GB2066397A - Hydraulic two-circuit brake systems - Google Patents

Abstract

An hydraulic two-circuit external force brake is proposed in which a brake valve (7) has two separate plunger-actuated valves (8, 9), one for each brake circuit and the two plungers (17, 18) are interconnected by way of a compensating lever or balance beam (14) and are in the form of outlet slide valves and have equal effective pressure surfaces each subjected in use to the pressure introduced by the respective separate valve. One plunger (17) has an associated piston (54) which is subjected to the pressure introduced by the respective plunger-actuated separate valve (8) and which acts upon the compensating lever (14) to upset the equilibrium whereby, upon application of the brakes, a higher pressure is introduced into the brake circuit (II) with which the non-assisted plunger (18) is associated than in the brake circuit (I) having the plunger (17) assisted by the piston (54). The brake circuit having the reduced pressure is the front axle brake circuit I, and the pressure lag in the front axle brake circuit is made up at full brake pressure. <IMAGE>

Description

SPECIFICATION Improvements in or relating to hydraulic two circuit external force brake systems The present invention relates to hydraulic two circuit external force brake systems.

One form of external force brake system is disclosed in German Offenlegungsschrift 2 335 424.

The establishment of satisfactory brake pressure radius is desirable in the lower part range of the brake pressure, particularly in commercial vehicles having drum brakes on the front and rear axles. In addition to proportioning the pedal travel against the brake pressure introduced, it is also advantageous to proportion the pedal force such that a large increase in the pedal force occurs in the lower range of brake pressure and a small increase in the pedal force occurs in the upper range of brake pressure. These desired ratios are achieved in compressed air brake systems by using so-called proportioning or pressure-ratio valves on the front axle.By virtue of these valves, the brake pressure at the front axle remains below the brake pressure at the rear axle in the lower range of pressure, picks up again from a predetermined pressure, and is finally equal to it again at full brake pressure. An essential feature is that improved distribution of the brake force between the front and rear axles is obtained by the pressure proportioning which is performed only at the front axle.

However, pressure proportioning of this kind by means of special proportioning or pressure ratio valves is complicated and expensive.

According to the present invention, there is provided an hydraulic two-circuit external force brake system in which a brake valve has two separate plunger actuated valves, one for each brake circuit, and the plungers of the two valve are operatively interconnected by way of a compensating lever, and the two plungers each have a pressure surface which is subjected in use to pressure introduced by the respective individual valve, and one of the two plungers has an associated piston which is subjected to the pressure introduced by the respective plunger actuated separate valve and which acts upon the compensating lever in the same manner as the plunger of that separate valve.

An hydraulic two-circuit external force brake system embodying the present invention can have the advantage that pressure proportioning is integrated in the brake valve.

The additional costs of a construction of this kind are so small that a considerable saving of cost is achieved compared with a construction having a separate adaptation valve.

The operational reliability of a brake system embodying the present invention can be increased, since, compared with a separate type of construction, fewer seals, fewer parts and fewer pipe screw connections are needed.

The invention will be further described by way of example with reference to the accompanying drawings illustrating one embodiment of the invention and in which Figure 1 shows diagrammatically a hydraulic two-circuit external force brake system of which a brake valve is shown in section, Figure 2 is a cross section through the brake valve of Fig. 1 taken on the line Il-li of Fig. 1, Figure 3 is a cross section taken on the line Ill-Ill of Fig. 1, and Figure 4 is a graph of pressure characteristics.

An hydraulic two-circuit external force brake system includes a pressure-supply arrangement 50 comprising a pump 1, two non-return valves 2 and 3, and two accumulators 4 and 5, one for each brake circuit I and II. An overflow valve 6 ensures a predetermined maximum level of the pressure available.

Two separate valves 8 and 9 disposed parallel to one another are arranged in a two-circuit brake valve 7 which is connected to the pressure supply arrangement 50 and which is actuable by way of a pedal 10 and interposed travel-extending spring unit 11. The travel-extending spring unit 11 has a thrust protrusion 12 against which a central projection 1 3 of a compensating lever 14 abuts.

Two ends of the compensating lever 14 are in contact with two valve plungers 17 and 1 8 which are parts of the separate valves 8 and 9.

The ends of the plungers 17 and 18 and the compensating lever 14 are disposed in a chamber 1 9 which has a relief port at 1 5. The two plungers 17 and 18 are provided with annular pressure surfaces 20 and 21 with which the plunger can move across radially disposed housing passages 22 and 23 in a slide-valve-like manner. These two passages 22 and 23 are connected to the chamber 19 by way of a common passage 24, so that respective outlet slide valves 20, 22 and 21,23 are formed by the pressure surfaces 20 and 21.

The two pressure surfaces 20 and 21 define in part respective valve chambers 25 and 26 which are connected by way of respective passages 27 and 28 to respective brake lines 29 and 30 of the brake circuits I and II which lead to respective wheel brake cylinders 31 and 32 of the front and rear axles respectively.

Respective valve springs 33 and 34 and respective valve plunger extensions 35 and 36 are disposed in the respective valve chambers 25 and 26. The respective plunger extensions 35 and 36 are intended for cooperation with respective spherical valve closure members 37 and 38 of respective inlet seat valves 39 and 40 which are urged against respective valve seats 43 and 44 by respective valve springs 41 and 42. The respective valve springs 41 and 42 are disposed in respective inlet chambers 45 and 46 which are connected to respective pressure lines 47 and 48 which lead from the pressure supply arrangement 50.

