JP2530333Y2 - Negative pressure booster - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION A. Object of the Invention (1) Field of Industrial Application The present invention relates to a vacuum booster used for operating a brake master cylinder of a vehicle, for example, a booster shell, and a booster shell. The booster shell reciprocates back and forth to partition the inside into a first working chamber connected to a negative pressure source and a second working chamber selectively connected to the first working chamber or the atmosphere via a control valve. A booster piston that is freely accommodated; a valve cylinder for a control valve fitted in a valve cylinder fitting hole located at the center of the booster piston and a rear surface of a retaining flange closely overlapped with a front surface of the booster piston; A locking plate that elastically engages with a plurality of locking claws of the booster piston so as to cooperate with the booster piston to lock the lock;
On the front surface of the booster piston, a substantially annular reinforcing rib is provided radially outward from a close overlap portion between the front surface and the rear surface of the retaining flange and radially inward from each of the locking claws. The present invention relates to a negative pressure type booster integrally protruded.

(2) Prior Art The present applicant has previously proposed an apparatus of this type disclosed in Japanese Patent Application Laid-Open No. 60-67251.

(3) Problems to be Solved by the Invention In the conventional device described above, a portion of the booster piston radially outward from the retaining flange may be bent forward due to a pressure difference generated between the two working chambers. Since the rear surface of the retaining flange is always in close contact with the front surface of the booster piston and the bending fulcrum of the piston is always located at the outer peripheral edge of the rear surface of the retaining flange, the booster piston corresponding to the outer peripheral edge of the rear surface is specified. The stress caused by the bending is concentrated at the location, and there is a possibility that the location may be damaged at an early stage.

The present invention has been proposed in view of the above, and a reinforcing rib provided conventionally on the front surface of a booster piston is used so that stress concentration can be effectively prevented from occurring at the above-described specific portion of the booster piston. It is an object of the present invention to provide a vacuum booster having a simple structure.

B. Configuration of the Invention (1) Means for Solving the Problems In order to achieve the above object, the present invention provides a negative pressure booster of the type described above, in which an outer peripheral portion of a rear surface of a retaining flange is provided.
A step is provided there to form a rearward supporting surface deflected to the front side from the rear surface, and the front end of a reinforcing rib disposed on the booster piston corresponding to the step is brought into contact with the supporting surface. The reinforcing ribs are formed in a cross-sectional arc shape so that the contact area of the reinforcing ribs with the support surface expands outward in the radial direction of the booster piston due to deformation of the reinforcing ribs due to the forward deflection of the booster piston. Further, the support surface is formed on a flat surface.

(2) Operation When the amount of forward bending of the booster piston increases, the contact area of the reinforcing rib, which deforms in response to the flat support surface, expands outward in the radial direction, and therefore, the corresponding area of the booster piston. Stress distribution in the vicinity of the contact area is achieved, and the moment applied to the booster piston when flexing is reduced.

Moreover, since the supporting surface to which the reinforcing rib is to be applied is formed on the step portion specially provided on the rear surface of the retaining flange, the reinforcing rib abuts on the rear surface of the retaining flange,
The close overlapping state of the portion of the front surface of the booster piston radially inside the reinforcing rib and the rear surface of the retaining flange is maintained.

(3) Embodiment Hereinafter, an embodiment of the present invention will be described with reference to the drawings. First, in FIGS. 1 and 2, a booster piston 2 is housed in a booster shell 1 so as to be able to reciprocate back and forth, and the booster piston 2 causes the inside of the booster shell 1 to have a first working chamber A at the front and a first working chamber A at the rear. And two working chambers B.

A diaphragm 3 is superimposed on the rear surface of the booster piston 2 and its outer peripheral bead 3a is formed on the peripheral wall of the booster shell 1 and therefore before forming the booster shell 1, the rear shell 1a, 1
The inner peripheral bead 3b is connected to the inner peripheral portion of the booster piston 2 and is connected to the annular groove 4 defined at the abutting portion b.
The binding structure of the inner bead 3b will be described later.

The first working chamber A is always in communication with an intake manifold (not shown) of the internal combustion engine, which is a negative pressure source, through a negative pressure introducing pipe 5, and the second working chamber B is connected to a first through a control valve 6 described later. The first working chamber A or the air introduction port 9 opened in the end wall 8 of the rear extension cylinder 7 of the booster shell 1 is selectively communicated.

