Abstract:
A two stage master brake cylinder uses a primary and secondary piston to displace an initial large volume of hydraulic fluid during a first stage and a second stage that continues brake actuation by displacing a reduced volume of hydraulic fluid determined by the secondary piston. A by-pass control valve is adjustable to vary transition from the first stage to the second stage.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a two-stage master cylinder for initially providing a large volume of braking fluid for actuation of brake cylinders and a second stage for finer control after a certain pressure has been reached. 
     For most brake applications two single piston calipers are controlled by one master brake cylinder. In demanding brake environments additional brake cylinders at the caliper are used to increase the force exerted on the brake pad and these additional cylinders increase the volume of braking fluid that must be provided under pressure by the master cylinder. The larger volume of brake fluid can be provided by a larger piston in the master cylinder however once the brakes are applied such a large piston does not have the degree of feel and the actual force that an operator must exert on a large piston is relatively high. These factors control the range of brake pedal travel and determine the force that an operator must exert on the brake pedal. If a small piston is used the brake pedal travel distance is greater, however the actual force the operator must exert on the brake pedal is reduced. If a large piston is used in the master cylinder the extent of movement of the brake pedal is reduced but the force exerted by the operator is increased. 
     The present invention overcomes a number of these problems and provides a system that allows adjustment of the forces that occur during brake actuation and control of the brake pedal travel distance. 
     SUMMARY OF THE INVENTION 
     A two stage master cylinder according to the present invention includes a primary piston and a secondary piston that are moved by a push actuator. During the first stage movement of the push actuator moves the primary and secondary pistons and displaces through an outlet of the master cylinder, a large volume of hydraulic fluid. A by-pass control valve is associated with the primary piston and automatically opens a vent path at a predetermined pressure at the outlet to define the second stage of operation. Further movement of the push actuator moves both pistons with only movement of the secondary piston increasing hydraulic pressure and/or movement of hydraulic fluid through the outlet. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Preferred embodiments of the invention are shown in the drawings wherein: 
         FIG. 1  is a vertical sectional view through the master cylinder having a two-stage piston design without a force being applied to the master cylinder; 
         FIG. 2  is a horizontal section through the brake cylinder in the condition of  FIG. 1 ; 
         FIG. 3  is a horizontal section through the master cylinder with the push rod closing a needle valve associated with the piston assembly; 
         FIG. 4  is a horizontal section through the brake cylinder with the brake actuator continuing to move the piston assembly; 
         FIG. 5  is a vertical section through the master brake cylinder where a preset pressure has been established in the brake cylinder and the effect of the large piston is now being bypassed; 
         FIG. 6  is a horizontal section through the brake cylinder in the condition of  FIG. 5 ; 
         FIG. 7  is a vertical section through the master cylinder with the brake actuator being moved to a non-operative position; 
         FIG. 8  is a horizontal section through the brake cylinder in the condition of  FIG. 7 ; 
         FIG. 9  is a sectional view through an alternate two stage master cylinder showing a variation of the master cylinder with a compensating valve replacing the needle valve; 
         FIG. 10  is a further sectional view through the alternate master cylinder showing details of the bypass control valve; 
         FIG. 11  is a sectional view through the alternate two stage master cylinder with the pistons in a non-braking application; 
         FIG. 12  is a sectional view through the alternate master cylinder showing details of the bypass control valve with the pistons in a non-braking position; 
         FIG. 13  is a sectional view through the alternate master cylinder with the pistons initially providing hydraulic fluid to the brake caliper for braking; 
         FIG. 14  is a sectional view through the alternate master cylinder with the bypass control valve shown and the pistons in an initial braking application; 
         FIG. 15  is a sectional view through the alternate master cylinder where only the small diameter piston is continuing to apply pressure to the brake caliper; 
         FIG. 16  is a view similar to  FIG. 15  but showing the blow-off valve whereby the large piston is effectively in communication with the reservoir through the modulator valve; 
         FIG. 17  shows details of the return of fluid to the secondary piston chamber and the larger piston chamber when pressure is released from the brake pedal; 
         FIG. 18   a  and  FIG. 18   b  show details of a brake caliper; and 
         FIG. 19  shows a schematic of a braking system using a pair of two stage master cylinders. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The two stage master cylinder  2  shown in  FIG. 1  includes a piston assembly  4  having a large (primary) piston  6  sliding within cylinder  8  and a small (secondary) piston  10  sliding in cylinder  12 . The small piston  10  includes an open center port  14  that can be selectively closed by the needle valve  16 . 
     The small piston  10  is coaxial with the large piston  6  and moves with the large piston. With the needle valve in an open position, both pistons are not under pressure. The push rod  20  is connected to the brake actuator and is shown in  FIG. 1  in a non actuating state. The needle valve  16  is biased by the valve release spring  56  to a clear position. Brake fluid can pass through the port  14  of the small piston and find its way through the outlet  30  to the brake reservoir. This particular flow path is shown as  50  in the horizontal section of  FIG. 2 . The large piston  6  is biased by the spring  18  to the one end of the master cylinder. Hydraulic fluid can flow through the valve  80  to the rear side of the large piston  6  on the return stroke. 
       FIG. 1  also includes the bypass control valve  40  in the form of a two diameter spool valve having a large head  41  having a pressure face exposed to the braking pressure at outlet  71 . The large head  41  slides within the cylinder  45  and the small head  43  slides within the cylinder  47 . The bypass control valve  40  is moveable within these cylinders from the closed position of  FIG. 1  to an open position as shown in  FIG. 5 . The small head  43 , in the position of  FIG. 1 , closes the bypass control valve  40  and when closed isolates the large piston  6  from the vent channel  62  connected to the reservoir outlet  30 . In  FIG. 5  the spool valve has moved to the right and opened the rear side of the large piston  6  to the reservoir through the vent channel  62  creating a pressure actuated by-pass associated with the large piston  6 . This vent channel preferably includes an adjustable modulator valve located to restrict the vent channel. 
     The bypass control valve  40  is biased by a primary spring  46  to the closed position as shown in  FIG. 1 . An adjusting spring  42  exerts a pressure on the large head  41 . The adjusting spring counteracts to a certain extent the primary spring  46  and allows adjustment of the pressure at which the control valve moves to the by-pass position. Basically, the pressure face  44  of the large head  41  is exposed to the braking pressure at outlet  71 . The opposite face of head  41  is exposed to a reduced pressure (pressure of the reservoir) when the small head closes the bypass control valve  40 . The rear face  48  of the small head  41  is always exposed to the pressure associated with the primary piston  6 . This pressure partially counteracts the pressure exerted on the pressure face  44  of the large head  41  creating a bias force tending to open the bypass control valve  40 . Both of the front and rear surfaces of head  41 , once the smaller head has opened the bypass control valve  40 , are exposed to pressure but a pressure differential exists as the bypass control valve is now open and bleeds off fluid to the reservoir. This is shown in  FIG. 5 . At a certain pressure at outlet  71 , the bypass control valve will move into the position shown in  FIG. 5  overcoming the spring bias force urging the bypass control valve to close. 
     The horizontal sectional view of  FIG. 2  shows the valve member  70  which is biased by spring  72  to a closed position as shown in  FIG. 2 . As the piston assembly  4  moves due to the movement of the push rod  20 , as shown in  FIGS. 3 and 5 , a pressure on the backside of the large piston  6  moves valve  72  to an open position and provides hydraulic fluid to the brake piston cylinders through outlet  71 . Both pistons initially act and provide a high volume of hydraulic fluid with initial movement of both pistons. 
     The operation of the two-stage master cylinder can be appreciated from a review of  FIGS. 3 through 6 . Initial movement of the push rod  20  to the left as indicated in  FIG. 3  causes the needle valve  16  to seal with the open port  14  of the small piston  10 . Further movement of the push rod  20  causes movement of the piston assembly and movement of the large piston  6  and the small piston  10 . The hydraulic fluid pushed by the small piston  6  is open to the port  71  as shown in  FIG. 4 . The large piston  6  pushes hydraulic fluid through the valve  70  to the outlet  71 . The large piston can be of a diameter of 1½ inches and the smaller piston of a diameter of approximately ¾ of an inch for example. A large amount of fluid is initially forced through the outlet  71  by the two pistons for movement of the piston cylinders at the brakes. 
     With this arrangement the amount of brake pedal travel to initially cause the brake piston to move the brake pads against the braking members is relatively low due to the combined area of the pistons. As can be appreciated, pressure will build within the master cylinder once the pistons at the brakes have moved the brake pads to engage the brake member. This occurs without excessive brake pedal travel as a relatively high volume is being displaced. 
     Further pressure exerted on the push rod  20  by the operator is shown in  FIGS. 5 and 6 . The movement of the piston assembly  4  has caused a certain hydraulic brake fluid pressure to be present adjacent the outlet  71 . This pressure acts on the head  41  of the bypass control valve  40 . This pressure also acts on the rear face  48  of the small head  41 . This pressure produces a bias force and if this force is sufficient, it moves the bypass control valve  40  towards the right overcoming the bias exerted by the spring  46  and assisted by the bias produced by the spring  42  to the left of the control valve. 
       FIG. 5  shows that the control valve has now moved and the hydraulic fluid under pressure by the large piston  6  is now allowed to vent to the reservoir along the fluid path shown in  FIG. 5  unless the pressure associated with the large piston  6  exceeds the pressure at outlet  71 . The hydraulic fluid passes through a small vent channel  62  having an adjustable restrictor  64 . At this point the effect of the large piston  6  has been generally eliminated. Further movement of the push rod  20  is opposed by the small piston  10  and the hydraulic pressure exerted on the small piston. This provides better control for the user of the brakes and any additional brake pedal movement is not substantial as the brake pads have already been engaged. It can also be appreciated that adjustment of the adjusting spring  42 , due to adjustment of the threaded case  39 , can increase or decrease the force exerted by the spring  42  on the bypass control valve  40 . This allows adjustment of the hydraulic pressure at which the control valve opens. The restrictor  64  provided in the channel  62  also allows an adjustable resistance with respect to the venting or the release of the pressure associated with the large piston  6 . This restriction reduces a sudden or abrupt change when the large piston has now been effectively bypassed. Preferably the bypass control valve opens at a pressure between 50 and 60 PSI. 
     In  FIGS. 5 and 6 , the second stage i.e. the small piston alone, is now producing the hydraulic fluid pressure for further actuation of the brakes. With this arrangement, initial brake pedal movement causes both pistons to move and displace a large amount of brake fluid for the brake pistons. Once a certain pressure is reached, the brakes are normally controlled by the small piston. The action of the large piston and the bypass valve and valve  70  are dynamic and there are circumstances where the large piston can reconnect with the outlet port  71 . For example, suddenly jamming on the brakes can reactivate the large piston that was previously bypassed. 
       FIGS. 7 and 8  show the master cylinder when the braking force exerted on the push rod  20  has been removed. In this case the needle valve  16  has moved under the force of its spring and the pressure differential to a clear position and hydraulic fluid can vent through the open port  14  of the small piston  10 . The large piston is returned to an initial position as shown in  FIG. 7  due to the biasing force exerted on it by spring  18 . Brake fluid is provided to chamber  90  associated with the large piston  6  through the valve element  80  which has now moved to a clear position as movement of the large piston creates a low pressure on that valve element. 
     With the present arrangement a large volume of hydraulic brake fluid is initially provided through the outlet port  71  by the combination of the large piston  6  and the small piston  10 . At a certain pressure that is adjustable by the operator the action of the large piston is generally eliminated by the movement of a bypass control valve. The second stage of the master brake cylinder continues to act due to the influence of the brake actuator, in this case push rod  20 , the position of the small piston  10 . Typically the brake pads engaging a braking member cause a build up in pressure sufficient to bypass the large piston. 
     As can be appreciated variations of this two-stage master cylinder can use different ratios of the large piston  6  to the small piston  10 . This allows for further variation. Preferably the cylinders in which these pistons slide are releasably secured in the body housing  5  of the master cylinder and are easily interchanged. 
     An alternate two-stage master cylinder  102 , shown in  FIGS. 9 through 18 , includes a piston assembly  104  having a large piston  106  and a smaller piston  110 . The large piston  106  slides within cylinder  108  and the smaller piston  110  slides within cylinder  112 . 
     A push rod actuator  120  is connected to the brake pedal by a linkage (see  FIG. 19 ) and applies a force against the large piston  106 . The small piston  110  includes a longitudinally extending chamber  113  that receives the stem  115  of the compensating valve  117  provided at one end of the master cylinder  102 . The compensating valve  117  shown in  FIG. 9  is in a release position as the stem  115  is controlled by the small piston  110  and moved to the release position. The piston return spring  119  draws both the large piston  106  and the small piston  108  to the release position. Compensating valve  117  includes a spring  121  which is being compressed. The stem  115  includes a shoulder region located within the chamber  113  of the small piston. The stem  115  is slideable within the chamber  113 . 
     In  FIG. 10 , details of the bypass valve are shown with the bypass valve being in a closed position and the compensating valve  117  also being in an open position. 
     As can be seen from a review of  FIG. 11 , the compensating valve  117  allows fluid from the caliper to be returned to the reservoir through the passage  123  when the compensating valve is open. The large piston is also in communication with the reservoir if the check valve indicated by the ball  127  is in a release position. This path provides additional fluid required to compensate for movement of the large piston when the brake is released. Brake fluid is returned from the calipers as shown in  FIG. 11 , through the compensating valve  117  as the valve  170  is closed. The check valve  127  can move to an open position to allow the large piston to return under the bias of spring  119 . 
     The check valve ball  127  is preferably made of a UHMW (ultra high molecular weight) polyethylene that is buoyant in brake fluid and floats. With the check valve orientated as shown in  FIG. 12 , the check valve ball  127  will float to the closed position and opens based on a pressure differential across the check valve ball. If a different orientation is used a light spring can be used to bias the check valve ball to the closed position. 
       FIG. 12  shows further details of the compensating valve. 
     The operation of the two stage master cylinder can be appreciated from a review of  FIG. 13 . The push rod  120  forces the large piston  106  and the small piston  110  from right to left. Fluid is forced towards the caliper through the outlet  171  and the compensating valve  117  moves to a closed position (due to the spring bias). The brake fluid associated with the large piston  106  has caused the check valve  127  to move to a closed position isolating the large piston from the reservoir. Brake fluid is forced through the valve  170  as it is able to overcome the spring bias associated with the small spring  172 . 
     In  FIG. 14  it can be seen that the bypass valve remains in the closed position as fluid is being provided to the brake pistons and pressure is only starting to build between the master cylinder pistons and the brake pistons. Once the brakes engage the caliper, pressure within the master cylinder (i.e. at outlet  171 ) builds and at a desired pressure, for example 50 PSI, the bypass valve  140  will open as indicated in  FIG. 16 . At this point in time further movement of the large piston generally causes fluid associated therewith to return to the reservoir through the vent path  150  having the modulator valve  152 . The small piston continues to provide fluid to the brake caliper and the second stage of the master cylinder is effectively being used. An operator will require less force to continue movement of the brake pedal. 
       FIG. 17  shows the alternate master cylinder when the brake pedal is released. Brake fluid is returned to the front face of the small piston  110  and in cooperation with the return spring  119 , forces the actuating rod to move as indicated in the drawing. The compensating valve  117  remains closed due to pressure in front of the small piston. Brake fluid returning from the caliper is also at a pressure higher than the pressure on the side of valve  117  exposed to the large piston and as such that valve is closed. With movement of the small piston and large piston to the right as indicated, additional fluid is provided to the operating face of the large piston as the check valve  127  has opened. 
       FIG. 18  shows a brake caliper  200  having opposed brake pads  204  and  206  for engaging a disc brake. The one brake pad  204  is connected to the actuating piston  208 . The brake pad  204  includes a support plate  210  as the size of the piston  212  is somewhat large. The piston  212  is moveable within the piston chamber  214  of the caliper. The piston  212  cooperates with a piston outer ring  216 . This piston outer ring  216  moves within the cylinder  214  and includes an o-ring seal  218 . The piston outer ring includes a shoulder  220  which contacts a wave washer  222  that in turn engages the split sleeve  224 . The wave washer  222  provides a preset return distance or retraction distance of the piston outer ring  216  relative to the split sleeve  224 . The split sleeve  224 , when pushed forward due to movement of the piston, effectively locks with the cylinder  214  and compensates for wear of the brake pads. The wave washer  222  provides a preferred positive preset retraction of the brake pad as the piston will return a preset distance and clear the brake member (disc, brake drum, etc) reducing drag and increasing fuel efficiency. 
     It has been found that a particular relationship between the effective size of the pistons of the master cylinder and the size of the pistons of the brake pads is desirable. There is a preferable 4:1 ratio of area whereby the brake pads may be positively retracted from the disc brake. The positive retraction is effectively provided by the wave washer, and while providing positive retraction, restricts or compensates for the wear of the brake pads. 
     A further preferred embodiment of the invention is shown in  FIG. 19  where two master cylinders  300  and  302  are shown connected to a common linkage associated with a vacuum booster  304  which is in turn connected to the brake actuator  306 . One of the two stage master cylinders operates to control the front disc brakes and the second master cylinder  302  is connected to the rear brakes. 
     Although preferred embodiments of the invention have been described here in detail it will be understood by those skilled in the art that variations may be made thereto without departing from the spirit of the invention and scope of the appended claims.