GB2054076A

GB2054076A - Pressure reducing device for vehicular hydraulic brake systems

Info

Publication number: GB2054076A
Application number: GB8019213A
Authority: GB
Original assignee: Alfred Teves GmbH
Current assignee: Continental Teves AG and Co OHG
Priority date: 1979-06-28
Filing date: 1980-06-12
Publication date: 1981-02-11
Also published as: IT8023029A0; FR2460237B1; GB2054076B; FR2460237A1; DE2926062C2; IT1131376B; DE2926062A1

Abstract

A pressure reducing device for vehicular hydraulic brake systems. A casing (1, 2) is provided with an inlet chamber (8) and an outlet chamber (10) separated by a movable piston (11) on a piston rod (13). The piston rod carries an annular gasket (12) which is prestressed to form a tight seal against the piston (11) and/or the piston rod (13). The piston is spring loaded (15) towards the outlet chamber (10) which is formed with axial by-pass grooves (21) extending only part-way towards the inlet chamber (8). The gasket (12) has an outwardly inclined flexible lip (22) which contacts the periphery of the outlet chamber. When fluid pressure is applied via the inlet (7) fluid passes through the grooves (21) to apply pressure at the outlet (9). When the pressure on the surface of the piston in the outlet chamber exceeds the pressure on the reduced surface of the piston in the inlet chamber by an amount greater than the spring pressure, the piston moves towards the inlet chamber until the flexible gasket lip (22) passes the end of the grooves (21) and seals the outlet chamber from the inlet chamber. When the inlet pressure is reduced fluid can flow back to the inlet chamber past the flexible lip (22). <IMAGE>

Description

SPECIFICATION Pressure reducing device for vehicular hydraulic brake systems This invention relates to a pressure reducing device for vehicular hydraulic brake systems, which device comprises a casing, which is provided with a cylinder chamber, and a piston which is unilaterally provided with a piston rod, the piston dividing the cylinder chamber into an outlet chamber and an annular smaller crosssection inlet chamber around the piston rod, the piston being loaded by a spring in the direction of the outlet chamber, said device further comprising at least one by-pass channel, which extends from the outlet chamber over a portion of the length of the cylinder chamber, and an annular seal at the piston, which seal will seal off the inlet and outlet chambers in respect of each other and form part of a check valve between the two chambers if the end of the by-pass channel is overridden.

In a known device of this type (U.S. Patent Specification No. 3,088,285) the annular seal has two parallel front faces, a cylindrical inside circumferential surface, and a slightly arched outside circumferential surface. The outside circumferential surface serves to override the end of the by-pass channel designed as bore, annular gap, or the like. The inside circumferential surface is slightly larger than the portion of the piston which takes up the annular seal. The annular seal is held between two flanges of the piston, the distance between them slightly exceeding the height of the annular seal. The flange adjacent to the inlet chamber has a bore, bevel, or the like leading to the inside diameter. During normal operation, with the inlet pressure predominating, the annual seal will be pressed sealingly against the flange adjacent to the outlet chamber.Thus, any communication between the inlet and outlet chambers is only possible via the by-pass channel.

If, however, the outlet pressure is predominating, the annular seal will be displaced axially, thus forming a return channel which extends along the one front face and the inside circumferential surface of the annular seal. Very close manufacturing tolerances have to be observed in this design as only little inaccuracies will be sufficient to keep the return channel permanently closed or to have the outside circumferential surface abut the circumference of the cylinder chamber with excessive force. Moreover, its life will be limited due to wear. The annular seal does not only perform a relative movement in respect of the casing, but also a relative movement in respect of the piston. The thus ensuing wear will in course of time lead to changes in the dimensions which will derogate from a reliable sealing action.

In another known pressure reducing device (German Printed Patent Application = DE-AS No.

1,530,148) the annular seal is held between a flange of the piston and a shoulder of the casing.

Its inside circumferential surface permanently keeps a distance in respect of the associated portion of the piston. The outside circumferential surface and the two front faces have naps in order to always maintain a defined distance in respect of the adjacent wall. At the transition from one front face to the inside circumferential surface the annular seal forms an annular valve seat cooperating with a circumferential rim at the piston and permitting passage of the hydraulic fluid from the inlet chamber into the outlet chamber. At the diagonally opposite corner of the cross-section of the annular seal there is a sealing lip which is abutting the circumferential wall of the cylinder chamber with prestress, lifting off this circumferential wall if the pressure in the outlet chamber exceeds the pressure in the inlet chamber.The shape of this annular seal is very complicated, involving the need for very close tolerances.

