Abstract:
A brake booster includes reaction transmitting means comprising a first and a second retainer, a coiled spring disposed between the both retainers, and a stop which prevents the both retainers from being spaced from each other beyond a given distance. The brake booster also includes a valve mechanism comprising an annular first valve seat formed on the inner periphery of the valve body, a second valve seat formed on the rear portion of the valve plunger at a location radially inward of the first valve seat, and a valve element adapted to be seated upon either valve seat. A backup plate having a diameter less than the internal diameter of the first valve seat is embedded into the valve element. This arrangement allows a sufficient hysteresis to be obtained in a region where the valve element causes an expansion and shrinkage of the coiled spring in the reaction transmitting means to provide a better brake feeling than a conventional arrangement.

Description:
FIELD OF THE INVENTION 
     The invention relates to a brake booster, and more particularly, to a brake booster including a reaction transmitting means having a spring. 
     DESCRIPTION OF THE PRIOR ART 
     A brake booster is known in the art which is designed to produce a reduced servo ratio during an initial phase of a braking operation of the brake booster while increasing the servo ratio during a later phase of the braking operation. 
     A brake booster of the kind described includes reaction transmitting means disposed between a reaction disc and a valve plunger and comprising a first and a second retainer disposed to be displaceable relative to each other, a spring disposed between the first and the second retainer and resiliently charged to a preset load to maintain the both retainers spaced apart normally, and a stop member which prevents the first and the second retainer from becoming spaced beyond a given distance (see Japanese Laid-Open Patent Application No. 85,442/1996). 
     With such an arrangement, if a brake pedal is depressed to operate the brake booster, the reaction transmitting means merely transmits a brake reaction which is transmitted from the reaction disc to the valve plunger through the first retainer, the spring and the second retainer until the spring in the reaction transmitting means is compressed, whereby a prevailing servo ratio can be chosen to be relatively low to improve the maneuverability in a range where a low braking force is required. 
     On the other hand, when the spring in the reaction transmitting means is compressed, the second retainer and the valve plunger are advanced relative to the first retainer and a plate plunger by an amount corresponding to the compression of the spring, and this is equivalent to a further depression of the brake pedal by an amount corresponding to the compression of the spring. This allows the prevailing servo ratio to be set to a relatively high value to allow a braking force of an increased magnitude to be obtained with a reduced force of depression. 
     In the brake booster employing the reaction disc, the reaction disc is formed of an elastic material such as rubber to cause a hysteresis, a phenomenon that the magnitude of an output from the brake booster becomes different between upon depression and release of the brake pedal if the force with which the brake pedal is depressed is maintained constant. 
     In the presence of the hysteresis, there results an advantage that the brake feeling is improved in that if a driver ceases to increases the force to depress the brake pedal and slightly decrease the force unintentionally, the braking force of an equal magnitude is obtained. 
     However, with the brake booster mentioned above which includes the reaction transmitting means, as the spring in the reaction transmitting means is compressed, a compressive deformation of the reaction disc is reduced in a relative sense, whereby the flow path switching action of a valve mechanism in the brake booster takes place almost entirely through the expansion and shrinkage of the spring. As a consequence, because the hysteresis which the reaction disc exhibits becomes reduced during the expansion and shrinkage of the spring, there results a disadvantage that a change in the output from the brake booster becomes sensitive to a change in the force with which the brake pedal is depressed. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing, it is an object of the invention to provide a brake booster which assures a good brake feeling if it includes reaction transmitting means having a spring. 
     Thus, the invention relates to a brake booster comprising a valve body slidably disposed within a shell, a valve mechanism disposed within the valve body, an input shaft for causing a valve plunger, which forms part of the valve mechanism, back and forth to switch a flow path in the valve mechanism, an output shaft slidably disposed on the valve body, a reaction disc interposed between one end of the output shaft and the valve plunger, and reaction transmitting means interposed between the reaction disc and the valve plunger for transmitting a brake reaction from the reaction disc to the valve plunger, and wherein the valve mechanism comprises an annular first valve seat formed on the inner periphery of the valve body, an annular second valve seat formed on a rear portion of the valve plunger at a location radially inward of the first valve seat, a valve element adapted to be seated upon either valve seat, and a backup plate formed by a rigid body embedded into the valve element. In accordance with the invention, the external diameter of the backup plate is chosen to be less than the internal diameter of the first valve seat and greater than the external diameter of the second valve seat. 