As will be seen from the sectional illustration of Figure 2, the compensating lever 14 is in the form of a plate and has a point of contact 51 for the plunger 18 at one side of the projection 13, although it has two points of contact 52 and 53 at the other side of the projection. As is shown in the sectional illustration of Figure 3, the plunger 1 7 acts upon one point of contact 52, and a piston 54 of two-part construction acts upon the other point of contact 53. One part 55 of the piston 54 is subjected to the brake pressure prevailing in the chamber 25 and a thrust member 56 transmits a corresponding force to the compensating lever 14.

A fettered spring 57 is disposed between the two parts 55 and 56 and permits a predetermined amount of compression of the piston 54. A stop 58 is disposed in the path of the piston part 55 and limits the stroke of the piston part 55 towards the compensating lever 14.

The system described operates as follows:- When the brakes are reieased i.e., the pedal 10 Is released, the pressure supply arrangement 50 is shut off and the wheel brake cylinders 31 and 32 are connected to the relief port 1 5 by way of the open outlet slide valves 8 and 9.

When the brakes are applied, the pedal 10 is depressed and exerts a force upon the compensating lever 14 by way of the travelextending spring unit 11. The compensating lever acts as a balance beam having two lever arms of equal length and urges the plungers 17 and 18 to the left as seen in Figure 1, so that the two outlet slide valves 8 and 9 are closed. The extensions 35 and 36 of the plungers then open the inlet seat valves 39 and 40 and a brake pressure is built up in the brake lines 29 and 30 from the pressure supply arrangement 50. The two inlet seat valves 39 and 40 close again when the pressure has risen in conformity with the pedal actuation force introduced. The so-called shut-off or central position is then reached in which both the inlet and the outlet valves are closed.

The pressure characteristic will now be further described with reference to the graphs of Figure 4 in which the rear and front axle brake pressures p, and p2 respectively (in bar) are plotted along the ordinate, and the actuation travel S (in mm) of the pedal 10 and the input force FE (in daN) are plotted along the abscissa on the left and right respectively. The symbols P, K applied to the broken line serve to indicate that normally with a pedal travel of approximately 4 mm (to the left) would set up a rear axle brake pressure p1 of 50 bar with an input force of approximately 80 daN (to the right). The straight line curve in the left and right hand halves denote conditions when p1 = P2.

By virtue of the additional use of the piston 54, the pressures p and p2 introduced are in reverse ratio to the associated area of the plunger pressure surface (21), diameter d1, on the one hand, and the area of the plunger pressure surface (20), diameter d2, together with the area of the pressure, surface diameter d3, of the piston 54 on the other hand. The fettered spring 57 in the piston 54 is thereby compressed to an extent corresponding to the front axle brake pressure p2 introduced into the brake line 29.

When, as a result of further depression of the pedal 10, the front axie brake pressure p2 increases to an extent that the piston part 55 of the piston 54 abuts against the stop 58, the characteristics p2 in the graph illustrated for two different area ratios (i) 1:1.25 and 1:1.6 reach discontinuity points. Upon a further rise in pressure, only the force on the plunger 1 7 pressure surface, diameter d2, and the force of the spring 57 reached at the discontinuity point acts upon the lower part of the balance-beam-like compensating lever 14. The characteristics thereby change their slope and, with corresponding design, again meet the characteristic p1 (rear axle) at maxirnum operating pressure at point Z or Z,.

It will be seen that, in this manner, there is a large increase in the pedal force in the low range of pressure and a smaller increase in the pedal force in the upper range of pressure. Also, the actuation travel S covered by the pedal is large in the first instance and then small. Furthermore, the brake pressure p2 in the front axle brake cylinders is lower than the brake pressure p1 in the rear axle brake cylinders in the lower range of pressure, increases again from a predetermined pressure, and is finally equal to the brake pressure n the rear axle brake cylinders again at full brake pressure. An improvement in the distribution of the brake force between the front and the rear axles is also ensured by pressure compensation cf this kind which is performed only in the front axle brake circuit I.

Claims (5)

1. An hydraulic two-circuit external force brake system, in which a brake valve has two separate plunger-actuated valves, one for each brake circuit and the plungers of the two valves are operatively interconnected by way of a compensating lever, and the two plungers each have a pressure surface which is subjected in use to pressure introduced by the respective separate valve, and one of the two plungers has an associated piston which is subjected to the pressure introduced by the respective plunger-actuated separate valve and which acts upon the compensating lever in the same manner as the plunger of that separate valve.

2. An hydraulic two-circuit external force brake system as claimed in claim 1, in which the piston is of two part construction with a fettered spring interposed between the two parts, and one part of the piston is subjected to said introduced pressure whilst the other part of the piston acts as a thrust member and abuts against the compensating lever.

3. An hydraulic two-circuit external force brake system as claimed in claim 1 or 2 in which the travel of the one part of the piston towards the compensating lever is limited by a stop.

4. An hydraulic two-circuit external force brake system as claimed in any of claims 1 to 3, in which the compensating lever acts as a balance beam whereby two different brake pressures which in use are introduced by the separate valves can be counterbalanced relative to one another.

5. An hydraulic two-circuit external force brake system having a brake valve constructed and arranged and adapted to operate substantially as hereinbefore particularly described with reference to and as illustrated in the accompanying drawings.