The booster piston 2 is always rebounded by a return spring 10 contracted into the first working chamber A, that is, is resiliently moved toward the second working chamber B. When the rearward limit is reached, a protrusion 11 formed on the rear surface of the diaphragm 3 is raised. It is regulated by contacting the inner surface of the rear wall of the shell 1.

The booster piston 2 is provided with a control valve valve cylinder 12 projecting rearward from the center thereof, and the valve cylinder 12 is slidably supported by a flat bearing 13 provided on the rear extension cylinder 7, It is opened toward the air inlet 9.

The control valve 6 is configured in the valve cylinder 12 as follows. That is, the first valve seat 14 _first annular are formed on the front inner wall of the valve cylinder 12, the front of the valve cylinder 12, valve piston 16 constituting the front end is engaged slidably coupled to the input rod 15, This valve piston 16
Second valve seat 14 of the annular be surrounded by the first valve seat 14 ₁ to the rear end ₂
Is formed.

A base end portion 18 of a cylindrical valve element 17 having both open ends is prevented from being removed from the inner wall of the valve cylinder 12 via a valve element holding cylinder 19 fitted to the valve cylinder 12. The valve body 17 is formed of an elastic material such as rubber, and a thin diaphragm 20 extends radially inward from a base end portion 18 thereof, and a thick valve portion 21 is continuously provided at the inner peripheral end. are, the valve unit 21 is opposed to ₂ wherein the first and second valve seat 14 _1, 14. Thus, the valve portion 21 can move back and forth by the deformation of the diaphragm 20, and can also contact the front end surface of the valve body holding cylinder 19.

Annular reinforcing plate 22 is embedded in the valve unit 21, which valve spring 60 so as to urge toward the valve section 21 in both valve seat 14 _1, 14 ₂ are connected to.

Outer portion of the first valve seat 14 ₁ via the through hole 23 of the valve cylinder 12 first
The working chamber A, also the first and second valve seat 14 _1, 14 middle portion of the ₂ second working chamber through another hole 24 B, further second valve seat
14 ₂ of the inner portion is always communicated respectively to the air inlet port 9 through the internal valve 17.

The valve cylinder 12 is provided with a large-diameter cylinder hole 25 opening at the center of the front surface thereof and a small-diameter cylinder hole 26 opening at an inner end face of the cylinder hole 25. An elastic piston 27 made of rubber or the like and an output piston 28 having the same diameter as the elastic piston 27 are sequentially slid, and a reaction piston 29 having a smaller diameter than the elastic piston 27 is slid into the small-diameter cylinder hole 26. The valve piston is inserted into a smaller diameter cylinder hole 26.
Small shaft 30 protruding from the front end face of 26 rushes and is a reaction piston
It faces the rear end face of 29. Output rod in front of output piston 28
An output rod 31 is connected to a working piston of a brake master cylinder (not shown).

The input rod 15 is always repelled by the return spring 32 in the retreating direction. The retraction limit is that the movable stopper plate 33 screwed and attached to the input rod 15 comes into contact with the inside of the end wall 8 of the rear extension cylinder 7. Regulated by When the movable stopper plate 33 is rotated, the screwing position between the movable stopper plate 33 and the input rod 15 changes.
The retraction limit of 15 can be adjusted back and forth. The fixing of the movable stopper plate 33 after the adjustment is performed by tightening a lock nut 34 which is also screwed to the input rod 15. Vent holes in the movable stopper plate 33 so that they do not block the air introduction port 9.
35 is formed.

Filters 36 and 37 that can purify the air introduced from the atmosphere inlet 9 and deform so as not to hinder the operation of the input rod 15 are attached to the outer end opening of the valve cylinder 12.

Next, the assembling structure of the valve cylinder 12 with respect to the booster piston 2 will be described. The booster piston 2 is formed in a substantially disc shape from a thin steel plate in order to reduce its cost, and a valve cylinder fitting hole 38 is formed at the center thereof. Then, an inner bead 3b of the diaphragm 3 is attached to the edge of the valve cylinder fitting hole 38.

The valve cylinder 12 is molded from a synthetic resin such as phenolic resin, and fits into the valve cylinder fitting hole 38.
And a stopper flange 40 projecting from the outer peripheral surface of the front end of the cylindrical body 39 and having a rear surface a overlapped with the front surface b of the booster piston 2.

