Abstract:
A pressure relief arrangement for a disc-type brake system in which a master cylinder assembly applies and releases the brake. The pressure relief arrangement is in the form of a pressure relief valve located between the piston and reservoir of the master cylinder assembly. The pressure relief valve provides normal operation of the brake upon extension and retraction of the piston when fluid pressure remains below a predetermined threshold. When fluid pressure exceeds the predetermined threshold, the pressure relief valve establishes communication between the reservoir and the fluid flow path, to relieve the excessive pressure. In this manner, the pressure relief valve reduces or eliminates the brake calipers from functioning improperly when the brakes are applied, which can result from the excess pressure caused by fluid expansion and dynamic braking effects that tend to move the caliper pistons.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims the benefit of U.S. provisional application serial No. 60/450,477, filed Feb. 27, 2003, the disclosures and teachings of which are herein incorporated by reference. 
     
    
     
       BACKGROUND AND SUMMARY OF THE INVENTION  
         [0002]    This invention relates to a brake system, and more particularly to an arrangement for preventing adverse effects which can occur when the brake system experiences excessive fluid pressure.  
           [0003]    Brake systems, as hydraulic systems, rely significantly on precise pressures that translate into precise movements of the components parts. This is particularly significant in the precise caliper piston movements required in disc brake systems. The thermal expansion of fluid in the system or from contact between the brake disc and the brake pads may translate into knock-back of the brake actuating piston.  
           [0004]    A disc brake system typically includes a master cylinder assembly which is responsive to an operator-controlled lever or pedal to control operation of a caliper, and thereby application and release of the brake. The master cylinder assembly includes a body defining a fluid reservoir, and a piston mounted within the master cylinder body. The piston is movable relative to the master cylinder body between an extended position and a retracted position. Typically, the master cylinder piston is moved to the extended position in response to actuation of the brake lever or pedal by an operator. Such extension of the piston is operable to displace a quantity of fluid from the master cylinder which causes movement of the caliper so as to apply the brake. When the operator releases the brake pedal or lever, the piston is moved to the retracted position under the influence of a spring. The piston includes a seal arrangement, and is exposed to fluid within the valve body. The seal arrangement is configured to cut off communication between the interior of the valve body and the fluid reservoir when the piston is moved to the extended position, and to establish communication between the reservoir and the interior of the valve body when the piston is moved to the retracted position. In this manner, the quantity of fluid that is displaced upon extension of the piston returns to the reservoir through the interior of the valve body when the piston is retracted.  
           [0005]    One problem is known as “knock-back”, in which the dynamic effects of a vehicle can tend to push the caliper pistons into their respective bores forcing fluid into the master cylinder reservoir. In most instances, this results in partial or complete loss of braking capability. The loss of fluid in the pressure circuit causes increased lever/pedal movement and in effect decreases the ability to achieve system pressure. Another result of knockback is an unnatural braking sensitivity, in which this increased lever/pedal travel reduces the ability to modulate braking power.  
           [0006]    In general, brake calipers that are hard mounted to a vehicle versus floating automotive type calipers experience this knock-back. More specifically, this occurs when caliper pistons are pushed into their respective bores due to the dynamic effects of a vehicle. When enough fluid is displaced to the master cylinder reservoir during this event, the brake lever/pedal can be completely actuate with little or no braking of vehicle.  
           [0007]    Under certain circumstances, an increase in pressure in the fluid flow path can result in undesirable effects on the brake system. For example, when the disc comes into contact with the brake pads, which is caused by an increase in pressure from thermal expansion of a fluid in the brake line, it tends to move the piston inwardly beyond its normal retracted position. This condition is commonly known as thermal lock, and creates the potential for the caliper piston to be moved such that premature braking can occur without actuation of the brake lever/pedal. In addition, thermal expansion of the brake fluid may cause extension of the piston even when the brake is not applied, resulting in drag on the brake disc.  
           [0008]    It is an object of the present invention to provide a pressure relief arrangement for a disc brake system, which prevents the adverse effects that can result from excessive pressure in the brake system caused by contact between the brake disc and the brake pads or thermal expansion of brake fluid in the system. It is a further object of the invention to provide such a pressure relief arrangement which is incorporated in the master cylinder body. A further object of the invention is to provide such a pressure relief arrangement that controls the flow of fluid between the fluid flow path and the fluid reservoir upon movement of the piston between the extended and retracted positions.  
