Abstract:
A dual circuit pilot master cylinder for a full power brake system provides a minimally sized package for installation in a vehicle cab interior. The master cylinder utilizes a side-by-side disposition of the primary and secondary cylinders and a brake pedal linkage to the pistons allowing flexibility in location of the device.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Technical Field  
         [0002]     The invention relates to vehicle brake systems and more particularly to a pilot master cylinder for the brake system which is positionable inside the vehicle cab.  
         [0003]     2. Description of the Problem  
         [0004]     Front engine busses pose a number of problems for engine and vehicle system layout not inherent in conventional or rear engine configurations. Front engine busses have a characteristic flat front or “snub nose” appearance, with the bus driver and engine positioned over the front wheels. The front dash panel is spaced from the front wall of the vehicle by only a few inches. Access to the engine is from the passenger compartment or through relatively small panels in the sides or front of the vehicle. The engine compartment is much smaller than in conventional engine vehicles and components that are located in the engine compartment of conventional engine vehicles, such as brake master cylinders, must frequently be relocated due to size considerations and to considerations relating to providing linkages between operator controls, such as between a brake pedal and the brake master cylinder. For example, in a conventional engine bus a standard horizontally inclined master cylinder may be used where in a front engine bus the master cylinder has frequently had to be relocated due to lack of under hood clearance. On front engine vehicles the brake master cylinder has commonly been located on the vehicle frame, where it is exposed to road splash which poses a corrosion threat.  
         [0005]     Recent developments in vehicle design favor the use of full power brake systems where the brake master cylinder, rather than providing the required pressure for actuating the brakes, provides a hydraulic pilot signal to an electronic controller, which in turn generates the signals for actuation, modulation and release of the individual wheel end brakes for normal braking, anti-lock braking, stability control and traction control. It would be advantageous to exploit the elimination of the need for power boost to configure a pilot master cylinder which could be repositioned off the frame of the vehicle, preferably to inside the passenger compartment or the under hood compartment to avoid corrosion exposure and improve accessibility.  
       SUMMARY OF THE INVENTION  
       [0006]     According to the invention there is provided a dual circuit pilot master cylinder for a brake system. A cylinder housing has top and bottom sides and includes fittings for attachment of the cylinder housing to a vehicle dash panel or other advantageous location. A pair of cylinders is disposed in the cylinder housing in parallel to one another and extending vertically into the cylinder housing from adjacent the top side. A piston is positioned in each cylinder for reciprocating motion. A linkage connects to the two pistons to a foot actuated lever for joint up and down movement. The linkage further includes a crank fulcrum positioned on the top side of the cylinder housing and a lever fulcrum depending from the bottom side of the cylinder housing. A crank having opposing ends is mounted for pivoting movement in the crank fulcrum. Piston rods extend from the pistons out of the cylinders for connection to ends of the crank on one side of the crank fulcrum. A brake pedal lever mounted in the lever fulcrum has ends on opposed sides of the lever fulcrum. A push rod extends from a point of connection with an end of the brake pedal lever below the bottom side of the cylinder housing to a point of connection with the an end of the crank above the top side of the cylinder housing. A pedal is disposed on the end of the lever opposite the end connected to the push rod. A working fluid reservoir is mounted on the outside face of the dash panel opposite the cylinder housing and is provided with couplings to the cylinders extending through the dash panel.  
         [0007]     Additional effects, features and advantages will be apparent in the written description that follows. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]     The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself however, as well as a preferred mode of use, further objects and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein:  
         [0009]      FIG. 1  is a perspective view of a front engine bus with which the invention is advantageously employed.  
         [0010]      FIG. 2  is a high level schematic of a vehicle brake system.  
         [0011]      FIG. 3  is a cross sectional view of a dual circuit pilot master cylinder.  
