Abstract:
A fast-fill braking system in one embodiment includes a master cylinder, a first chamber within the master cylinder, a second chamber within the master cylinder located forward of the first chamber, a fast-fill piston within the first chamber, a primary piston within the first chamber and movable with respect to the fast-fill piston, and a secondary piston within the second chamber.

Description:
FIELD 
       [0001]    The invention relates to braking systems, and in particular to a fast-fill master cylinder. 
       BACKGROUND 
       [0002]    A braking system typically includes a master cylinder which is fluidly coupled to downstream braking circuits. During an initial period of actuation, the master cylinder generates fluid pressure in downstream braking circuits and displaces fluid in order to place friction members of the braking system, e.g., brake pads, against complementary surfaces, e.g., a rotor or a drum. In certain circumstances, brake pads may be displaced away from the rotor, thereby generating a gap between the brake pads and the rotor. Thus, when actuation is first initiated the brake pads are not in physical contact with the rotor of the wheel. 
         [0003]    This lack of physical contact between a brake pad and a rotor precludes any physical braking until the brake pads are repositioned into contact with the wheel rotors. Moreover, since there is no significant resistance in the system, there is only minimal pressure buildup in the downstream braking circuits. Consequently, in addition to the lack of braking, an operator of the vehicle may receive a different pedal feedback when braking is first initiated as compared to the pedal feedback the operator receives once the brake pads are in contact with the rotor. This difference in the pedal feedback can be unsettling to the operator. 
         [0004]    One way to shorten the lack of braking and reduce the unsettling difference in the pedal feedback when the actuation is first initiated is to displace a larger quantity of fluid within the braking system in order to quickly take up the gap, described above. Such systems are typically referred to as a “fast-fill” braking systems. In order to transfer the larger quantity of fluid, the braking system may include an actuating piston in the master cylinder with a larger diameter as compared to an actuating piston in a braking system which is not designed to provide the desired fast-fill function. A larger diameter piston moves a larger volume of fluid, thereby quickly filling the downstream braking circuits. 
         [0005]    A larger piston, however, requires a larger force to move. While during the initial period of actuation the force required to move the larger piston is relatively low, after the initial period of actuation a larger force is required to move the piston than is needed in a system with nominally sized piston. This additional force necessitates a larger boost system. 
         [0006]    Therefore, it is highly desirable to provide a master cylinder construction which can minimize the lack of braking and reduce the unsettling difference in the pedal feedback when the actuation is first initiated by rapidly increasing pressure in the downstream braking circuits, and without the need to use a larger boost system. 
       SUMMARY 
       [0007]    According to one embodiment of the present disclosure, there is provided a fast-fill braking system which includes a master cylinder, a first chamber within the master cylinder, a second chamber within the master cylinder located forward of the first chamber, a fast-fill piston within the first chamber, a primary piston within the first chamber and movable with respect to the fast-fill piston, and a secondary piston within the second chamber. 
         [0008]    According to another embodiment a fast-fill braking system includes a master cylinder, a first chamber within the master cylinder, a second chamber within the master cylinder located forward of the first chamber, a fast-fill piston within the first chamber, a primary piston slidably received within a cavity in the fast-fill piston, and a secondary piston within the second chamber. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  depicts a partial cross sectional view of a tandem braking system including a primary piston assembly located within a cavity of a fast-fill piston, and a secondary piston assembly; 
           [0010]      FIG. 2  depicts a cross sectional view of the master cylinder of the braking system of  FIG. 1 ; 
           [0011]      FIG. 3  depicts a cross sectional view of the fast-fill piston of  FIG. 1 ; 
           [0012]      FIG. 4  depicts a cross sectional view of the primary piston assembly of  FIG. 1 ; 
           [0013]      FIG. 5  depicts a cross sectional view of the secondary piston assembly of  FIG. 1 ; 
           [0014]      FIG. 6  depicts a partial cross sectional view of the braking system depicted in  FIG. 1  after a fast-fill stroke; and 
           [0015]      FIG. 7  depicts a partial cross sectional view of the braking system depicted in  FIG. 1  after a full stroke. 
       
    
    
     DESCRIPTION 
       [0016]    For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the embodiments illustrated in the drawings and described in the following written specification. It is understood that no limitation to the scope of the invention is thereby intended. It is further understood that the present invention includes any alterations and modifications to the illustrated embodiments and includes further applications of the principles of the invention as would normally occur to one of ordinary skill in the art to which this invention pertains. 
         [0017]    Referring to  FIG. 1 , a partial cross sectional view of a tandem braking system  100  is depicted. The braking system  100  includes a master cylinder  102  which includes a bore  104 . A fast-fill piston  106 , a primary piston assembly  108 , and a secondary piston assembly  110  are located at least partially within the bore  104 . 