For many decades gaskets have been used in vehicular hydraulic brake systems, e.g. as primary seal in master cylinders of such brake systems.

According to this invention there is provided a pressure reducing device for vehicular hydraulic brake systems, which device comprises a casing, which is provided with a cylinder chamber, and a piston which is unilaterally provided with a piston rod, the piston dividing the cylinder chamber into an outlet chamber and an annular smaller crosssection inlet chamber around the piston rod, and the piston being loaded by a spring in the direction of the outlet chamber, said device further comprising at least one by-pass channel, which extends from the outlet chamber over a portion of the length of the cylinder chamber, and an annular seal at the piston, which seal will seal off the inlet and outlet chambers in respect of each other and form part of a check valve between the two chambers if the end of the by-pass channel is overridden, the annular seal being a gasket with an outer sealing lip directed towards the inlet chamber and an inner collar permanently resting in a sealing manner at the piston and/or piston rod.

This design combines both control or valve functions in one common spot, i.e. at the sealing lip. As both end faces and the inside circumferential surface do not participate in the control and valve functions they will be completely uncritical in respect of manufacturing tolerances.

It will be sufficient for one of these surfaces to rest sealingly at the piston and/or piston rod. But even in respect of manufacturing accuracy of the sealing lip no high requirements will have to be made, the pressure which prevails in the inlet chamber pressing the sealing lip outwards. Even the wear of the annular seal will be less since now but a relative movement between the gasket and the casing will have to be considered. However, wear of the sealing lip may be tolerated up to a considerable extent as the pressure prevailing in the inlet chamber will effect an automatic compensation of the easily deformable sealing lip.

Consequently, its life will also be very long.

It will be of particular advantage to hold the inner collar with radial prestress on the piston rod.

Instead, or simultaneously, it may be possible for the inner collar to be held with axial prestress between the piston and a supporting flange. Both will result in the desired sealing action and will very easily be achieved by selecting the inside diameter of the collar slightly smaller than the associated portion of the piston or rather by selecting the distance between the two end faces of the sealing collar to be slightly smaller than the distance between piston and supporting flange.

Any differences in tolerances in this design at most will lead to variations in the prestress.

It will further be advantageous for the outer sealing lip to extend from gasket portion in a slightly outwardly inclined manner, said gasket portion having a smaller diameter than the cylinder chamber. Thus it will be safeguarded that upon an excessive pressure in the outlet chamber the sealing lip will lift off the cylinder chamber wall over its entire circumference, thus bringing about a quick pressure compensation.

In a further embodiment, at its end, the by-pass channel may pass via an inclined surface over into the circumferential surface of the cylinder chamber. This will ensure that the front rim of the sealing lip will not be impaired by a sharp-edged passage at the end of the by-pass channel.

In a simple embodiment, the by-pass channel is formed by at least one groove. In this arrangement, the outer circumferential rim of the sealing lip may rest at the circumferential surface of a cylinder chamber.

Embodiments of the present invention will now be described by way of examples illustrated in the drawing, wherein.

Fig. 1 shows a longitudinal section taken through a first embodiment, the piston being in its rest position; Fig. 2 is a longitudinal section taken through a second embodiment, the piston being in its operating position; and Fig. 3 is a diagram showing the dependence of the outlet pressure PA on the inlet pressure PE.

In the embodiment according to Fig. 1, a casing 1 is screwed together with a cover 2 by means of a thread 3, a flat annular seal 4 being inserted between casing 1 and cover 2. At the same time, this serves to keep a cylindrical insert 5 in the casing 1, said insert 5 having a seal 6 on the outside.

In the casing, an inlet connection 7 is provided which is communicating with an annular inlet chamber 8. In the cover, there is an outlet connection 9 communicating with an outlet chamber 10.

The inlet chamber 8 and the outlet chamber 10 are separated from each other by means of a piston 11 with a gasket 1 2. The piston is connected with a piston rod 13 having a supporting flange 14 acted upon by a spring 1 5 and passed outwards through a gasket 1 6.

The gasket 12 has an inner collar 1 7 held with radial prestress on the piston rod 13. Further, the two end faces of this collar portion may be held with axial prestress between the piston 11 and the supporting flange 14. The outer circumferential rim 1 9 of an outer sealing lip 1 8 of the gasket 12 rests at the inner circumferential surface of the insert 5, said inner circumferential surface defining a cylinder chamber 20. In this circumferential surface, several axial grooves 21 are provided which form by-pass channels. The ends 22 of said by-pass channels are formed by inclined surfaces which may be overridden by the sealing lip 1 8.

