GB2074273A

GB2074273A - Improvements in and Relating to Brakes

Info

Publication number: GB2074273A
Application number: GB8111537A
Authority: GB
Original assignee: Kelsey Hayes Co
Current assignee: Kelsey Hayes Co
Priority date: 1980-04-11
Filing date: 1981-04-13
Publication date: 1981-10-28
Also published as: DE3112925C2; AR224192A1; GB8403330D0; SE8102349L; SE449834B; IT8121021A1; JPS5718547A; ES501257A0; BR8101987A; FR2485457A1; IT1194780B; JPH026665B2; MX152855A; ES8306978A1; GB2140517B; GB2140517A; DE3112925A1; GB2074273B; FR2485457B1; CA1180038A

Abstract

A hydraulic braking system for vehicles which experience variable loading conditions comprises two proportioning valves 14 and 16, hydraulically in series with one another, one, 16, of which includes a spring axle driven stepped cam 25 which, in response to a predetermined compression of the vehicle suspension system, holds the valve 16 open. The output pressure characteristic of the first valve 14 is suitable for a fully laden vehicle but is not effective until the second valve 16 is held open by a cam 25 when its lightly laden characteristic is no longer effective. <IMAGE>

Description

SPECIFICATION Improvements In and Relating to Brakes The present invention relates to improvements in load sensing hydraulic brake pressure control apparatus for use in the hydraulic circuit between the master cylinder and the rear wheel brake cylinders.

It is known that changes in vehicle loading cause changes in braking capability. For example, when a vehicle is fully loaded, the rear wheels will have nearly the same braking capability as the front wheels. However, when the vehicle is lightly loaded, the rear wheels may exhibit less braking capability than the front wheels. Thus the potential for premature rear wheel lock up is much greater when stopping the lightly loaded vehicle than when stopping the fully loaded vehicle. In order to compensate for the inherent imbaiance between front and rear braking action, it has been customary in past years to provide a proportioning valve which restricts fluid communication to the rear wheel brake cylinders after a predetermined pressure level is generated.

However, such proportioning valves represent a compromise between the desirable system characteristics for the full load condition and those for the light load condition. Thus the selected proportioning valve characteristic is neither suitable for the full load condition nor the light load condition. Many load sensing or vehicle height sensing valve mechanism have been proposed in the past, but they have tended to be unnecessarily complex or otherwise unsuitable for modern vehicle use. Such mechanisms are disclosed, for example, in United States Patent Nos. 3,362,758; 3,503,657; 3,649,084; 3,684,329; 3,734,574; 3,768,876; 3,848,932; 4,150,855; and 4,159,855.

The present invention provides load responsive hydraulic brake pressure control apparatus which is placed in the hydraulic circuit upstream of the rear wheels and senses changes in the distance between the chassis and the axle of an automotive vehicle and controls the hydraulic pressure delivered from the master cylinder to the rear wheel brake cylinders in response to such changes, comprising first and second proportioning valve assemblies connected hydraulically in series with each other. The first proportioning valve assembly is positioned downstream of the master cylinder and the second valve assembly is positioned between the first valve assembly and the vehicle rear brakes.

The first proportioning valve produces an output pressure suitable for a vehicle under a full load condition. The second proportioning valve, which receives the first valve's output pressure as input pressure, acts to modify or proportion the pressure received from the first valve producing an output pressure suitable for a lightly loaded vehicle.

The second proportioning valve assembly is rigidly attached to the vehicle frame and includes a rotatable digital cam driven by mechanical linkage attached to the vehicle axle. As the vehicle is loaded compression of the suspension system reduces the distance between the vehicle frame and the axle. The mechanical linkage in response to the reduction in distance rotates the digital cam to a position in which the second proportioning valve mechanism is disabled. Thus the output pressure of the first proportioning valve is transmitted unchanged through the second proportioning valve assembly to the rear wheel brakes.

