Abstract:
A master cylinder is disclosed which is switched from a normal operating mode to a secondary mode when available vacuum is insufficient to operate the booster or at booster run-out. In these circumstances where the normal power assist from the booster is not available, the effective area of the input plunger of the master cylinder is reduced to reduce the manual actuating force required to generate braking pressure, thereby enabling the vehicle operator to more easily generate braking pressure.

Description:
TECHNICAL FIELD  
         [0001]    This invention relates to a master cylinder which is switchable between normal and booster failed modes and which requires a lower force for actuation in the booster failed mode.  
         BACKGROUND OF THE INVENTION  
         [0002]    Modern passenger cars and light trucks are normally provided with a vacuum powered brake booster which amplifies the force applied by the vehicle operator to actuate the vehicle master cylinder, which develops braking pressure to actuate the vehicle brakes. The master cylinder develops pressure in each of a pair of braking circuits, and is normally mounted directly on the brake booster, so that a “push through” link may be provided between the master cylinder and the brake pedal so that the vehicle operator may directly actuate the master cylinder (but without the power assist provided by the booster) upon failure of the booster.  
           [0003]    Boosters commonly fail because the vehicle engine stalls. Since boosters commonly use engine manifold vacuum as a power source and provide a vacuum reserve sufficient for only a very few actuations when the vehicle engine is not running, manual operation of the master cylinder must be accommodated. However, since it is desirable to provide short pedal travel during normal power operation of the brake system, braking systems are designed to use the higher force output of modern brake boosters to shorten pedal travel while providing the force necessary to operate the vehicle brakes. However, during the aforementioned manual operation, the force required to actuate the brakes may be excessive. Accordingly, expensive fail safe measures have been provided to reduce failed mode actuation forces to an acceptable level.  
           [0004]    Manual actuation of the master cylinder also occurs when the brake application forces generated by the vehicle operator exceed the maximum output force that the booster can supply. This maximum force is commonly referred to as booster “run-out”. Accordingly, it is also desirable to minimize manual actuation forces at booster run-out.  
         SUMMARY OF THE INVENTION  
         [0005]    According to the present invention, the master cylinder primary piston, which develops pressure in the primary braking chamber, is provided with a shoulder opposing the face of the primary piston exposed to pressure in the primary chamber. The shoulder cooperates with the master cylinder housing to define a pressure cavity. During normal operation of the booster, the cavity is communicated to the master cylinder reservoir, so that the primary piston has an effective area equal to the entire cross-sectional area of the primary piston. Upon booster failure, or upon booster run-out, the cavity is communicated to the primary braking circuit, thus reducing the effective area of the plunger by the cross-sectional area of the shoulder. The smaller effective area of the primary piston during operation of the master cylinder in the booster failure mode or upon booster run-out correspondingly reduces the input forces required of the vehicle operator during booster failure or run out or generates a higher pressure in the master cylinder with the same input force. Since the available brake pedal travel cannot be used fully without vacuum in the booster, the working area of the master cylinder may be reduced and the resulting increase in pedal travel be used to lower actuation forces. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]    [0006]FIG. 1 is a cross-sectional view of a master cylinder made pursuant to the present invention and a fragmentary portion of the brake booster upon which the master cylinder is installed, the master cylinder being illustrated in the rest or brake released position;  
         [0007]    [0007]FIG. 2 is a view similar to FIG. 1, but with the master cylinder illustrated in the brake applied position. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0008]    Referring now to the drawings, a master cylinder generally indicated by the numeral  10  made pursuant to the present invention includes a housing  12  having a mounting flange  14  which is secured to the rear portion  16  of a conventional vacuum booster. The vacuum booster amplifies the force applied by the vehicle operator and includes an output member (not shown), the end of which is received within socket  18  of a master cylinder input plunger  20 , which is slidingly received within bore  22  defined within master cylinder housing  12 . The booster normally operates the plunger  20  to effect a brake application, and also provides a direct link between the brake pedal and the plunger for manual actuation of the plunger  20  during booster failure due to lack of engine vacuum. The plunger  20  is an integral part of a primary master cylinder piston  34 , which will be described in detail hereinafter. A pin  24  is received in an oversized transverse bore  26  in plunger  20 . As will be described hereinafter, the pin  24  is urged against a circumferentially extending stop washer  28  when the master cylinder is in the brake released or rest position as illustrated in FIG. 1. The wall of the bore  22 , the outer surface of the plunger  20 , the stop washer  28 , and shoulder  35  on piston  34  define a circumferentially extending cavity  30 . A circumferentially extending seal  32  circumscribes the plunger  20  at stop member  28 .  
