Abstract:
A fast fill braking system includes a master cylinder, a primary pressure chamber having a first chamber portion located forwardly of a second chamber portion, the first chamber portion having a diameter larger than a diameter of the second chamber portion, and a primary piston including a first piston portion positioned within the primary pressure chamber and a second piston portion extending out of the primary pressure chamber, the first piston portion having a diameter (i) larger than a diameter of the second piston portion, and (ii) complementary to the diameter of the second chamber portion.

Description:
FIELD 
     The invention relates to braking systems, and in particular to a fast-fill master cylinder. 
     BACKGROUND 
     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. When fully actuated, the brake pads are in contact with the rotor, and thereafter the brake pads perform the desired braking function. When actuation is first initiated, however, the brake pads are not in physical contact with the rotor. This lack of physical contact results in minimal pressure buildup in the downstream braking circuits, which results in lack of braking. In addition to the lack of braking, an operator of the vehicle may receive a different pedal feedback when the actuation 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. 
     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. This method is typically referred to as a fast fill braking system. 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. 
     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, known in the art. 
     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 
     According to one embodiment of the present disclosure, there is provided a fast fill braking system. The fast fill braking system includes a master cylinder, a primary pressure chamber having a first chamber portion located forwardly of a second chamber portion, the first chamber portion having a diameter larger than a diameter of the second chamber portion, and a primary piston including a first piston portion positioned within the primary pressure chamber and a second piston portion extending out of the primary pressure chamber, the first piston portion having a diameter (i) larger than a diameter of the second piston portion, and (ii) complementary to the diameter of the second chamber portion. 
     According to one embodiment of the present disclosure, there is provided a fast fill braking system. The fast fill braking system includes a brake cylinder and a primary pressure chamber. The primary pressure chamber is fixedly defined within the cylinder and includes a first chamber portion having a first diameter, and a second substantially cylindrical chamber having a second diameter. The second diameter is less than the first diameter. The fast fill braking system further includes a primary piston which includes a first large diameter portion positioned within the primary pressure chamber, and a second small diameter portion. The second small diameter portion extends rearwardly of the first large diameter portion and has a diameter smaller than a diameter of the first large diameter portion. The fast fill braking system further includes a first annular seal mounted on the first large diameter portion and (i) configured to sealingly engage the second substantially cylindrical chamber when the first large diameter portion is within the second substantially cylindrical chamber and the primary piston is moving in a forward direction, and (ii) configured to not sealingly engage the first chamber portion when the first large diameter portion is within the first chamber portion and the primary piston is moving in the forward direction. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  depicts a partial cross sectional view of a braking system including a master cylinder assembly, a primary piston assembly, a secondary piston assembly, poppet valve assemblies, and a sleeve assembly, shown in a first position; 
         FIG. 2  depicts a cross sectional view of the primary piston assembly, depicted in  FIG. 1 ; 
         FIG. 3  depicts a cross sectional view of the secondary piston assembly, depicted in  FIG. 1 ; 
         FIG. 4  depicts a cross sectional view of one of the poppet valve assemblies, depicted in  FIG. 1 ; 
         FIG. 5  depicts a partial cross sectional view of the braking system depicted in  FIG. 1  in an initial activation position; 
         FIG. 6  depicts a partial cross sectional view of the braking system depicted in  FIG. 1  in a first subsequent activation position; and 
         FIG. 7  depicts a partial cross sectional view of the braking system depicted in  FIG. 1  in a second subsequent activation position. 
     
    
    
     DESCRIPTION 
     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. 
     Referring to  FIG. 1 , a partial cross sectional view of a braking system  100  is depicted. The braking system  100  includes a master cylinder assembly  102 , a primary piston assembly  104 , a first poppet valve assembly  106 , a secondary piston assembly  108 , and a second poppet valve assembly  110 . A reservoir (not shown) is fluidly coupled to the master cylinder assembly  102  via fluid channels  114 ,  116  and  118 . The master cylinder assembly is in fluid communication with downstream braking circuits (not shown) via fluid channels  121  and  122 . The master cylinder assembly  102  includes a bore  103 . 
