Abstract:
A master cylinder includes a cylinder bore, a first piston positioned within the cylinder bore, a second piston positioned within the bore and located forwardly of the first piston, a first pressure chamber within the cylinder bore and defined in part by a rear portion of the second piston and a forward portion of the first piston, a first pressure chamber seal extending radially outwardly from the rear portion of the second piston, a first reservoir inlet configured to provide fluid communication between a reservoir and the cylinder bore at a location forwardly of the first pressure chamber seal, and a first groove extending axially along the cylinder bore and positioned such that (i) when the second piston is in a released position, the first groove is located directly outwardly of the first pressure chamber seal and the reservoir is in fluid communication with the first pressure chamber through the first groove, and (ii) when the second piston is in an activated position, the first pressure chamber seal isolates the reservoir from the first pressure chamber at a location forwardly of the first groove.

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. 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. 
         [0003]    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 include a check valve, which allows a fast refill of the downstream braking circuits from a reservoir followed by isolating the braking circuits from the reservoir. The check valve, however, adds cost and may be unreliable in a harsh environment associated with typical braking systems. 
         [0004]    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. 
       SUMMARY 
       [0005]    According to one embodiment of the present disclosure, there is provided a master cylinder. The master cylinder includes a cylinder bore, a first piston positioned within the cylinder bore, a second piston positioned within the bore and located forwardly of the first piston, a first pressure chamber within the cylinder bore and defined in part by a rear portion of the second piston and a forward portion of the first piston, a first pressure chamber seal extending radially outwardly from the rear portion of the second piston, a first reservoir inlet configured to provide fluid communication between a reservoir and the cylinder bore at a location forwardly of the first pressure chamber seal, and a first groove extending axially along the cylinder bore and positioned such that (i) when the second piston is in a released position, the first groove is located directly outwardly of the first pressure chamber seal and the reservoir is in fluid communication with the first pressure chamber through the first groove, and (ii) when the second piston is in an activated position, the first pressure chamber seal isolates the reservoir from the first pressure chamber at a location forwardly of the first groove. 
         [0006]    According to one embodiment of the present disclosure, there is provided a braking system. The braking system includes a master cylinder including an inner wall defining a portion of a bore, a primary piston positioned within the bore, a secondary piston positioned within the bore and located forwardly of the primary piston, a primary pressure chamber within the bore and defined in part by a rear portion of the secondary piston and a forward portion of the primary piston, a secondary pressure chamber within the bore and defined in part by a forward portion of the secondary piston, a first seal extending radially outwardly from the second piston, the first seal isolating the secondary pressure chamber from a low pressure secondary area, a second seal extending radially outwardly from the second piston, the second seal positioned between the low pressure secondary area and the primary pressure chamber, a reservoir in fluid communication with the low pressure secondary area, and a first groove extending axially along the bore and positioned such that (i) when the secondary piston is in a released position, the first groove is located directly outwardly of the second seal and the reservoir is in fluid communication with the primary pressure chamber through the first groove, and (ii) when the secondary piston is in an activated position, the second seal sealingly engages the inner wall to isolate the reservoir from the primary pressure chamber. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  depicts a fragmentary cross sectional view of a braking system including a reservoir and master cylinder assembly, shown in a released position; 
           [0008]      FIG. 2  depicts a cross sectional view of a first piston assembly of the master cylinder assembly, depicted in  FIG. 1 ; 
           [0009]      FIG. 3  depicts a cross sectional view of a second piston assembly of the master cylinder assembly, depicted in  FIG. 1 ; 
           [0010]      FIG. 4  depicts a cross sectional view of a poppet assembly of the master cylinder assembly, depicted in  FIG. 1 ; 
           [0011]      FIG. 5  depicts a fragmentary cross sectional view of the master cylinder assembly depicted in  FIG. 1  in an activated position; and 
           [0012]      FIG. 6  depicts a fragmentary cross sectional view of the master cylinder assembly depicted in  FIG. 1  in a full travel position. 
       
    
    
     DESCRIPTION 
       [0013]    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. 
