Abstract:
A brake booster includes a housing configured for mounting to the master cylinder of a braking system. The housing defines a bore for receiving a power piston and an end cap, with a return spring disposed between the head of the piston and end cap. The housing and end cap define a recess and a rib sized to be received within the rib but configured for an interference fit between the end cap and housing outside the recess. The interference fit is calibrated to require a force to push the end cap into the bore that exceeds the force generated by the return spring when it is compressed between the end cap and the piston head. The recess and rib at least temporarily retains the spring within the bore until the brake booster is mounted to the master cylinder.

Description:
BACKGROUND 
     The present disclosure relates to hydraulic braking systems and more particularly to a brake booster used with a braking master cylinder. 
     Generally, hydraulic braking systems include an element, such as a drum, associated with a vehicle wheel, brake shoes, a backing plate, and a hydraulic cylinder. The drum includes a braking surface against which the brake shoes bear when the braking system is activated. The backing plate does not rotate with the wheel or drum, but instead supports the hydraulic cylinder, the brake shoes, and the other braking components. 
     The hydraulic cylinder, or master cylinder, incorporates one or more pistons which force the brake shoes against the braking surface of the drum to stop or to limit the rotation of the drum. To activate the braking system an actuator, such as a foot pedal, is moved, thereby causing an increased hydraulic pressure in the master hydraulic cylinder. This increased pressure forces the piston(s) to extend from the master cylinder to drive the brake shoes. 
     In certain heavier vehicles, the hydraulic braking system may include a booster that is coupled to the master cylinder to magnify the hydraulic pressure applied to the brake shoes. A typical booster scavenges pressure from a source outside the braking system, such as from the power steering hydraulics of the vehicle, and uses that hydraulic pressure to drive a power piston within the booster. The power piston applies a force to the master cylinder piston(s) to thereby increase the braking force available through the braking system. 
     By way of example,  FIG. 1  depicts a brake booster  10  coupled to a master cylinder  12  of a hydraulic braking system. The booster housing  11  is mounted to the housing  13  of the master cylinder with the power piston  16  of the booster slidably disposed within a bore  11   a  and axially aligned with the primary piston  18  of the master cylinder. The booster  10  includes a throttle  19  connected to the braking actuator, such as the foot pedal, that is operable to activate the power piston  16 . The booster  10  includes a return spring  24  that is operable to return the power piston  16  to the neutral position shown in  FIG. 1  when the braking actuator has been released. One end of the spring  24  bears against the head  16   a  of the piston disposed at one end, or piston end, of the bore  11   a . The opposite end of the return spring must bear against a reaction surface, which includes the master cylinder housing  13  when the booster  10  is coupled to the master cylinder  12  at the opposite end, or mating end  11   b , of the bore  11   a.    
     However, prior to mounting the booster  10  it is necessary to retain the return spring  24  within inner bore  11   a  of the booster housing as the booster  10  is being assembled. Consequently, an end cap  20  has been included in prior booster devices that provides the reaction surface to retain the spring. The end cap  20  is supported on the power piston  16  at the mating end  11   b  of the bore  11   a , as shown in the enlarged view of  FIG. 2 . The end cap  20  includes an inner hub  21  that may be sized for a close running fit with the outer surface of the piston. Alternatively, a seal  22  may be engaged over the inner hub  21  to provide a tight fit between the hub and the piston; however, the tight fit provided by the seal  22  is not sufficient to resist the force of the return spring  24 . Consequently, a retaining ring  25  is provided that is retained in a circumferential groove  26  formed near the end of the power piston. The retaining ring  25  extends radially outward to contact a mating face  23  of the end cap  20 . The retaining ring may be in the form of a snap ring or a lock washer that snaps into the groove  26  in the power piston. 
     The retaining ring  25 , mounted to the end of the power piston  16 , thus traps the return spring  24  within the inner bore  11   a  of the brake booster  10 . One end of the spring bears against the head  16   a  of the piston while the other end bears against the end cap  20  held by the retaining ring  25 . However, as shown in the detail view of  FIG. 2 , when the brake booster  10  is mounted to the master cylinder  12 , the inner mating surface  14  of the master cylinder housing  13  provides the reaction surface against which the end cap  20  bears under the force of the return spring  24 . The housings  11  and  13  are typically toleranced so that when the housings are coupled together the mating face  14  of the master cylinder housing  13  essentially pushes the end cap  20  away from the retainer ring  25 . In other words, when the booster is assembled with the master cylinder the retainer ring  25  becomes superfluous since it can no longer contact the end cap  20 . 