The digital cam is rotatingly seated upon an axial drive shaft so as to allow relative rotation therebetween. A torsional spring fixed to the digital cam has one leg anchored thereon and the other leg engaging a flat diametric camming surface provided in the drive shaft. Thus the digital cam is caused to rotate in concert with the drive shaft. However by reason of the torsion spring a unique drive mechanism is provided which accommodates relative motion between the vehicle frame and the axle during vehicle operation by permitting relative rotation between the cam and the driveshaft whenever rotation of the cam is restricted by the functional operation of the proportioning valve mechanism.

Although the load sensing proportioning valve assembly is herein described as being in series with a first proportioning valve assembly it is to be understood that the load sensing valve may be used alone in systems where the master cylinder output pressure is suitable, without an intervening proportional valve, for direct transmission to the vehicle brakes in the heavily loaded condition.

One form of brake proportioning system constructed in accordance with the present invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a diagram of a hydraulic brake system incorporating a load sensing proportioning valve; Figure 2 is a graph of the performance of a brake proportioning system incorporating the present invention; Figure 3 is a perspective view of a load sensing proportioning valve installed in a vehicle; Figure 4 is a partial cross-sectional view of the load sensing proportioning valve; Figure 5 is a cross-section along the line 5-5 in Figure 4; Figure 6 is a partial cross-section along the line 6-6 in Figure 4; Figure 7 is a partial cross-section along the line 7-7 in Figure 4; Figure 8 is an exploded view of part of the proportioning valve;; Figure 9 is a perspective view of the digital cam of the proportioning valve; Figure 10 is a schematic illustration of the load sensing proportioning valve configuration when the vehicle is lightly loaded; Figure 11 is a schematic illustration of the load sensing proportioning valve configuration when the vehicle is heavily loaded; Figures 12 and 13 are schematic illustrations of load sensing proportioning valve configurations accommodating over-rotation of the digital cam driveshaft; and Figure 14 is a partial cross-section similar to Figure 6, but showing the digital cam mechanism in a different orientation.

Referring to the drawings a vehicle hydraulic braking system includes a master cylinder 11 which in use supplies brake activating hydraulic fluid pressure to front wheel brakes 1 3L and 1 3R through a conduit F and a metering valve assembly, not shown, contained in a combination valve 12. A conduit R similarly provides an independent source of brake activating hydraulic fluid pressure to a first proportioning valve assembly 14, shown schematically in the combination valve 12, for supply to rear wheel brakes 1 5L and 15R.

The proportioning valve 14 may be of conventional design, for example, as shown in United States Patent No. 3,423,936, having a single split point relationship between its input hydraulic pressure and its output hydraulic pressure. The proportioning valve 14 is designed to produce an output pressure to input pressure relationship as shown by the line -marked "LOADED" in Figure 2. The split point at which the valve 14 begins proportioning is indicated by the letter L. The line marked "LOADED" in Figure 2 represents a master cylinder to rear brake pressure relationship acceptable for a vehicle loaded beyond a given mid-load condition and up to its full gross vehicle weight (GVW). The output hydraulic fluid pressure from the proportioning valve 14 is transmitted to the vehicle rear brakes by conduits R1 and R2 passing through a load sensing proportioning valve (LSPV) device 20.

The LSPV 20 includes a second proportioning valve assembly 16, hereinafter described in greater detail, similar to the proportioning valve assembly 14 contained in the combination valve 12. The second proportioning valve assembly 16,.

when permitted to function, operates upon the output hydraulic pressure received from the proportioning valve 14 such that the relationship between the master cylinder pressure (the input to the proportioning valve 14) and the rear brake pressure (the output from the proportioning valve 16) corresponds to the line marked "EMPTY" in Figure 2. The "EMPTY" line shown in Figure 2 represents a master cylinder to rear brake pressure relationship acceptable for a vehicle load condition below the above-mentioned given midload condition.

A digital cam mechanism 25 is provided within the LSPV 20 to selectively disable proportioning valve assembly 1 6 in the fully open configuration vwhen the vehicle is heavily loaded. Thus when the vehicle is loaded beyond the selected mid-load condition, the proportioning valve 16 is deactivated by the digital cam 25 thereby permitting, undisturbed, the transmission of hydraulic pressure therethrough and resulting in the desired "LOADED" pressure relationship shown in Figure 2. However, when the vehicle is lightly loaded the proportioning valves 14 and 16 function and produce the "EMPTY" master cylinder pressure to rear brake pressure relationship shown in Figure 2.