         [0018]    With reference to  FIG. 2 , the bore  104  includes a large diameter portion  120 , a medium diameter portion  122 , and a small diameter portion  124 . Seal wells  126  and  128  are located within the large diameter portion  120  and an O-ring well  130  is located at a forward end of the large diameter portion  120 . A vent chamber  132  located between the seal wells  126  and  128  is in fluid communication with a vent port  134  which further vents the O-ring well  130 . 
         [0019]    The large diameter portion  120  is separated from the medium diameter portion  122  by a sealing lip  136 . A primary chamber outlet  138  is located at a forward portion of the medium diameter portion  122 . Seal wells  140  and  142  are located within the small diameter portion  124  and a vent chamber  144  located between the seal wells  140  and  142  is in fluid communication with a vent port  146 . A secondary chamber outlet  148  is located near a forward wall portion  150  of the medium diameter portion  122 . 
         [0020]    The seal wells  126 ,  128 ,  140 , and  142  receive seals  152 ,  154 ,  156 , and  158 , respectively (see  FIG. 1 ). The seals  152  and  154  sealingly engage the fast-fill piston  106  which is also shown in  FIG. 3 . The fast-fill piston  106  includes an inner chamber or cavity  160  which opens to a forward facing end portion  162  through an opening  164 . A cut-off seal  166  is located at the forward facing end portion  162 . The cut-off seal  166  is sized complementary to the sealing lip  136  (see  FIG. 2 ). The cut-off seal  166  thus has an outer diameter that is larger than the diameter of the medium diameter portion  122  of the bore  104  and smaller than the diameter of both the fast-fill piston  106  and the large diameter portion  120  of the bore  104 . A lock ring  168  is located at the end of the inner chamber  160  opposite to the opening  164 . 
         [0021]    The inner chamber  160  is sized to slidingly receive the primary piston assembly  108  which is depicted in  FIG. 4 . The primary piston assembly  108  includes a primary piston  170  with a body portion  172  and a forward portion  174 . A primary spring  176  is located about the forward portion  174 . The body portion  172  is sized slightly smaller than the diameter of the inner chamber  160  and larger than the inner diameter of the lock ring  168 . A seal  178  mounted in the body portion  172  provides a sealing engagement with the wall of the inner chamber  160  and a cavity  180  is configured to be operatively connected with a brake pedal (not shown). 
         [0022]    The forward portion  174  of the primary piston  170  is sized to pass through the opening  164  of the fast-fill piston  106  (see  FIG. 3 ) and to be received within a rearward facing cavity  190  of the secondary piston assembly  110  which is shown in  FIG. 5 . The rearward facing cavity  190  is defined by a body  192  of a secondary piston  194 . The secondary piston  194  further defines a forward facing cavity  196  and a spring step  198 . A secondary spring  200  is positioned at least partially within the forward facing cavity  196  and extends between a base flange  202  and a base flange  204 . The base flange  202  is affixed to the body  192  of the secondary piston  194  while the flange  204  is affixed to the forward wall portion  150  as seen in  FIG. 1 . 
         [0023]    When the braking system  100  is assembled and in a rest position as depicted in  FIG. 1 , the secondary spring  200  biases the secondary piston  194  to a location spaced apart from the front wall portion  150  of the master cylinder  102  with the opening of the rearward facing cavity  190  proximate the rear of the medium diameter portion  122  of the bore  104  (see also  FIG. 2 ). Seal  156  located within the seal well  140  sealingly engages the secondary piston  194  and defines a forward end of a primary chamber while seal  158  located within the seal well  142  sealingly engages the secondary piston  194  and defines a rear end of a secondary chamber. 
         [0024]    An O-ring  210  is positioned within the O-ring well  130  and a fast-fill spring  212  mounted on the spring step  198  biases the fast-fill piston  106  to a location within the large diameter portion  120  that is spaced apart from the sealing lip  136 . The primary spring  176  biases the primary piston  170  rearwardly to a location abutting the lock ring  168 . The primary piston  170  and the fast-fill piston  106  are coaxial with the forward portion  174  of the primary piston  170  aligned with the opening  164  in the fast-fill piston  106  and the rearward facing cavity  190  of the secondary piston. 
