GB1602024A - Hydraulic brake boosters for vehicle brake system - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO HYDRAULIC BRAKE BOOSTERS FOR VEHICLE BRAKE SYSTEM (71) We, ROBERT BOSCH GMBH, a German company of Postfach 50, 7 Stuttgart 1, Federal Republic of Germany, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to hydraulic brake boosters for vehicle brake systems.

A conventional hydraulic brake booster for vehicle brake systems is adapted to be actuated by means of a brake pedal and is provided with a control valve which controls communication between a source of pressure and a brake circuit by way of a booster cylinder and communication between the brake circuit and a point of pressure relief by way of the booster cylinder.

One known brake booster operates without a reactive effect. Thus, the driver's attention is not drawn to a badly vented brake circuit.

A rise in the brake pressure is delayed when a fault of this type occurs. A phase shift of this type is very undesirable during braking, since the driver then depresses the brake pedal with a greater force and the booster valve consequently introduces a brake pressure which is higher than that required for retardation.

A limit pressure at which one or more wheels would be locked can thereby frequently be exceeded, at least on a wet roadway, and this can cause critical travelling states particularly at high speed. Although this disadvantage might be eliminated by corresponding dimensioning of the cross section in the booster valve, this would result in too great a rate of the pressure rise when commencing to apply the brakes with a satisfactorily vented brake.

According to the present invention there is provided a hydraulic brake booster for a vehicle brake system, which booster is adapted to be actuated by means of a brake pedal and is provided with a control valve which controls communication between a source of pressure and a brake circuit by way of a booster cylinder, and communication between the brake circuit and a point of pressure relief by way of the booster cylinder, in which a pressure line leading from the source of pressure to the brake booster incorporates a restrictor whose flow through cross section is automatically adjustable in dependence upon the brake pressure prevailing in a brake line of the brake circuit.

A hydraulic brake booster embodying the present invention can have the advantage that the rate of pressure rise in a range of high pressure is reduced. Consequently a rise in pressure beyond an intended level when there are air bubbles in the brake line and the brake pedal is depressed can be avoided. A limit pressure at which one or more wheels would be locked can than not so readily be exceeded, and critical travelling states then can no longer occur.

The invention will be further described with reference to the accompanying drawings, in which: Fig. 1 shows diagrammatically and partly in section an hydraulic brake booster having a restrictor and actuating piston according to one embodiment of the invention; Fig. 2 is a graph showing a rime lag in operation of the booster of Fig. 1.

An hydraulic brake booster 1 is arranged between a pedal 2 and an hydraulic twocircuit master cylinder 3 of tandem construction. The brake booster 1 has an actuating rod 4 which can act upon a plunger 6 of a double valve 7, 8 by way of a spring 5. One valve 7 of the double seat valve is an inlet valve. It is arranged coaxially of the actuating rod 4 and has a spherical valve closure member 9 which normally abuts against its valve seat 11 under the force of a spring 10 and which may be moved off its seat by means of a plunger extension 12 of an outlet valve closure member 13. The other valve 8 is the outlet valve of the brake booster 1 and is connected upstream of and coaxially with the inlet valve 7.

The seat of the inlet valve 7 is arranged in a widened portion of a passage 18, which passage is provided in a booster piston 30 and leads into a control pressure chamber 25 surrounding the plunger extension 12 of the out let valve closure member 13, and a radial passage 26 leads from the control pressure chamber 25 to an annular chamber 27 on the booster piston 30. The annular chamber 27 communicates with a brake circuit II by way of a brake line 20. The passage 18 leads to a supply pressure chamber 21. A plunger 28 mounted on the booster piston 30 passes through the pressure chamber 21 and through a partition 29 in part defining the supply pressure chamber 21. A strong spring 31 is located between the partition 29 and the booster piston 30 and surrounds the plunger 28 coaxially.The supply pressure chamber 21 is connected to the pressure side of a pump 24 and to a reservoir 24' by way of a pressure line 22 and a non-return valve 23.

A free end 32 of the plunger 28 extends into a primary pressure chamber 33 of the master cylinder 3, the pressure chamber 33 being in part defined by a piston 34 of a double piston 34, 35. The pressure chamber 33 communicates with the brake circuit II by way of a branch line 36.

The two pistons 34 and 35 are rigidly interconnected by means of an intermediate member 38 whose portion of smaller diameter is presented to the secondary piston 35. A return spring 39 is mounted on the piston 35 and the other end of the return spring 39 abuts against the end of the master cylinder 3. A chamber 40 disposed between the end of the master cylinder and the piston 35 is the secondary pressure chamber of the master cylinder 3. A line 41 is conected to the chamber 40 and leads to a brake circuit I. The chamber 40 communicates with a topping-up reservoir 50 by way of a port 49.

The brake circuit I is the front axle brake circuit, and the circuit II is the rear axle brake circuit. The suction side of the pump 24 is connected to a reservoir 43 which communicates with a connection 44 of the brake booster 1 by way of a line 43'. The connection 44 communicates with an annular cylindrical recess 45 in the booster piston 30, the recess 45 being permanently connected to a toppingup reservoir 46 and to a relief chamber 47 in the booster piston 30. The relief chamber 47 accommodates the spring 5 for the actuating rod 4.

The pressure line 22 leading to the brake booster 1 from the pump 24 and the reservoir 24', acting as a source of pressure medium, incorporates a throttle 51 which controls the passage through the pressure line 22 by means of a movable throttle member 52. The throttle member 52 is connected to an actuating piston 53 which is movable in a cylinder 54. A working chamber 55 of the cylinder 54 communicates with the brake line 20 by way of a branch line 56. A return spring 57 is located at the other side of the piston 53 and abuts at one end against the piston 53 and at the other end against the end of the cylinder 54.

