearward of the atmospheric valve 10 (hereinafter referred to as "tubular space rearward region 37a") communicates with the atmosphere through an opening of the tubular portion 39.
A valve spring 15 is interposed between the input rod 7 and the valve body 6. A poppet spring 16 is interposed between the input rod 7 and the distal end of the poppet valve member 12.
The valve spring 15 presses the input rod 7 toward the proximal end side thereof (rightward in Fig. 4) relative to the valve body 6. The poppet spring 16 presses the distal end of the poppet valve member 12 against the plunger 8 (and hence the atmospheric valve seat 20). The vacuum valve 11 is positioned with its center axis coincident with the center axis of the atmospheric valve 10 (i.e. in coaxial relation to the atmospheric valve 10) and outside the atmospheric valve 10 (i.e. the vacuum valve 11 is larger in diameter than the atmospheric valve 10).
When the pneumatic booster 1 is in an inoperative

state, both the atmospheric valve 10 and the vacuum valve 11 are closed. When a brake pedal (not shown) connected to the input rod 7 is depressed, the input rod 7 advances against the forces of the valve spring 15 and the poppet spring 16, causing the atmospheric valve 10 to open. Consequently, the tubular space rearward region 37a and the variable-pressure chamber 4 communicate with each other, and atmospheric air is introduced into the variable-pressure chamber 4. The introduction of atmospheric air into the variable-pressure chamber 4 produces a pressure difference between the constant-pressure chamber 3 and the variable-pressure chamber 4. The pressure difference propels the power piston 5, causing thrust to be output to an output rod 22 through a reaction disk 21. Meanwhile, a part of reaction force from the output rod 22 to the reaction disk 21 is transmitted to the input rod 7.
The atmospheric valve 10 is closed when the input rod 7 stops advancing and when the valve body 6 advances by a predetermined amount relative to the input rod 7 as the result of the advance of the power piston 5 (valve body 6). Thus, a balanced state is reached.
When the brake pedal is released, the input rod 7 is retracted by the spring force of the valve spring 15, and the plunger 8 also retracts. Consequently, the poppet valve member 12 is pushed by the plunger 8 to lift from a valve seat 23 of the vacuum valve 11 (hereinafter referred to as "negative-pressure valve seat 23") that is provided

inside the valve body 6. As a result, a negative pressure is introduced into the variable-pressure chamber 4 through a negative-pressure passage 24 and an atmospheric air passage 25, and thus the above-described pressure difference is canceled. Thereafter, the valve body 6 is retracted by the spring force of a return spring 26 provided in the constant-pressure chamber 3, and the power piston 5 returns to its original position where a stopper key 27 inserted into a radial hole 27a formed in the valve body 6 abuts against a stepped surface 29 in a rear shell 28. At the same time, the plunger 8 also returns to its original position. Thus, the vacuum valve 11 is also closed.
The negative-pressure passage 24 is formed in the valve body 6. One end of the negative-pressure passage 24 communicates with the constant-pressure chamber 3. The other end of the negative-pressure passage 24 opens on the inner peripheral surface of the tubular portion 39. An opening 24a at the other end of the negative-pressure passage 24 (hereinafter referred to as "negative-pressure passage opening 24a") is disposed to face the outer periphery of the poppet valve member 12. The reaction disk-side edge of the negative-pressure passage opening 24a is formed to define as the negative-pressure valve seat 23 of the vacuum valve 11. The outer peripheral portion of the valving portion 12c liftably rests on the negative-pressure valve seat 23. A space 70 formed by the tubular portion 39 and the poppet valve member 12

(hereinafter referred to as "poppet outer peripheral space 70") communicates with the negative-pressure passage 24 in a state where the valving portion 12c rests on the negative-pressure valve seat 23.
As has been stated above, the atmospheric valve 10 opens in response to the advance of the input rod 7. When this is to be achieved, the force applied to the input rod 7 must exceed a basic force required to begin opening of the atmospheric valve 10 (hereinafter referred to as "ineffective input"), which is determined by the set loads (spring forces) of the valve spring 15 and the poppet spring 16. Furthermore, the valve spring 15 and the poppet spring 16 have set loads (spring forces) set so as to effectively act against the pressure difference between the atmospheric pressure and the pressure in the constant-pressure chamber 3 placed under a negative pressure or the pressure in the variable-pressure chamber 4 supplied with a negative pressure.
Incidentally, pneumatic boosters have recently been demanded to have an increased stroke in accordance with the increase in the stroke of master cylinders used in combination with them. Achievement of an increased stroke for a pneumatic booster requires a reduction of the pedal lever ratio in order to maintain a favorable relationship between the stroke with respect to the associated master cylinder and the brake pedal operating quantity. That is, it is necessary to push the input rod 7 at a position as far away from the center of the pedal pivot as possible.