         [0014]    Referring to  FIG. 1 , a fragmentary cross sectional view of a braking system  100  is depicted. The braking system  100  includes a reservoir  102  and a master cylinder assembly  104 . The reservoir  102  is fluidly coupled to the master cylinder assembly  104 . 
         [0015]    The reservoir  102  includes two outlets  106  and  108  for communicating fluid within the reservoir  102  to the master cylinder assembly  104 . Generally, fluid within the reservoir  102  is under low pressure as compared to fluid within the master cylinder assembly  104  during braking operations. The reservoir  102  includes a sensor (not shown) to provide a signal to an electronic control unit (ECU) (not shown) corresponding to the fluid level within the reservoir  102 . 
         [0016]    The master cylinder assembly  104  includes a cylinder  110 , a first piston assembly  112 , a second piston assembly  114 , a first poppet assembly  116 , a second poppet assembly  118 , a first spring  120 , and a second spring  122 . The first poppet assembly  116  is disposed between the first piston assembly  112  and the second piston assembly  114 . The second poppet assembly  118  is disposed between the second piston assembly  114  and an end  158  of the cylinder  110 . The first piston assembly  112  is coupled to and biased away from the second piston assembly  114  by the first spring  120 . The second piston assembly  114  is biased away from the end  158  of the cylinder  110  by the second spring  122 . The first piston assembly  112 , the second piston assembly  114 , and the cylinder  110  define a first pressure chamber  124 . The second piston assembly  114  and the end  158  of the cylinder  110  define a second pressure chamber  126 . The first pressure chamber  124  is in fluid communication with a first downstream circuit (not shown) via a first outlet  128 . The second pressure chamber  126  is in fluid communication with a second downstream circuit (not shown) via a second outlet  130 . Also depicted in  FIG. 1  is an input rod  132  which is coupled to a brake pedal (not shown). The input rod  132  is coupled to the first piston assembly  112  with a ball-and-socket interface  134 . 
         [0017]    The cylinder  110  includes a bore  136 . The bore  136  provides a generally continuous inside diameter that interfaces with the first and second piston assemblies  112  and  114 . A discontinuity  138 , however, is provided in the bore which defines a beginning of a groove  140 . The groove  140  may be an axial groove or a radial groove, or a combination of axial and radial grooves. The groove  140  generally includes a first portion  142  and a second portion  144 , and is terminated at a second discontinuity  146  in the bore  136 . The first portion  142  defines a diameter slightly larger than the diameter of the bore  136 . The second portion  144  defines a diameter larger than the first portion  142 . Therefore, the first portion  142  provides a transition between the second portion  144  and the bore  136 . The groove  140  is formed about the first outlet  128 , which is in fluid communication with the first downstream braking circuit (not shown), and extends in a forward direction from the first portion  142  to the first outlet  128 . 
         [0018]    The cylinder  110  also defines reservoir inlets  150  and  152  in fluid communication with fittings  154  and  156 , respectively. The fittings  154  and  156  couple to complementary fittings corresponding to the outlets  106  and  108 , respectively. The reservoir inlet  150  is configured to establish fluid communication between the reservoir  102  and the first pressure chamber  124  through the second piston assembly  114  when the second piston assembly  114  and the first poppet assembly  116  are in the position as depicted in  FIG. 1 , as described more fully below. 
         [0019]    Referring to  FIG. 2 , a cross sectional view of the first piston assembly  112  is depicted. The first piston assembly  112  includes a first piston body  202 , a first spring bracket  204 , a radial boss  206 , and seals  208  and  210 . The first piston body  202  defines a front-facing cavity  212  and the ball-and-socket interface  134  at opposite ends thereof. The first piston body  202  couples with the first spring bracket  204  at a radial groove  214  formed at the left end of the first piston body  202 . The seals  208  and  210  seal the first piston body  202  against the bore  136  of the cylinder  110  (see also  FIG. 1 ). 