     With the described arrangement, as a brake pedal is depressed to drive the valve plunger and the valve element which is seated on the second valve seat which is formed on the valve plunger forward, the valve element will become seated upon the first valve seat on the valve body which remains stationary at a forward position and become removed from the second valve seat, thereby allowing the brake booster to be operated in the similar manner as in the conventional arrangement. 
     The valve element is not provided with a backup plate in an area thereof which is adapted to be seated upon the first valve seat, whereby the elastic deformation becomes increased in comparison to the elastic deformation which occurs when such area is provided with a backup plate. 
     As a consequence, the hysteresis that an output from the brake booster has different magnitudes between upon depression and release of the brake pedal can be enhanced by the area of the valve element which is adapted to be seated upon the first valve seat, thereby allowing a change in the output which has been sensitive to a change in the force with which the brake pedal is depressed to be moderated in a region where the expansion and shrinkage of the spring in the reaction transmitting means occurs. In this manner, an improved brake feeling can be obtained as compared with the conventional arrangement. 
     Above and other objects, features and advantages of the invention will become apparent from the following description of an embodiment thereof with reference to the attached drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross section of a first embodiment of the invention; 
     FIG. 2 is a cross section, to an enlarged scale, of part shown in FIG. 1; 
     FIG. 3 is an enlarged cross section of a valve element  20 ; 
     FIG. 4 is a cross section, to an enlarged scale, illustrating the seating of the valve element  20  on a first valve seat  16 ; and 
     FIG. 5 is a cross section, to an enlarged scale, illustrating a compression of a resilient member  46 . 
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Referring to the drawings, the invention will now be described with reference to an embodiment shown therein. Referring to FIG. 1, a tandem brake booster  1  includes a shell  2 , the interior of which is partitioned by a center plate  3  into a front chamber  4  and a rear chamber  5  thereacross. A tubular valve body  6  slidably extends through a rear portion of the shell  2  and through the center plate  3 , with seal means  7 ,  8  maintaining a hermetic seal thereat. 
     A front power piston  10  and a rear power piston  11  are connected around the outer periphery of the valve body  6  at locations which are situated within the front chamber  4  and the rear chamber  5 , respectively, and a front diaphragm  12  and a rear diaphragm  13  are applied to the back surfaces of the power pistons  10 ,  11 , respectively. A constant pressure chamber A and a variable pressure chamber B are defined across the front diaphragm  12  in the front chamber  4  while a constant pressure chamber C and a variable pressure chamber D are defined across the rear diaphragm  13  in the rear chamber  5 . 
     A valve mechanism  15  which switches a communication between the constant pressure chambers A, C and the variable pressure chambers B, D is disposed within the valve body  6 . Referring to FIG. 2, the valve mechanism  15  comprises an annular first valve seat  16  formed on the valve body  6 , an annular second valve seat  18  formed on a rear portion of a valve plunger  17  which is slidably mounted on the valve body  6  at a location radially inward of the first valve seat  16 , and a valve element  20  which is urged by a poppet return spring  19  to be seated upon either valve seat  16  or  18 . 
     The valve element  20  includes a mount  20   a  which is mounted on the inner peripheral surface of a terminal tubular portion  6 A of the valve body  6 , a curved portion  20   b  of a reduced thickness which is curved radially inward from the mount  20   a  and then extends forwardly, and a seating portion  20   c  formed on the distal end of the curved portion  20   b . A ring-shaped backup plate  21  formed by a rigid body is embedded into the seating portion  20   c . One end of the popped return spring  19  abuts against the rear end face of the backup plate  21 , thus urging the seating portion  20   c  to the left. 
