GB2223548A - Anti-skid brake pressure modulator - Google Patents

Abstract

Tho modulator has a plunger (40) slidingly sealed within a cylinder (12), an inlet (50) and outlet (51) adjacent one end of the cylinder (12) and a valve assembly (45, 47) to control flow of fluid from the inlet (50) to the outlet (51) via the cylinder (12), controlled by the end of the plunger (40), a piston (72) movable coaxially of the plunger (40) being provided adjacent the other end of the cylinder (12), sealingly mounted within a casing (71) to divide it into two fluid tight chambers (81 and 82), means being provided to control the pressure of fluid in the two chambers (81 and 82) so that a pressure differential may be established across the piston (72) to move the piston (72), resilient means (86) mounted on the piston (72) frictionally engaging the plunger (40) so that upon movement of the piston (72) a frictional force will be applied to the plunger (40) inducing it to move with the piston (72). <IMAGE>

Description

BRAKE PRESSURE MODULATORS This invention relates to brake pressure modulators and in particular, although not exclusively, to brake pressure modulators for use in controlling the flow of brake fluid in antilock braking systems.

Brake pressure modulators, for example of the type used in the vehicle antilock braking system disclosed in British Patent Application GB 2,174,775A, comprise a cylinder body having a plunger slidably located therein, said plunger being arranged to control a ball valve which in turn controls flow of fluid from a brake master cylinder to one or more wheel cylinders. Typically, movement of the plunger is controlled by a vacuum actuator in which, a flexible diaphragm is connected between the outer periphery of a piston and a casing to define two fluid tight chambers. The piston abuts one end of the plunger and spring means is provided to urge the piston and plunger towards the ball valve. The chamber on the side of the piston from which the spring means acts is connected to vacuum and the other chamber is selectively connected to vacuum or to atmosphere, by for example, a solenoid valve.

The pressure differential across the piston may thus be controlled by the solenoid valve to control movement of the piston.

With such brake pressure modulators, the plunger merely abuts the piston of the vacuum actuator and movement of the plunger with the piston is due to the pressure of brake fluid acting upon the opposite end of the plunger and spring means acting upon the valve element.

Seals are however provided between the plunger and cylinder and these seals apply a frictional drag on the plunger. As a result of this frictional drag, as the plunger moves to reduce pressure in the braking system, the brake pressure will be in excess of the pressure differential across the piston of the vacuum actuator. This difference in pressure will depend on the frictional drag which in turn depends upon the speed of movement of the piston and hence it will be difficult to accurately control the rate of pressure reduction in the braking system.

The present invention provides means for applying frictional engagement between the piston and plunger to overcome the frictional drag of the seals on the plunger.

According to one aspect of the present invention, a brake pressure modulator comprises a cylinder, a plunger slidingly sealed within the cylinder, an inlet and outlet being provided adjacent one end of the cylinder and a valve assembly being provided to control flow of fluid from the inlet to the outlet via the cylinder, the adjacent end of the plunger being adapted to control the valve assembly, a piston being provided adjacent the other end of the cylinder, said piston being mounted within a casing for movement coaxially of the plunger, the piston being sealed with respect to the casing to divide it into two fluid tight chambers, means being provided to control the pressure of fluid in the two chambers, so that a pressure differential may be established across the piston to move the piston, resilient means mounted on the piston frictionally engaging the plunger, so that upon movement of the piston a frictional force will be applied to the plunger inducing it to move with the piston.

In accordance with the present invention, the frictional force applied between the piston and plunger will balance, at least in part, the frictional drag of the seals on the plunger, thus reducing the pressure excess in the braking system due to the frictional drag.

Frictional engagement between the plunger and piston will however permit relative movement therebetween if forces acting on the plunger in the direction opposite to the direction of movement of the piston are greater than the frictional force between the plunger and piston. For example, if the pressure in the brake circuit is reduced to say zero, suction on the plunger may be sufficient to exceed the frictional force applied between the piston and plunger so that the piston will move without the plunger, thus avoiding the establishment of a vacuum in the braking circuit.