so that the input rod 7 will be displaced to a larger extent for the same amount of pivotal movement of the pedal.
However, a reduction of the pedal lever ratio weakens the force gain effect of the lever and hence results in an increase in the initial pedal pressure (and hence an increase in the ineffective input) in pneumatic boosters with the conventional structure. Consequently, the pedal operability is degraded correspondingly, causing inconvenience to users.
It is conceivable to minimize the ineffective input for the purpose of improving the pedal operability degraded as stated above. However, the greater part of the ineffective input is the set loads (spring forces) of the valve spring and poppet spring set so as to effectively act against the pressure difference between the atmospheric pressure and the pressure in the constant-pressure chamber placed under a negative pressure or the pressure in the variable-pressure chamber supplied with a negative pressure, as stated above. It has heretofore been difficult to reduce the ineffective input appropriately.
The present invention was made in view of the above-described circumstances. Accordingly, an object of the present invention is to provide a pneumatic booster capable of minimizing the spring force of the poppet spring and hence capable of reducing the ineffective input.

The present invention provides a pneumatic booster including a shell body and a power piston that divides the interior of the shell body into a constant-pressure chamber and a variable-pressure chamber. A valve body connected to the power piston has a tubular portion projecting rearward from the shell body. A valve mechanism is provided in the valve body. The valve mechanism is actuated in association with the movement of a plunger connected to an input rod. A negative-pressure passage is provided in the valve body. The negative-pressure passage communicates at one end thereof with the constant-pressure chamber. The other end of the negative-pressure passage opens in the inner surface of the tubular portion as a negative-pressure passage opening. The valve mechanism is actuated in response to the advance of the input rod to introduce atmospheric air into the variable-pressure chamber through an opening of the tubular portion, thereby producing a pressure difference between the constant-pressure chamber and the variable-pressure chamber, and the power piston is propelled by the pressure difference. The negative-pressure passage opening opens toward the proximal end side of the input rod. The valve mechanism has an annular atmospheric valve seat formed on the rear end of the plunger, and a negative-pressure valve seat formed on the peripheral edge of the negative-pressure passage opening. The valve mechanism further has a poppet valve member provided to liftably rest on the atmospheric valve seat and the negative-pressure valve

seat, and a poppet spring that presses the poppet valve member in a direction for resting on the atmospheric valve seat and the negative-pressure valve seat. The poppet valve member opens or closes the negative-pressure passage opening by lifting from or resting on the negative-pressure valve seat. A poppet back space is defined by the tubular portion and the poppet valve member in an area at the back of the poppet valve member that faces the negative-pressure passage opening. The poppet back space communicates with the variable-pressure chamber.
In a first preferred example of the present invention, the poppet valve member has an annular poppet proximal end portion secured to the tubular portion of the valve body, and an approximately tubular bellows portion connected to the poppet proximal end portion. The poppet valve member further has an annular valving portion connected to the distal end of the bellows portion. The valving portion liftably rests on the atmospheric valve seat and the negative-pressure valve seat. The distal end of the bellows portion is connected to an inner peripheral portion of the valving portion.
In a second preferred example of the present invention, the poppet valve member has an annular poppet proximal end portion secured to the tubular portion of the valve body through an annular retainer provided inside the tubular portion, and an approximately tubular bellows portion connected to the poppet proximal end portion. The poppet valve member further has an annular valving portion