     In this prior brake booster, the interface between the end cap  20  and the booster housing  11  is configured to facilitate insertion of the end cap into the inner bore  11   a . Thus, as shown in the detailed view of  FIG. 3 , the end cap includes an outer circumferential rim  30  that is sized for a close running fit with the inner bore  11   a . In certain boosters, a seal  34  may be engaged over the outer rim  30  to provide a tight fit between the rim and the housing. As with the seal  22  described above, the tight fit provided by the seal  34  is not sufficient to resist the force of the return spring  24 . The end cap  20  includes a spring retainer surface  32  that provides the reaction surface for the spring  22  and helps retain the spring in axial alignment with the power piston  16 . 
     Also shown in  FIG. 3  is the relationship between the master cylinder housing  13  and the end cap  20 . In particular, the outer mating surface  15  of the master cylinder housing  13  contacts the booster housing  11 . The inner mating surface  14  is offset in relation to the outer mating surface  15  to account for the offset of the end cap mating face  23  relative to the end of the outer rim  30 . The tolerance stack between the master cylinder housing mating surfaces  14  and  15  and the booster housing  11  is such that the master cylinder housing  13  slightly compresses the end cap  20  against the return spring  24  when the two housings are coupled. This arrangement renders the retaining ring  25  no longer necessary, as explained above. 
     SUMMARY 
     A braking booster is provided including a housing configured to be coupled to the housing of a master cylinder. The housing defines a bore for receiving a power piston with the piston head disposed at one end of the bore. An end cap is slidably disposed within and at an opposite mating end of the bore, and is used to trap a return spring between the piston head and the end cap. In one feature, the end cap and the housing define a recess and a rib adjacent the mating end of the bore. The rib is configured to be received within the recess and is further configured for an interference fit between the end cap and the housing outside the recess, and more particularly between the recess and the mating end of the bore. The recess and rib at least temporarily hold the end cap within the bore to thereby retain the return spring within the bore until the braking booster is mounted to the master cylinder. 
     In one aspect, the recess is defined adjacent the mating end of the bore of the housing and the rib is defined on the end cap. For a cylindrical bore in the booster housing, the recess is a circumferential recess within the bore and the rib is a circumferential rib. The end cap may include a circumferential rim sized for a close running fit with the bore, the rib extending radially outward from the rim. In order to facilitate introduction of the end cap into the bore and ultimately into the recess, the housing may define a circumferential chamfer at the mating end of the bore, the chamfer terminating adjacent the recess. 
     In the brake booster, the return spring has an initial state in which the spring is compressed between the end cap and the piston head to generate an initial spring force and an operative state in which the spring is further compressed to generate an operative spring force greater than the initial spring force. In one feature, the interference fit is calibrated to require a predetermined force greater than the initial spring force to push the end cap into the mating end of the bore until the rib is received within the recess. This predetermined force is sufficient to hold the end cap within the bore, trapping the return spring even under the initial spring force, at least until the braking booster is mounted to the master cylinder. 
     The recess may include an end face closest to the mating end of the bore, in which the rib and the end face are configured to resist expulsion of the rib toward the mating end of the bore by operation of the return spring. The rib and the end face may be configured to require a predetermined force greater than the operative spring force to expel the end cap from the recess toward the mating end of the bore. 
     In another feature, the end cap includes a circumferential rim and a resiliently compressible seal mounted on the rim. The compressible seal is configured to form at least a close running fit with the bore. The rib to be disposed within the recess is formed by an enlarged portion of the seal. 
     A method for assembling a brake booster including a housing defining a bore, a power piston having a piston head, an end cap, and a return spring disposed between the piston head and the end cap, comprises providing the housing with a circumferential recess defined in the bore adjacent a mating end of the bore, and providing a circumferential rib on the end cap, the rib sized to be received within the recess and for an interference fit with the bore between the mating end and the recess. The method further includes introducing from the mating end of the bore the power piston into the bore with the piston head disposed at the opposite end of the bore, and then introducing from the mating end of the bore the return spring over the piston and in contact with the piston head. The end cap is then introduced into the bore from the mating end of the bore. The end cap is pressed through the interference fit until the rib is disposed within the recess. 
