GB2098294A

GB2098294A - Brake master cylinder

Info

Publication number: GB2098294A
Application number: GB8211178A
Authority: GB
Original assignee: Alfred Teves GmbH
Current assignee: Continental Teves AG and Co oHG
Priority date: 1981-05-04
Filing date: 1982-04-16
Publication date: 1982-11-17
Also published as: JPS57186568A; ZA822761B; IT1151151B; SE8202355L; DE3117551A1; IT8221051A0; GB2098294B; BR8202504A; FR2504877A1; ES511882A0; ES8303989A1; DE3117551C2

Abstract

The master cylinder piston (3) in the master cylinder of a hydraulically- actuated motor vehicle brake includes in its front wall a compensating bore (18) communicating with the piston rear side and adapted to be closed by a valve body (14) movable relative to the master cylinder piston (3). The valve body (14) is provided at the end of a central stem member (11) connecting the two spring collars (8, 10) of a piston return spring (9) of the master cylinder piston (3). The valve body (14) is urged by spring 20 against the compensating bore (18) as soon as the piston return spring (9) is loaded in compression. <IMAGE>

Description

SPECIFICATION Brake master cylinder This invention relates to a brake master cylinder for the hydraulically actuated brakes of an automotive vehicle, of the kind including at least one master cylinder piston with a piston return spring anchored by a central clamping member and a valve body which is slidable relative to the master cylinder piston and arranged to open a compensating port at the master cylinder piston which the piston return spring is relieved.

In hydraulically-actuated motor vehicle brakes, provisions must be made to ensure that the master cylinder pressure chamber and the whole brake system are automatically topped up with hydraulic fluid which is drawn from a reservoir when required. In this regard, it is generally known to provide in the cylinder wall immediately in front of the master cylinder piston in the latter's inactive position a compensating bore through which the pressure chamber of the master cylinder communicates with the reservoir in the inactive position. Topping up with brake fluid is thereby ensured; in addition, it is possible to achieve a pressure balance between the brake system and the reservoir, for example when the brake fluid in the brake system expands as a result of heating.

Actuation of the master cylinder piston will cause it to close the compensating bore in the initial phase of the piston stroke, enabling the pressure required for a braking action to be built up in the brake system.

The opening of the compensating bore into the cylinder internal chamber represents an interruption in the smooth cylinder running surface over which the piston seal has to travel with each stroke of the master cylinder piston. This puts a load on the piston seal.

In order to avoid an interruption in the cylinder running surface brushed over by the pistori seal, attempts have already been made to use, instead of the compensating bore in the cylinder wall, a compensating port at the master cylinder piston which is closed on actuation of the master cylinder piston by a valve body.

In such a known design (French Patent No.

1,571,663), the master cylinder piston possesses a central compensating bore. The valve body closing this compensating bore is provided at the front end of a piston actuating rod which is axially slidable relative to the master cylinder piston by a small amount. This amount, which is unavoidable, represents an undesired clearance in the actuation of the brake. The return spring necessary for the return motion of the master cylinder piston does not act on the master cylinder piston itself but on the piston-actuating rod. By this arrangement it is not ensured that the compensating bore is opened in time when the load on the piston is removed; there exists the risk of at least a temporary vacuum being formed in the brake system which may cause air to enter the master cylinder.

In another known design (German Patent No.

819,354), the master cylinder piston is provided with a piston head which is axially slidable relative thereto, carries the piston seal and opens, when displaced, radial channels forming the compensating port in a central frontal bore of the master cylinder piston. As in the design referred to above, the construction of the master cylinder piston and its members is relatively complicated.

To seal off the compensating port, the rear side of the piston seal is used which is thereby subjected to particularly high loads.

In this known master brake cylinder there exists in particular the danger that in the case of an overfilled brake system due to rapidly succeeding actuations of the master cylinder piston, the returning fluid is still at such a high pressure when the master cylinder piston reaches its end position that the friction of the piston seal is not sufficient to open the compensating valve in the master cylinder piston. This prevents a full release of the brake system.

In a master brake cylinder of the type initially referred to (German Patent No. 1,243,539), the valve body is formed by an O-ring seal interposed between a piston return spring seat close to the piston and a ring bearing against the piston via a compression spring in the opposite direction. With this arrangement in the relieved state, the compensating port is located between the O-ring and a conical outer surface at the piston front end.

Here, too, the construction of the piston with the valve device for closing the compensating port is complicated by comparison. The O-ring is required to fulfil a dual function as both a piston seal and a valve body. The piston and the members surrounding it cannot be pre-assembled but have to be inserted into the master cylinder one after the other, resuiting in a relatively complicated assembly.

Based on this state of the art, it is an object of the present invention to provide a master cylinder of the kind initially referred to which, while affording a relatively simple design, enables its parts to be pre-assembled to a high degree and ensures reliable opening of the compensating port closed by the valve body, independently of the ratios of friction occurring during operation.