Figure 3 shows a typical vehicle installation of the load sensing proportioning valve 20. The LSPV 20 is rigidly mounted on a sprung portion of the vehicle frame 35. A driveshaft 50 of the cam mechanism 25 is firmly attached to a linkage 30 so that, as the linkage 30 rotates, the driveshaft 50 rotates the digital cam 25 by a drive mechanism hereinafter described in greater detail.

The linkage 30 is firmly attached to a vehicle axle tube 31 or any other suitable element of the unsprung portion of the rear wheel assembly.

The digital cam 25, through the action of the linkage 30 attached to the vehicle axle 31, responds to compression or expansion of the vehicle suspension system. When the linkage is extended, as indicated at 30 in Figure 1, the vehicle is lightly loaded and the proportioning valve 1 6 is permitted to function. However, when the linkage is compressed, as indicated at 30' in Figure 1, the vehicle is heavily loaded and digital cams 25 is rotated into a position in which it prevents the operation of the proportioning valve 16.

Referring to Figure 5, the proportioning valve assembly 1 6 as shown and described herein is merely representative of known proportioning valve mechanisms. Any known proportioning valve mechanism which may be arranged to function as herein described can be used with the present invention, and the operation of proportioning valve assembly 1 6 will be described only to the extent necessary to understand its interrelationship with the digital cam 25 and its function with respect to the overall brake hydraulic system.

The proportioning valve assembly 16 comprises a valve piston 40 positioned axially within a bore 45 and extending into a bore 45a of smaller diameter which in turn opens through a still narrower bore 49 into a cavity 70 containing the digital cam mechanism 25. An O-ring seal 47 is provided to seal the bore 45 from the bore 45a hydraulically to prevent the flow of hydraulic fluid into the bore 45a. The piston 40 is provided with a pin-like extension 48 projecting into the bore 49. The piston 40 can move axially within the bore 45a to such an extent that the pin 48 may project into the digital cam cavity 70.

The opposite end of the piston 40 extends into a bore 45b of smaller diameter than the bore 45 and includes a valve head 43 which is of smaller diameter than the bore 45b thus permitting the unrestricted flow of hydraulic fluid past the valve head. The piston 40 is further provided with an extension cap 41 having in it a notch 42. The piston 40 is normaliy biased, to the left as seen in Figure 5, by action of a spring 46 such that the extension 41 is urged into abutment with the end of the bore 45b. Hydraulic fluid entering through an inlet port R1 in the side of the bore 45 can thus pass freely between the piston 40 and an elastomeric valve seat 44, past the valve head 43, through the notch 42 and out through an outlet port R2.Thus in the configuration as shown in Figure 5 the fluid pressure at the outlet port R2 will be equal to the fluid pressure at the inlet port R1.

During brake application the above described fluid path through proportioning valve 1 6 remains open until the fluid pressure delivered at inlet port R1 attains a predetermined level at which the valve head 43 will close against the valve seat 44.

The pressure at which this occurs is dependent upon the force exerted by the spring 46 and upon the effective area of the valve piston 40 acted upon by the inlet fluid pressure in a direction opposing the spring 46. This effective area is determined by the diameter D of the piston 40 since the right hand end of the piston projecting into the bore 45a is sealed off from the inlet fluid pressure by the O-ring seal 47 while the inlet fluid pressure acts against all of the remaining portions of the piston.

