GB2209570A - Hydraulic anti-skid braking systems - Google Patents

Abstract

In an hydraulic anti-skid braking system of the pump and de-boost type, bleeding of a modulator (2) and pump assembly (3) in a common housing (1) is achieved by a bleed screw (27) disposed between a de-boost chamber (22) and a reservoir (11), and a plunger (30) of differential outline which works in a stepped bore (31) and controls communication between the master cylinder (7) and a circuit containing the pump (3). The plunger (30) has a pressure-responsive face (36) over which pressure from the master cylinder (7) acts following closure of the bleed screw (27) to return the plunger (30) automatically into a normal brake-applying position with the master cylinder (7) isolated from the pump circuit. <IMAGE>

Description

IMPROVEMENTS IN HYDRAULIC ANTI-SKID BRAKING SYSTEMS FOR VEHICLES This invention relates to hydraulic anti-skid braking systems for vehicles of the kind in which a supply of operating fluid from a supply to a vehicle brake is modulated by a modulator assembly in accordance with skid signals from skid sensing means,and an hydraulic pump incorporating at least one plunger working in a bore has a working chamber which is in communication with the modulator assembly to control brake re-application following skid correction,the modulator assembly comprising a bore in which works a de-boost piston for co-operation with a control valve assembly adapted to control communication between the supply of operating fluid and the brake through an expansion chamber defined in the bore between the piston and the control valve assembly, normally the piston being held in an inoperative advanced position by a trapped volume of fluid supplied by the pump and,in this position,the valve assembly is fully open and the effective volume of the expansion chamber is at a minimum, the trapped volume of fluid being released in response to a skid signal which permits the de-boost piston to move into a retracted position, initially to permit the valve assembly to close and isolate the supply from the brake, and subsequently to increase further the effective volume of the expansion chamber, whereby to relieve the brake-applying pressure, and following correction of the skid, the pressure from the pump being operative to urge the de-boost piston towards its advanced position, initially to-apply the brake by pressurising the fluid in the expansion chamber, and subsequently to open the valve assembly to re-establish communication between the supply and the brake.

Known anti-skid braking systems of the kind set forth form the subject of numerous patents including GB-A-2 029 914 and GB-A-2 069 640.

It is a problem in anti-skid braking systems of the kind set forth to ensure that bleeding of the system can be carried out effectively.

Some known systems of the kind set forth include two manually operable bleed screws which are both opened so that bleeding of the system can take place by operation of the supply of operating fluid, suitable a pedal or lever-operated hydraulic master cylinder. The first bleed screw is located above the second bleed screw, between the upper end of the de-boost piston which is remote from the valve assembly and a reservoir from which the pump draws fluid, and the second bleed screw is disposed between the pump circuit and the master cylinder.

Should the first bleed screw accidently remain open with the second bleed screw closed following bleeding, there will be reaction at the pedal or lever but the brakes cannot be applied due to the absence of fluid for holding the de-boost piston in its advanced position. Similarly, should the second bleed screw accidently remain open with the first bleed screw closed, then fluid will be present to hold the de-boost piston in its operative position. However the brakes cannot be re-applied following a skid signal since the pump is unable to generate pressure for returning the de-boost piston to its advanced position, as the pump will draw fluid from the master cylinder and pump it to the reservoir, so causing the brake pedal to "bottom out.

According to our invention, an anti-skid hydraulic braking system of the kind set forth is provided with first and second bleed devices of which the first device is manually operable and is disposed between the upper end of the de-boost piston and a reservoir from which the pump draws fluid, and the second device comprises a plunger working in a bore to control communication between the supply of operating fluid and a circuit including the pump, the plunger having a pressure-responsive face, and being manually displaceable into an open bleed position to place the supply in communication with the pump circuit, the piunger being automatically movable into a closed position to isolate the supply from the pump circuit following closure of the first device and in response to a force comprising the supply pressure acting over the pressure-responsive face.

This ensures that bleeding is terminated automatically following manual closure of the first device and there is no possibility of the second device remaining in its open bleed position. In any case bleeding cannot be terminated until the first device is closed which, in consequence, ensures that the first device will not remain open if the braking system is to be effective.

