GB2070167A

GB2070167A - Hydraulic Braking Force Booster

Info

Publication number: GB2070167A
Application number: GB8102707A
Authority: GB
Original assignee: ZF Friedrichshafen AG
Current assignee: ZF Friedrichshafen AG
Priority date: 1980-02-22
Filing date: 1981-01-29
Publication date: 1981-09-03
Also published as: ES8200605A1; FR2476573A1; IT1142319B; JPS56124543A; IT8147853A0; BR8100995A; ES498639A0; DE3006610A1

Abstract

To prevent negative pressure in the pressure chamber 9 of a booster on failure in the booster pressure supply a non-return valve 28 is connected, directly or indirectly, between the sides of the servo piston 8. The servo piston 8 divides a servo- cylinder 4 into a pressure medium supply chamber 9 and a spring chamber 10 in which a compression spring 11 is guided by a thrust rod 12 of the piston 8 which actuates a piston 13 of a master brake cylinder 14. Slidably connected to the servo piston 8 is a pedal actuated control piston 7 with control edges 23. A pin 18 of the servo piston 8 engages a compression spring 20 accommodated in a bore 19 of the piston 7. The chamber 9 of the servo- cylinder 4 is supplied with pressure oil by means of line 21, duct 22 and control edges 23 from a pump 1 either directly or via a hydraulic accumulator 3. The non-return valve 28 either is disposed in an equalising line 27 between the supply line 21 and a return line 26 extending to a reservoir 2 or alternatively is constituted by a through bore 27% in the servo piston 8 cooperating with a resilient plate 28% secured to the piston 8. <IMAGE>

Description

SPECIFICATION Hydraulic Braking Force Booster This invention relates to a hydraulic braking force booster, of the kind comprising a control piston which is connected to an actuating rod and controls via control edges the supply of a hydraulic liquid to a servocylinder chamber, a servo piston in said chamber which servo piston acts via a connecting member on a master brake cylinder, a supply duct for supplying hydraulic liquid to said chamber, and a return duct extending from said chamber to a reservoir.

Braking force boosters of the foregoing kind are used to facilitate the actuation of motor vehicle brake systems. In addition to the small compact construction of the hydraulic booster, this apparatus has the advantage over the known negative pressure brake apparatuses of shorter response times and a substantially higher operating pressure. Even if the energy supply (pressure oil supply) fails, several braking operations can still be carried out with full boosting force.

In the conventional manner a hydraulic braking force booster installation comprises a hydraulic booster with flanged-on master brake cylinder, a pressure-controlled accumulator-charging valve with a hydraulic accumulator, and a pump with a hydraulic fluid reservoir.

When the brake pedal is operated, an actuating rod moves a control piston forward until at control edges the pressure oil flows metered through a then opened control gap to the rear side of a servo piston. The pressure oil force is transmitted to the master cylinder via a thrust rod connected to the servo piston. The pressure simultaneously exerted on the actuating rod gives the driver a feeling of the strength of the braking. The boosting factor of the apparatus is the result of the ratio between the servo piston area and the actuating piston area.

However, one disadvantage of such an apparatus is that under some circumstances a negative pressure may build up in the servocylinder chamber and produce a counterforce, because if sufficient pressure oil from the pump or an accumulator is not available when the brake is applied, during the inoperative travel of the brake pedal a negative pressure builds up in the servo-cylinder chamber. As a result the driver not only lacks the hydraulic boosting, but in addition he must overcome the negative pressure. There is a risk of a negative pressure more particularly when starting the vehicle while applying the brake at the same time, and also if the pump fails.

For this reason hitherto pumps with a high delivery rate have been used in order to ensure a rapid build-up of pressure, more particularly when starting the vehicle. However, one disadvantage is that the pumps are very expensive and relatively heavy. Furthermore, they require a high power and their pressure control system is very expensive.

It is therefore an object of the invention to provide a hydraulic braking force booster of the foregoing kind which obviates the occurrence of the negative pressure in the servocylinder chamber.

Accordingly, the present invention consists in a hydraulic braking force booster, comprising a control piston which is connected to an actuating rod and controls via control edges the supply of a hydraulic liquid to a servocylinder chamber, a servo piston in said chamber which servo piston acts via a connecting member on a master brake cylinder, a supply duct for supplying the hydraulic liquid to said chamber, and a return duct extending from the servocylinder to a reservoir, characterised in that the servocylinder chamber is directly or indirectly connected to the reservoir via an equalising duct, in which a non-return valve opening in the direction of the servocylinder chamber is disposed.

In the arrangement of the invention, if the pump fails to deliver enough pressure oil, during the idle travel of the brake pedal, pressure oil is sucked in from the reservoir via the non-return valve. For this purpose the non-return valve may be connected to the return duct at any place.

Since a negative pressure is prevented from building up, at the end of the pedal idle travel, even a flow of small volume from the pump is enough to build up in the filled servocylinder chamber a servo force for the master brake cylinder. The non-return valve is then closed.

According to one embodiment of the invention a spring chamber is disposed downstream of the servo piston and has extending through it a thrust rod as connecting member between the servo piston and the piston of the master brake cylinder, the spring chamber being connected via an oil line to the reservoir, the equalising duct being connected to said oil line.

Since normally the spring chamber is connected either via an oil line or by an oil return line after a braking operation to the reservoir, the equalising line with the non-return valve may be connected to the reservoir in a very simple manner by a line extending into the oil line.