         [0009]    The primary master cylinder piston  34  and a secondary master cylinder piston  36  are slidably mounted in the bore  22  and are coaxial with each other. The secondary piston  36  includes opposite end portions  38 , 40  which are rigidly connected by a slotted portion  42  having a longitudinally extending slot  46 . A stop pin  48  is secured in a bore  50  extending from bore  22  and extends through slot  46  and into a compensating port  52  which communicates with the fluid within master cylinder reservoir  54 . Accordingly, the reservoir  54  is communicated with the cavity circumscribing the slotted portion  42  at all times. The end portion  38  cooperates with closed end  56  of bore  22  to define a secondary pressure chamber  58  therebetween, which is communicated to the vehicle brake actuators (not shown) actuated by the vehicle secondary braking circuit  60 , which communicates with secondary pressure chamber  58 . A spring loaded compensating valve  62  is mounted in the end portion  38  and includes a stem  64  which extends through the end portion  38  and is stopped by the stop pin  48  when the secondary piston  36  is in the rest or brake release position as illustrated in FIG. 1 to thereby open the valve. However, when a brake application is effected (FIG. 2), the stem  64  is moved away from the stop pin  48 , permitting the compensating valve  60  to close. The end portion  38  is urged toward stop pin  48  by a secondary return spring  66 .  
         [0010]    The end portion  40  of secondary piston  36  cooperates with the primary piston  34  to define a primary master cylinder chamber  68  therebetween. Primary chamber  68  is communicated with primary braking circuit  70 , which communicates braking pressure to the brake actuators (not shown) controlled by the primary braking circuit  70 . A sleeve  72  extends from the secondary piston  36  toward the primary piston  34 , and another sleeve  74  extends from the primary piston  34  toward the secondary piston  36 . Sleeve  72  terminates in a radially inwardly extending stop surface which is urged against radially outwardly extending stop surface on sleeve  74  by return spring  76  when the master cylinder is in the rest or brake released position. A compensating valve  78  similar to compensating valve  62  is mounted in the primary piston  34 , and includes a stem  80  which is stopped by the pin  24  engaging stop washer  28 , which permits the primary piston  34  to move relative to pin  24  a distance equal to the clearance between the pin  24  and transverse bore  26  as the primary piston approaches the rest or brake release condition to thereby open the compensating valve  78 . However, when the pin  24  is moved away from the stop washer  28  when a brake application is effected, the spring closes the compensating valve.  
         [0011]    The primary and secondary pistons function in a manner well known to those skilled in the art to assure that braking pressure is available in at least one of the primary or secondary braking circuits in the event of failure of the other braking circuit. For example, in the case of failure in the secondary braking circuit  60 , the portion  40  of secondary piston  36  bottoms out on stop pin  48  to permit the primary piston  34  to generate braking pressure in primary chamber  68  and primary braking circuit  70 . In case of failure of the primary braking circuit  70 , spring  76  collapses to permit the sleeve  74  to engage the end portion  40  of secondary piston  36 , thus forming a rigid link between the primary piston  34  and secondary piston  36  to permit generation of braking pressure in the secondary chamber  58  and secondary braking circuit  60 .  
         [0012]    The cavity  30  is communicated to the master cylinder reservoir  54  by a first flow path  88 , which includes a conventional normally open electrically actuated solenoid valve generally indicated by the numeral  90 . The cavity  30  is also communicated to the primary braking circuit  70  through a second flow path  92 , which includes a conventional, normally closed, electrically actuated solenoid valve  94 . When a brake application is effected, movement of the plunger  20  and primary piston  34 , and resulting movement of secondary piston  38  into secondary chamber  58  builds braking pressure in the primary and secondary braking circuits  70 ,  60  to effect a brake application. In this normal brake application, in which a power assist is provided by the vacuum brake booster, the pressure in cavity  30  is at reservoir pressure, since valve  90  is open to permit communication of reservoir pressure through the flow path  88  into cavity  90  and communication through flow path  92  is prevented by valve  94 . Accordingly, the effective area of the plunger  20  will be the same as the diameter of bore  22 , indicated as area A 1 . In this normal operating mode, full braking pressure is developed relatively quickly while requiring a relatively large activating force, which is readily available from the brake booster. However, this force may be too great to be easily generated by a normal vehicle operator in the event of booster vacuum failure.  
         [0013]    According to the invention, a conventional vacuum sensor (not shown) is provided to generate an electrical signal when the booster is in a failure mode due to lack of engine manifold vacuum. When this occurs, electrically actuated solenoid valves  90 , 92  are operated to close off communication between cavity  30  and the reservoir  54  and to open communication between cavity  30  and the primary brake circuit  70 . Accordingly, a pressure level is communicated to the cavity  30  that is substantially the same as that in the primary chamber  68 . Since the pressure of the primary braking circuit  70  in cavity  30  acts on shoulder  35  which opposes the pressure in the primary chamber  68 , the effective area of the plunger is reduced by the area of the shoulder  35 . This reduced effective area is indicated at A 2  in the Figures. Since the area is reduced, the actuating force necessary to develop a desired braking force will also be reduced. When the brakes are released, both solenoid valves are de-energized. The solenoid valves may also be actuated at booster run-out, to thereby also reduce the actuation force that would otherwise be required to increase braking pressure beyond that available at booster run-out.  
         [0014]    Although the invention has been described as incorporating two separate electrically actuated solenoid valves  92 ,  94 , since both of the valves are actuated and de-actuated at the same time, the flow paths  88  and  92  may be re-routed so that only a single solenoid actuator is required to actuate both the valves  90 , 92 .