     The poppet valve assembly  106  is biased away from the secondary piston assembly  108  by a spring  112 . The secondary piston assembly  108  is biased away from the poppet valve assembly  110  by a spring  113 . The first and second springs  112  and  113  may have the same or different spring constants. In addition, these springs  112  and  113  may be constructed to provide uniform spring stiffness (i.e., a constant spring constant over the compression range of the spring) or non-uniform spring stiffness (i.e., varying spring constants over the compression range of the spring). 
     A sleeve assembly  120  is sealingly coupled to the bore  103  via seals  152 ,  164 ,  166 ,  168 , and  170 . The sleeve assembly  120  includes seal housings for seals  150 ,  152 ,  154 ,  156 ,  158 ,  160 ,  162 ,  164 ,  166 ,  168 , and  170 . The seals  150 ,  154 , and  156  seal the primary piston assembly  104  against the sleeve assembly  120 . Also, the seals  158 ,  160 , and  162  seal the sleeve assembly against the secondary piston assembly  108 . 
     The master cylinder assembly  102 , the primary piston assembly  104 , and the sleeve assembly  120  define a primary pressure chamber  124 . The primary pressure chamber  124  is divided into two chamber portions  128 / 130  as depicted by the phantom line in  FIG. 1 . The first chamber portion  128  is defined by a larger diameter of the sleeve assembly  120 . The second chamber portion  130  is defined by a smaller diameter of the sleeve assembly  120 . As depicted in  FIG. 1 , the first chamber portion  128  is positioned forwardly of the second chamber portion  130 . 
     Similarly, the master cylinder assembly  102 , the secondary piston assembly  108 , and the sleeve assembly  120  define a secondary pressure chamber  126 . The secondary pressure chamber  126  is divided into two chamber portions  132 / 134  as depicted by the phantom line in  FIG. 1 . The first chamber portion  132  (also referred to as the third chamber portion) is defined by a larger diameter of the sleeve assembly  120 . The second chamber portion  134  (also referred to as the fourth chamber portion) is defined by a smaller diameter of the sleeve assembly  120 . As depicted in  FIG. 1 , the third chamber  132  is positioned forwardly of the fourth chamber  134 . 
     The primary and secondary pressure chambers  124  and  126  are in continuous fluid communication with the downstream braking circuits (not shown), and in selective fluid communication with the reservoir (not shown). As described further below, these chambers  124  and  126  are configured to pressurize fluid therein, thereby pressurizing fluid within the downstream braking circuits (not shown), in response to leftward movement of the primary piston assembly  104  (as depicted in  FIG. 1 ). 
     The primary piston assembly  104  defines a cavity  107 . The cavity  107  is configured to receive the first poppet valve assembly  106 . The first poppet valve assembly  106  isolates the primary pressure chamber  124  from the reservoir (not shown) by sealing against the primary piston assembly  104 , as further described below. The secondary piston assembly  108  also defines a cavity  111 . The second poppet assembly  110  is aligned with the second cavity  111 . Similarly, the second poppet valve assembly  110  isolates the secondary pressure chamber  126  from the reservoir (not shown) by sealing on an end wall  172 . 
     The sleeve assembly  120  includes diameter transition points  174  and  176 . The diameter of the sleeve assembly  120  changes at these diameter transition points. For example, the diameter increases from right to left across the diameter transition point  174  from a smaller diameter  178  to a larger diameter  180 . Similarly, the diameter increases from right to left across the diameter transition point  176  from a smaller diameter  182  to a larger diameter  184 . The first and second chamber portions  128  and  130  or the third and fourth chamber portions  132  and  134  are also defined about the diameter transition points  174  and  176 , respectively. The sleeve assembly  120  may be constructed in a uniform manner about the diameter transition points  174  and  176  (i.e., a uniformly circular construction with two diameters), or may be, as depicted in  FIG. 1 , constructed in a non-uniform manner (i.e., only annular portions of the inside of the sleeve assembly  120  formed with the larger diameter). 