A pair of notched recesses 41 are formed in the outer peripheral portion of the retaining flange 40 so as to be point-symmetric with respect to the center of the retaining flange 40, and a pair of convex portions 42 engaging with both the concave portions 41 are provided with the booster piston 2. Is bulged and formed on the front surface b. The concave portion 41 and the convex portion 42 constitute a concave / convex engaging portion for stopping the valve cylinder 12 from rotating with respect to the booster piston 2.

As clearly shown in FIG. 2, a straight line XX connecting the two convex portions 42 is formed.
Both sides of the booster piston 2, each of the first pair of hook so as to be positioned around the retaining flange 40, the second locking claw 43 _1, 43 ₂ is formed by a cut and raised adjacent.

The linear X-X both first locking claw 43 ₁ other and both second engagement claw 43 ₂ mutually existing on both sides of which a point-symmetrical about the center of the booster piston 2, across the straight line X-X the second engagement claw 43 ₂ of both the distal end of the first locking claw 43 ₁ and the other side of the one side
44 _1, 44 ₂ are adapted to face each other.

Each first and second locking claws 43 _1, 43 _2, reinforcing ribs from the tip portion 44 _1, 44 ₂ so as to extend to the flat portion 45 of the booster piston 2
46 _1, 46 ₂ are bulged shaped.

The holding plate 46 is formed of a steel plate, and is attached to the booster piston 2 by rotating clockwise around the center line of the retaining flange 40 in FIG.
The stopper flange 40 is clamped under the cooperation of the stopper plate body 47 and the clamping plate body 47 overlapping the front surface c of the stopper flange 40, and opposed to the outer peripheral portion of the stopper plate body 47. A pair of bow-shaped bent portions 48 formed and loosely fitted to the outer peripheral surface of the retaining flange 40, and retaining flanges from the outer peripheral edge at the tip of each bent portion 48.
40 projecting outward in the radial direction, each of the first and second locking claws
43 _1, 43 ₂ of the front end portion 44 _1, a first pair of elastic engagement 44 ₂ inner surface,
And second locking pieces 49 ₁ and 49 ₂ .

As clearly shown in FIG. 3, each of the first engagement pieces 49 ₁ is provided on a side edge on the rear side in the rotation direction of the holding plate, and each of the second engagement pieces 49 ₂ is provided on a front side in the rotation direction of the holding plate. At the rims, there are locking stoppers 50 ₁ , 50 ₂ that bend inward so as to approach the flat part 45 of the booster piston 2, respectively. Flat part 45 of booster piston 2
And extends in a direction intersecting the rotation direction of the
Locking claws 43 _1, 43 ₂ of the cut-and-raised holes 51 _1, 51 ₂ each pair of reinforcing ribs 52 ₁ sandwiching the, 52 ₂ are bulged shaped, each first engagement piece 49 ₁ of the latch
50 _{1 The} tip is one of the reinforcing ribs 5
Facing the 2 ₁ and the latch 50 ₂ tip of each second engagement piece 49 _2, between the second engagement claw 43 _{and second} clamping stop plate rotation direction front side edge and the flat portion 45 of the booster piston 2 It abuts against the side wall 53 to be connected. The sidewalls 53 are simultaneously molded with the second engagement claw 43 ₂ cut-and-raised.

This way both the latch 50 _1, 50 ₂ and one of the reinforcing ribs 52 ₁ and the side wall 53 in cooperation with Kyotomeban 46 retaining flange
Rotation in the detaching direction around the center line 40, that is, clockwise and counterclockwise directions in FIG. 2 is prevented.

Each pair of reinforcing ribs 52 ₁ , 52 ₂ cuts and raises and effectively reinforces the periphery of the holes 51 ₁ , 51 ₂ , and the reinforcing ribs 52 ₁ existing on the rear side in the rotation direction of the pinching plate come off the pinching plate 46. It also serves as a stop.

The inner bead 3b of the diaphragm 3 is provided with a valve cylinder fitting hole 38.
It is connected between the rim and the outer surface of the valve cylinder 12, thereby forming a valve fitting hole.
38 is sealed. A pair of windows 54 is formed in the holding plate main body 47, and one of the windows 54 faces the opening of the through hole 23 in the valve cylinder 12.