         [0020]    The first spring bracket  204  includes an inward ring portion  216 , a tab  218 , and an outward ring portion  220 . The outward ring portion  220  is fixedly coupled to the radial boss  206 . The first spring bracket  204  is assembled about the first piston body  202  in a snap-fit manner (i.e., the tab  218  springs outward as the first spring bracket  204  is brought into contact with the first piston body  202 , and then the tab  218  snaps into the radial groove  214 ). When assembled, the outward ring portion  220  is substantially perpendicular to the first piston body  202  in an axial direction. Similarly, the inward ring portion  216  is substantially perpendicular to the first piston body  202  in the axial direction. The inward ring portion  216  provides a termination plane for the front-facing cavity  212 , and defines an opening  222 . 
         [0021]    Referring to  FIG. 3 , a cross sectional view of the second piston assembly  114  is depicted. The second piston assembly  114  includes a rear piston body  252 , a front piston body  253 , a second spring bracket  254 , a radial boss  256 , and seals  258  and  260 . The front piston body  253  defines a front-facing cavity  262 , while the rear piston body  252  defines a sealing surface  263 , at opposite ends of the second piston assembly  114 . Also, the rear piston body  252  defines a central bore  274 . A radial opening  276  is formed between the rear piston body  252  and the front piston body  253 . The front piston body  253  couples with the second spring bracket  254  at a radial groove  264  formed at the left periphery of the front piston body  253 . The seal  258  seals the rear piston body  252  against the bore  136 , while the seal  260  seals the front piston body  253  against the bore  136  of the cylinder  110  (see also  FIG. 1 ). 
         [0022]    The second spring bracket  254  includes an inward ring portion  266 , a tab  268 , and an outward ring portion  270 . The outward ring portion  270  is fixedly coupled to the radial boss  256 . The second spring bracket  254  is assembled about the front piston body  253  in a snap-fit manner. When assembled, the outward ring portion  270  is substantially perpendicular to the front piston body  253  in an axial direction. Similarly, the inward ring portion  266  is substantially perpendicular to the front piston body  253  in the axial direction. The inward ring portion  266  provides a termination plane for the front-facing cavity  262 , defining an opening  272 . 
         [0023]    Referring to  FIG. 4 , a cross sectional view of the first poppet assembly  116  is depicted. While the description of  FIG. 4  is directed to the first poppet assembly  116 , the same description is also applicable to the second poppet assembly  118 . The first poppet assembly  116  includes a poppet body  302 , a poppet spring bracket  304 , a poppet spring  306 , a poppet seal  308 , and a poppet actuation rod  310 . As depicted in  FIG. 1 , the poppet spring bracket  304  is biased forward against the second piston assembly  114  by the first spring  120  (or in the case of the second poppet assembly  118  against the end  158  of the cylinder  110  by the second spring  122 ). The poppet spring bracket  304  includes an outward ring portion  312  with a radial boss  314 . The radial boss  314  couples to the first spring  120 . The poppet spring bracket  304  is configured to slidably interface with the poppet body  302 . 
         [0024]    The poppet spring  306  of the first poppet assembly  116  has a lower spring constant than the first spring  120 . Similarly, the poppet spring  306  of the second poppet assembly  118  has a lower spring constant than the second spring  122 . When the poppet spring bracket  304  is firmly held in place against the second piston assembly  114  by the first spring  120 , the poppet spring  306  biases the poppet body  302  toward the sealing surface  263  of the second piston assembly  114  (or in the case of the second poppet assembly  118  toward the end  158  of the cylinder  110 , see also  FIGS. 1 and 3 ). The poppet seal  308  seals the poppet body  302  against the sealing surface  263  (or in the case of the second poppet assembly  118  against the end  158  of the cylinder  110 ). 
         [0025]    The poppet body  302  is fixedly coupled to the poppet actuation rod  310 . In one embodiment, the poppet actuation rod  310  is press fit inside the poppet body  302 . The poppet actuation rod  310  includes a head portion  316  that is located within the front-facing cavity  262  (see  FIG. 1 ) and interfaces with the inward ring portion  216  (or in the case of the second poppet assembly  118  with the inward ring portion  266 ). 