After the valve head 43 closes against the valve the valve seat 44, if the fluid pressure at the inlet port R1 is further increased the increased pressure will act against the piston 40 over an effective area having a diameter equal to the area of the valve head 43 inside the seal with the seat 44 less the crosssectional area of the piston 40 extending into the bore 45a. This produces a force acting upon the piston 40 in the same direction as the spring 46 to reopen the valve so as to deliver at least a portion of the increased fluid pressure to the outlet port R2. However, any increased fluid pressure delivered to outlet port R2 creates an opposing force upon the piston 40. The opposing force tends to reclose the valve head 43 against the valve seat 44.The opposing forces tend to keep the valve head 43 close to the valve seat 44, restricting the flow of fluid from the inlet port R1 to the outlet port R2 and causing the pressure at the outlet port R2 to increase at a lower rate than the pressure at the inlet port R1 The ratio of the pressures is determined by the relationship of the effective areas previously referred to and hence the fluid pressure passing through proportioning valve 1 6 may be caused to follow a predetermined relationship.

During that portion of a brake application in which the applied pedal effort is reduced subsequent to a brake application of sufficient intensity to have moved piston 40 to the restricted flow position, the forces tending to move the piston 40 to the left as seen in Figure 5 are reduced and the piston 40 moves to the right under the influence of the pressure at the outlet portR2. As the piston 40 moves to the right the valve head 43 is permitted to slide within the inner peripheral surface of the valve seat 44, thereby increasing the available volume of the fluid at the rear brake cylinders 1 5L and 15R and accomplishing a reduction in the pressure at the outlet port R2.The pressure at the outlet port R2 can never be greater than the pressure at the inlet port R1 because the valve seat 44 also acts as a fluid check-valve permitting the flow of fluid from the port R2 into the bore 45.

A more detailed description of proportioning valve operation and the design of particular proportioning valve elements is given in United States Patent No. 3,423,936 issued on January 28, 1969.

Referring now to Figures 4 to 9, a housing 19 of the LSPV 20 includes a stepped bore 60. A floor 69 of the bore 60 has in it a semicircular slot 67 and a journal recess 68. The cam driveshaft 50 is supported and retained as shown in Figure 4, with a journal 51 of the driveshaft journalled in the journal recess 68. The shaft 50 extends generally normai to the bore floor 69 passing through and being rotationally supported by an end cap 61. The end cap 61 is snugly retained within the wider portion 60a of the bore 60 and against the step 62 by a snap ring 63. An O-ring 55 is provided to seal the digital cam chamber 70 from the entrance of any contamination thereto.

The cam driveshaft 50 protrudes far enough outside the end cap 61 to permit rigid engagement thereof by the linkage 30 (see Figure 3). Thus the driveshaft 50 is caused to rotate through the same angle as the linkage 30.

The digital cam 25 is supported on a cam journal 52 of the driveshaft 50 such that the cam can rotate relative to the driveshaft. The cam 25 is provided with a peripheral recess 26 and axially aligned knurls 24 over at least the working peripheral portion of the cam. The working portion of the cam 25 will become apparent as the function and operation are further described hereinafter. A pin 32 projects axially from the cam 25 into and slidably engages the slot 67 in the bore floor 69 thereby limiting the angular rotation of the cam 25 to that arc spanned by the slot 67.

The inboard side 22 of the cam 25 is milled providing an inboard-facing stepped surface 27. A circular recess 21 extends axially through the cam 25 from the outboard surface 28 and slightly past the inboard-facing stepped surface 27 thereby providing a passageway 23 between the outboard surface 28 and the inboard surface 27. A mandrel 38 is axially positioned within the circular recess 21 extending outboard and slightly past the outboard surface 28. A torsion spring 34 is seated about the mandrel 33, the helical portion of the torsion spring being seated within the circular recess 21 such that the inboard leg 34a of the spring extends through the passageway 23 over the inboard-facing stepped surface 27 and engages in a spring retention hole 29.The outboard leg 34b of the spring 34 extends over the outboard surface 28 of the cam 25 and into a slot 54 in the driveshaft 50 and engages a flat camming surface 53. In their normal assembled state as described above and as shown in Figure 6, the legs 34a and 34b of the torsion spring 34 are spring loaded so as to apply an angularly outward force to the spring retention hole 29 and the flat camming surface 53 of the driveshaft 50.

A slot 56 is provided at the external and outboard end of the cam driveshaft 50 to permit external adjustment.