As shown in FIG. 4 (a), an annular step 58 is formed on the outer periphery of the rear surface a of the retaining flange 40 so as to cross both the recesses 41 and deviate forward from the rear surface a. 58 is the front surface b of the booster piston 2 in a portion other than the two concave portions 41.
And has a flat surface 58a as a support surface facing the light emitting device. On the flat portion 45 of the booster piston 2, a pair of semi-annular reinforcing ribs 59, both ends of which are connected to the biconvex portions 42, are formed so as to protrude forward, and each reinforcing rib 59 is formed in an arc-shaped cross section. The top, ie, the front end, is in contact with the flat portion 58a. Thus, each reinforcing rib 59 is elastically deformed such that the contact area with the flat portion 58a expands outward in the radial direction of the booster piston 2 with the forward bending of the booster piston 2.

The reinforcing ribs 59 Thus, the booster piston front b and and each locking claw 43 ₁ radially outward of closely overlapping portion of the surface a following retaining flange 40 of the 2, 43 radially inside than ₂ And reinforces that portion of the booster piston 2. Assembling of the holding plate 46 to the booster piston 2 is performed as follows.

That is, as shown by the chain lines in FIGS. 2 and 3, the clamping plate body 47 is overlapped on the front surface c of the retaining flange 40, and each bent portion 48 is loosely fitted on the outer peripheral surface of the retaining flange 40 so that each of the first the engagement piece 49 ₁ above the respective reinforcing ribs 52 ₁ Kyotome plate rotation direction rear side, also the first respective second engagement pieces 49 ₂ Aitonaru, second locking claws 43 _1, 43 ₂ between In each case. Then, the clamping plate 46 is rotated in the clockwise direction in FIG. 2 while pressing the retaining flange 40 thereby compressing the inner peripheral bead 3b of the diaphragm 3 to rotate the first and second engaging pieces.
49 _1, 49 ₂ each first forcibly sliding engagement with the second engagement claw 43 _1, 43 ₂ of the front end portion 44 _1, 44 ₂ inner surface.

In this case, since each concave portion 41 of the retaining flange 40 is engaged with each convex portion 42 of the booster piston 2, the rotation of the valve cylinder 12 with respect to the booster piston 2 is reliably performed.
When engaging the second engagement piece 49 _1, 49 ₂ each of the first, the second locking claw 43 _1, 43 _2, there is no twisting of the inner peripheral bead 3b of the diaphragm 3.

Also the latch 50 ₁ of the first engagement piece 49 ₁ is easily move over the respective reinforcing ribs 52 ₁ by its elasticity, so that each reinforcing rib 5
2 ₁ to permit rotation of the direction of engagement Kyotomeban 46, each on its first
1, Guides 55 ₁ and 55 ₂ are formed on the side edges of the second locking claws 43 ₁ and 43 ₂ on the rear side in the rotation direction of the pinching plate so as to bend outwardly away from the flat portion 45 of the booster piston 2. And each first
Guides 56 which are bent inward so as to approach the flat portion 45 of the booster piston 2 are formed on the front edge of the engagement piece 49 _{1 on} the front side in the rotation direction of the holding plate, and the second engagement pieces 49 ₂ are disengaged. Stop 50 ₂
There therefore form a guiding action, the first, second locking claws 43 _1, 43 each for ₂ first, second engagement piece 49 _1, 49 ₂ of the sliding case carried out smoothly.

As clearly shown in FIG. 4 (a), the outer peripheral portion of the front surface c of the retaining flange 40 is formed as a tapered surface 57 which expands from the front surface c toward the rear surface a, and the tapered surface 57 is opposed to the tapered surface 57. An arcuate space S is defined between the holding plate 46 and the rear surface of the engagement piece connecting portion 46a. Each first by the space S, the fulcrum f of the deflection in the second engagement piece 49 _1, 49 ₂ becomes the front edge of the tapered surface 57, the fulcrum f and the first, second engagement piece 49 ₁ , 49 since the distance between the _two becomes long, the first, second engagement piece 49 _1, 49 ₂ of the spring action is increased, thereby the first, each relative to the second locking claw 43 _1, 43 ₂ a 1, the sliding engagement of the second engagement pieces 49 ₁ and 49 ₂ becomes easy,
The respective first, second engagement piece 49 _1, 49 ₂ of the elastic engagement force is increased.