connected to the distal end of the bellows portion. The valving portion liftably rests on the atmospheric valve seat and the negative-pressure valve seat. The poppet spring is provided between the valving portion and the retainer.
In a third preferred example of the present invention, the poppet spring is provided in the poppet back space.
In a fourth preferred example of the present invention, the poppet proximal end portion is fitted to the inner periphery of the retainer and has an outer diameter smaller than the inner diameter of the poppet spring.
In a fifth preferred example of the present invention, the distal end of the bellows portion is connected to an inner peripheral portion of the valving portion.
According to the present invention, when a boosting action is to be initiated, the negative-pressure passage has been placed under a negative pressure and cut off from the poppet back space. When the atmospheric valve opens in response to the advance of the input rod, atmospheric air is introduced into the variable-pressure chamber to propel the power piston. Meanwhile, atmospheric air is introduced into the poppet back space. Consequently, the poppet valve member is urged in a direction for closing the negative-pressure passage, i.e. the negative-pressure valve seat. Accordingly, the spring force of the poppet

spring required to keep the vacuum valve closed can be reduced correspondingly, and hence the ineffective input can be minimized.
According to the first example of the present invention, the distal end of the bellows portion is connected to an inner peripheral portion of the valving portion. Therefore, the poppet back space can be widened.
According to the second example of the present invention, the poppet spring is interposed between the valving portion and the retainer. Therefore, the spring force of the poppet spring surely acts as pressing force for the negative-pressure valve seat. Thus, satisfactory sealing properties can be ensured. Because the spring force of the poppet spring does not act on the input rod, the ineffective input to the input rod can be minimized.
According to the third example of the present invention, the poppet spring is provided in the poppet back space. Therefore, atmospheric air does not pass through the poppet spring when introduced into the variable-pressure chamber. Hence, air-passing noise can be reduced.
According to the fourth example of the present invention, the poppet proximal end portion has an outer diameter smaller than the inner diameter of the poppet spring. Therefore, the poppet spring can be readily inserted to the back of the poppet valve member.
According to the fifth example of the present invention, the distal end of the bellows portion is

connected to an inner peripheral portion of the valving portion. Therefore, the poppet back space can be widened.
Fig. 1 is a sectional view schematically showing a pneumatic booster according to one embodiment of the present invention.
Fig. 2 is an enlarged view of an essential part of Fig. 1.
Fig. 3 is a front view showing a part of a valve body in Fig. 2 that includes negative-pressure passage openings.
Fig. 4 is a sectional view schematically showing a conventional pneumatic booster.
A pneumatic booster 1A according to one embodiment of the present invention will be described below in detail with reference to Figs. 1 and 2.
In Figs. 1 and 2, the pneumatic booster 1A has a shell body 2 including a front shell 30 and a rear shell 28. A power piston 5 is provided in the shell body 2 to divide the interior thereof into a constant-pressure chamber 3 and a variable-pressure chamber 4. The power piston 5 is held by a diaphragm 35 and provided in its center with a valve body 6 having an axial hole 36 and a cylindrical space 37.
The valve body 6 has a large-diameter body portion (hereinafter referred to as "valve body portion") 38 in which the axial hole 36 and a part of the cylindrical

space 37 are formed, and a small-diameter tubular portion 39 where the greater part of the cylindrical space 37 is formed. The valve body portion 38 and the tubular portion 39 are contiguous with each other. The tubular portion 39 extends air-tightly and slidably through a small-diameter tubular portion 40 at the rear of the rear shell 28. The tubular portion 39 extends rearward beyond the tubular portion 40. The tubular portion 39 includes a tubular distal end portion 39A closer to an opening thereof (hereinafter referred to as "tubular portion opening") 39a, and a tubular proximal end portion 39B remote from the tubular portion opening 39a. The tubular proximal end portion 39B is contiguous with the valve body portion 38. The tubular proximal end portion 39B has a larger thickness than that of the tubular distal end portion 39A.
The valve body 6 is provided with a negative-pressure passage 71 (constant-pressure passage) that communicates at one end thereof with the constant-pressure chamber 3. The other end of the negative-pressure passage 71 opens on the inner surface of the tubular portion 39. An opening 72 at the other end of the negative-pressure passage 71 (hereinafter referred to as "negative-pressure passage opening 72") projects from the tubular proximal end portion 39B toward the tubular distal end portion 39A in an approximately tubular shape and opens toward the proximal end side of an input rod 7 (rightward in Fig. 2; toward the obverse side of the plane of Fig. 3). The negative-pressure passage opening 72 is positioned to face