         [0026]    The operation of the braking system  100  is described with reference to  FIGS. 1 , and  5 - 6 . In operation, with the brake pedal (not shown) in the released position, the input rod  132  is in a rest position, as depicted in FIG. In  FIG. 1  the first spring  120  biases the second piston assembly  114  away from the first piston assembly  112 , and the second spring  122  biases the second piston assembly  114  away from the end  158  of the cylinder  110 . Therefore, the first and second piston assemblies  112  and  114  are positioned in a rearward position wherein the second piston assembly  114  is spaced from the end  158  of the cylinder  110  and the first piston assembly  112  is spaced from the second piston assembly  114 . 
         [0027]    Since the poppet springs  306  bias the poppet bodies  302  leftward and since the actuation shafts  310  are fixedly coupled to the poppet bodies  302 , the head potions  316  are also biased leftward. The leftward movement of the head portions  316  are, however, limited by the inward ring portions  216  and  266 . As a result, the head portions  316  cause the poppet springs  306  to be compressed. The compressed states of the poppet springs  306 , therefore, generate gaps between the poppet assemblies  116  and  118  and the second piston assembly  114  and the end  158  of the cylinder  110 , respectively, as depicted in  FIG. 1 . 
         [0028]    The gap between the first poppet assembly  116  and the second piston assembly  114  allows fluid communication between the reservoir  102  and the first pressure chamber  124  through the reservoir inlet  150 , the radial opening  276 , and the central bore  274  (see also  FIG. 3 ). Similarly, the gap between the second poppet assembly  118  and the end  158  of the cylinder  110  allows fluid communication between the reservoir  102  and the second pressure chamber  126  through the reservoir inlet  152 . The state of the braking system depicted in  FIG. 1  may hereinafter be referred to as the “released position.” 
         [0029]    When braking is first initiated (i.e., when the operator of the vehicle applies a force to the brake pedal (not shown)), the input rod  132  travels leftward, with reference to  FIG. 1 . The leftward travel of the input rod  132  moves the first piston assembly  112  leftward. Depending on spring constants of the first and second springs  120  and  122 , leftward travel of first piston assembly  112  causes leftward movement of the second piston assembly  114  and/or the first piston assembly  112 . For example, if the spring constants of the first and second springs  120  and  122  are equal, then for every unit of leftward travel of the first piston assembly  112  the second piston assembly  114  travels leftward ½ the same unit. 
         [0030]    With the first and second piston assemblies  112  and  114  moving leftward, the spaces between the first and second piston assemblies  112  and  114  and between the second piston assembly  114  and the end  158  of the cylinder  110  reduce, as the first and second springs  120  and  122  are partially compressed. As a result, the poppet springs  306  of the first and second poppet assemblies  116  and  118  are allowed to expand and the poppet bodies  302  seal against the second piston assembly  114  and the end  158  of the cylinder  110 , respectively, as depicted in  FIG. 5 . Once the first poppet assembly  116  is sealed against the sealing surface  263  of the second piston assembly  114 , fluid communication between the reservoir  102  and the first pressure chamber  124  through the first poppet assembly  116  is cutoff. Specifically, fluid communication through the second piston assembly  114  (i.e., through the central bore  274 ) and around the first poppet assembly  116  is cutoff. In addition, fluid communication between the reservoir  102  and the first pressure chamber  124  about the groove  140  is cutoff as the leftward movement of the second piston assembly  158  moves the seal  258  against the bore  136  of the cylinder  110  at the first portion  142  of the grove  140 . The state of the braking system depicted in  FIG. 5  may hereinafter be referred to as the “activated position.” 
         [0031]    Once the braking system is in the condition depicted in  FIG. 5 , further leftward movement of the first piston assembly  112 , results in further leftward movement of the second piston assembly  114 . As discussed above, once the seal  258  passes by the first portion  142  of the groove  140  and the first poppet assembly  116  seals against the second piston assembly  114 , the first pressure chamber  124  is isolated from fluid communication with the reservoir  102 . Additional leftward movement of the first piston assembly  112  after this point results in fluid being pressurized within the first and second pressure chambers  124  and  126 . 