     Referring to  FIG. 2 , a cross sectional view of the primary piston assembly  104  is depicted. The primary piston assembly  104  includes a rear portion  252 , a central portion  254 , and a forward portion  256 . The rear portion  252  is connected to the central portion  254  providing a gap  258  between the two portions. The central portion  254  includes a bore  260  thereby providing a fluid channel  262 . 
     The rear portion  252  defines a cavity  266  with an interface  268  for receiving an input shaft (not shown). The input shaft (not shown) and the cavity  266  interface in a fixed manner, e.g., in a press fit manner. Therefore, movement of the input shaft (not shown) generates movement of the primary piston assembly  104  in response thereto. 
     The forward portion  256  includes a seal housing for the seal  156 . The interface between the central portion  254  and the forward portion  256  defines the cavity  107  which is in fluid communication with the fluid channel  262 . A sealing surface  264  is provided at a forward area of the central portion  254  (see also  FIG. 1 ). The central portion  254  also defines an outer diameter  270  which is smaller than an outer diameter  272  of the forward portion  256 . 
     Referring to  FIG. 3 , the secondary piston assembly  108  is depicted. The secondary piston assembly  108  includes a body portion  300 , a forward portion  302 , a rear member  304 , and a front member  306 . While the rear member  304 , the body portion  300 , and the forward member  306  are depicted as three separate members, the reader should appreciate that these members may be integrally formed as a single component. 
     The rear and forward members  304  and  306  include shoulders  305  and  307 , respectively, for receiving ends of the springs  112  and  113  (shown in phantom). Therefore, a biasing force generated by the spring  112  may be exerted on the shoulder  305  of the rear member  304  which is configured to transfer the biasing force to the body portion  300  and to the forward member  306 . Similarly, a biasing force generated by the spring  113  may be exerted on the shoulder  307  of the front member  306  which is configured to transfer the biasing force to the body portion  300  and to the rear member  304 . 
     The rear and front members  304  and  306  include cavities  308  and  310  for receiving head portions  362  of shafts  360  of the poppet valve assemblies  106  and  110 , respectively, as described in further detail below with reference to  FIG. 4 . The rear member  304  terminates in a washer  312  which defines an opening  314 . The inner diameter of the opening  314  is larger than the outer diameter of the shaft  360  and smaller than the diameter of the head portion  362  of the poppet valve assemblies  106 / 110 . Similarly, the front member  306  terminates in a washer  316  which defines an opening  318 . The inner diameter of the opening  318  is larger than the outer diameter of the shaft  360  and smaller than the diameter of the head portion  362  of the poppet valve assemblies  106 / 110 . 
     The body portion  300  defines an outer diameter  320  which is smaller than an outer diameter  322  of the front portion  302 . The difference between these outer diameters defines the cavity  111  (see  FIG. 1 ). The front portion  302  defines a seal housing for the seal  162 . 
     Referring to  FIG. 4 , a cross sectional view of the poppet valve assemblies  106 / 110  is depicted. The poppet valve assembly  106 / 110  includes a valve body  350 , a valve bracket  352 , and a valve spring  354 . The valve body  350  includes an outfacing shoulder  368 . The valve bracket  352  defines an outfacing shoulder  356  and an in-facing shoulder  370 . The outfacing shoulders  356  of the poppet valve assemblies  106  and  110  are configured to receive one end of the springs  112  and  113 , respectively. The outfacing shoulder  368  and the in-facing shoulder  370  are configured to receive ends of the valve spring  354 . Therefore, the spring  354  biases the in-facing shoulder  370  of the valve bracket  352  away from the outfacing shoulder  368  of the valve body  350 . 