an annular valving portion 57 of a poppet valve member 12A. As shown in Fig. 3, the valve body 6 is provided with two negative-pressure passages 71, and two negative-pressure passage openings 72 are provided in correspondence with them. A ring-shaped edge of each negative-pressure passage opening 72 forms a negative-pressure valve seat 73 on which the valving portion 57 can liftably rest. In other words, the valving portion 57 opens or closes the negative-pressure passage openings 72 by lifting from or resting on the negative-pressure valve seats 73.
A space (hereinafter referred to as "poppet back space") 70A is defined by the tubular portion 39, an inner tubular member 74 and the poppet valve member 12A in an area at the back of the valving portion 57 (poppet valve member 12A) that faces the negative-pressure passage openings 72. The poppet back space 70A communicates with the variable-pressure chamber 4 through an atmospheric air passage 25. Communication between the poppet back space 70A and the negative-pressure passages 71 is cut off when the valving portion 57 rests on the negative-pressure valve seats 73.
The inner tubular member 74 is fitted to the tubular distal end portion 39A. The inner tubular member 74 consists essentially of a tubular body 75. a tubular thick-walled portion 76 (annular retainer) contiguous with one end of the tubular body 75 and having a larger thickness than that of the tubular body 75, and a tubular portion first flange 77 projecting radially outward from

the other end of the tubular body 75 to abut against the tubular portion opening 39a. The tubular portion first flange 77 is used for positioning with respect to the valve body 6.
An annular groove 79 is formed on the outer periphery of the tubular thick-walled portion 76 to accommodate and secure an O-ring 78.
A radially inwardly projecting tubular portion second flange 80 is formed forward (leftward in Fig. 2) of the tubular thick-walled portion 76 to seal an annular poppet proximal end portion 55 of the poppet valve member 12A fitted to the inner periphery of the tubular thick-walled portion 76. The tubular portion second flange 80 also prevents dislodging of the poppet valve member 12A.
The tubular portion second flange 80 has an annular projection 81 projecting forward (leftward in Fig. 2) to retain one end of a poppet spring 16A.
A poppet retainer 82 is disposed in the tubular portion 39 to retain the poppet proximal end portion 55 of i the poppet valve member 12A from the inner peripheral side thereof. The poppet retainer 82 secures the poppet proximal end portion 55 to the tubular thick-walled portion 76 and also retains a valve spring 15.
The valve body 6 is normally urged toward the rear > side (rightward in Fig. 1) by a return spring 26 provided in the constant-pressure chamber 3. When the pneumatic booster 1A is in an inoperative state, the valve body 6 is placed in its original position where a stopper key 27

inserted in the radial hole 27a abuts against an inner stepped surface 29 of the small-diameter tubular portion 40 of the rear shell 28.
The constant-pressure chamber 3 is supplied with an engine negative pressure, for example, through an inlet port 41 provided in the front shell 30. The negative pressure is also supplied into the variable-pressure chamber 4 through the negative-pressure passages 71 and the atmospheric air passage 25 by the operation of the valve mechanism 9 actuated in response to the movement of the input rod 7 extending from a brake pedal (not shown).
A noise-reducing filter 42 and a dust-removing filter 43 are provided in the inner tubular member 74 disposed in the tubular portion 39 at a side thereof closer to the tubular portion opening 39a. Atmospheric air is introduced into the cylindrical space 37 through the filters 42 and 43. The atmospheric air is supplied into the variable-pressure chamber 4 through the atmospheric air passage 25 by the operation of the valve mechanism 9. The noise-reducing filter 42 and the dust-removing filter 43 have a ring shape. The input rod 7 extends through the respective bores of the filters 42 and 43. The filters 42 and 43 are prevented from dislodging by a dust boot 44 in the shape of a tube, one end of which is closed. The dust boot 44 has its brim fitted onto the small-diameter tubular portion 40 of the rear shell 28 and covers the tubular portion 39 of the valve body 6 and the inner tubular member 74.