     The mount  20   a  is fixedly mounted within the terminal tubular portion  6 A of the valve body in a hermetically sealed manner by a step  6 B formed on the internal peripheral surface of the terminal tubular portion  6 A and a retainer  22  formed rearward of the mount  20   a.    
     The seating portion  20   c  comprises an annular first seat S 1  (FIG. 3) which contacts the first valve seat  16  and a second seat S 2  (FIG. 3) which contact the second valve seat  18  at a location radially inward of the first seat S 1 . A space located radially outward of the first seat S 1  communicates with the constant pressure chamber A through an axial constant pressure passage  23  formed in the valve body  6 , and the constant pressure chamber A communicates with the constant pressure chamber C through another axial constant pressure passage  24  formed in the valve body  6 . The constant pressure chamber A communicates with an intake manifold of an engine, not shown, through a tubing  25  mounted on the shell  2  for introducing a negative pressure, whereby a negative pressure is normally introduced into the constant pressure chambers A and C. 
     A space located between the first seat S 1  and the second seat S 2  communicates with the variable pressure chamber D through a radial variable pressure passage  26  formed in the valve body  6 , and the variable pressure chamber D communicates with the variable pressure chamber B through an axial variable pressure passage  27  formed in the valve body  6 . 
     A space located radially inward of the second seat S 2  communicates with the atmosphere through a pressure passage  28  disposed in the terminal tubular portion  6 A, with a filter  29  being disposed in the pressure passage  28 . 
     As shown in FIG. 3, the backup plate  21  which is embedded in the seating portion  20   c  of the valve element  20  has an external diameter D 1  which is less than the internal diameter D 2  of the first valve seat  16  and greater than the external diameter D 3  of the second valve seat  18 . In the present embodiment, the first seat S 1  has an increased wall thickness than the remainder by an amount corresponding to the absence of the backup plate  21  embedded therein. 
     Accordingly, when the first seat S 1  becomes seated upon the first valve seat  16 , it undergoes an elastic deformation which is increased by an amount corresponding to the absence of the backup plate  21  embedded therein, whereby its front end face will be greatly depressed. 
     While the first seat S 1  has an increased wall thickness in the present embodiment, it should be understood that the invention is not limited thereto, but that it may be formed to a reduced thickness. In such instance, the first seat S 1  will spring back rearwardly when it becomes seated upon the first valve seat  16 . 
     The right end of the valve plunger  17  is pivotally connected with a distal end of an input shaft  31 , and a valve return spring  32  having a greater resilience than the poppet return spring  19  is disposed between the input shaft  31  and the retainer  22  for normally causing the second seat S 2  on the valve element  20  to be seated upon the second valve seat  18  on the valve plunger  17  and for causing the first seat S 1  on the valve element  20  to be removed from the first valve seat  16  on the valve body  6 . The other end of the input shaft  31  is mechanically coupled to a brake pedal, not shown. 
     A withdrawal of the valve plunger  17  is prevented by a key member  33 , which is movable back and forth in the axial direction of the valve body  6  and which abuts against the internal surface of the shell  2  when the brake booster  1  is inoperative to maintain the valve plunger  17  at an advanced position relative to the valve body  6  to permit a lost motion of the input shaft  31  to be reduced at the commencement of operation of the brake booster  1 . 
     Disposed forwardly of the valve plunger  17  are reaction transmitting means  34  to be described later, a plate plunger  35  and a reaction disc  36  in a sequential manner. The plate plunger  35  is slidably fitted into a holder  37  which is secured to the front end face of the valve body  6  while maintaining a hermetic seal thereat while the reaction disc  36  is received in a recess formed in one end of an output shaft  38 . The recess formed in one end of the output shaft  37  is slidably fitted around the outer peripheral surface of the holder  37  while the other end or the distal end of the output shaft  38  projects externally of the shell  2  while maintaining a hermetic seal by means of a seal member  39 , for connection with a piston of a master cylinder, not shown. 