           [0009]    A still further object of the invention is to provide such a pressure relief arrangement which functions as a valve that selectively establishes communication between the valve interior and the fluid reservoir when pressure in the fluid flow path exceeds a predetermined threshold. Yet another object of the invention is to provide such a pressure relief arrangement which allows the brake to be operated in the same manner as in the prior art while preventing the adverse effects that can occur from excessive pressure in the system. A still further object of the invention is to provide such a pressure relief arrangement which is relatively simple in its components, construction and operation, yet which is effective to control the flow of fluid between the valve interior and the fluid reservoir while preventing the adverse effects that can occur from excessive pressure in the valve interior.  
           [0010]    In accordance with one aspect of the present invention, a disc brake system includes a brake disc and a caliper, and an actuator for selectively moving the caliper to apply and release the brake. The brake actuator includes a master cylinder body defining a fluid reservoir, and a valve body mounted within a passage defined by the master cylinder body. A piston is mounted for movement between an extended position and a retracted position. In a typical configuration, movement of the piston to the extended position displaces a quantity of fluid from the interior to operate the caliper so as to apply the brake, and movement of the piston toward the retracted position returns a quantity of fluid toward the fluid reservoir through the interior, to release the brake. A pressure relief valve arrangement is interposed between the interior and the reservoir, for controlling the flow of fluid upon movement of the piston between the extended and retracted positions. The pressure relief valve arrangement is configured and arranged to selectively vent fluid from the to the fluid reservoir in the event pressure in the fluid flow path exceeds a predetermined threshold. In the disclosed embodiment, the pressure relief valve arrangement includes a valve body defining an internal cavity in communication with the interior, and the valve body includes a relief port in communication with the fluid reservoir. The pressure relief valve arrangement further includes a biased valve member movably mounted with the internal cavity, which normally seals the relief port to prevent communication between the internal cavity and the reservoir. During normal operation, brake release causes the piston to move from the extended position to the retracted position to return fluid to the interior, which causes fluid to be displaced into the internal cavity and results in movement of the valve member to move to an expanded position against the bias. When the brake is applied, the piston is moved toward the extended position to displace fluid from the interior, and the valve member is opened to the reservoir and is moved to an exhaust position under the influence of the bias to discharge fluid from the internal cavity into the fluid reservoir through a series of ports associated with the master cylinder body. The valve body is configured such that the valve member normally prevents communication between the internal cavity and the fluid reservoir through the relief port when the valve member is in both the expanded position and the exhaust position. The valve member is further movable from the expanded position to a relief position against the bias when pressure in the fluid flow path exceeds the predetermined threshold. When in the relief position, the valve member is positioned such that the relief port is opened so as to establish communication between the internal cavity and the fluid reservoir. This functions to relieve pressure in the internal cavity and thereby in the interior, to prevent movement of the piston beyond its normal retracted position in the event of contact between the brake disc and the brake pads which otherwise may result in knock-back of the piston, and to prevent piston extension which may otherwise result from thermal expansion of fluid in the system.  
           [0011]    The invention contemplates a brake actuator or master cylinder assembly incorporating a pressure relief arrangement, as well as a method of relieving excessive pressure in a brake system, substantially in accordance with the foregoing summary. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    A preferred exemplary embodiment of the invention is illustrated in the accompanying drawings in which like reference numerals represent like parts throughout, and in which:  
         [0013]    [0013]FIG. 1 is a sectional view schematically illustrating a brake system that includes a master cylinder assembly incorporating the pressure relief valve arrangement of the present invention, in which the piston of the master cylinder assembly is shown in a retracted position and the valve member of the pressure relief valve arrangement is shown in an expanded position;  
         [0014]    [0014]FIG. 1A is an exploded view of the pressure relief valve arrangement in accordance with one aspect of the present invention;  
         [0015]    [0015]FIG. 2 is a sectional view showing the master cylinder assembly as in FIG. 1, in which the piston of the master cylinder assembly is shown in an extended position for applying the brake and the valve member of the pressure relief valve arrangement is shown in an at rest position; and  
         [0016]    [0016]FIG. 3 is a sectional view similar to FIG. 2, showing the piston in the retracted position and the valve member of the pressure relief arrangement moved to a relief position for relieving excessive pressure in the master cylinder assembly. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0017]    [0017]FIG. 1 schematically illustrates a disc-type brake system incorporating an actuator in the form of a master cylinder assembly  10  which includes the pressure relief valve arrangement of the present invention. In accordance with conventional construction of a brake system of this type, master cylinder assembly  10  is interconnected with an operator-controlled actuating lever shown at  12 , and functions to control movement of a caliper shown schematically at  14 . Caliper  14  closes upon brake application to apply pressure to a brake disc or rotor  16 , and opens upon release to relieve pressure on rotor  16 . Master cylinder assembly  10  is particularly well suited for use in a fixed caliper or hard-mounted (vs. floating automotive type) application, such as is commonly incorporated in a vehicle such as a motorcycle, snowmobile or all terrain vehicle (ATV), although it is understood that master cylinder assembly  10  may be used in other types of braking arrangements.  