The valve mechanism 9 has a plunger 8 slidably fitted in the axial hole 36 formed in the valve body portion 38. As shown in Fig. 2, a distal end portion (no reference numeral) of the input rod 7 extending through the noise-reducing filter 42 and the dust-removing filter 43 is inserted into and connected to a rear end portion 45 of the plunger 8 (hereinafter referred to as "plunger-input rod joint 45") and a cavity (no reference numeral) formed in a plunger body portion 48 (described later). The input rod 7 advances leftward in Fig. 1 in response to stepping on the brake pedal, and the plunger 8 also advances together with the input rod 7 as one unit.
The plunger 8 has the plunger-input rod joint 45 and the plunger body portion 48 contiguous therewith. The plunger body portion 48 is larger in diameter than the plunger-input rod joint 45. A shaft portion (hereinafter referred to as "plunger shaft portion") 49 extends from the forward end of the plunger body portion 48. The plunger shaft portion 49 is smaller in diameter than the plunger-input rod joint 45. The plunger shaft portion 49 is inserted into the axial hole 36 and positioned so that the distal end surface of the plunger shaft portion 49 can abut against a reaction disk 21.
As shown in Fig. 2, the valve mechanism 9 has an annular atmospheric valve seat 20 formed on the rear end of the plunger 8 (i.e. plunger-input rod joint 45). The valve mechanism 9 further has the above-described negative-pressure valve seats 73, the poppet valve member

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12A disposed in the tubular portion 39 (cylindrical space 37) to liftably rest on the valve seats 20 and 73, and the poppet spring 16A provided between the valving portion 57 of the poppet valve member 12A and the tubular thick-walled portion 76 to normally press the valving portion 57 in a direction for resting on the valve seats 20 and 70. As shown also in Fig. 2, the poppet spring 16A is disposed in the poppet back space 70A at the back of the poppet valve member 12A.
The poppet valve member 12A consists essentially of the following portions: the annular poppet proximal end portion 55 secured to the valve body 6 with the inner tubular member 74 interposed therebetween; an approximately tubular bellows portion 56 contiguously connected at the proximal end thereof to the inner peripheral end side of the poppet proximal end portion 55; and the above-described annular valving portion 57 provided at the distal end of the bellows portion 56. The bellows portion 56 is gently enlarged in diameter from the distal end toward the proximal end thereof and disposed between the poppet back space 70A and the cylindrical space 37. The poppet proximal end portion 55 is secured to the inner peripheral side of the tubular thick-walled portion 76, as stated above. Therefore, the poppet proximal end portion 55 is secured at a more radially inward side of the tubular portion 39 than in the conventional structure. The outer diameter of the poppet proximal end portion 55 is smaller than the inner diameter

of the poppet spring 16A, so that the poppet spring 16A can be readily inserted to the back of the poppet valve member 12A. It should be noted that, in this embodiment, the poppet proximal end portion 55, the bellows portion 56 and the valving portion 57 are integrally formed from the same flexible material.
The valve mechanism 9 has an atmospheric valve 10 formed from the atmospheric valve seat 20 and an inner peripheral side portion (no reference numeral) on the front side (left-hand side in Fig. 2) of the valving portion 57 of the poppet valve member 12A that separably abuts against the atmospheric valve seat 20. The valve mechanism 9 further has a vacuum valve 11A formed from the negative-pressure valve seats 73 and an outer peripheral side portion (no reference numeral) on the front side (left-hand side in Fig. 2) of the valving portion 57 of the poppet valve member 12A that separably abuts against the negative-pressure valve seats 73. The variable-pressure chamber 4 is supplied with atmospheric air or a negative pressure selectively according to whether the atmospheric valve 10 or the vacuum valve 11A opens.
The atmospheric valve 10, which is arranged as stated above, selectively allows or blocks communication between the variable-pressure chamber 4 and the atmosphere. The vacuum valve 11A, which is arranged as stated above, selectively allows or blocks communication between the constant-pressure chamber 3 and the variable-pressure chamber 4.