     The spring  112  biases the outfacing shoulder  356  of the poppet valve assembly  106  away from the valve body  350  until the outfacing shoulder  356  is firmly seated on the sealing surface  264  of the forward area of the central portion  254  (see also  FIG. 2 ). Similarly, the spring  113  biases the outfacing shoulder  356  of the poppet valve assembly  110  away from the valve body  350  until the outfacing shoulder  356  is firmly seated on end wall  172  (see also  FIG. 1 ). 
     The poppet spring  354  of the poppet valve assembly  106  has a lower spring constant than the spring  112 . Similarly, the poppet spring  354  of the poppet valve assembly  110  has a lower spring constant than the spring  113 . 
     The valve body  350  defines a bore  358  which partially extends an axial length of the valve body  350 . The bore  358  is configured to receive a portion of the shaft  360  in a press fit manner, or any other manner in which the shaft  360  is fixedly coupled to the valve body  350 , e.g., by using a set screw. 
     The valve body  350  also includes a housing  364  for a seal  366 . The seal  366  of the poppet valve assembly  106  is configured to make contact and thereby seal against the sealing surface  264  of the central portion  254  (see also  FIGS. 1 and 2 ). Similarly, the seal  366  of the poppet valve assembly  110  is configured to contact and thereby seal against the sealing surfaces formed by the end wall  172  of the master cylinder assembly  102  on two sides of the opening defined by the fluid channel  114 . 
     The operation of the braking system  100  is described herein with initial reference to  FIG. 1 . In operation, the input shaft (not shown) is coupled to a boost system (not shown). The input shaft (not shown) is configured to convey movement of a brake pedal (not shown) to a linear movement of the input shaft (not shown) coupled to the rear portion  252  of the primary piston assembly  104  with assistance of the boost system (not shown). With the brake pedal (not shown) in a released position (i.e., in an unapplied position), the braking system is as depicted in  FIG. 1 , and is hereinafter referred to as the “rest” position. In  FIG. 1  the spring  112  biases the secondary piston assembly  108  away from the outfacing shoulder  356  of the poppet valve assembly  106  (see  FIG. 4 ), thereby causing the outfacing shoulder  356  to be firmly seated on the sealing surface  264  of the central portion  254  of the first piston assembly  104  (see  FIG. 2 ). Similarly, the spring  113  biases the secondary pitons assembly  108  away from the outfacing shoulder  356  of the poppet valve assembly  110 , thereby causing the outfacing shoulder  356  to be firmly seated on the end wall  172  of the master cylinder assembly  102 . 
     At the same time, the valve springs  354  of the poppet valve assemblies  106 / 110  bias the outfacing shoulders  368  of the valve bodies  350  away from the in-facing shoulders  370  of the valve brackets  352 . These biasing forces tend to move the valve bodies  350  and the shafts  360  fixedly coupled to the valve bodies  350  and the integrally formed head portions  362  rightward (the poppet valve assembly  106 ), and leftward (the poppet valve assembly  110 ), with reference to  FIG. 1  (see also  FIG. 4 ). 
     Since the washers  312 / 316  defining the openings  314 / 318  have an inner diameter that is smaller than the outer diameter of the head portions  362 , movement of the head portions  362  is limited by the washers  316 . As a result, the seals  366  are not able to seal against their complementary sealing surfaces, described above. Therefore, the fluid channel  262 , which is in continuous fluid communication with the reservoir (not shown), is in fluid communication with the primary pressure chamber  124 . Similarly, the secondary pressure chamber  126  is in fluid communication with the reservoir (not shown) via the fluid channel  114 . 
     Since the primary pressure chamber  124  is in continuous fluid communication with a primary downstream braking circuit (not shown), the pressure therein is the same as the pressure of the reservoir (not shown). Similarly, since the secondary pressure chamber  126  is in continuous fluid communication with a secondary downstream braking circuit (not shown), the pressure therein is the same as the pressure of the reservoir (not shown). Therefore, with the braking system  100  in the rest position, no braking is generated at the downstream braking circuits (not shown). 