Preferably the plunger is of differential outline and the bore in which it works is of complementary stepped outline provided with first and second axially spaced radial passages communicating with the supply and with the pump circuit respectively, the larger diameter portion of the plunger carrying a first seal slidably engageable at all times with the larger portion of the bore which is disposed on the side of the first passage remote from the second passage, and the smaller diameter portion of the plunger carrying at least one second seal movable with the plunger between a bleed position providing flow through the second passage when the plunger is in the open bleed position and a closed position in which the second passage when the plunger is in the open bleed position and a closed position in which the second seal engages sealingly with the smaller portion of the bore when the plunger is moved into the closed position by the supply acting over a shoulder on the plunger at the step in diameter which defines the pressure-responsive face.

When the pump itself includes a first one-way valve incorporated in the pump plunger and through which fluid is drawn into a pump chamber from the reservoir, only a single second seal is carried by the smaller diameter portion of the plunger of the second bleed device. However, when the pump plunger is imperforate, and the first one-way valve of the pump is remote from the pump plunger itself and is housed in the second passage, the smaller diameter portion of the plunger carries two axially spaced second seals which are alternately engageable sealingly with lands defined by parts of the smaller diameter portion of the bore disposed on opposite sides of the second passage wher.

the plunger is in its open bleed position and its closed position respectively.

Two embodiments of our invention are illustrated in the accompanying drawings in which: Figure 1 is a transverse section through combined modulator and pump assembly for an hydraulic anti-skid braking system suitable for a motor-ccle or light passenger car or van; Figure 2 is a view similar to Figure 1 but showing a bleed device in an open bleed position; Figure 3 is a transverse section similar to Figure 1 but showing a modified combinated modulator and pump assembly; and Figure 4 is a view similar to Figure 3 but showing a bleed device in an open bleed position.

The assembly illustrated in Figures 1 and 2 of the drawings comprises a housing 1 incorporating a modulator assembly 2, an hydraulic pump assembly 3, and pressure dump valve 4. A longitudinally extending shaft 5 projecting at opposite ends through the housing 1 is coupled at one end to a wheel to be braked and at the other end carries skid sensing means (not shown).

The dump valve 4 and the skid sensing means may be of any of the forms disclosed in GB-A-2 029 914, and the pump assembly 3 forms the subject of GB-A-2 069 640. These mechanisms need not to be described further herein except to mention that the pump 3 incorporates a plunger 9 which is urged in one direction by an eccentric 6 on the shaft 5 and in the opposite direction by pressure from a pedal or lever operated master cylinder 7 which acts over an operating piston 8. A one-way valve 10 is disposed between the plunger 9 and the piston 8 and through which fluid is drawn from a reservoir 11 and into a pumping space 12 between the plunger 9 and the piston 8 as the plunger 9 and the piston 8 are displaced relatively towards the shaft 5.

The modulator assembly 2 comprises a bore 13 extending from the dump valve 4 and in which works a de-boost piston 14. The piston 14 is normally urged into an inoperative position against a stop comprising a wall 15 at the closed end of a sleeve 16 substantially of cup-shaped outline by means of a spring 17.

A control valve assembly 18 housed in the sleeve 16 controls communication between the master cylinder 7 and a wheel brake 19 through an expansion chamber 20 defined in the bore 13 between the piston 14 and the control valve assembly 18 itself.

In the normal operative position shown in the drawing the pump valve 4 is closed so that the piston 18 is held in an inoperative advanced position in which the valve assembly 18 is open by fluid trapped in a de-boost chamber 22 by the pump 3.

When the brake is to be applied by operation of the master cylinder 7, hydraulic fluid is supplied to the brake 19 through radial ports 21 in the wall of the sleeve, and through the open valve assembly 18 to the expansion chamber 20. Thus there is a substantially unrestricted flow of fluid to the brake.