If the piston moves excessively relative to the plunger, upon reapplication of the brakes there will be a significant delay while the piston moves back into engagement with the plunger, before the pressure in the brake circuit begins to increase. This problem may be overcome in accordance with a preferred embodiment of the invention, in which the means for applying the frictional engagement between the piston and plunger also acts as a sealing element which controls a bypass from one fluid tight chamber to the other. In this manner, when the plunger moves out of frictional engagement with the piston the fluid tight chambers will be interconnected and the pressure differential across the piston will be destroyed, thus preventing further movement of the piston.

An embodiment of the invention is now described, by way of example only, with reference to the accompanying drawing which shows in sectional elevation, a brake pressure modulator in accordance with the present invention.

The brake pressure modulator illustrated has a valve assembly 11 as disclosed in International Patent Application WO 89/00522. The valve assembly 11 has a cylindrical portion 12 and a valve assembly housing 13, which are secured together by inter-engaging screw threads and sealed by means of O-ring 14.

The cylindrical portion 12 and valve assembly housing 13 define an axial stepped bore 15. The end of the bore 15 defined by the valve assembly housing 13 is closed by a plug 16 which defines an inlet port 17. A valve seat 18 is secured in the bore 15 by means of plug 16, said valve seat 18 having a control port 19 which may be closed by a ball 20, the ball 20 being positioned between the inlet port 17 and seat 18 and resiliently loaded towards seat 18 by means of a compression spring 21. An outlet port 22 is provided from the bore 15 adjacent the plug 16 but on the opposite side of the seat 18 to the inlet port 17.

A control plunger 25 is slidingly located in the bore 15 by engagement of a necked portion 26 towards the end of the valve assembly housing 13 which is connected to the cylindrical portion 12. Said plunger 25 is sealed with respect to the bore 15 by means of a pair of sealing rings 27 and 28, each located against one of the ends of the necked portion 26, thus defining a fluid tight chamber 30 with inlet port 17 and outlet port 22. The end of the plunger 25 adjacent the plug 16 is slidingly supported against the wall of bore 15 by means of a bearing ring 31, the bearing ring 31 being located in a circumferential groove 32 on the plunger 25. Axial grooves are provided on the internal diameter of the bearing ring 31 to permit passage of fluid into the chamber 30.

The plunger 25 is provided with a small diameter axial extention 35 which may extend, with clearance, through the control port 19 and, in the extreme position illustrated in the drawing, engage and unseat the ball 20.

A second control plunger 40 is slidingly located in the portion of the bore 15 defined by cylindrical portion 12, by engagement of a reduced diameter portion 41 adjacent the open end of bore 15. The plunger 40 is sealed with respect to the bore 15 by means of sealing ring 42, which is located against the reduced diameter portion 41 and thus, with sealing ring 28, defines a second fluid tight chamber 43. The end of plunger 40 abuts the end of plunger 25 which extends through the necked portion 26.

The valve assembly housing 13 defines a valve seat 45 with control port 46 which opens into chamber 43 adjacent the end thereof defined by the sealing ring 28. The control port 46 is inclined to the axis of the bore 15 at an abtuse angle away from the end of the bore 15 defined by the cylindrical portion 12. A ball 47 is resiliently loaded towards seat 45 by means of a compression spring 48 which acts against a plug 49 defining an inlet port 50. An outlet 51 is also provided from the chamber 43 adjacent sealing ring 28. The end of plunger 40 adjacent plunger 25 is slidingly supported against the wall of bore 15 in a manner similar to plunger 25, by means of bearing ring 44.