         [0012]      FIG. 4  is a cross sectional view taken at a right angle with respect to  FIG. 3  of the dual circuit pilot master cylinder. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0013]     Referring to the drawings, and particularly to  FIG. 1 , a bus  10  on which the present invention is advantageously employed is illustrated. Bus  10  is a front engine bus with a square front end  13  which maximizes the portion of vehicle length available for passenger seating and which positions a driver close to the front end to improve sight lines. A penalty of this arrangement is more difficult access to the vehicle&#39;s engine, which is located behind a flat front wall  12  and which is accessed for checking fluid levels through access doors  14 . The dual circuit pilot master cylinder  20  may of course be used with other types of vehicles, but its novel and non-obvious features lend it particularly useful in vehicles of the type depicted.  
         [0014]      FIG. 2  illustrates a full power vehicle braking system  15  as may be advantageously used with bus  10  of  FIG. 1 . Braking system  15  incorporates as many wheel end brake units  32  as there are ends of axles on the vehicle. For bus  10  four wheel end brake units  32  are employed. Braking system  15  is actuated by a driver using a dual circuit pilot master cylinder  20  which is mounted on the inside of the vehicle cab from a dash panel  16 . A hydraulic fluid reservoir  22  is mounted on the opposite side of dash panel  16  in a gap  18  between vehicle front wall  12  and the dash panel. A brake pedal  24  depends from dual circuit pilot master cylinder  20  being positioned below the master cylinder and having an actuation arc A in a plane that is vertical and perpendicular with respect to dash panel  16 . Dual circuit pilot master cylinder  20  is connected to a full power brake module  30  by a primary hydraulic pilot circuit  26  and a secondary hydraulic pilot circuit  28 . Full power brake module  30  generates control signals for the wheel end brake units  32  in response to the signals received on the pilot circuits and to sensor data (e.g. vehicle speed, wheel lock up, parking brake status, etc.) in accord with the requirements of stopping the vehicle at a rate proportional to the pressure of the pilot signals while maintaining control, avoiding skidding and maintaining vehicle stability. Operation of full power brake module  30  is supported by a full power brake fluid reservoir  122 .  
         [0015]     Referring now to  FIGS. 3 and 4 , dual circuit pilot master cylinder  20  is illustrated in detail. Dual circuit pilot master cylinder  20  comprises a housing  120  which encloses primary and secondary circuit cylinders  38  and  40 . The housing  120  is, in a preferred application, hung from its back face  95  on the inside face of a dash panel  16 . A back side  95  of housing  120  is contoured to fit the shape of the face of dash panel  16  and includes flanges  97  which extend beyond the dimensions of housing  120 . Conventional fasteners may be fitted through flanges  97  to hold housing  120  to dash panel  16 . Primary circuit cylinder  38  is disposed upright and parallel to secondary circuit cylinder  40 , the cylinders being located in a vertical plane spaced away from dash panel  16 .  
         [0016]     Brake fluid is supplied to primary and secondary circuit cylinders  38  and  40  from a reservoir  22  located in gap  18  between dash panel  16  and front wall  12 . The fluid is delivered through a conduit  90  to inlets  99  into the cylinders located about half way between the tops and the bottoms of the cylinders. Reservoir  22  is filled with replacement fluid through a fill point  34  accessed via an access door  14  in the vehicle&#39;s front wall  12 . Reservoir  22  is conventionally fabricated from plastic and divided by an internal baffle for the primary and secondary circuits. Reservoir fluid level is marked by full and low lines.  
         [0017]     Brake fluid is displaced from cylinders  38 ,  40  by downward movement of pistons  42 ,  82  in cylinders  38 ,  40 . Brake fluid moves out through outlets  89  near the bottommost points in the cylinders through outlet channels  92  to delivery outlets  96  for and into primary and secondary hydraulic pilot circuits  26 ,  28  (fluid delivery from cylinder  40  to the secondary hydraulic pilot circuit  28  is not shown but is identical to that for the primary hydraulic pilot circuit  26 ). Fluid returns to the cylinders  38 ,  40  with upward movement of pistons  42 ,  82 . Delivery outlet  96  is tapped by an upwardly slanted primary (and identical secondary) pressure switch port  96 . Both the delivery port  94  and pressure switch port  96  for both the primary and secondary systems pass through the dash panel  16  for connections inside gap  18 . The areas around all outlets are bossed to extend through openings in dash panel  16  to ease making the appropriate switch and hydraulic circuit connections.  