In another constructionally very simple embodiment of the invention the equalising line is formed by a through bore with non-return yalve in the servo piston. A resilient plate is conveniently disposed as the non-return valve upstream of the through bore in the servo-cylinder chamber.

Another possible way of connecting the servocylinder chamber to the reservoir is that the equalising duct is connected to the supply duct.

In order that the invention may be more readily understood, reference is made to the accompanying drawings which illustrate diagrammatically and by way of example an embodiment thereof, and in which: Fig. 1 is a sectional view of a hydraulic braking force booster, and Fig. 2 shows a servo piston with a non-return valve.

The hydraulic braking force booster is of a basically known construction, so that it is merely illustrated and described in principle. A pump 1 sucks oil from a reservoir 2 and either charges a hydraulic accumulator 3 or pumps the oil directly into a braking force booster 4.

The braking force booster 4 is actuated by a brake pedal 5 via an actuating rod 6 which carries a control piston 7. Inside the braking force booster 4 a servo piston 8 separates a servo piston chamber 9 from a chamber 10 accommodating a return spring 11. Extending through the spring chamber 10 is a thrust rod 12 which is connected to the servo piston 8 and the piston 13 of a master brake cylinder 14. From the master brake cylinder 14 hydraulic lines 15 extend to brakes 1 6.

A control pin 1 8 is connected to the servo piston 8 and extends into a bore 1 9 in the control piston 7. A spring 20 is also disposed in the bore 19 between the rear end wall of the control pin 18 and the control piston 7.

The pressurised oil passes via a supply line 21 into a duct 22 in the control piston 7. The pressure in the servocylinder chamber 9 is controlled by means of a supply control edge 23 and a return control edge 24. After a braking operation, the servocylinder chamber 9 is emptied via a duct 25 inside the control pin 18, the spring chamber 10, an oil line 30 and a return line 26 into the reservoir 2.

Disposed between the supply line 21 and the return line 26; 30 is an equalising line 27 with a non-return valve 28 which opens in the direction of the supply line 21 and therefore of the servocylinder chamber 9. The non-return valve 28 prevents the occurrence of negative pressure in the servocylinder chamber 9.

In a normal braking operation, the control piston 7 is pushed downward, as viewed in the drawing, via the actuating rod 6, so that pressure oil passes via the duct 22 and the opening supply control edges 23 into the servocylinder chamber 9. As a result the pedal force is correspondingly boosted hydraulically. If for any reason the servocylinder chamber 9 receives no pressure oil from the pump 1 or the hydraulic accumulator 3, during the inoperative travel of the control piston 7 a negative pressure would be set up in the servocylinder chamber 9. However, in that case the non-return valve 28 in the equalising line 27 opens, and the right quantity of pressure oil flows out of the return line 26 via the supply line 21 to prevent a negative pressure from building up in the servocylinder chamber 9.

By virtue of the non-return valve 28, a smaller pump can be substituted for a large pump intended to prevent the occurrence of a negative pressure. Instead of an accumulator-charging valve, therefore, pressure control can also be effected in a very simple manner via an excess pressure valve 29.

Instead of the equalising line 27 being connected to the supply line 21, the equalising line 27 can be connected directly to the servocylinder chamber 9. In that case the line 27 (Fig. 1) issues above the non-return valve 28, not into the supply line 21, but laterally directly into the servocylinder chamber 9.

The spring chamber 10 is also connected via the oil line 30 to the return line 26, and the oil line 30 represents the return line, since the oil is discharged from the servocylinder chamber 9 via the duct 25 and the spring chamber 10.

Fig. 2 shows a further embodiment of the invention. The equalising line with the non-return valve takes the form of a through bore 27' which extends transversely through the servo piston 8 and in this way connects the servocylinder chamber 9 to the spring chamber 10. The nonreturn valve used is a resilient plate 28' which is disposed upstream of the bore 27' in the servocylinder chamber 9 and is retained by a circlip 31.

If there is a risk of a negative power building up in the servocylinder chamber 9, the resilient plate 28' lifts off from the servo piston 8 and thereby opens up the connection between the spring chamber 10 in which there is hydraulic oil, and the servocylinder chamber 9.

Claims

1. A hydraulic braking force booster, comprising a control piston which is connected to an actuating rod and controls via control edges the supply of a hydraulic liquid to a servocylinder chamber, a servo piston in said chamber which servo piston acts via a connecting member on a master brake cylinder, a supply duct for supplying the hydraulic liquid to said chamber, and a return duct extending from the servocylinder to a reservoir, characterised in that the servocylinder chamber is directly or indirectly connected to the reservoir via an equalising duct, in which a nonreturn valve opening in the direction of the servocylinder chamber is disposed.

2. A hydraulic braking force booster as claimed in claim 1, wherein a spring chamber is disposed downstream of the servo piston and has extending through it a thrust rod as connecting member between the servo piston and the piston of the master brake cylinder, the spring chamber being connected via an oil line to the reservoir, the equalising duct being connected to said oil line.

3. A hydraulic braking force booster as claimed in claim 2, wherein the equalising duct is formed by a through bore with non-return valve in the servo piston.

4. A hydraulic braking force booster as claimed in claim 3, wherein a resilient plate is disposed as the non-return valve, upsteam of the through bore, in the servocylinder chamber.

5. A hydraulic braking force booster as claimed in claim 1, wherein the equalising duct is connected to the supply duct for the hydraulic liquid.

6. A hydraulic braking force booster, substantially as herein described with reference to and as shown in the accompanying drawings.