         [0018]    Master cylinder assembly  10  includes a body  18  that defines a passage  20  within which a piston  22  is mounted. Master cylinder body  18  further includes a fluid reservoir  24  sealed by a cap  26  which includes a diaphragm  28 , in a known manner. Reservoir  24  is defined by side walls such as shown at  30  and a bottom wall  32 , a portion of which defines passage  20 .  
         [0019]    In accordance with known construction, piston  22  is reciprocally movable within a cartridge  34  located within passage  20 , between a retracted position as shown in FIGS. 1 and 3 in which brake caliper  14  is released, and an extended position as shown in FIG. 2 in which brake caliper  14  is applied.  
         [0020]    Cartridge  34  includes a passage  36  defined by an annular cartridge side wall  38 , and piston  22  is movably mounted within passage  36 . A timing port  40  is formed in cartridge side wall  38 , and establishes communication between cartridge passage  36  and a recess  42  formed in the outer surface of cartridge side wall  38 . In addition, a backup port  44  is formed in cartridge side wall  38  at a location spaced from timing port  40 , and establishes communication between cartridge passage  36  and recess  42 . Cartridge  34  further includes a reservoir port  46  that communicates between cartridge passage  36  and a recess  48  formed in the outer surface of cartridge side wall  38 . An O-ring seal  50  is seated within a groove formed in cartridge side wall  38  between recesses  42  and  48 , and engages the inwardly facing surface of cartridge side wall  38  to establish a fluid-tight seal therebetween.  
         [0021]    Piston  22  includes a spool section  52  having a recess  54  located between a pair of cup-type seals  56 ,  58  that contact the inwardly facing surface of cartridge side wall  38  to establish a fluid-tight seal therebetween. In addition, piston  22  includes a forward extension section  60  that extends from spool section  52 . A cup-type seal  62  is mounted to extension section  60 , and engages the inwardly facing surface of cartridge side wall  38  to form a fluid-tight seal therebetween.  
         [0022]    Piston  22  defines a passage  64  within which a return spring  66  is mounted. The forward end of return spring  66  is supported by a collar  68 , in accordance with known construction. Return spring  66  is operable to bias piston  22  toward the retracted position, in a manner as known in the art.  
         [0023]    Bottom wall  32  of master cylinder body  18 , which forms a part of fluid reservoir  24 , is formed with a return opening  64  that communicates between fluid reservoir  24  and master cylinder passage  20 . Forwardly of return opening  64 , a pressure relief valve assembly  66  is mounted to bottom wall  32 . In a manner to be explained, pressure relief valve assembly  66  is operable to control fluid flow into and out of reservoir  24 , and to relieve excessive pressure that may be experienced in the pressure vessel or chamber located in the forward portion of cartridge passage  36  forwardly of piston seal  62 .  
         [0024]    Pressure relief valve assembly  72  includes a valve body or cylinder  74  having a side wall  76  and an end wall  78  that cooperate to define an internal cavity  80 . A valve member  82  is movably mounted within internal cavity  80 . Valve member  82  includes a body section  84  that carries a cup-type seal  86  which contacts the inner surface of side wall  76  to form a fluid-tight seal therebetween. Valve member  82  further includes a shoulder section  88  which is configured to maintain seal  86  in position, and a head section  90  that extends from shoulder section  88 .  
         [0025]    Side wall  76  of pressure relief valve cylinder  74  is formed with a valve relief opening or port  92 , and seal  86  is normally engaged with the area of cylinder side wall  76  below the lowermost extent of relief port  92 .  
         [0026]    A cap  94  is secured to the upper end of valve cylinder  74 . Cap  94  includes an outwardly extending flange  96  engaged within a groove  98  formed in the inner surface of side wall  76 , which functions to mount cap  94  to valve cylinder  74 .  
         [0027]    Cap  94  is formed with a recess  100  in its inner surface, which faces and is in communication with internal cavity  80  of valve cylinder  74 . Recess  100  is formed to define an inner mounting boss  102 , and one end of a low-force inner spring  104  is engaged with mounting boss  102 . The opposite end of low-force inner spring  104  is engaged with head section  90  of valve member  82 . Inner spring  104  is operable to apply a biasing force on valve member  82  that tends to urge valve member  72  toward end wall  78  of valve cylinder  74 .  