    
    
     
       DESCRIPTION OF THE FIGURES 
         FIG. 1  is a schematic view of a brake booster coupled to a master cylinder in a hydraulic braking system. 
         FIG. 2  is an enlarged cross-sectional view of the area A in  FIG. 1 . 
         FIG. 3  is an enlarged cross-sectional view of the area B in  FIG. 1 . 
         FIG. 4  is an enlarged view of a modified booster housing and end cap according to the present disclosure. 
         FIG. 5  is an enlarged view of another modified booster housing and end cap according to the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     In accordance with the present disclosure, the retaining ring  25  is eliminated in favor of a modified interface between the end cap and the booster housing. Thus, as shown in  FIG. 4 , the modified end cap  20 ′ includes an outwardly projecting circumferential rib  40  defined in the outer rim  30 ′. The remainder of the end cap  20 ′ may be configured like the end cap  20  shown in  FIGS. 1-3 . 
     The booster housing  11 ′ is modified to incorporate a circumferential recess  42  adjacent the mating end  11   b  of the bore  11   a . The recess  42  is sized to receive the rib  40 ′ of the end cap  20 ′. Except for the rib  40 , the outer rim  30 ′ is sized for a close running fit within the inner bore  11   a  of the housing  11 ′. The rib, on the other hand, is sized for an interference fit with the bore  11   a , except at the recess  42 . In the position shown in  FIG. 4 , the rib  40  is engaged within the recess  42  and resists expulsion even under the force of the return spring against the end cap  20 ′. 
     In order to facilitate assembly of the end cap, the housing  11 ′ is provided with a circumferential chamfer  44  at the mating end  11   b  of the bore. The chamfer may be offset from the recess  42 , as depicted in  FIG. 4 . The end cap  30 ′ is shown in  FIG. 4  in phantom lines entering the inner bore  11   a . As the end cap approaches the housing  11 ′, the circumferential rib  40  bears against the chamfer  44  and follows this angled surface into the bore  11   a . The chamfer  44  may be at an angle of less than about 25°. As the rib rides the chamfer, the outer rim  30 ′ is uniformly radially compressed. When the end cap has been advanced far enough into the bore, the rib encounters the recess  42  so that the outer rim  30 ′ can return to its original shape and radius. The rib  40  may be generally circular in cross-section, as shown in  FIG. 4  to reduce the friction between the rib and the housing as the end cap is pushed into place. 
     The maximum radial dimension of the circumferential rib  40  is sized relative to the radius of the inner bore  11   a  to provide an interference fit to produce a calibrated force necessary to push the end cap into the position shown in  FIG. 4 . In particular, this “push-in” force must be greater than the force generated by the return spring in its initial or installed state. It is understood that in most brake boosters the return spring will be slightly compressed so that the spring will generate a return force throughout its entire stroke. In one specific example, the return spring in its installed state generates a spring force of under 17 lbs. Thus, the “push-in” force for the end cap must exceed that spring force. In a specific example, the circumferential rib diameter was sized to produce a nominal interference fit of 0.015 in. which led to a “push-in” force of about 20 lbs. that exceeds the installed state spring force. 
     The recess  42  may be provided with a distal face  45  farthest from the mating end  11   b  of the bore  11   a  that is generally parallel to the chamfer  44 , or more specifically at an angle of less than about 25°. The relationship between the rib  40  and the distal face  45  is such that the rib  40  may be dislodged inwardly from the recess  42  if the end cap  20 ′ is pushed deeper into the inner bore  11   a . However, once the “push-in” force is removed, the return spring  24  will push the end cap outward until the rib re-engages within the recess  42 . 
     In one configuration, the maximum radius of the rib  40  is equal to the maximum radius of the recess  42  so that the outer rim  30 ′ is not deformed when the rib is within the recess. Thus, the recess may have a depth equal to the height or prominence of the rib relative to the outer diameter of the outer rim, which corresponds to the nominal interference fit, or about 0.015 in. in the example discussed above. Since the return spring  24  will necessarily be compressed during actuation of the power piston  16 , it may be desirable for the interface between the circumferential rib  40  and the recess  42  to hold the end cap  20 ′ in place within the recess when subjected to a greater force than the installed state spring force. For instance, in a specific example, the operationally compressed return spring  24  may generate a force of about 35 lbs. when the throttle  19  is actuated. This value exceeds the 20 lb. “push-in” force as facilitated by the chamfer  44  in the housing  11 ′. However, unlike the entry to the recess, the exit from the recess is not chamfered. In particular, the end face  47  of the recess  42  closest to the mating end  11   b  of the bore  11   a  is substantially perpendicular to the surface of the bore. Thus, rather than sliding along a chamfered surface, the rib  40  must “climb” the end face  47  to be dislodged from the recess. In the specific example described above, the depth of the recess and the height of the rib require a force of over 50 lbs before the rib exits the recess. This force exceeds the spring force of 35 lbs for the operational state of the return spring  24 . Thus, the end face  47  is sufficient to hold the rib within the recess, and consequently the return spring within the booster housing  11 ′ under all conditions. 