Fluid from the master cylinder 7 also acts on the piston 8 to bias it towards the eccentric 6 but the plunger 9 is held away from the eccentric 6 in a known manner by fluid trapped between the plunger 9 and the de-boost piston 14.

When a skid signal is received the dump valve 4 opens to release the volume of fluid trapped in the de-boost chamber 22 so that the piston 14 can retract against the force in the spring 17 initially to allow the valve assembly 18 to close. This cuts-off communication between the master cylinder 7 and the brake 19, and the retraction of the piston 14 continues to increase the effective volume of the expansion chamber 20, whereby to relieve the pressure applied to the brake 19.

Opening the dump valve 4 also unbalances the pump 3 causing it to pump fluid in a closed pump circuit into the bore 13 from the reservoir 11 to which it is returned, through the dump valve 4. Since communication between the 8 and the master cylinder 7 is unrestricted, the pump 3 can move freely.

At the termination of the skid signal the dump valve 4 closes to isolate the bore 13 from the reservoir 11 and the pump 3 is then operative to increase the pressure in the bore 13, with the result that the piston 14 is urged towards its inoperative, retracted, position. Initial movement of the piston 14 in this direction re-applies the brake 19 by pressurising the volume of fluid trapped in the expansion chamber 20, and subsequent movement opens the valve assembly 18.

The modulator assembly is provided with first and second bleed devices 25, 26. When installed in a vehicle, the bleed device 25 is positioned above the bleed device 26.

the bleed device 25 comprises a screw 27 which co-operates at its inner end with a seating 28 disposed between the de-boost chamber 22 and a passage 29 leading to the reservoir 11. In a closed position shown in Figure 1 the screw 27 engages with the seating 28 to isolate the de-boost chamber 22 from the reservoir 11.

The bleed device 26 comprises a plunger 30 of differential outline which works in a stepped bore 31.

The inner end of the bore 31 which is of smaller diameter communicates with the pumping space 12 through a radial passage 32 and the piston of the bore 31 which is of larger diameter communicates with the master cylinder 7 through a radial passage 33. The portion of the plunger 30 which is of smaller diameter carries a seal 34 adapted for sealing engagement with the smaller diameter portion of the bore, and the larger diameter portion of the plunger 30 carries a seal 35 in sealing engagement at all times with the portion of the bore which is of larger diameter. A pressure responsive face 36 is defined by a shoulder at step in diameter of the plunger 30.

To bleed the system, as shown in Figure 2, the screw 27 is unscrewed to place the de-boost chamber 22 in communication with the reservoir 11, and the plunger 30 is urged inwardly so that the seal 34 enters the inner end of the passage 32 to provide communication from the master cylinder 7 to the passage itself and the pump circuit.

During bleeding fluid through the valve assembly 18, past the plunger 30, through the pump 3 to the de-boost chamber 22, and finally through past the screw 27 to the reservoir 11.

When bleeding is complete, the screw 27 is screwed home to isolate the de-boost chamber 22 from the reservoir 11. The master cylinder 7 is then operated, and the pressure so generated acts over the pressure-repsonsive face 36 to apply a net force to the plunger 30 in a direction to return it automatically to the position shown in Figure 1 with the seal 34 sealing against the bore 31 to isolate the master cylinder 7 from the pump circuit, and movement of the plunger arrested by a circlip 37.

In the construction illustrated in Figures 3 and 4 of the drawings, the pump plunger 9 is imperforate and the one-way valve 10 is located in the passage 32. The smaller diameter portion of the plunger 30 is provided with two axially spaced seals 38 and 39 which are alternatively sealingly engage able with portions of the bore 31 on the opposite sides of the passage 32. In the normal brake-applying position shown in Figure 3 the seal 39 seals with the bore 31, and the seal 38 is spaced from the bore 31 so that fluid from the reservoir 11 can be drawn by the pump 3 into the pumping space 12 through a passage 40 in communication with the inner end of the bore 31.

When bleeding, as in the embodiment of Figures 1 and 2, the screw 27 is opened. The plunger 30 is urged inwardly which causes the seal 38 to seal against the bore 31 to isolate the reservoir 11 from the pumping space 12 which, in turn, is placed in communication with the master cylinder 7.