In operation, the cam 25 is caused to rotate with the cam driveshaft 50 by the torsion spring 34 applying a spring force upon the camming surface 53 of the shaft 50. However should the cam 25 be prevented from rotating because of interference between the pin 32 and the slot 67 or between the cam 25 and the pin 48 on the valve piston 40, the cam driveshaft 50 may rotate relative to the cam 25, further compressing the torsion spring 34. Thus a spring drive mechanism is provided between the cam driveshaft 50 and the digital cam 25 which allows for over-travel of the shaft 50 when rotation of the cam 25 is otherwise restricted.

Figures 3, 5, 6, 7 and 10 show the LSPV 20 under conditions of light vehicle loading. The vehicle frame 35 is riding relatively high with respect to the suspended axle 31. Thus the linkage 30 positions digital cam 25 such that the peripheral recess 26 permits the pin 48 of the piston 40 to move axially into and out of the digital cam chamber 70. The second proportioning valve 16 is permitted to function freely resulting in a master cylinder pressure to rear brake pressure relationship as shown by the line marked "EMPTY" in Figure 2.

So long as the vehicle is lightly loaded the proportioning valve 16 is operative. The peripheral slot 26 accommodates operation of the valve 16. However, should the valve piston pin 48 protrude in the cam chamber 70 as a result of vehicle braking and the vehicle encounter an extreme road condition causing the cam driveshaft 50 to over-rotate momentarily because of excessive compression of the vehicle suspension system, the cam 25 will momentarily engage the valve piston pin 48 stopping the cam's rotation. However, the cam driveshaft 50 is permitted to continue its rotation by compressing the torsion spring 34. Such a condition is illustrated in Figure 1 3.

When the vehicle is heavily loaded the suspension system is compressed and the vertical separation between the frame 30 and the axle 31 is reduced. The linkage 30 assumes a configuration as shown in Figure 11 thereby rotating the digital cam 25 counterclockwise as shown. In this configuration the outermost periphery of the cam 25 is rotated into a position that deactivates the proportioning valve 16 by preventing axial movement of the piston 40. Thus in the loaded condition, as illustrated in Figure 11, the master cylinder pressure to rear brake pressure relationship is as shown by the line marked "LOADED" in Figure 2. So long asthe vehicle is in the loaded condition the outer periphery of cam 25 will remain in the valve piston obstructing configuration as shown in Figures 11 and 12.In this configuration and when the applied braking load is such that the valve piston 40 attempts to move to the right, the valve piston pin 48 abuts against the cam 25 and engages the axial knurls 24 on the outer periphery of the cam. Thus the cam 25 is restricted from freely rotating. Any further rotation of the cam driveshaft 50 resulting from road-induced vacillations of axle 31 will be accommodated by compression of the torsion spring 34 as shown in Figure 12.

The initial value A of the angle (see Figure 6) between the centreline of the pin 48 and the radius of the digital cam 25 passing through the step 26a at the end of the recess 26 determines the vehicle load condition at which the proportioning valve 16 becomes inoperative, and therefore it is necessary that this angle be accurately fixed. The angle A is determined for an unloaded vehicle and represents that angle through which driveshaft 50 will rotate as the vehicle is loaded to that mid-load condition at which it is desired to change from the "EMPTY" curve to the "LOADED" curve as shown in Figure 2. The step 26b at the other end of the recess 2b is located so as not to interfere with the operation of the proportioning valve 16; the pin 32 and the slot 67 may also be configured so as to limit the clockwise rotation of cam 25 to prevent the step 26b interfering with the operation of the proportioning valve 16.

The LSPV as illustrated in Figures 1 to 13 is arranged to be operated by counterclockwise rotation of the cam driveshaft 50 (as seen in Figure 5) upon compression of the vehicle suspension system. However, the LSPV may be easily adapted to be operated by rotation in the opposite sense as illustrated in Figure 14. By relocation of the slot 67 as shown in Figure 14 the mechanism is adapted for rotation clockwise as seen in Figures 5 and 14.