     With reference to  FIG. 5 , when braking is first initiated (i.e., when the operator of the vehicle applies a force to the brake pedal (not shown)), the input shaft (not shown) coupled to the rear portion  252  of the primary piston assembly  104 , travels leftward which causes the primary piston assembly  104  to travel leftward. The leftward travel of the primary piston assembly  104  compresses the springs  112  and  113 , which thereby increases the biasing force generated by these springs  112  and  113 . Compressions of the springs  112 / 113  generate the potential for (i) for the seals  366  to firmly seat against their respective seating surfaces, described above, and (ii) for the head portions  362  of the shafts  360  to move within their respective cavities  308  and  310 . Initially the first option (i.e., seating of the seals  366  on their respective seating surfaces) occurs. After the seals are firmly seated, continued compressions of the springs  112 / 113  result in the second option. 
     Movements of the primary and secondary piston assemblies  104 / 108  are dependent on the spring constants of the springs  112  and  113 . For example, if the spring constants of the springs  112  and  113  are equal, then for every unit of leftward travel of the primary piston assembly  104  the distance between the primary and secondary piston assemblies  104 / 108  is reduced by half (½) the same unit. Also, with equal spring constants for the springs  112 / 113 , the poppet valve assemblies  106  and  110  travel equally with respect to the primary and secondary piston assemblies  104  and  108 . 
     With the poppet valve assemblies  106 / 110  sealed against their respective sealing surfaces, fluid communication between the reservoir (not shown) and the primary pressure chamber  124  is cutoff. Specifically, fluid communication through the fluid channel  118  (i.e., through the bore  260  of the central portion  254  of the primary piston assembly  104 ) and around the poppet valve assembly  106  is cutoff. In addition, fluid communication between the reservoir (not shown) and the secondary pressure chamber  126  is cutoff. Specifically, fluid communication through the fluid channel  114  and around the poppet valve assembly  110  is cutoff. 
     Once the primary and secondary pressure chambers  124  and  126  are isolated from the reservoir (not shown), further leftward movement of the primary and secondary piston assemblies  104  and  108  transfer fluids from the primary and secondary pressure chambers  124 / 126  to the downstream braking circuits (not shown) via the fluid channels  121  and  122 . 
     Referring to  FIG. 6 , a first subsequent activation position of the braking system  100  is depicted in response to application of additional force to the brake pedal (not shown) by the operator. The additional leftward movement of the primary and secondary piston assemblies  104 / 108  as compared to the positions of the assemblies depicted in  FIG. 5  has brought the seal  156  and  162  proximate to the diameter transition points  174 / 176 . In doing so, two cavities  400 / 402  are formed between the primary piston assembly  104  and the secondary piston assembly  108 . While the seals  156 / 162  are positioned to seal against the smaller diameters  178 / 180 , the cavities  400 / 402  are in fluid communication with the reservoir (not shown) via the fluid channels  118  and  116 . Therefore, the pressure within these cavities  400 / 402  is approximately the same as the pressure within the reservoir (not shown). 
     With the seals  156 / 162  to the right of the diameter transition points  174 / 176 , large amounts of fluid transfer occurs between the master cylinder assembly  102  and the downstream braking circuits via the fluid channels  121  and  122  in response to leftward movement of the primary and secondary piston assemblies  104 / 108 . Because of the larger diameters  180 / 184 , larger fluid quantities are transferred to the downstream braking circuits (not shown) as compared to a braking system that is based on smaller diameters  270 / 320 , as further described below. The larger fluid quantities provide the desired fast fill function of the braking system  100 . The fast fill function reduces the difference in the feel of the brake pedal (not shown) during the initial activation period when pressure buildup in the braking system is significantly reduced until friction members of the braking system come into contact with their complementary braking surfaces. 