A sleeve 60 is slidingly located on the end portion of plunger 25 that extends into chamber 43. A spring 61 acts between a flange portion 62 at one end of the sleeve 60 and a washer 63 which is trapped between the end of cylindrical portion 12 and valve assembly housing 13, to resiliently urge the sleeve 60 into abutment with the adjacent end of plunger 40. The end of the sleeve 60 remote from flange portion 62 defines a collar 64. The collar 64 engages a pin 65 which is located in the control port 46 and engages ball 47, so that in the extreme position illustrated the pin 65 will hold the ball 47 clear of the seat 45. The pin 65 is provided with flats, so that when the ball 47 is unseated, fluid may flow from the inlet 50 into chamber 43 and from chamber 43 through outlet 51.

A vent 29 is provided in the necked portion 26 between chambers 30 and 43. The vent 29 is closed by a loose rubber plug to prevent ingress of dirt but will permit fluid leaking past seal 27 or 28 to leak out, to provide an indication of seal failure, which may otherwise go undetected, if fluid were permitted to leak from one chamber to the other.

The cylindrical portion 12 of the valve assembly 11 is mounted through a closure member 70 of a casing 71, so that it projects coaxially into the casing 71. A piston 72 has a central tubular supporting portion 73 which is slidably located on the cylinder portion 12. Axially extending grooves 39 are provided along the surface of the cylindrical portion 12 engaged by the tubular supporting portion 73.

A helical compression spring 74 acts between the end of casing 71 and the piston 72, to urge the piston 72 towards the closure member 70. An annular elastomeric diaphragm 75 is secured at its inner periphery to the piston 72, an inner peripheral bead 76 being retained in a recess 77 in the piston 72 by spring washer 78. An outer peripheral bead 79 on the diaphragm 75 is trapped between the closure member 70 and casing 71 in a peripheral groove 80 in the closure member 70. The diaphragm 75 and piston 72 thereby divide the casing 71 into two fluid tight chambers 81 and 82.An inlet (not shown) is provided to the chamber 81 by means of which it may be connected to a source of low pressure, for example vacuum, and a solenoid valve (not shown) is provided for selectively connecting chamber 82 via port 83, to the source of low pressure or to a source of high pressure, for example atmosphere.

An annular retaining member 84 is located in the end of the tubular supporting portion 73 of piston 72. Said retaining member 84 has a circumferential groove 85 on its internal diameter in which is located an elastomeric O-ring 86. The plunger 40 extends beyond the end of cylindrical portion 12 and when both the piston 72 and plunger 40 are hard over to the right as illustrated, the end of the plunger 40 extends through the O-ring 86 to provide a seal and frictional engagement between the plunger 40 and piston 72. An aperture 87 is provided in the end of the retaining member 84.

The brake pressure modulator is connected in a brake system with inlet ports 17 and 50 connected to the outlet ports of a dual master cylinder and outlet ports 22 and 51 connected to separate circuits each including one or more brake actuators.

Under normal operation, the solenoid valve will connect chamber 82 to the low pressure source (vacuum) so that there will be no pressure differential across diaphragm 75 and; piston 72 and plungers 25 and 40 will be forced hard over to the right, as illustrated, by the spring 74. In this position, the plungers 25 and 40 will unseat balls 20 and 47 so that the master cylinder is connected to the brake actuator(s).

Upon antilock operation, the solenoid valve connects chamber 82 to the high pressure source (atmosphere). A pressure differential is thus established across the diaphragm 75 which opposes a force applied to the piston 72 by spring 74. Under the influence of this pressure differential together with the pressure of brake fluid and springs 21, 48 and 61 acting on the plungers 25 and 40, piston 72 will move to the left (as illustrated) and plungers 25 and 40 will move with the piston 72. Movement of the plungers 25 and 40 will allow balls 20 and 47 to close control ports 19 and 46, thus interrupting communication between the master cylinder and brake actuator(s). Continued movement of the plungers 25 and 40 will then permit brake fluid to flow from the brake actuator(s) back into the cylindrical bore 15, thus reducing the braking effort.The rate at which braking effort is reduced will depend upon the rate of movement of the plungers 25 and 40 which may be controlled, in known manner, by controlling the rate at which high pressure fluid is introduced into chamber 82. Frictional engagement between the plunger 40 and O-ring 86, will pull the plunger 40 with the piston 72 and balance the frictional drag imposed on the plunger 40 by seal 42. Movement of the plunger 40 will thus be accurately controlled by movement of the piston 72.