A stop 58 is provided for limiting the stroke of the piston 53 in the throttling direction.

The system described operates in the following manner: When the brake is released, the movable parts of the brake booster assume the positions shown in the drawings.

When the pedal 2 is actuated for the purpose of braking, and after effecting the amount of travel determined by the spring 5, the outlet valve 8 is first closed by means of the plunger 6. Thus, communication between the brake circuit II and the topping-up reservoir 46 and the reservoir 43 is interrupted.

Upon a further movement of the pedal 2, the extension 12 of the outlet valve closure member 13 opens the inlet valve 7. Pump pressure can now be propagated by way of the control pressure chamber 25 into the line 20 leading to the brake circuit II. However, this pressure is also effective in the primary pressure chamber 33 by way of the line 36. The double piston 34, 35 is displaced to the left against the force of the spring 39. After passing over the port 49, the hydraulic fluid in the chamber 40 is introduced into the brake circuit I by way of the line 41. The pressure in the brake circuit I is thereby dependent upon the pressure in the brake circuit II.

Fig. 2 shows a pressure curve A which is obtained when the brake line 20 is satisfactorily vented i.e. bled or deaerated. It will be seen that the rate of the brake pressure is relatively great, and the brake pressure p, plotted against time t, rises very rapidly.

A second pressure curve B shows the pressure characteristic when the brake is badly vented, that is, when air bubbles or vapour bubbles (in the case of thermal overheating) are located in the system. The time lag of the rise in pressure will be seen. This phase shift would be particularly troublesome during commencement of braking. This fault in the higher range of pressure is negligible only when the occlusions of gas are compressed, that is, in excess of approximately 10 bar. It is only then that the starting gradient in the pressure rise is attained.

It is at this juncture that the invention takes effect by virtue of the fact that the throttle 51 receives, by way of the branch line 56, information concerning the brake pressure attained in the brake line 20, and correspondingly influences the inflow of pressure medium.

Curve A' shows the time characteristic which is substantially independent of the vented state of the brake circuit.

This throttling function can be effected continuously or, alternatively, in stages. However, in accordance with the present day state of knowledge, single-stage control is suSicient.

The throttling action preferably has an exponential characteristic.

The build-up of pressure remains unthrottled up to a control pressure of, for ex ample, 10 bar. At this pressure value, the enclosed volume of air is compressed to 1/10; so that fixed throttling can be effected in the following higher range of pressure. A substantially complete solution to the problem would be that of non-linear throttling which has certain advantages in the range of high pressure. In this range, the differential pressure between the reservoir and the control pressure is smaller, so that the rate of pressure rise becomes smaller. If the control valve is in the form of a slide valve, the corresponding cross section has to be taken into account by a specific curve shape.

WHAT WE CLAIM IS:- 1. A hydraulic brake booster for a vehicle brake system, which brake booster is adapted to be actuated by means of a brake pedal and is provided with a control valve which controls communication between a source of pressure and a brake circuit by way of a booster cylinder, and communication between the brake circuit and a point of pressure relief by way of the booster cylinder, in which a pressure line leading from the source of pressure to the brake booster incorporates a restrictor whose flow-through cross section is automatically adjustable in dependence upon the brake pressure prevailing in a brake line of the brake circuit.

2. A brake booster as claimed in claim 1, in which the restrictor has a movable throttle member which is coupled to an actuating piston which is subjected on one side to the brake pressure and on the other side to the force of a spring.

3. A brake booster as claimed in claim 2, in which a stop for the actuating piston is provided which can be reached by the piston after a specific amount of spring travel has been covered.

4. A brake booster as claimed in any of claims 1 to 3, in which the throttling action is carried out in accordance with an exponential characteristic.

5. A hydraulic brake booster for a vehicle brake system, constructed and arranged and adapted to operate substantially as hereinbefore particularly described with reference to and as illustrated in the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (5)

**WARNING** start of CLMS field may overlap end of DESC **. ample, 10 bar. At this pressure value, the enclosed volume of air is compressed to 1/10; so that fixed throttling can be effected in the following higher range of pressure. A substantially complete solution to the problem would be that of non-linear throttling which has certain advantages in the range of high pressure. In this range, the differential pressure between the reservoir and the control pressure is smaller, so that the rate of pressure rise becomes smaller. If the control valve is in the form of a slide valve, the corresponding cross section has to be taken into account by a specific curve shape. WHAT WE CLAIM IS:-

1. A hydraulic brake booster for a vehicle brake system, which brake booster is adapted to be actuated by means of a brake pedal and is provided with a control valve which controls communication between a source of pressure and a brake circuit by way of a booster cylinder, and communication between the brake circuit and a point of pressure relief by way of the booster cylinder, in which a pressure line leading from the source of pressure to the brake booster incorporates a restrictor whose flow-through cross section is automatically adjustable in dependence upon the brake pressure prevailing in a brake line of the brake circuit.

2. A brake booster as claimed in claim 1, in which the restrictor has a movable throttle member which is coupled to an actuating piston which is subjected on one side to the brake pressure and on the other side to the force of a spring.

3. A brake booster as claimed in claim 2, in which a stop for the actuating piston is provided which can be reached by the piston after a specific amount of spring travel has been covered.

4. A brake booster as claimed in any of claims 1 to 3, in which the throttling action is carried out in accordance with an exponential characteristic.

5. A hydraulic brake booster for a vehicle brake system, constructed and arranged and adapted to operate substantially as hereinbefore particularly described with reference to and as illustrated in the accompanying drawings.