At the completion of bleeding, the screw 27 is closed, and the pressure from the master cylinder again acts over the face 36 to return the plunger 30 to its normal position with the seal 39 engaging with the bore 31 to isolate the master cylinder 7 from the pump circuit, and the pumping space 12 being placed in communication with the reservoir 11 through the one-way valve.

Should the seal 39 become damaged, then this will be immediately obvious due to lack of reaction or "feel" at the lever or pedal when the master cylinder 7 is operated. This is because fluid leaks back to the reservoir 11 and not to the pump circuit where it would otherwise be trapped.

In the constructions described above each of the seats 34, 35, 38 and 39 conveniently comprises an 'O' ring, and the passage 32 where it enters the bore 31 is provided with inclined annular shoulders to facilitate the movement of the seal 34, or each seal 38, 39, into and out of engagement with the bore 31.

Claims (12)

1. An anti-skid hydraulic braking system of the kind set forth provided with first and second bleed devices of which the first device is manually operable and is disposed between the upper end of the de-boost piston and a reservoir from which the pump draws fluid, and the second device comprises a plunger working in a bore to control communication between the supply of operating fluid and a circuit including the pump, the plunger having a pressure-responsive face, and being manually displaceable into an open bleed position to place the supply in communication with the pump circuit, the plunger being automatically movable into a closed position to isolate the supply from the pump circuit following closure of the first device and in response to a force comprising the supply pressure acting over the pressure-responsive face.

2. An anti-skid hydraulic braking system according to claim 1 wherein the plunger is of differential outline, and the bore is of a complementary stepped outline, with first and second axially spaced radial passages, the first passage communicating with the supply and the second passage communicating with the pump circuit.

3. An anti-skid hydraulic brake system according to claim 2 wherein a larger diameter portion of the plunger carries a first seal slidably engaged at all times with the larger portion of the bore which is disposed on the side of the first passage remote from the second passage, and the smaller diameter portion of the plunger carries at least one second seal, the second seal being moveable with the plunger between a bleed position providing flow through the secona passage and a closed position in which the second seal engages sealingly with the smaller portion of the bore.

4. An anti-skid hydraulic braking system according to claim 3 wherein a shoulder at the step in diameter of the plunger defines the pressure-responsive face.

5. An anti-skid hydraulic braking system according to any preceding claim wherein the pump plunger incorporates a first one-way valve through which fluid is drawn from the reservoir and into a pump chamber, and the smaller diameter portion of the plunger of the second bleed device carries only a single second seal.

6. An anti-skid hydraulic braking system according to any one of claims 1 to 4 wherein the pump plunger is imperforate, the smaller diameter portion of the plunger carries two axially spaced second seals, and a first one-way valve of the pump is located in the second passage, remote from the pump.

7. An anti-skid hydraulic braking system according to claim 6 wherein the two second seals are alternately sealingly engageable with lands defied by parts of the smaller diameter portion of the bore disposed on opposite sides of the second passage when the plunger is in its open bleed position and its closed position.

8. An anti-skid hydraulic braking system according to any preceding claim wherein bleeding of the system is initiated by operation of the first device to open communication between the pump circuit and the reservoir and the plunger is urged inwardly so that the second seal enters the end of the second passage to provide cormlunication between the master cylinder and the pump circuit.

9. An anti-skid hydraulic braking system according to claim 8 wherein when bleeding is complete the first device is closed and operation of the master cylinder generates a pressure which acts over the pressure-responsive face of the plunger to return it to its closed position, isolating the master cylinder from the pump circuit.

10. An anti-skid hydraulic circuit according to claim 9 wherein the return movement of the plunger is arrested by a circlip.

11. An anti-skid hydraulic braking system of the kind set forth substantially as described herein with reference to and as illustrated in Figures 1 and 2 of the accompanying drawings.

12. An anti-skid hydraulic braking system of tile kind set forth substantially as described herein itb reference to and as illustrated in Figures 3 and 4 of the accompanying drawings.