If movement of the plunger is sufficient to reduce the fluid pressure in the brake actuator(s) to zero, for example when an antilock operation is triggered under light braking, the suction force applied to the piston 40 will be sufficient to overcome the frictional engagement between plunger 40 and piston 75. The piston 75 will consequently move relative to the plunger 40, the plunger 40 sliding through O-ring 86. When the plunger 40 moves out of engagement with the O-ring 86, chamber 81 will be interconnected with chamber 82 via aperture 87, the bore of retaining member 84 and the axial grooves 39.

Interconnection of chambers 81 and 82 will permit the pressure differential across diaphragm 75 to reduce, so that displacement of the piston 72 away from the plunger will be maintained at a minimum and the piston 72 will be ready to move plungers 40 and 25 to the right, to reapply braking effort, as soon as the antilock operation is terminated.

Various modifications may be made without departing from the invention. For example, while a vacuum operated brake pressure modulator has been described above, the low pressure source could for example be atmosphere and the high pressure source compressed air. The bypass through O-ring 86 and retaining .ring 84 is also optional and if this feature is not adopted, any suitable means may be used to apply frictional engagement between the plunger 40 and piston 72. For example, the O-ring 86 may be replaced by a metal circlip. The plungers 25 and 40 may also be rigidly or resiliently interconnected so that the positive control of movement of plunger 40 with piston 72 may be transmitted to plunger 25. Alternatively, the brake pressure modulator of the present invention may comprise a single plunger controlled ball valve controlling flow of fluid between one inlet and one outlet.

Claims (7)

1. A brake pressure modulator comprising a cylinder, a plunger slidingly sealed within the cylinder, an inlet and outlet being provided adjacent one end of the cylinder and a valve assembly being provided to control flow of fluid from the inlet to the outlet via the cylinder, the adjacent end of the plunger being adapted to control the valve assembly, a piston being provided adjacent the other end of the cylinder, said piston being mounted within a casing for movement coaxially of the plunger, the piston being sealed with respect to the casing to divide it into two fluid tight chambers, means being provided to control the pressure of fluid in the two chambers, so that a pressure differential may be established across the piston to move the piston, resilient means mounted on the piston frictionally engaging the plunger, so that upon movement of the piston a frictional force will be applied to the plunger inducing it to move with the piston.

2. A brake pressure modulator according to Claim 1 in which the resilient means mounted on the plunger comprises an annular element which is an interference fit about the plunger, so as to impose a frictional load thereon.

3. A brake pressure modulator according to Claim 2 in which the annular element is retained in a circumferential groove in the internal diameter of a tubular portion of the piston, said tubular portion being coaxial of the plunger.

4. A brake pressure modulator according to Claim 3 in which the cylinder extends into the casing and the tubular portion of the piston is slidingly located on the external surface of the cylinder.

5. A brake pressure modulator according to any one of Claims 1 to 4 in which the resilient means provides a seal between the plunger and piston, said seal controlling a bypass between the fluid tight chambers, so that relative movement between the piston and plunger when a load is applied to the plunger in excess of the frictional force between the piston and plunger will cause the plunger to move out of engagement with the resilient means, thereby permitting interconnection of the chambers.

6. A brake pressure modulator according to Claim 5 when taken with Claim 4, in which the bypass is provided by one or more axial grooves in the surface of the cylinder engaged by the tubular portion of the piston.

7. A brake pressure modulator substantially as described herein with reference to and as shown in the accompanying drawings.