         [0028]    An outer, high force preloaded spring  106  is seated within cap recess  100  outwardly of mounting boss  102 . The lowermost extent of outer spring  106 , shown at  108 , is provided with an elongated configuration, and the ends of lower turn  108  of outer spring  106  are engaged with opposed seat surfaces  110  which are formed in the inner surface of valve cylinder side wall  76 . With this construction, outer spring  106  is preloaded when cap  94  is engaged with valve cylinder  74 , via seating of the lower turns  108  of outer spring  106  in seat surfaces  110  and compression that is applied to outer spring  106  when cap  94  is secured to valve cylinder  74 .  
         [0029]    Valve cylinder  74  is mounted within reservoir  24  via an extension  112  of end wall  78  that is received within an opening  114  formed in bottom wall  32 . Representatively, a threaded connection may be provided between extension  112  and opening  114 , although it is understood that any other satisfactory mounting arrangement may be employed. A passage  116 , which includes a frustoconical inlet, is formed in end wall  78  and extension  112 , and establishes communication between internal cavity  80  of valve cylinder  74  and recess  42  defined by cartridge  34 .  
         [0030]    [0030]FIG. 1A is an exploded view of the pressure relief valve arrangement  200  in accordance with one aspect of the present invention. The valve arrangement  200  includes a valve body  202 , a piston  204 , and a generally u-shaped cup seal  206 , with these items defining a pressure vessel  208 . The valve arrangement further includes a cap  210 , a small, low installed force, spring  212  and a larger, higher installed force spring  214 .  
         [0031]    In one embodiment, a pressure relief valve assembly and a master cylinder combination is disclosed. On such master cylinder for use in the combination is the master cylinder shown and described with respect to FIGS. 1-3. The combination comprises: a master cylinder having a master cylinder reservoir; and a pressure relief valve assembly positioned within the master cylinder reservoir. The pressure relief valve assembly itself comprises: a pressure vessel for displacing a fluid. The pressure vessel itself comprises: a valve body; a generally u-shaped cup seal disposed adjacent the valve body; and a piston in sealing contact with the generally u-shaped cup seal. The valve arrangement further comprises: a first spring located adjacent the pressure vessel piston; a second spring adjacent the pressure vessel piston and concentric with the first spring; and a cap in engagement with the first spring. The second spring is preloaded to have a higher installed force and a higher spring rate than the first spring. When the piston moves to contact the first spring, compress the first spring, and compress the second spring, fluid is drawn into the pressure vessel, thereby relieving pressure from the master cylinder reservoir.  
         [0032]    A method of reducing caliper piston knockback using a brake master cylinder is also disclosed. The method comprises: displacing a pressure relief valve assembly piston to contact and compress a first spring and a second spring concentric with the first spring, thereby increasing the pressure required to force caliper pistons into their respective bores, thereby reducing caliper piston knockback.  
         [0033]    During every braking event the system stabilizes itself when the valve discharges all its fluid on a brake application. The valve is designed to release pressure before caliper pistons can be actuated due to thermal expansion of the fluid.  
         [0034]    In operation, valve member  82  is normally in an expanded position as shown in FIG. 1 when the brake is released and piston  22  is in the retracted position. The brake is applied by operating brake actuating lever  12  in a known manner, to move piston  22  from the retracted position of FIG. 1 to the extended position of FIG. 2. Such movement of piston  22  displaces a quantity of fluid from the pressure chamber of cartridge passage  36  outwardly to brake caliper  14 , to apply pressure on brake disc  16  in a known manner. Such movement of piston  22  to the extended position functions to cut off communication of timing port  40  with the pressure chamber of cartridge passage  36 , and also positions the rearward cup seal  58  forwardly of both timing port  40  and backup port  44 .  
         [0035]    During such movement of piston  22  to the extended position, internal cavity  80  of valve cylinder  74  is open to reservoir  24  via passage  116 , backup port  44 , spool recess  54 , reservoir port  46 , cartridge recess  48  and return opening  70 . Such communication between valve cylinder internal cavity  80  and reservoir  24  allows valve member  82  to be moved under the influence of inner spring  104  from its expanded position of FIG. 1 to its exhaust position of FIG. 2, to discharge fluid from valve cylinder internal cavity  80 . When the operator releases actuating lever  12 , piston  22  returns to the retracted position of FIG. 1 under the influence of retraction spring  66 , which causes introduction of a volume of fluid into the pressure chamber of cartridge passage  36  and movement of cartridge seals  58 ,  62  to the position as shown in FIGS. 1 and 3, in which timing port  40  is opened to the pressure chamber of cartridge passage  36  and backup port  44  is sealed. In this manner, fluid that is introduced to cartridge passage  36  upon retraction of piston  22  is supplied through timing port  40  to cartridge recess  42 , and through passage  116  of valve cylinder  74  into internal cavity  80 . The fluid pressure is sufficient to cause valve member  82  to return to the expanded position of FIG. 1, against the bias of inner spring  104 .  