The sealing lip projects from a gasket portion 23 outwards at an inclined angle, the diameter of said gasket portion 23 being smaller than the inside diameter of the insert 5 as a result of which an annular chamber 24 is formed.

In the embodiment according to Fig. 2, which is a more schematical representation, the corresponding parts have reference numerals increased by 100. What is of essential difference is that the spring 1 15 is accommodated in a spring casing 125 seated outside on the casing 101, and that the spring 1 15 is acting on the piston rod 113 via a spring plate 126. A further difference is that an annular groove 121 serves as by-pass channel, the end 122 of said annular groove 121 being designed as conical inclined surface.

Both embodiments will lead to a characteristic curve of the outlet pressure PA prevailing at the outlet connection 9 in dependence on the inlet pressure PE prevailing at the inlet connection 7 as represented in the diagram of Fig. 3. The outlet pressure will follow the inlet pressure over a first section I. Subsequently, the outlet pressure will follow the inlet pressure with a reduced value, i.e.

with a pressure reduction, in accordance with section 11. This will result from the following mode of operation: Starting from the rest position of the piston 11 in Fig. 1, the outlet chamber 10 will be communicating with the inlet chamber 8 via the by-pass channel grooves 21. The outlet pressure will equal the inlet pressure. In the outlet chamber 10, this pressure will act on the entire surface F, of the piston 11, while in the inlet chamber 8 it will act on the piston surface reduced by the piston rod cross-section F2. As soon as the value PE F2 thus exceeds the prestress force K of the spring 15, the piston 11 will move to the righthand side in Fig. 1. If the sealing lip 1 8 overrides the end of the by-pass channel grooves 21, i.e. if the outlet chamber 10 is separated from the inlet chamber 8, there will result a new balance of forces in accordance with the formula: PA F1 = PE(Fl - F2) + K. This will lead to section II with the reduced outlet pressure PA.

If, because of an increase of the inlet pressure, the brake cylinders communicating with the outlet connection 9 need more pressure fluid, the sealing lip 1 8 will slightly open the end of the by-pass channel grooves 21, thus some pressure fluid can pass whereupon the pressure balance will immediately restore itself, the by-pass being closed.

If the inlet pressure PE goes down so that the outlet pressure surpasses the inlet pressure, the outlet pressure prevailing in the annular chamber 24 will press the sealing lip 18 inwards and a pressure compensation is effected immediately, the piston being able to return to its illustrated rest position.

The embodiment of Fig. 2 will operate in a similar manner. Variations are possible in many respects. It will, for example, be possible to use but one spring for the loading of the pistons of two pressure reducing devices if the pistons are arranged in a horizontally opposed manner, i.e. in one axis, yet pointing in opposite directions, or if the pistons are featuring a twin arrangement, i.e. if they are placed side by side in a parallel manner.

Claims

1. A pressure reducing device for vehicular hydraulic brake systems, which device comprises a casing, which is provided with a cylinder chamber, and a piston which is unilaterally provided with a piston rod, the piston dividing the cylinder chamber into an outlet chamber and an annular smaller cross-section inlet chamber around the piston rod, and the piston being loaded by a spring in the direction of the outlet chamber, said device further comprising at least one by-pass channel, which extends from the outlet chamber over a portion of the length of the cylinder chamber, and an annular seal at the piston, which seal will seal off the inlet and outlet chambers in respect of each other and form part of a check valve between the two chambers if the end of the by-pass channel is overridden, the annular seal being a gasket with an outer sealing lip directed towards the inlet chamber and an inner collar permanently resting in a sealing manner at the piston and/or piston rod.

2. A pressure reducing device as claimed in claim 1, wherein the inner collar is held with radial prestress on the piston rod.

3. A pressure reducing device as claimed in claim 1 or claim 2, wherein the inner collar is held with axial prestress between the piston and a supporting flange on the piston rod.

4. A pressure reducing device as claimed in any of claims 1-3, wherein the outer sealing lip extends from a gasket portion in an outwardly inclined manner, said gasket portion having a smaller diameter than the cylinder chamber.

5. A pressure reducing device as claimed in any of claims 1-4, in which, at its end, the by-pass channel passes via an inclined surface over into the circumferential surface of the cylinder chamber.

6. A pressure reducing device as claimed in any of claims 1-5, wherein the by-pass channel is formed by at least one groove.

7. A pressure reducing device as claimed in claim 6, wherein the outer circumferential rim of the sealing lip is resting at the circumferential surface of a cylinder chamber.

8. A pressure reducing device substantially as described with reference to Fig. 1 or Fig. 2 of the accompanying drawing.

9. A vehicular brake system incorporating a pressure reducing device as claimed in any preceding claim.