Claims

1. A vehicle including a hydraulic brake system comprising a master cylinder, wheel braking means, fluid transmission means for conveying hydraulic pressure from the master cylinder to the wheel braking means, first and second proportioning valve means connected in series between the master cylinder and the wheel braking means, the vehicle load sensing means arranged to lock the second proportioning valve means in an open configuration in which it transmits hydraulic fluid pressure unaltered when the vehicle is loaded beyond a predetermined value.

2. A vehicle as claimed in claim 1, wherein the vehicle load sensing means senses the relative positions of a sprung and an unsprung part of the vehicle.

3. A vehicle as claimed in claim 2, wherein the said wheel braking means brakes at least one wheel on the said unsprung part.

4. A vehicle as claimed in any one of claims 1 to 3, wherein the arrangement is such that the load sensing means cannot cause the second proportioning valve means to become or cease to be locked while the braking means is in operation.

5. A proportioning valve for a vehicle hydraulic brake system having a lock out mechanism whereby said valve may be selectively rendered inoperative, comprising, valve means including piston means responsive to hydraulic pressure for operating the valve means, rotatable cam means so arranged and configured as to restrict movement of the piston means when the cam means is rotated to a given angular position, locking the valve means in an inoperative configuration.

6. A proportioning valve as claimed in claim 5, comprising a rotatable shaft so coupled to the cam means that the cam means tends to rotate in response to rotation of the shaft.

7. A proportioning valve as claimed in claim 6, wherein the rotatable cam is provided with stop means arranged to limit the angle of rotation of the cam means to a predetermined arc.

8. A proportioning valve as claimed in claim 7, wherein the coupiing between the shaft and the cam means comprises resilient means arranged to permit the rotatable shaft to rotate through an angle of rotation greater than that of said rotatable cam.

9. A proportioning valve as claimed in any one of claims 6 to 8, wherein the rotatable cam is coaxial with and rotatable about the rotatable shaft, the shaft having a D-shaped axially extending portion thereof and the coupling means comprises a torsional spring having one leg thereof secured to the rotatable cam and the other leg thereof drivingly engaging the flat surface of the said D-shaped portion of the rotatable shaft, the arrangement being such that the rotatable shaft can rotate relative to the rotatable cam whenever the torque in the rotatable shaft is sufficient to cause the said torsion spring to yield.

10. A proportioning valve as claimed in any one of claims 6 to 9, comprising, a housing having a first bore and a second bore the axes of which bores intersect each other at a right angle, the first bore having therein proportioning valve means including the piston means extending axially along the first bore and into the second bore, the rotatable shaft coaxial with and extending axially through the second bore, the rotatable cam coaxial with the second bore and extending radially outward from the rotatable shaft.

11. A proportioning valve as claimed in any one of claims 5 to 10, wherein the said rotatable cam is a stepped cam, and an axial end of the piston means is arranged to abut the stepped surface of the cam.

12. A proportioning valve as claimed in claim 11, wherein the rotatable cam has only two radii in that portion of its surface that can be engaged by the piston means.

13. A proportioning valve as claimed in claim 12, wherein when the rotatable cam is in the said given angular position a portion of the cam of the greater radius is engaged by the piston means and prevents movement of the piston into a position in which the proportioning valve is closed.

14. A proportioning valve as claimed in any of claims 11 to 13, wherein at least a portion of the surface of the rotatable cam of greater radius is so formed with a serrated surface, and the said axial end of the piston means is so arranged to engage the serrations, that the rotatable cam cannot so rotate that the piston means passes from the portion of greater radius to a portion of less radius while hydraulic pressure is being applied to the piston means.

1 5. A proportioning valve as claimed in claim 5 and substantially as hereinbefore described with reference to, and as shown in, Figs. 3 to 14 of the accompanying drawings.

16. A vehicle as claimed in any one of claims 1 to 4, wherein the second proportioning valve means is a proportioning valve as claimed in any one of claims 5 to 1 5.

1 7. A vehicle as claimed in claim 1 and substantially as hereinbefore described with reference to, and as shown in, Figs. 1 and 2 of the accompanying drawings.