     The fluid transfer based on the larger diameters  180 / 184  may require additional force applied to the input shaft (not shown), however. As described above, the input shaft (not shown) is coupled to the boost system (not shown) in order to assist moving the primary piston assembly  104 . The larger force required to move the primary and secondary piston assemblies  104 / 108  may require the boost system (not shown) to be dimensioned so that it can provide the assist as compared to a braking system that is based only on the smaller diameters  270 / 320 . 
     The reader should note, however, that during the initial activation period, since the pressure buildup is significantly reduced, the forces required to move the primary and secondary piston assemblies  104 / 108  are smaller. Therefore, the braking system  100  may be so dimensioned that the desired fast-fill is completed as soon as the seals  156 / 162  reach the diameter transition points  174 / 176 . As a result, the boost system (not shown) need not be dimensioned to be able to provide the larger force, described above. 
     Further application of force to the brake pedal (not shown) by the operator results in further leftward movement of the primary piston assembly  104  from what is depicted in  FIG. 6 . Referring to  FIG. 7 , a second subsequent activation position of the braking system  100  is depicted. Since the primary and secondary pressure chambers  124  and  126  remain isolated from the reservoir (not shown), the additional leftward movement of the primary piston assembly  104  and the secondary piston assembly  108  results in additional pressurization of fluid within these chambers  124  and  126 . 
     As depicted in  FIG. 7 , the seals  156  and  162  have crossed the diameter transition points  174  and  176 , respectively. Therefore, these seals  156 / 162  provide no further sealing function (i.e., sealing the primary and secondary piston assemblies  104 / 108  against the sleeve assembly  120 ). Once the seals  156 / 162  cross the diameter transition points  174 / 176 , only the seals  154 / 160  provide the sealing function provided previously by the seals  156 / 162 . The seals  154 / 160 , however, seal against diameters  270 / 320  of the primary and secondary piston assemblies  104 / 108 . 
     Therefore, fluid transfer from the master cylinder assembly  102  to the downstream braking circuits (not shown) after the seals  156 / 162  cross the diameter transition points  174 / 176  is based on the smaller primary and secondary piston diameters  270 / 320 . Because of the smaller diameters  270 / 320 , and thereby smaller quantities of fluid transfer, the force required to move the primary and secondary piston assemblies  104 / 108  and which is provided by the boost system (not shown) is smaller. 
     Immediately after the seal  156 / 162  cross the diameter transition points  174 / 176 , the previously formed cavities  400 / 402  which were at or slightly below the pressure of the reservoir (not shown) are integrated with the larger primary and secondary pressure chambers  124 / 126 . Therefore, the fluid volumes that were collected in the cavities  400 / 402  are further added to the downstream braking circuits (not shown), additionally providing the desired fast fill function. 
     The reader should note that the head portions  362  of the shafts  360  are disposed within the cavities  308  and  310 , as depicted in  FIGS. 6 and 7 . The head portions  362  slide into the cavities  308 / 310  as the primary and secondary pistons assemblies  104 / 108  move further leftward. 
     When the operator of the vehicle partially releases the brake pedal (not shown), the input shaft (not shown) moves rightward, moving with it the rear portion  252  of the primary piston assembly  104 . Fluid within the downstream braking circuits (not shown) returns to the master cylinder assembly  102  via fluid channels  121 / 122  based on the primary and secondary piston diameters  270 / 320  until the seals  156 / 162  cross the diameter transition points  174 / 176 . At that point, fluid is transferred between the downstream braking circuits (not shown) and the master cylinder assembly  102  based on the larger diameters  180 / 184 . Also, fluids within the cavities  400 / 402  are returned to the reservoir (not shown) via fluid channels  118  and  116 . 
     Further release of the brake pedal (not shown) allows the seals  366  to unseat from their respective sealing surfaces, as described above, due to further rightward movement of the primary and secondary piston assemblies  104 / 108 . The braking of the seals  366  places the primary and secondary pressure chambers  124  and  126  in fluid communication with the reservoir (not shown) via the fluid channels  114 / 118 . 
     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.