The poppet valve member 12A, which constitutes the valve mechanism 9, is secured to the valve body 6 by a poppet retainer 82 retaining one end of a valve spring 15 that urges the input rod 7 toward the rear side. The other end of the valve spring 15 is retained by a collar portion 66 integrally provided on the input rod 7. Thus, the input rod 7 is normally urged in a return direction toward the rear side. The plunger 8 moves in association with the movement of the input rod 7. The plunger 8 is limited on the range of movement relative to the valve body 6 by the stopper key 27 inserted into the radial hole 27a of the valve body 6. When the pneumatic booster 1A is in an inoperative state, as shown in Fig. 1, the return end of the plunger 8 is defined by the stopper key 27, and the poppet valve member 12A maintains the valve-closing position where it abuts against the atmospheric valve seat 20 and the negative-pressure valve seats 73.
At the front end side of the valve body 6, a reaction disk 21 and a proximal end cup portion 68 of an output rod 22 are positioned so that the distal end of the plunger shaft portion 49 of the plunger 8 faces the back of the reaction disk 21 across a slight gap. The distal end of the output rod 22 extends through the constant-pressure chamber 3 and through the front shell 30 and operatively coupled with a piston in a master cylinder (not shown) . It should be noted that the proximal end cup portion 68 of the output rod 22 and the reaction disk 21 are prevented from dislodging by a retainer 69 pressed and

secured to the front end of the valve body 6 by the return spring 26.
The operation of the pneumatic booster 1A will be explained below.
The pneumatic booster 1A is mounted on a vehicle body (not shown) by using a plurality of stud bolts 80 provided on the rear side of the rear shell 28, while the master cylinder (not shown) is connected to the pneumatic booster 1A by using stud bolts 81 provided on the front side of the front shell 30.
When the brake pedal (not shown) is depressed, the input rod 7 advances, and the plunger 8 also advances, causing the atmospheric valve 10 at the rear end of the plunger 8 to separate from the poppet valve member 12A. Thus, the atmospheric valve 10 opens, and atmospheric air flows into the valve body 6 through the noise-reducing filter 42 and the dust-removing filter 43. The atmospheric air is introduced into the variable-pressure chamber 4 from the atmospheric valve 10 through the atmospheric air passage 25. As a result, a pressure difference is produced between the constant-pressure chamber 3 supplied with a negative pressure and the variable-pressure chamber 4 supplied with atmospheric air. The pressure difference propels the power piston 5, together with the valve body 6. Thus, thrust is output from the valve body 6 to the master cylinder through the output rod 22. Meanwhile, a part of reaction to the output is transmitted from the output rod 22 to the input

rod 7 through the reaction disk 21 and the plunger 8. Thus, a boosting action in which the output increases according to the increase in the input takes place.
Incidentally, during the operation of the pneumatic booster, the valve body 6 and the plunger 8 move relative to each other and are subjected to reaction force. This may cause the valve body 6 and the plunger 8 to be skewed with each other. Accordingly, the position at which the valving portion 57 rests on the negative-pressure valve seats 73 may be displaced. If the displacement occurs, air-tightness between the valving portion 57 and the negative-pressure valve seats 73 may be degraded. Therefore, conventional practice is to increase the spring force of the poppet spring to maintain the required air-tightness. In the pneumatic booster 1A, when the atmospheric valve 10 opens at the beginning of a boosting action, the negative-pressure passages 71 have beer placed under a negative pressure and cut off from the poppet back space 70A. Meanwhile, atmospheric air is introduced into the poppet back space 70A. A differential pressure between the negative-pressure passages 71 and the poppet back space 70A urges the valving portion 57 in a direction for closing the negative-pressure passages 71, i.e. the negative-pressure valve seats 73. As a result, the force pressing the valving portion 57 against the negative-pressure valve seats 73 increases by an amount corresponding to a rise in pressure caused by the introduced atmospheric air. thereby allowing the required

air-tightness to be maintained. Accordingly, the spring force of the poppet spring 16A required to close the vacuum valve 11A can be reduced correspondingly, and hence the ineffective input can be minimized. In other words, because the spring force of the poppet spring 16A does not act on the input rod 7, the ineffective input to the input rod 7 can be minimized. A reduction in the spring force of the poppet spring 16A also allows a reduction in the spring force of the valve spring 15 that has to pull back the input rod 7 against the spring force of the poppet spring 16A.
Because the ineffective input can be minimized as stated above, the pedal pressure need not be increased even if the pedal lever ratio is reduced, which is regarded as required in connection with the achievement of an increased stroke. In other words, it becomes possible to attain a longer stroke without increasing the pedal pressure (and hence possible to achieve a reduced pedal lever ratio).
Further, because the distal end of the bellows portion 56 is connected to the inner peripheral portion of the valving portion 57, the poppet back space 70A can be widened. Thus, it is possible to ensure a sufficiently wide space for accommodating the poppet spring 16A at the back of the poppet valve member 12A. In addition, because the effective radial dimension of the bellows portion 56 can be made smaller than in the conventional structure, the area of differential pressure acting on the bellows