     Accordingly, a brake reaction which is transmitted from the piston of the master cylinder is transmitted through the output shaft  38  to the reaction disc  36 , and thence transmitted through the holder  37  to be accepted by the valve body  6  and also transmitted through the plate plunger  35 , reaction transmitting means  34 , the valve plunger  17  and the input shaft  31  to a brake pedal, not shown. 
     The valve body  6  and the power pistons  10 ,  11  are urged by a return spring  40  disposed between the shell  2  and the valve body  6  to be normally maintained in their inoperative positions shown. 
     As shown to an enlarged scale in FIG. 2, the reaction transmitting means  34  comprises a first retainer  42  and a second retainer  43 , a coiled spring  44  disposed between the both retainers  42  and  43 , and a stop  45  which prevents the withdrawal of the first retainer  42  from within the second retainer  43 . 
     The first retainer  42  which is disposed rearwardly comprises an inner tubular portion  42   a  which is fitted around the base end of a projection  17 A formed on the distal end of the valve plunger  17 , a radial portion  42   b  extending radially outward from the rear end of the inner tubular portion  42   a  and abutting against a stepped end face on the valve plunger  17 , and an outer tubular portion  42   c  extending forwardly from the outer periphery of the radial portion  42   b.    
     On the other hand, the second retainer  43  which is disposed forwardly comprises an outer tubular portion  43   a  which surrounds the first retainer  42 , a radial portion  43   b  extending radially inward from the front end of the outer tubular portion  43   a  and abutting against a stepped end face on the plate plunger  35 , and an inner tubular portion  43   c  extending axially rearward from the inner periphery of the radial portion  43   b  and through which a projection  35 A on the plate plunger  35  slidably extends. 
     The coiled spring  44  mentioned above is disposed between the radial portion  42   b  of the first retainer  42  and the radial portion  43   b  of the second retainer  43 , and while the coiled spring  44  is compressed to a given degree, the ring-shaped stop  45  which is secured to the outer tubular portion  43   a  of the second retainer  43  is brought into abutment against the rear side of the first retainer  42 , thus assembling the first retainer  42 , the second retainer  43 , the coiled spring  44  and the stop  45  in an integral manner. 
     Accordingly, a brake reaction from the plate plunger  35  is transmitted through the second retainer  43 , the coiled spring  44  and the first retainer  42  to the valve plunger  17 . 
     A disc-shaped resilient member  46  is mounted on the free end of the projection  17 A of the valve plunger  17  which projects through the inner tubular portion  42   a  of the first retainer  42  and has a similar diameter as the projection  17 A. In the inoperative condition, the resilient member  46  is spaced from the projection  35 A on the plate plunger  35  which projects through the inner tubular portion  43   c  of the second retainer  43 . 
     Accordingly, when a brake reaction from the plate plunger  35  exceeds the resilience of the coiled spring  44  to reduce the spacing between the first retainer  42  and the second retainer  43 , the resilient member  46  ultimately abuts against the projection  35 A of the plate plunger  35 . 
     With the described construction, when a brake pedal is depressed to drive the input shaft  31  and the valve plunger  17  forward, the reaction transmitting means  34  is integrally driven forward while maintaining the condition illustrated in FIG. 2, and hence the plate plunger  35  is also driven forward until the force of depressing the brake pedal exceeds the preset load to which the coiled spring  44  is charged. 
     As the valve plunger  17  is driven forward, the valve element  20  which is seated on the second valve seat  18  formed thereon is driven forward, and becomes seated upon the first valve seat  16  formed on the valve body  6  which remains stationary at a forward position, and is then moved away from the second valve seat  18 . This introduces the atmosphere into the variable pressure chambers B and D, producing a pressure differential between the constant pressure chambers A, C and the variable pressure chambers B, D. The pressure differential is effective to drive the power pistons  10 ,  11  and the valve body  6  forward, whereby the output shaft  38  is driven forward to produce a braking liquid pressure in the master cylinder. 