         [0025]    Upon initial application of a force on the primary piston  170  in the direction of the arrow  220  of  FIG. 1 , force is transferred from the primary piston  170  through the primary spring  176  to the fast-fill piston  106 . The fast-fill piston  106  in turn passes the applied force to the fast-fill spring  210  which presses against the spring step  198  causing the secondary piston  194  to place a compressive force on the secondary spring  200 . The spring constant of fast-fill spring  212  is selected to be greater than the spring constant of the secondary spring  200  and less than the primary spring  176 . Accordingly, the secondary spring  200  begins to compress, allowing the secondary piston  194  to move forward toward the forward wall portion  150 . Movement of the secondary piston  194  forces fluid out of the secondary chamber outlet  148  and into downstream braking circuits (not shown). As the secondary piston  194  moves forward, the fast-fill piston  106  and the primary piston  170  move forward as a unit. 
         [0026]    Movement of the fast-fill piston  106  and the primary piston  170  displaces a large amount of fluid from the large diameter portion  120  into the medium diameter portion  122  and out of the primary chamber outlet  138 . The large amount of fluid is located within a fast-fill chamber defined by the seals  178 ,  154 , and  156 . The large flow of fluid rapidly fills the downstream braking circuits (not shown) in fluid communication with the primary chamber outlet  138 . Forward movement of the fast-fill piston  106 , the primary piston  170 , and the secondary piston  194  continues until completion of a fast-fill stroke at which time the braking system  100  has moved from the configuration of  FIG. 1  to the configuration of  FIG. 6 . 
         [0027]    In  FIG. 6 , the fast-fill piston  106 , the primary piston  170 , and the secondary piston  194  have moved closer to the forward wall portion  150  as compared to the rest configuration of  FIG. 1 . Additionally, the cut-off seal  166  has been moved into contact with the sealing lip  136 . The seal  178  thus defines a rear portion of a primary chamber which includes a portion of the inner chamber  160  and the medium diameter portion of the bore  104  up to the seal  156  while the cut-off seal  166  defines an outer portion of the primary chamber. 
         [0028]    Moreover, further movement of the fast-fill piston  106  in the direction of the arrow  220  is precluded by the contact between the cut-off seal  166  and the sealing lip  136 . Consequently, continued application of a force on the primary piston  170  in the direction of the arrow  220  of  FIG. 6  results in compression of the primary spring  176  and movement of the primary piston  170  toward the forward wall portion  150  thereby increasing the pressure within the primary chamber between the seal  178  and the seal  156 . 
         [0029]    The increased pressure within the primary chamber is transferred to the secondary piston  194  to place further compressive force on the secondary spring  200 . Consequently, the secondary spring  200  is compressed as pressure within the secondary chamber, which extends from the seal  150  to the forward wall portion  150 , increases. Forward movement of the primary piston  170  and the secondary piston  194  continues until completion of a full stroke at which time the braking system  100  has moved from the configuration of  FIG. 6  to the configuration of  FIG. 7 . 
         [0030]    In  FIG. 7 , the secondary piston  194  is adjacent to the forward wall portion  150  with the secondary spring  200  fully compressed. Additionally, the forward portion  174  of the primary piston has moved through the bore  164  and into the rearward facing cavity  190  as compared with the configuration of  FIG. 6  and the primary spring  176  is fully compressed. The full stroke condition of  FIG. 7  thus provides a high pressure in both the primary and the secondary chambers. 
         [0031]    When pressure in the direction of the arrow  220  is removed from the primary piston  174 , the foregoing description is substantially reversed. Thus, the primary spring  176  acts against the primary piston  174  to move the primary piston  174  away from the forward wall portion  150 . Movement of the primary piston  174  away from the forward wall portion  150  reduces the pressure in the primary chamber (between the seals  130 ,  178  and  156 ) allowing the secondary spring  200  to decompress. As the secondary spring  200  decompresses, the secondary piston  194  is biased away from the forward wall portion  150  reducing the pressure in the secondary chamber (between the seal  158  and the forward wall portion  150 ). 
         [0032]    The rearward movement of the primary piston  174  and the secondary piston  194  continues until the braking system  100  is in the condition depicted in  FIG. 6 . From the condition of  FIG. 6 , the absence of a force applied to the primary piston  174  allows the primary spring  176  to decompress until rearward movement of the primary piston  174  is arrested by the lock spring  168 . The resulting reduction in pressure in the primary chamber (between the seals  130 ,  178  and  156 ) allows the secondary spring  200  to decompress thereby forcing the fast-fill spring  212  against the fast-fill piston  106 . The spring  200  and the fast-fill spring  212  thus bias the fast-fill piston  106  away from the forward wall portion  150  until the braking system  100  arrives at the condition depicted in  FIG. 1 . 
         [0033]    While the invention has been illustrated and described in detail in the drawings and foregoing description, the same should be considered as illustrative and not restrictive in character. It is understood that only the preferred embodiments have been presented and that all changes, modifications and further applications that come within the spirit of the invention are desired to be protected.