         [0018]     Cylinders  38 ,  40  are located in a side by side arrangement in housing  120  which are outwardly displaced from dash panel  16  and located near the front face  101  of the housing. Cylinders  38 ,  40  are vertically oriented and pistons  42 ,  82  are disposed in the cylinders for reciprocating up and down movement. Extending upwardly from pistons  42 ,  82  are piston rods  44 ,  84 , respectively. Piston rods  44 ,  84  extend through openings  78 ,  80  in an upper cover plate  36  to pin connections  54 ,  154  in tines  74 ,  76  of a forked bell crank  48 . Pistons  42 ,  82  are upwardly biased in position by rebound compression springs  46 ,  86  located in cylinders  38 ,  40  between the pistons and the bottoms of the cylinders. Pistons  42 ,  82  are of piston and follower design to better maintain alignment and relieve stress from piston seals  98 .  
         [0019]     Pistons  42 ,  82  are manually operated by force applied to a brake pedal  24  hung below housing  120 . Pedal  24  swings in an arc A (shown in  FIG. 2 ) which is located in a vertical plane perpendicular to dash panel  16 . Brake pedal  24  is connected to pistons  42 ,  82  by a linkage which assures that pistons  42 ,  82  move in unison. The linkage comprises: piston rods  44 ,  84 ; forked bell crank  48 ; a push rod  56 ; and a pedal lever  64 . Forked bell crank  48  is supported at a fixed point above housing  120  on upper cover plate  36  by a crank fulcrum  50 . Pedal lever  64  hangs from a fixed point below housing  120  from a pedal lever fulcrum  68  extending from bottom side  70  of the housing. Forked bell crank  48  is connected to crank fulcrum  50  on a pin  52  through a central member  53  of the forked bell crank which allows the forked bell crank to rock back and forth from end to end. Similarly, pedal lever  64  is supported by a pin  88  through a centered location allowing the pedal lever to pivot. Pins  52  and  88  are parallel to back side  95  of housing  120  and to one another. Each is positioned horizontally making the planes of rotation of forked bell crank  48  and pedal lever  64  coincident. Forked bell crank  48  and pedal lever  64  each have inner ends close to dash panel  16  and outer ends distal to the dash panel. Forked bell crank  48  divides into two tines  74  and  76  on its side distal to dash panel  16 . Brake pedal  24  is mounted to the outer end of pedal lever  64  distal to dash panel  16 . Piston rods  44 ,  84  are pivotally pinned to forked bell crank  48  away from dash panel  16  to take stress off of piston seals  98 .  
         [0020]     Pedal lever  64  is linked to forked bell crank  48  by a push rod  56  positioned to pass through a space in the back of housing  120  between the housing and dash panel  16 . Push rod  56  is connected to the inner ends of forked bell crank  48  and pedal lever  64 . Pivot pins  62  and  66  provide the means of connection between the respective ends of push rod  56  and forked bell crank  48  and pedal lever  64 , respectively. Depression of brake pedal  24  thus is transmitted to forked bell crank  48  by upward movement of push rod  56 , and corresponding, joint downward movement of piston rods  44 ,  84 . The motion is coupled to the pair of pistons  42 ,  82  imparting to the pistons joint up and down motion in the cylinders with the foot actuated pedal lever  64  having a motion in a vertical plane perpendicular to the dash panel  16 .  
         [0021]     Forked bell crank  48  divides into first and second tines  74 ,  76  on the end away from the point of connection to push rod  56 . This allows pedal lever  64  to be centered between cylinders  38 ,  40 .  
         [0022]     The invention provides for an internally mountable dual hydraulic circuit pilot master cylinder for a motor vehicle brake system that fits into a foot print as small as those for an air brake system valve package. The system reservoir is readily positioned for ease of access on a variety of vehicle types. The arrangement of the system also makes it largely self bleeding and provides a readily accessible point for mounting of a pressure switch and for final bleeding of both pilot delivery circuits.  
         [0023]     While the invention is shown in only one of its forms, it is not thus limited but is susceptible to various changes and modifications without departing from the spirit and scope of the invention.