     It can be appreciated that the relative dimensions of the rib  40  and recess  42  are dictated by the spring force that the rib-recess engagement must resist. For instance, if the installed spring force is greater the interference dimension between rib and recess may be increased. In addition, the general circular configuration of the rib  40  may be modified particularly to increase the maximum spring force that can be sustained before the rib is dislodged from the recess. For example, the trailing face of the rib  40  that faces the end face  47  when the rib is in the recess can be modified to present a sharper profile, rather than the generally circular profile shown in  FIG. 4 . Manufacturing considerations notwithstanding, a generally perpendicular trailing face of the rib abutting the generally perpendicular of the recess could almost permanently capture the end cap. However, it may be more beneficial to configure the rib and recess to permit removal of the end cap when needed for servicing the brake booster  10 . 
     It can be appreciated that the circumferential rib and recess interface between the end cap and the booster housing is sufficient to hold the booster assembly together prior to coupling the booster to the master cylinder. Once the master cylinder is added the mating surface  14  provides the reaction surface for the return spring, by way of the end cap, and the rib  40  and recess  42  become superfluous, just like the retainer ring  25 . However, the rib and recess components of the modified booster  11 ′ eliminates the more complicated machining operation to form the snap ring groove in the power piston, in favor of the more easily machined chamfer  44  and recess  42 . Moreover, eliminating the retainer ring removes a labor-intensive step in the assembly of the brake booster of engaging the retainer ring within the snap ring groove while holding the end cap against the force of the return spring. 
     With respect to the end cap, if the end cap is molded or forged production of the modified end cap  20 ′ is no more involved than production of the end cap  20  used in the prior booster designs. The modified end cap  20 ′ does not require the seal  22  of the prior end cap design. The modified end cap leads to a reduction in manufacturing and assembly costs and assembly time. 
     In an alternative configuration shown in  FIG. 5 , a booster housing  11 ″ includes a recess  42 ″ that is modified from the recess shown in  FIG. 4 . The end cap  20  is configured substantially like the end cap in the prior brake booster designs. However, in this alternative configuration, a modified seal  34 ″ is engaged about the outer rim  30  in frictional contact with the inner bore  11   a  of the housing  11 ″. The modified seal  34 ″ includes an enlarged portion  37  that is resiliently compressible when pushed into the inner bore  11   a . A chamfer  44 ″ may be added to facilitate compressing the seal as it squeezes between the outer rim  30  and the inner bore  11   a . With sufficient radial pressure, the outer rim  30  may also deform. Once the enlarged portion  37  of the seal  34 ″ reaches the recess  42 ″ the seal resiliently expands to fill the recess. 
     Like the rib-recess interface of  FIG. 4 , the enlarged seal and recess interface is sized to require a “push-in” force that exceeds the installed spring force of the return spring  16 . In this embodiment, the enlarged seal portion  37  may not need to be sized to prevent dislodgement under operating spring loads, since the master cylinder housing  13  will be available to provide a reaction surface for the return spring. 
     In the configuration shown in  FIG. 4 , the male component of the rib-recess interface (i.e., the rib  40 ) is part of the end cap  30 ′, while the female component (i.e., the recess  42 ) is part of the booster housing  11 ′. In an alternative configuration, the male and female components may be reversed. In this configuration, the rib is a radially inwardly projecting circumferential rib formed on the inner bore  11   a . The recess, then, is defined in the outer rim  30 ′ of the end cap  20 ′. The interference fit relationship between the rib and recess described above may be maintained to provide the same “push-in” force and the same resistance to the operationally loaded return spring. 
     It will be appreciated that various of the above-described features and functions, as well as other features and functions, or alternatives thereof, may be desirably combined into many other different systems or applications. Various presently unforeseen or unanticipated alternatives, modifications, variations or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims.