CA1048085A - Fluid pressure control device - Google Patents

Abstract

Abstract of the Disclosure An inertia responsive movable ball valve of a braking pressure control device is supported by barrier means lacking in an aperture penetrating its end portion for preventing the flow of a fluid pressure from striking a front surface of the ball valve and from exerting thereon a thrust which biases the ball valve to a position closing an inlet port of the fluid pressure to biasing means and for diverting the flow of the fluid pressure to passage means formed in the circumferential portion of the barrier means to cause the fluid pressure to pass through the circumference of the ball valve to the inlet port.

Description

s The present invention relates generally to a brakins pres~ure control device which regulates the output fluid pressure for the wheel cylinders of the motor vehicle to increase at the same rate as the input fluid pressure from the master cylinder when the input fluid pressure is below a critical fluid pressure and to increase at a rate small than that of increase in the input fluid pres~ure when the input fluid pres~ure i8 above the critical fluid preqsure and further which varies the critical fluid pressure in accordance with variation in the weight of the;motor vehicle and par-ticularly to a braking pressure control device of this type which is improved to comprises barrier means for preventing the flow of the input fluid pressure from striking the inertia re~pon~ive movable ball valve and from exerting thereon a thrust or dynamic pressure which moves the valve to the valve seat to close the port thereof and for cau~in~ the input fluid pressure to flow through the surroundings of the valve to the valve seat.
As is well known in the art, usual motor vehicle hydraulic braking systems are such that the brakes are applied to the front and rear wheels concurrently. In thi~ instance~ if an exce~s amount of braking force i~
applied to the front wheels, the front wheels are

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locked earlier than the rear wheelA to make it impossi-ble for the driver to handle the motor vehicle. On the contrary, if the rear wheels are braked excessively, they are locked prior to the front wheels to cau~e the rear portion of the motor vehicle to swing tran~versely to the longitudinal direction thereof. Accordingly, in order to assure and increa~e the safety and stability of the vehicle during the braking operation, it iq necessary to effect the distribution of the braking forces to lock the frvnt and rear wheels concurrently.
When the vehicle is braked, the 40-called nose-dive phenom~non take3 place in which the vehicle weight applied on the front wheels increase~ and the vehicle weight applied on the rear wheel# decreases. According-ly, it is necessary for concurrently locking the frontand rear wheels to distribute to the front wheels a braking force greater than a braking force di~tributed to the rear wheel~. It is al~o necessary that the distribution of the braking forces to the front and rear wheels is varied in accordance with ~ariation in the vehicle weight. Thus, ideal characteristics of di~tribution of the bralcing forces to the front and rear wheels, when are illustrated on oblique coordinates having the axes of abscissa and ordinate indicating respectively thereon the ratios (deceleration rate ratios) ~f/W and -- 3 ~

~C~4~ 35 Br/W of the braking forces Bf and Br on the front and rear wheels versus the vehicle weight W, are expressed by curves having tangen-t3 the angles of inclination of which are relatively large within a range of the origin to a certain value and are relatively small outside the range. Furthermore, the heavier the vehicle weight is, the higher the ideal characteristics curve is located on the coordinate~.
It is accordingly neces~ary for providing the dis-tribution of the braking forces which i9 close to the ideal characteristics curve to feed to the rear wheel cylinders a fluid pre~ure increasing at a rate Rmaller than that of increase in a fluid pressure fed to the front wheel cylinders or at a rate of zero when the fluid pressure fed to the front wheel cylinder~ exceed~
a predetermined or critical fluid pres~ureO A~ ~n expedient for solvin~ the problem, a limiting valve, proportioning ~alve or G valve wa~ disposed in a rear braking circuit leading to the rear wheel cylinders.
The limiting valve generates an output fluid pres~ure increasing at a rate of ~ero when an input fluid pres-sure exceeds a critical fluid pressure. The proportion-ing valve generates an output fluid pre~qure increasing at a rate lower than that of increa~e in an input f~uid pre~sure when the input fluid pressure exceeds a ~)41~Q85 critic~l fluid pressureO The G valve generate~ an output fluid pressure increasing at a rate less than an input fluid pressure when a pradetermined rate of deceleration is attained. However, the output fluid pressure generated by these valves merely carried out the distribution of the braking force~ approximating to a single ideal characteristics curve which accordingly, corresponds to a predetermined vehicle weight and, when the vehicle weight is varied,~effected a distribution of the braking forces which largely deviated from an ideal characteristics curve corresponding to the vehicle weight varied~
On the other hand, most motor vehicles are in recent years provided with a hydraulic braking system of the tandem type which comprises front and rear braking circuits leading respectively~separately to the front and rear wheel cylinders. However1 a braking pressure control valve which i9 disposed in the rear braking circuit generated the same output fluid presiure as in the event of no failure of the fluid pressure in the front braking circuit in the event of the failure of the fluid pres~ure in the front braking circuit and a8 a result caused the deficiency of the braking force.
Thus, the applicants have proposed a braking pres-sure control device comprising a fluid pressure regulating ~O~EiO15 5 valve serving as a proportionillg valve or limitingvalve, biasing means which urges the fluid pressure regulating valve and to which the master cylinder fluid pressllre is fed to control the force of the biasing ~neans,. and an inertia responsive movable ball valve responsive to a predetermined rate of deceleration to close the path of flow of the fluid pressure to the biasing means to maintain the fluid pressure having been fed to the biasing means at a predetermined value so thHt the critical fluid pressure i8 varied in accord-ance with variation in the vehicle weight to generate the output fluid pressure which provides the distribution~
of the braking forces which approximate to the ideal characteristics curves corresponding to the vehicle ,~ 15 weight variedO The fluid pressure regulating valve is also biased by the fluid pressure in the front br~k-ing circuit in a direction oppo~ite to the biasing direction by the biasing means so that in the event of - the failure of the fluid pressure in the front braking circuit the critical fluid pressure i9 sufficiently .increased to generate the output fluid pressure hi~h to compensate the deficiency of the braking force.
~ owever, the conventional braking pressure-control device has had a drawback in that support means for supporting the ball valve is formed through its central ~fL041~QEi 5 end portion with an aperture for passing the master cylillder fluid pressure to the bia~ing means so that the f`low of the fluid pressure strikes the ball valve to exert a thrust thereon which moves the ball valve into a position to clo~e an inlet port to the biasing means to hinder the braking pressure control deYice from exhibiting its desired function completely.
It is~ therel`ore, an object of the invention to provide an improved braking pressure control device in which the flow of the maqter cylinder fluid pressure is prevented from striking the ball valve and from exertl.ng thereon a thrust which moves the ball valve into a po-sition to close an inlet port to the biasing means to have the braking pressure control device exhibit its desired function completely by providing barrier support means for supporting the ball valve which mean~ lack an aperture penetrating its central end portion to prevent the master cylinder fluid pressure from passing therethrough and which diverts the flow of the fluid pressure to the periphery of the barrier means and by forming passage means in the periphery of the barrier means aIId the ball valve which passes to the biasing means therethrough the fluid pressure diverted by the barrier means.
This and other objects and advantages of the ~o~

invention will become more apparent from the followirlg detailed description taken in connection with the accompanying drawings in which:
Fig. -l is a graphic representation of the ideal characteristics curves of the distribution of the brak-ing forces to the front and rear wheels;
Fig. 2 is a schematic view of a motor vehicle hydraulic braking system incorporating thereinto a braking pressure control device according to the in-vention;
Fig. 3 is a schematic cross sectional view of a preferred embodiment of a braking pres~ure control device accordin~ to the invention;
Fig. 4 is a schematic pPrspective view of~barrier support means forming part of the braking pressure control deYice ~hown in Fig. 3;
Fig~ 5 is a graphic representation of the relation-ship between the input fluid presqure and the output fluid pressure of the braking pressure control device shown in Fig. 3; and Fig. 6 is a graphic representation of the relation-ship between the critical fluid pre~sure of the braking pressure control device shown in Fig. 3 and the vehicle weight.
~eferring to Fig. 1 of the drawings, the ideal 1~4~85 characteristics curves a1 and a2 as per the introduction of the specification of the distribution of the braking forces to the front and rear wheel~ are illustrated on oblique coordinates having the axe~ of abscissa and ordinate indicating respectively thereon the ratios (deceleration rate ratios) Bf/W and Br/W of the braking forces Bf and ~r on the front and rear wheels versus the vehicle weight W. The curves a1 and a2 are the ideal characteristics curves at the time when the weight of the vehicle is W1 (no load3 and W2 (the vehicle carries load), respectively. Generally, the heavier the vehicle weight is, the higher or the more the ideal characteristics curve is positioned or extends steeply from the origin 0 in the graph of Fig. 1.
As is apparent from the graph, the angle of in-clination of a tangent of each of the curves a1 and a2 is relatively large wi-thin a range of the origin 0 to a certain value and is relatively small outside the rangeO In the graph o~ Fig. 1, there is also illustrated the characteristics lines b1 and b2 of the distribution of the braking forces to the front and rear wheels which distribution is provided to approximate respectively to the ideal characteristics curves a1 and a2 by a motor vehicle hydraulic braking system incorporating therein a braking pressure control device accolding to the ~04~ilQ~3S

invelltion.
Referring to Fig. 2 of the drawings, there is S}lOWIl a motor vehicle hydraulic braking system incorpo-rating therein a braking pressure control device or valve according to the invention. The hydraulic braking system, generally designated by the reference n~neral 10, includes a master cylinder 12 operated from a brake pedal 14. First and ~econd hydraulic fluid circuits 16 and 18 lead from the master cylinder 12. 1`he fir~t fluid line 16 i~ connected to front wheel cylinders 20 cooperating with brakes (not shown) of front wheels 22 of a motor vehicle, while the second fluid line 18 i~
connected to the control device, generally designated by the reference numeral 24, which is connectsd through a fluid line 26 to rear wheel cylinders 28 cooperating with brakes (not shown) of rear wheels 30 and 32 of the vehlcle. The first and second fluid line~ ~6 and 18 are further connected to the control device 24 through branch lines 34 and 36, respectively. The control val~e 24 is mounted on the body (not show~) of the vehicle to have its axis 38 inclined at an angle of e from the horizontal plane 40 so that the forward end portion of the control valve 24 is positioned above the rearward end portion thereof.
~efexring to Figo 3 of` the drawings, the detailed ~09~8~3S
construction of the braking pressure control valve 24 according to the invention is shown. The control valve 2li comprises a housing 42 having a first cavity 44 and inlet and outlet ports 46 and 48 which are formed in its front portion 49. The inlet and outlet ports 46 and 48 are connected respectively to the second fluid line 18 and the fluid line 26~ An annular ~ealing member 50 such as a lip type seal is fixedly attached to a wall defining the cavity 44 and divides the same into first and second chamber~ 52 and 54 into which the lnle-t and outlet port~ 46 and 48 open, respec-tively.
The annular sealing member 50 h~s formed therethrough an aperture 56~ A plunger 58 extends through the aperture 56 and is axially movable in the first and second chambers 52 and 54. The aperture 56 provides an annular clearance between the annular sealing member 50 and the plunger 58 to provide fluid communication between the first and second chambers 52 and 54. A
plug member 60 is firmly fitted in a bore 61 formed ~ in the forward end portion 62 of the housing 42 and closes the forward end portion 62. The plug member 60 has formed therein an inlet port 64 connected to the branch line 34 of the first fluid circuit 16~ and a bore 65 communicating with the port 64. The plunger 58 has a stem portion 66 located in the first chamber ~4~85 52, all annular projection 68 having a cross sectional a.rea of A1, and forward and rearward end portions 70 and 72 having cross sectional area~ of A2 and A3, respectively, both of which are smaller than A~. The annular projection 68 is located in the second chamber 54 and is engageable with the annular sealing member 50 to obstruct fluid communication between the first and seconed chamber~ 52 and 54. The forward end portion 70 is connected to the annular projection 68 and is slidably supported in an aperture 74 formed through the forward end wall 76 of the cavity 44 and extends into the bore 65 of the closure member 60 through the aperture 74. The bore 65 is sealed from the second chamber 54 by a seal member 78. The rearward end portion 72 i9 connected to the stem portion 66 and is slid~bly supported in an aperture 80 formed through a rearward end wall 82 of the cavity 44. The rearward end portion 72 is formed therein with a blind bore 84 in which a pu~h rod 86 is received.
The housing 42 further has a ~econd cavity 88 formed in its mid portion 89, and two oppo~ite bores 90 and 9Z formed in opposit~ end walls 94 and 96 of the cavity 88 and both opening into the cavity o8.
Two pistons 98 and 100 are slidably fitted in the bores 90 and 92, respectively. The push rod 86 extends ~0~8~5 from the bore 84 of the plunger 58 into the bore 90 and engages the plunger seat 98. A spring seat 102 is s:l.idably fitted in the cavity 88 and is in abutting engagement with the end wall 96 or the piston 100. An inner compression spring 104 is located between the piston 98 and the spring ~eat 102 to urge these two members in opposite directions. An outer compression spring 106 is l.ocated between the end wall 94 and the spring seat 102 to urge the latter against the end wall 96 or the piston 100. The pi~ton 100 has a cross sectional a:rea of A4. The bore 90 i~ sealed from the firs-t chamber 52 by a qeal member 107. A chamber 108 is defined in the bore 92 between the piston 100 and an end wall 110 of the bore 92.
The housing 42 further has a third cavity llZ and a bore 114 which are formed in its rearward end portion 116, and a bore llB formed in an end wall 120 of the cavity 112. An inertia re~ponsive movable ball member 122 is rotatably or rollably fitted in the cavity 112.
A valve seat member 124 is firmly fitted in -the bore 118 and has formed therethrough an aperture 126 opening into the cavity 112 and communicating with the chamber 108 through a passage 128. The ball member 122 con-stitutes an inertia respon~ive valve 129 together with the seat member 121~ and i~ seated on the valve seat 124 ~4~itQ8S
in response to a predetermined rate of deceleration or inertia force to obstruct fluid communication between the chamber 108 and the inlet port 136. The cavity 112 has a groove 130 formed around the ball member 122. A
plug member 132 is threaded in the bore 114 to close the rearward end portion 116 of the housing 42 and is fo:rmed therein with a bore 134 and an inlet port 136 communicating with the bore 134 through an orifice 137 and connected to the branch line 36 of the second flu:id circuit 18. A ball support member 138 is press fitted in the bore 134 and is formed in its circumfer-. ential surface 139 with an axial groove or grooves 140 as shown in Fig~ 4 of the drawings which communicateA
with the inlet port 136. The ball support member 138 is not formed with an aperture pa~sing from its outer end 142 to its inner end 144 but the groove 140 provides fluid communication between the groove 130 and the inlet port 136 to cause the pressurized hydraulic fluid from the inlet port 136 to flow into the aperture 126 through the grooves 130 and 140 to prevent the flow of the hydraulic fluid from striking the ball member 122 and from exerting thereon a thrust which moves the ball member 122 into a po~ition to close the valve seat member 124. It is preferable that the ball support member 138 is formed in the outer end 142 with radial _ 14 -4~8S
grooves (llOt shown) leading to the grooves 140 for preventing or minimize pressure loss of the hydraulic fluid from the inlet port 136.
The braking pressure control device 24 thus far descrit)ed is operated as follow~:
When the brake pedal 14 i9 depreM~ed,the mn~ter cylinder 12 delivers a hydr~ulic fluid pressur~ Pm int~
the first and second fluid lines i6 and lfl. The fluid pressure Pm in the first fluid line 16 i9 fed into the front wheel cylinders 20 and through the inlet port 64 into the bore 65 of the pressure control valve 24. The fluid pres~ure Pm in the second fluid line 18 i9 fed into the first chamber 52 of the pressure control valve 24 through the inlet port 46 and is then delivered into the second chamber 54 through the aperture 56 of the annular sealing member 50 as an outlet hydraulic fluid pressure Pr which has been modulated or unmodulated.
The outlet fluid pre~ure Pr in the second chamber 54 is fed into the rear wheel cylinder~ 28 through the outlet port 48. The fluid pres~ure Pm in the second fluid circuit 18 is also fed into the chamber 108 of the pressure control valve 24 through the inlet port 136, the grooves 140 and 130, and the aperture 126 of the seat member 124.
When the inlet fluid pressure Pm is less than a critical fluid pressure Ps, the outlet fluid pressure Pr in the second chamber 54!is equal to the inlet fluid pressure ~m, that i~, Pr = Pm In this condition, the fluid pressure Pm in the bore 65 exerts on the forward end portion 70 of the plunger 58 a force Pm x A3 which urges the plunger 58 rearwardly.
When the inlet fluid pressure Pm is increa~ed to the critical fluid pressure Ps, the force P9 X A3 exceeds the force F1 of the inner spring 104 to move the plunger 58 into a closed position in which the annular projection 68 engages or is pressed against the annular sealing member 50 to obstruct f`luid communication between the first and second chambers 52 and 54. At this time, the following relation is defined:
Ps x A3 = Ft Accordingly, the critical fluid pressure P~ is expressed as P9 F1/A3 Eq. 1 In this instance, since the di~placement of the plunger 5O is extremely small, an increase in the force of the spring 104 is so little as to be neglected.
When the input fluid pressure Pm is subsequently further increased, the fluid pressure Pm in the first chamber 52 exerts on the annular projection 68 a forcs ~4~ S
which urges the plunger 58 into an open position to ullseat the annular projection 68 from the annular ~eal-ing member 50. When the annular projection 68 is unseated from the annular sealing member 50, the fluid pressure Pm in the first chamber 52 is allowed to flow into the second chamber 54 to cause an increase in the outp~t fluid pressure Pr. At this time, i.e., when Pm _ Ps, the following equilibrium equation is established:
2+Pr(Al-A2)=Pm(Al-A3)+F1 Eq. 2 Accordingly, the output ~luid pressure Pr is expreqsed as A1-A3-A2 F1 Eq. 3 The output fluid pressure Pr delivered from the outlet port 48 under the control of the braking pressure control valve 24 is in this manner given by either of the Equations 1 and 3 in accordance with the valve of the input fluid pressure Pm. Thus, when the input fluld pressure Pm increaqes from zero, the output fluid pressure Pr increases At the same rate as the input fluid pressure ~n until the input fluid pressure P~
reaches the critical fluid pressure Ps. When the input fluid pressure Pm increases above the critical fluid pressure Ps, the output fluid pressure Pr increases at the rate of m rwherein m = (A1-A3-A2)/(A1 A2)]

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sma.Ller than the rate of increa~e in the input fluid pressure Pm~
On the other hand9 when the braking force B on the wheels increases with increases in the input fluid pres-sure Pm, the ratio of the rate a of deceleration versusa gravitational acceleration g also increases. This deceleration rate ratio a/g is equal to the ratio of the braking force B versus the overall weight W of the motor vehicle as follows:

a g W Eq. 4 The braking force B is proportional to the input fluid pressure Pm as follows:
B = C~n (wherein C is a constant) Eq. 5 When the deceleration rate ratio a/g reaches a predetermined Yalue of (a/g)e which is a function f(3) of the angle e of inclination of the pressure control val~e 24, the ball member 122 of the inertia responsive valve 129 rolls forwardly in response to the force of inertia to seat on the valve seat member 124 to close the aperture 126 to obstruct fluid communication between the chamber 108 and the inlet port 136. Thu~, even if the inlet fluid pressure Pm subsequently increases, the fluid pressure in the chamber 108 is maintained at a fluid pressure Pg which is the input ~5 fluid pressure Pm at the moment when the aperture 126 ~4~ 3S

of the seat member 124 has been closed by the b~ll member 122. The fluid pressure Pg i9 expressed from the EqsD 4 and 5 and the Eqb 6 t(a/g)~=f(e)] as pg = Wc f(e) Eq. 7 At this time, from the condition of equilibrium of the piston 100 and the ~q. 7 the following equation is obtained:

F1 + F2 = Pg-A4 = f(~) A~-W Eq. 8 Where F2 i3 the force of the outer spring 106.
The forces F1 and F2 of the inner and outer springs 104 and 106 are expre~ed respectively as the sumY of the amounts f1 and f2 of preset or initial loads of the springs 104 and 106 and the products of the amounts of deflection or ~hrinkage of the springs 104 and 106 by a compressive force from the piston loo and the spring constants K1 and K2 of the ~prings 104 and 106. In this instance, since the amounts of deflection of the springs 104 and 106 are equal to each other, the following equation is obtained:

F2 = f2 + K1 (Fl 1) E~. 9 From the Eqs. 8 and 9, the force F1 of the spring 104 is - 19 _ L~8S
obtained as C ~4W ~ (f2 - K fl) ~1 K -- EqO 10 1 + _ Substitution of the Eq. 10 into the Eqs. 1 and 3 results in C A4W ~ (f2 ~ ~ fl) Ps = ~ Eq. 11

3 ( K1 ) When Pm ~ P9 F~
Pr = mPm + - -C A4W ~ (f2 ~ K f1) = mPm ~ K
(A1-A2)~ K2 ) It is apparent from the Eq. 11 that by selecting the variables in the EqO 11 in a manner to make the ~alue of (f2-f~oK2/K1) po~itive, a relation between the critical fluid pressure P~ and the overall vehicle weight W as shown in Fig. 4 of the drawings is obtained in which relation the critical fluid pressure Ps in-creases at a rate greater than the rate of increa~e in the vehicle weight W when the vehicle weight increases.
As a result, it is possible to make the characteristics ~!30135 of fluid pressure or br~king force distribution close to t.he ideal characteristics curves a, a2 of Fig. 1 in accordance with increases in the vehicle weight W0 In the event of the failure of the input fluid pres~ure Pm in the first fluid circuit 16, since PmA2 = 0 in the Eq. 2, the following equation is obtained:
Pr(Al-A2)=Pm(Al-A3)tFl Accordingly, the output fluid pre3sure Pr is obtained as Pr = Pm +
A1-A2 A1-Az A1 ~2 = m'~ m In this instance, between th~e braking force B on the wheels and the input fluid pressure ~n the following relation is provided; B = C'Pm where C~< C. Hence, the force F1~ of the spring 104 is expressed as f(~)A4W ~ (f2 ~ X1 1 1 + _ When the .input fluid pressure Pm i~ at a critical fluid pressure Ps ', the following equation i~ defined:

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pS~(A3-A2) = Fl Accoi-dingly, the critical f`luid pressure Rs' is oltaltled as C' A4W - (fz ~ K fl) Ps' = K~
(A3-A2)(1 + K
where PY ~ ~ Ps .
Accordingly, it ls apparent that the critical i`luid pressure Ps' is increased to a conslderably high value 10 which provides a braking force 80 great as to compensate tl-e failure of the lnput fluid pressure Pm in the first f`luid circuit 16.
Since the support member 138 serving as the barrier means prevents the flow of the fluid pressure Pm from the inlet port 136 from striking the rear surface 146 of the ball member 122 and from exerting on the ball member 122 a thrust which moves the same toward the valve seat 124, the ball member 122 does not impede the admission of the flu~d pre~ur~ Pm into the fluid chamber lOo to provide the forces of the springs 104 and 106 which forces are controlled by the fluid pres-sure Pm through the piston 100 to a desired or predetermined value and the ball member 122 moves to the valve seat 124 in response to a de~ired or pre-determ1lled decelerat~on rate accurately to have the 8~

control device 2~ perform its deslred functlon or operation accurately.
It wlll be appreciated that the invention provides a braking pre~ure control devlce comprlsing support and baffle means for dlverting the flow of the fluid pressure thereto to the periphery or edge thereof and :f`or preventlng the flow of the fluid pressure from ~striking the rear end of a ball valve and from exerting thereon a thrust which produces a bad influence upon the admission of the fluid pressure into a piston chamber and hinders the ball valve moving to close an inle-t port to the pi~ton chamber i.n response to a predetermined deceleration rate and pa.~sage means for passing the diverted flow of the fluld pressure to the inlet port through the circumference or circumferential surroundings of the ~upport and baffle means and the ball valve so that the braking pre~sure control device exhibits lts desired functlon completely or best.
It will be appreciated that the invention provides a braking~pre~sure control device in which support and baffle means is press fitted in a closure member for the rearward end of the device body to be integral with the closure member so that its furni~hing to the device body is easy as compared with;a conventional ball sup-port means held between a ball valve body and a c].osure ~4~

member and so that the support and barrier mean~ can he easily exchanged for a new one without removing the whole o:f the control device from the vehicle body or at a state in whlch the control device remains mounted on the vehicle body.
Although the invention ha~ been described as being applied to a braking fluid pressure control device com-prisi.ng a proportioning valve, the invention can be applied to a braking fluld pressure control device compri.sing a lim~t~ng valve 1n place of a proportioning valve.