         [0036]    In the event brake disc  16  contacts the brake pads of caliper  14  during rotation of brake disc  16 , which presents the potential for knock-back of piston  22 , an increase in fluid pressure is experienced in the pressure chamber of cartridge passage  36 . The pressure increase is transferred through timing port  40  and cartridge recess  42  into valve cylinder internal cavity  80  through passage  116 , to engage valve member  82  with the lower end of outer spring  106 . If the increase in pressure is above a predetermined threshold at which knock-back normally occurs, as governed by the preload on outer spring  106 , the increased pressure is operable to force valve member  82  upwardly against the bias of outer spring  106 , as shown in FIG. 3, to a relief position in which the lip of valve seal  86  is located above the lowermost extent of relief port  92 . Such movement of valve member  82  functions to establish communication between valve internal cavity  80  and reservoir  24  through relief port  92 . In this manner, the fluid pressure in the pressure chamber of cartridge passage  36  is relieved. When pressure in the pressure chamber of cartridge passage  36  falls below the threshold established by the force of outer spring  106 , valve member  82  is returned to the expanded position as shown in FIGS. 1 and 3 under the influence of both inner spring  104  and outer spring  106 .  
         [0037]    Valve member  82  can also be moved to the relief position of FIG. 3 when pressure in the pressure chamber of cartridge passage  36  increases above the predetermined threshold for any other reason, e.g. as a result of thermal expansion of the brake fluid which otherwise may result in drag of the pads of caliper  14  on brake disc  16 .  
         [0038]    Generally, when the system is first assembled its “at rest” assembled state is where the piston is fully extended within the valve. The “at rest” operational state (i.e., when the system is attached to the vehicle) is with the piston compressing the small spring but not contacting the large spring.  
         [0039]    During a braking event, the valve works in the following manner (assuming the valve starts from its “at rest” position on vehicle): When the brake lever/pedal is actuated, the master cylinder piston and primary cup travel forward, thereby closing off the port timing hole and creating pressure in the brake caliper. At the same time, fluid in the valve is directed into the master cylinder reservoir through the backup port, over the primary backup cup, and through the reservoir port. When the brake is released, fluid travels over the primary cups (the amount dependant upon return velocity) and the master cylinder piston stops at its home position. With both cups behind their respective ports, the last event is the retraction of the caliper pistons into their bores. When this occurs, the fluid displaced in the caliper pushes the piston in the valve back to its home (at rest) position. The retraction of the caliper pistons is the last event in the braking sequence.  
         [0040]    This occurs every braking event. It is important that the valve be able to fully discharge during a braking event, otherwise the valve will pump up until the valve piston hits the large spring, which eliminates the calipers ability to retract the pistons and will cause the brake pads to drag.  
         [0041]    During a knock back event, the valve works in the following manner: the caliper piston experiences an external force wanting to push it into its respective bore. The valve piston moves slightly contacting the large preloaded spring. Due to the increased pressure needed to move the valve piston further against the spring, the force needed to push the piston into the bore is increased. If the force is great enough, with enough displacement to overcome the large spring force in the valve, the valve can dump this pressure to the master cylinder reservoir as the valve U-cup pass this port. This pressure relief is also used in thermal expansion of the liquid.  
         [0042]    While the invention has been shown and described with respect to a specific embodiment, it is understood that various alternatives and modifications are contemplated as being within the scope of the invention. For example, and without limitation, while valve cylinder  74  is illustrated as being a separate member that is mounted to the valve body, it is also understood that the valve cylinder may be formed integrally with the material of the master cylinder body during manufacture. Moreover, it is important to note that the placement of the valve body and associated valve arrangement components described herein are shown in a particular region of the master cylinder shown and described. However, it is contemplated that the location or placement of the arrangement can vary depending on a number of constraints, for example, the size and dimension (and in general the make and model) of the master cylinder utilized. Thus, the changes in the exact location of the valve arrangement are contemplated, provided that the fluid flow and pressure relief objectives are met. The preferred location disclosed herein is chosen for its ability to be easily bled with the system (versus separately) and to provide a direct path for the fluid to be dumped from the valve into the master cylinder reservoir, reducing the need for extra lines and to reduce overall complexity.  
         [0043]    Various alternatives and embodiments are contemplated as being within the scope of the following claims particularly pointing out and distinctly claiming the subject matter regarded as the invention.