         [0032]    Additionally, since the poppet bodies  302  are seated against the second piston assembly  114  and the end  158  of the cylinder  110 , and the actuation shafts are fixedly coupled to the poppet bodies  302 , the actuation shafts  310  of the first and second poppet assemblies  116  and  118  slide within the front-facing cavities  212  and  262  of the first and second piston assemblies  112  and  114 , respectively. Also, the spaces between the first and second piston assemblies  112  and  114  and between the second piston assembly  114  and the end  158  of the cylinder  110  continue to be reduced, as the first and second springs  120  and  122  continue to be compressed. 
         [0033]    Continued leftward movement from the activated position of  FIG. 5  leads to a “Full Travel” state depicted in  FIG. 6 . In  FIG. 6 , the actuation shafts  310  of the first and second poppet assemblies  116  and  118  are nearly at the ends of the front-facing cavities  212  and  262 , respectively. The first and second springs  120  and  122  are depicted in a nearly compressed state. Pressure within the first and second pressure chambers  124  and  126 , and thereby at the first and second downstream braking circuits (not shown), are at the highest level, providing maximum braking function. 
         [0034]    When the operator of the vehicle partially releases the brake pedal (not shown), the input rod  132  moves rightward. With the first spring  120  in nearly a fully compressed position, rightward movement of the input rod  132  allows the first piston assembly  112  to move rightward from its full travel state, depicted in  FIG. 6 . Similarly, with the second spring  122  in nearly a fully compressed position, rightward movement of the first piston assembly  112  allows the second piston assembly  114  to move rightward from its full travel state, depicted in  FIG. 6 . As a result of the rightward movements of the first and second piston assemblies  112  and  114 , pressures within the first and second pressure chambers  124  and  126  decrease. Since the poppet springs  306  bias their respective poppet bodies  302  forward toward the second piston assembly  114  and the end  158  of the cylinder  110 , while the actuation shafts  310  of the first and second poppet assemblies  116  and  118  move relative to the front-facing cavities  212  and  262 , the poppet assemblies  116  and  118  remain sealed against the sealing surface  263  of the second piston assembly  114  and the end  158  of the cylinder  110 . 
         [0035]    The sealed relationship between the first poppet assembly  116  and the second piston assembly  114 , and the second poppet assembly  118  and the end  158  of the cylinder  110 , remains until the head portions  316  reach the inward ring portions  216  and  266 . At this point the poppet springs  306  are allowed to be compressed since the poppet bodies  302 , which are fixedly coupled to the actuation shafts  310 , move rightward as the first and second piston assemblies  112  and  114  continue to travel rightward, and further since the spring constants of the first and second springs  120  and  122  are higher than the poppet springs  306 . As the poppet springs  306  begin to be compressed, the seals between the first and second poppet assemblies and the second piston assembly  114  and the end  158  of the cylinder  110  break and fluid communication is reestablished between the reservoir  102  and the first and second pressure chambers  124  and  126 , as depicted in  FIG. 1 . 
         [0036]    Once the seals  308  of the first and second poppet assemblies  116  and  118  break from the sealing surface  263  and the end  158  of the cylinder  110 , the first and second pressure chambers  124  and  126  are placed in fluid communication with the reservoir  102 . At the same time, fluid is also allowed to transfer from the reservoir  102  to the first pressure chamber  124  around the seal  258  and about the groove  140 . Since the first pressure chamber  124  is in fluid communication with the first downstream braking circuit (not shown) through the first outlet  128 , the fluid path about the groove  140  allows for a large unimpeded fluid flow from the reservoir  102  to the first downstream circuit (not shown) to provide the desired fast-fill function. 
         [0037]    The aforementioned flow of fluid over the seal  258  of the second piston assembly  114  and about the groove  140  eliminates the need for a check valve in order to provide the desired fast fill function. In other words, the interface between the seal  258  of the second piston assembly  114  and the groove  140  allows the first downstream braking circuit (not shown) to be filled quickly from the reservoir  102 , and thereafter, as described above, isolate the first braking circuit (not shown) from the reservoir  102  in the next braking cycle, as depicted in  FIG. 5 . In one embodiment, a flow of about 13.9 cubic centimeters per second is provided to the first downstream braking circuit (not shown) when the braking is first initiated. 
         [0038]    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.