Claims (6)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A fluid pressure control device for a motor vehicle hydraulic braking system, comprising first valve means taking a first position generating, when an input fluid pressure is less than a critical fluid pressure, an output fluid pressure equal to said input fluid pressure and movable into a second position generating, when said input fluid pressure is higher than said critical fluid pressure, an output fluid pressure less than said input fluid pressure, and first valve control means comprising a fluid chamber, biasing means for urging said valve means away from said fluid chamber, said fluid chamber receiving a fluid pressure for controlling the force of said biasing means, second valve means operable for, in response to a predetermined deceleration rate, closing an inlet port of said fluid pressure to said fluid chamber to maintain the pressure of fluid in said fluid chamber at a predetermined value, passage means communicating through said inlet port with said fluid chamber to conduct said fluid pressure thereinto, and barrier means which is located in said passage means to support said second valve means, and which lacks an aperture penetrating its end wall portion to prevent the flow of said fluid pressure from striking said second valve means and from exerting thereon a thrust which moves said second valve means into a position closing said inlet port, and which diverts the flow of said fluid pressure to a peripheral portion of said barrier means, said passage means being formed in said peripheral portion of said barrier means and around said second valve means to pass the diverted fluid pressure to said inlet port.

2. A fluid pressure control device for a motor vehicle hydraulic braking system, comprising first valve means comprising a first chamber into which an input fluid pressure is delivered, a second chamber into which an output fluid pressure is generated, and a plunger extending axially movably in said first and second chambers and having an annular projection alter-natively providing and interrupting fluid communication between said first and second chambers, said plunger taking a first open position in which said annular projection provides said communication and said output fluid pressure generated is equal to said input fluid pressure when said input fluid pressure is lower than a critical fluid pressure, a closed position in which said annular projection interrupts said com-munication when said input fluid pressure is equal to said critical fluid pressure, and a second open position in which said annular projection provides said communication and said output fluid pressure generated is less than said input fluid pressure when said input fluid pressure is larger than said critical fluid pressure, and plunger control means comprising a piston, biasing means inter-posed between said plunger and said piston for urging them in opposite directions, a fluid chamber receiving a fluid pressure said piston having at a side thereof said fluid chamber and being biased by said fluid pressure in said fluid chamber toward biasing means, second valve means operable for, in response to a predetermined deceleration rate, closing an inlet port of said fluid pressure to said fluid chamber to maintain the pressure of fluid in said fluid chamber at a predetermined value, passage means communicating through said inlet port with said fluid chamber to conduct said fluid pressure thereinto, barrier means which is located in said passage means to support said second valve means, and which lacks an aperture penetrating its end wall portion to prevent the flow of said fluid pressure from striking said second valve means and from exerting thereon a thrust which moves said second valve means into a position closing said inlet port, and which diverts the flow of said fluid pressure to a peripheral portion of said barrier means, said passage means being formed in said peripheral portion of said barrier means and around said second valve means to pass the diverted fluid pressure to said inlet port.

3. A fluid pressure control device as claimed in Claim 2, in which said barrier means comprises a disk member formed in its peripheral portion with a plurality of grooves forming part of said passage means.

4. A fluid pressure control device as claimed in Claim 2, in which said second valve means comprises a ball.

5. A fluid pressure control device as claimed in Claim 3, in which said control means further comprises a closure member which is firmly fitted in a bore in an end portion of a body of said control device to close said end portion and which is formed therein with a bore, said disk member being firmly fitted in said bore in said closure member.

6. A hydraulic braking system for a motor vehicle, comprising a matter cylinder of a tandem type, front wheel cylinders, rear wheel cylinders, a fluid pressure control device as claimed in Claim 2 and comprising bore means defining a bore, a front braking circuit leading from said master cylinder and communicating with said front wheel cylinders and with said bore of said control device, and a rear braking circuit leading from said master cylinder and with said first chamber of said control device and with said passage means thereof, said second chamber of said control device communicat-ing with said rear wheel cylinders, said plunger having an extension which extends from said second chamber into said bore and on which a fluid pressure from said master cylinder acts to urge said plunger toward said biasing means.