     At this time, the first seat S 1  on the valve element  20  which is seated upon the first valve seat  16  under the resilience of the valve return spring  19  is not provided with the backup plate  21  embedded therein, and accordingly, the front end face of the first seat S 1  undergoes a large elastic deformation, followed by removal of the second valve seat  18  on the valve plunger  17  from the valve element  20  (see FIG.  4 ). 
     If an increase in the force with which the brake pedal is depressed is now ceased, the pressure differential between the constant pressure chambers A, C and the variable pressure chambers B, D drives the power pistons  10 ,  11  and the valve body  6  forward, and the valve element  20  which is seated on the first valve seat  16  eventually catches up with the second valve seat  18  on the valve plunger  17  to be seated thereon. In this manner, the valve element  20  is seated on both the first valve seat  16  and the second valve seat  18  to achieve a servo balance condition. 
     If the force with which the brake pedal is depressed is decreased under the servo balance condition, the valve element  20  will be caused to retract by the second valve seat  18  on the valve plunger  17 , but because the first seat S 1  on the valve element  20  has undergone a large elastic deformation as mentioned above, the first seat S 1  is restored to its original condition from the large elastic deformation before it is removed from the first valve seat  16 . 
     In other words, the servo balance condition continues from the time when an increase in the force with which the brake pedal is depressed is ceased until a reduction in the depressing force causes the valve element  20  to be removed from the first valve seat  16 , and this is reflected as the hysteresis. In the present embodiment, the backup plate  21  is omitted from the first seat S 1  as mentioned above so that the first seat S 1  undergoes a large elastic deformation, and accordingly, the degree of the hysteresis can be enhanced in comparison to the prior art in which the such backup plate is also embedded in the first seat. 
     During the operation of the brake booster, a brake reaction from the braking liquid pressure is transmitted through the output shaft  38  to the reaction disc  36 , and thence transmitted through the plate plunger  35 , the reaction transmitting means  34 , the valve plunger  17  and the input shaft  31  to the brake pedal. 
     When the brake reaction exceeds the preset load to which the coiled spring  40  in the reaction transmitting means  34  is charge d, the first retainer  42  and the valve plunger  17  will be relatively advanced with respect to the second retainer  43  and the plate plunger  35 , as shown in FIG. 5, whereby the output from the brake booster  1  rises with a greater servo ratio than before. 
     When the valve plunger  17  is relatively advanced and the spacing between the plate plunger  35  and the valve plunger  17  is reduced, the resilient member  46  mounted on the valve plunger  17  eventually abuts against the projection  35 A on the plate plunger  35  to be compressed between this projection and the valve plunger  17 . 
     The reaction disc  36  is compressed by the brake reaction during the operation of the brake booster, and hence exhibits the hysteresis which depends on the characteristic of rubber used therefor. However, in the present embodiment, in addition, the resilient member  46  is compressed in a region of a greater brake reaction, and the hysteresis is also produced at this area. Thus, in addition to the hysteresis of a greater magnitude which results the large elastic deformation of the first seat S 1  on the valve element  20 , the hysteresis produced by the resilient member  46  in a region of a greater brake reaction is also obtained. In this manner, in a higher output region where the coiled spring  44 , in particular, becomes compressed, a change in the output which has been sensitive to a change in the force with which the brake pedal is depressed can be moderated, allowing an improved brake feeling to be obtained. 
     It should be noted that the resilient member  46  described above in connection with the embodiment may be omitted. 
     The magnitude of the hysteresis can be arbitrarily established by choosing the external diameter D 1  of the backup plate  21  suitably with respect to the internal diameter D 2  of the first valve seat  16 , but the external diameter D 1  of the backup plate  21  must not be less than the external diameter D 3  of the second valve seat  18 . 
     In the described embodiment, a clearance is provided between the resilient member  46  and the projection  35 A of the plate plunger  35  when the brake booster is inoperative, but the invention is not limited thereto and the resilient member may be initially disposed in abutment against the plate plunger. 
     While the invention has been shown and described above in connection with a preferred embodiment thereof, it should be understood that a number of changes, substitutions and modifications are possible therein without departing from the spirit and scope of the invention defined by the appended claims.