Next, the operation of this embodiment will be described. The state shown in FIG. 1 shows a non-operating state, in which the valve piston 16 and the input rod 15 connected to each other come into contact with the end wall 8 where the movable stopper plate 33 is fixed. held with a spring force of the return spring 32 to a predetermined retracted position, and the valve piston 16 presses the front surface of the second valve seat 14 ₂ via valve 21, lightly contacting it in front of the valve body the holding cylinder 19 To the first valve seat
Forming a small gap g between 14 ₁ and the valve unit 21. Such a state can be easily obtained by adjusting the movable stopper plate 33 described above.

As described above, the first working chamber A, which always stores the negative pressure during the operation of the engine, communicates with the second working chamber B through the through hole 23, the gap g, and the through hole 24, and the front opening of the valve portion 21. since is closed by the second valve seat 14 _2, the second working chamber B has a negative pressure is transmitted both working chamber a of the first working chamber a, pressure B is balanced. Accordingly, the booster piston 2 also occupies the retracted position shown in FIG.

Now, if a brake pedal (not shown) is depressed to brake the vehicle and the input rod 15 and the valve piston 16 are advanced,
The valve portion 21 urged forward by the valve spring 60 has a valve piston 16.
Although follow-up to move forward, since the gap g between the first valve seat 14 ₁ and the valve portion 21 is very narrow, as described above, the valve unit 21, both working chambers immediately seated on the first valve seat 14 ₁ a, cuts off the communication between B, to communicate with the air introduction port 9 at the same time the second valve seat 14 ₂ through the internal hole 24 and the valve body 17 of the second working chamber B away from the valve unit 21. Therefore, the atmosphere is quickly introduced into the second working chamber B, and the pressure of the chamber B becomes higher than that of the first working chamber A.
The booster piston 2 returns spring due to the pressure difference between them.
Since the output rod 31 is moved forward through the elastic piston 27 and moves forward, the working piston of the brake master cylinder is driven forward, and the vehicle is braked.

When the small shaft 30 of the valve piston 16 moves forward, the
When the elastic piston 27 comes into contact with the elastic piston 27 through the valve 29, a part of the reaction force is swelled toward the reaction piston 29 due to the operation reaction of the output rod 31, and a part of the reaction force is transmitted to the brake pedal through the valve piston 16. This allows the operator to sense the output of the output rod 31, that is, the braking force.

As shown in FIG. 4 (b), when the booster piston 2 moves forward when the booster piston 2 bends forward due to a pressure difference generated between the two working chambers A and B, the reinforcing ribs 59 move outward of the booster piston 2 in the radial direction. As the amount of deformation increases as the amount of deflection of the booster piston 2 increases, each of the reinforcing ribs 59 supporting the deflection of the booster piston 2.
The contact area of the booster piston 2 with respect to the flat portion 58a expands outward in the radial direction of the booster piston 2. As a result, stress caused by the bending of the booster piston 2 does not act intensively at one place, and the booster piston 2 Is also reduced, whereby the durability of the booster piston 2 can be improved.

Next, when releasing the depression force of the brake pedal, the input rod 15 is retracted by first elastic force of the according to the valve piston 16 the reaction force and the return spring 32, thereby the second valve seat 14 ₂ to the valve portion 21 Since the valve portion 21 is seated and the valve portion 21 is brought into contact with the front surface of the valve body holding cylinder 19, the valve portion 21 undergoes a compressive deformation in the axial direction by receiving the retreating force of the input rod 15. As a result, the first valve seat 14
_Since a gap larger than the initial gap g is formed between ₁ and the valve portion 21, if the pressures of the two working chambers A and B are quickly balanced with each other through the large gap, and if the pressure difference between them disappears, The booster piston 2 is retracted by the elastic force of the return spring 10, and the projection 11 of the diaphragm 3 comes into contact with the inner surface of the rear wall of the booster shell 1 and stops. When the input rod 15 abuts against the end wall 8, the valve unit 21 is released from the retracted force input rod 15 is restored to the original shape, the small gap g again the gap between the first valve seat 14 ₁ Can be narrowed.