portion 56 decreases, and the differential pressure reduces correspondingly. Accordingly, the spring force of the valve spring 15 can be reduced, and, consequently, the ineffective input can be minimized. In this embodiment, the poppet spring 16A is placed in the widened poppet back space 70A to extend between the tubular thick-walled portion 76 and the valving portion 57. Therefore, the spring force of the poppet spring 16A surely acts on the vacuum valve 11A as pressing force. Thus, satisfactory sealing properties can be ensured. Further, atmospheric air does not pass through the poppet spring 16A when introduced into the variable-pressure chamber 4. Therefore, air-passing noise can be effectively reduced. In the foregoing embodiment, the pneumatic booster 1A is of the type that has a single power piston 5. The present invention, however, is not necessarily limited thereto but may be arranged in the form of a tandem type having two power pistons.

We claim:
1. A pneumatic booster comprising:
a shell body;
a power piston provided in said shell body to divide an interior of said shell body into a constant-pressure chamber and a variable-pressure chamber;
a valve body connected to said power piston, said valve body having a tubular portion extending through and projecting rearward from said shell body;
a valve mechanism provided in said valve body, said valve mechanism being actuated in association with movement of a plunger connected to an input rod; and
a negative-pressure passage provided in said valve body, said negative-pressure passage communicating at one end thereof with said constant-pressure chamber, the other end of said negative-pressure passage opening in an inner surface of said tubular portion as a negative-pressure passage opening;
wherein said valve mechanism is actuated in response to advance of said input rod to introduce atmospheric air into said variable-pressure chamber through an opening of said tubular portion, thereby producing a pressure difference between said constant-pressure chamber and said variable-pressure chamber, and said power piston is propelled by the pressure difference;
wherein said negative-pressure passage opening opens toward a proximal end side of said input rod;
said valve mechanism having:

an annular atmospheric valve seat formed on a rear end of said plunger;
a negative-pressure valve seat formed on a peripheral edge of said negative-pressure passage opening;
a poppet valve member provided to liftably rest on said atmospheric valve seat and said negative-pressure valve seat; and
a poppet spring that presses said poppet valve member in a direction for resting on said atmospheric valve seat and said negative-pressure valve seat;
wherein said poppet valve member opens or closes said negative-pressure passage opening by lifting from or resting on said negative-pressure valve seat; and
wherein a poppet back space defined by said tubular portion and said poppet valve member in an area at a back of said poppet valve member communicates with said variable-pressure chamber.
2. A pneumatic booster according to claim 1, wherein said poppet valve member has:
an annular poppet proximal end portion secured to the tubular portion of said valve body;
an approximately tubular bellows portion connected to said poppet proximal end portion; and
an annular valving portion connected to a distal end of said bellows portion, said valving portion liftably resting on said atmospheric valve seat and said negative-pressure valve seat;
wherein the distal end of said bellows portion is

connected to an inner peripheral portion of said valving portion.
3. A pneumatic booster according to claim 1, wherein
said poppet valve member has:
an annular poppet proximal end portion secured to the tubular portion of said valve body through an annular retainer provided inside said tubular portion;
an approximately tubular bellows portion connected to said poppet proximal end portion; and
an annular valving portion connected to a distal end of said bellows portion, said valving portion liftably resting on said atmospheric valve seat and said negative-pressure valve seat;
wherein said poppet spring is provided between said valving portion and said retainer.
4. A pneumatic booster according to claim 3, wherein said poppet spring is provided in said poppet back space.
5. A pneumatic booster according to claim 3 or 4. wherein said poppet proximal end portion is fitted to an inner periphery of said retainer and has an outer diameter smaller than an inner diameter of said poppet spring.
6. A pneumatic booster according to any one of claims 3 to 5, wherein the distal end of said bellows portion is connected to an inner peripheral portion of said valving portion.