C. Effects of the Invention As described above, according to the present invention, on the front surface of the booster piston, radially outward from the close overlap portion between the front surface and the rear surface of the retaining flange, and from the respective locking claws of the booster piston. In a radially inwardly protruding substantially annular reinforcing rib integrally provided, a step portion is provided on an outer peripheral portion of a rear surface of a retaining flange to form a rearwardly facing support surface deflected to the front side from the rear surface there. A front end of the reinforcing rib disposed on the booster piston corresponding to the step portion is brought into contact with the supporting surface, and the reinforcing rib is deformed by the forward bending of the booster piston, thereby supporting the reinforcing rib. So that the contact area with the surface extends radially outward of the booster piston,
Since the reinforcing ribs are formed in an arc shape in cross section and the supporting surface is formed in a flat surface, when the amount of forward deflection of the booster piston increases, the reinforcing ribs deformed correspondingly with respect to the flat supporting surface. The contact area expands outward in the radial direction of the booster piston, so that stress distribution can be achieved in the vicinity of the contact area of the booster piston, and the moment applied to the booster piston at the time of the bending can be reduced. Together with the reinforcing effect, it can greatly contribute to improving the durability of the booster piston.

Further, since the reinforcing rib, which is a reinforcing means for the radially intermediate portion of the front surface of the booster piston, can also be used as the bending fulcrum moving means of the booster piston as described above, the structure is simplified and the cost is reduced. be able to. In addition, since the supporting surface to which the reinforcing rib is to be applied is formed on the step portion specially provided on the rear surface of the retaining flange, the reinforcing rib is provided on the front surface of the booster piston despite the contact of the reinforcing rib with the rear surface of the retaining flange. It is possible to maintain a close overlap between the radially inner portion and the rear surface of the retaining flange, stabilize the support of the inner peripheral portion of the booster piston, and reduce the size of the overlapping portion of the booster piston and retaining flange in the axial direction. Can be achieved.

[Brief description of the drawings]

The drawings show an embodiment of the present invention. In particular, FIG. 1 is a longitudinal sectional view of the entirety, which corresponds to a section taken along line II of FIG. 2, and FIG. FIG. 3 is a cross-sectional view taken along the line III-III of FIG. 2, and FIG. 4 is an enlarged vertical cross-sectional view of an assembly portion of the valve cylinder with respect to the booster piston. b) shows a state in which the booster piston is bent. A, B ... first and second working chambers, a ... rear surface, b ... front surface, 1
... Booster shell, 2 ... Booster piston, 6 ...
Control valve, 12 ... valve cylinder, 38 ... valve cylinder fitting hole, 40 ... retaining flange, 43 ₁ , 43 ₂ ... first and second locking claws, 46 ... clamping plate, 58 ... Step, 58a ... Plane as support surface, 59 ...
... reinforcing ribs

Continuation of the front page (56) References JP-A-60-67248 (JP, A) JP-A-60-67251 (JP, A) JP-A-47-39870 (JP, A) , U) JP-B-49-18676 (JP, B1) JP-B-45-4452 (JP, B1)

Claims (1)

(57) [Scope of request for utility model registration] 1. A first working chamber (A) connected to a booster shell (1), a first working chamber (A) connected to a negative pressure source in the booster shell (1), and a control valve (6). Or a booster piston (2) housed in the booster shell (1) so as to reciprocate back and forth so as to be divided into a second working chamber (B) selectively communicated with the atmosphere; )
A control valve valve cylinder (12) fitted in a valve cylinder insertion hole (38) located at the center of the booster piston (2) and having a rear surface closely overlapped with a front surface (b) of the booster piston (2); A clamping plate elastically engaged with a plurality of locking claws (43 ₁ , 43 ₂ ) of the booster piston (2) so as to clamp the retaining flange (40) in cooperation with the booster piston (2). (46); the front surface (b) of the booster piston (2) is located more radially outward than the closely overlapping portion of the front surface (b) and the rear surface (a) of the retaining flange (40). And a substantially annular reinforcing rib (59) integrally protruding radially inward of each of the locking claws (43 ₁ , 43 ₂ ). 40) A step portion is formed on the outer peripheral portion of the rear surface (a) so as to form a rearward-facing support surface (58a) deflected forward from the rear surface (a). 58), and the front end of the reinforcing rib (59) disposed on the booster piston (2) corresponding to the step (58) is brought into contact with the support surface (58a) of the booster piston (2). ) Deforms the reinforcing ribs (59) due to the forward bending of the booster piston (2) so that the contact area of the reinforcing ribs (59) with the support surface (58a) expands outward in the radial direction of the booster piston (2). A negative pressure type booster, wherein the reinforcing ribs (59) are formed in an arc shape in cross section, and the support surface (58a) is formed in a flat surface.