According to the invention in its broadest aspect, a brake master cylinder for the hydraulically actuated brakes of an automotive vehicle of the kind referred to is characterised in that the clamping member for the piston return spring carries at its end close to the master cylinder piston the valve body which in the inactive position is at a small distance from the compensating port designed as a compensating bore in the front wall of the master cylinder piston, and in that the clamping member is arranged to pull the valve body into its opening position in the master cylinder piston's inactive position and constitutes a stop for the valve body.

The master cylinder piston is extremely simple in its design, providing seatings and bores which are easy to machine from the outside. Considering that the compensating bore may be of a very small design having, for example, a diameter of between 0.5 and 0.8 mm, the hydraulic force acting in the closing direction of the valve is so low that it can be reliably overcome by the piston return spring in the position of release of the master cylinder piston, thus ensuring reliable opening of the compensating bore.

Independently of the type of brake actuation and the building up of pressure in the brake system, it is ensured that the valve body releases the compensating port against the force of the hydraulic pressure, because the force of the piston return spring or the retaining force originating from a stop at the housing or the intermediate piston of a tandem master cylinder is transmitted to the valve body via the clamping member In an embodiment of the invention, a valve closure spring acting upon the clamping member is a compression spring inserted between the valve body and a spring collar close to the piston to seat the piston return spring.

Because the relative movement of the valve body is very small, only a very small compression spring with a very short spring travel can be accommodated in this area. Instead, it is also possible to arrange the valve closure spring as a compression spring between the end of the clamping member remote from the valve body and the associated spring collar of the piston return spring or the cylinder bottom. This solution is preferred if only very little space is available to accommodate a compression spring in the area between the valve body and the spring collar close to the piston.

The valve body is suitably guided in an endwise central bore of the master cylinder piston, with clearance being left for passageways. Thereby centering of the valve body in the master cylinder piston is accomplished by particularly simple design measures.

In a further embodiment of this invention, the anchoring device has its end remote from the valve body secured to the cylinder bottom or to the intermediate piston. This provides the possibility of designing the effective area of the compensating valve large enough to enable the inner rim of the cup seal to be employed at the same time as the sealing member for the compensating valve. Large-area compensating valves further afford the advantage of requiring only a short axial actuating travel so that the lost travel occurring on actuation of the master brake cylinder is low.

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a longitudinal section of a tandem master cylinder of a hydraulically actuatable motor vehicle brake; Figure 2 is an enlarged partial view, equally in longitudinal section, of the master cylinder piston of Figure 1, including its piston return spring and the device for closing the compensating bore: Figure 3 is a longitudinal section of a tandem master cylinder according to another embodiment; and Figure 4 is a similar longitudinal section of a tandem master cylinder further modified relative to the above embodiment.

The tandem master cylinder shown in Figure 1 includes a cylinder housing 1 having a cylinder bore 2 in which a master cylinder piston 3 and an intermediate piston 4 are guided in a slidable and sealed relationship. Supply channels 5 from a reservoir terminate in the chambers located behind the master cylinder piston 3 and the intermediate piston 4, respectively. Figure 1 does not show in more detail that the chamber located behind the master cylinder piston 3 is sealed relative to the right. Because the intermediate piston 4 with its associated parts corresponds in its design to the master cylinder piston 3, only the master cylinder piston 3 will be described in the following.

The master cylinder piston 3 carries as a piston seal a primary seal 6 which surrounds an endwise projecting cylindrical section 7 of the master cylinder piston 3. Bearing against this section 7 is a spring collar 8 of a piston return spring 9 which is designed as a compression spring and bears with its other end against another spring collar 1 0.

The piston return spring 9 is anchored by a rodshaped central clamping member 11 which extends axially movably through the spring collars 8 and 10 and abuts against their outer ends with heads 12 and 13, respectively.

One head 13 of the clamping member 11 is connected to a valve body 14 which extends into an endwise central bore 1 5 of the master cylinder piston 3 in which it is guided with clearance for lateral passageways 1 6 being provided. The valve body 14 carries a central seal 1 7 arranged at a short axial distance in front of a central compensating bore 1 8 in the master cylinder piston 3, the compensating bore communicating through a lateral bore 1 9 with the chamber behind the master cylinder piston 3 and thus with the supply channel 5. The outer diameter of the seal 1 7 which ends flush with the end surface of the valve body 14 is greater than the diameter of the compensating bore 18.

A compression spring 20 serving as a valve closure spring is arranged between the valve body 14 and the spring collar 8 close to the piston.

The drawing shows the master cylinder piston 3 in its inactive position. When the master cylinder piston is moved to the left in its cylinder bore 2 by an actuating force introduced from the right, the spring collar 10 of the piston return spring 9 will bear against the rear end of the intermediate piston 4. In a corresponding manner, the return spring of the intermediate piston 4 will bear against the cylinder housing 1 at the end of the cylinder bore.

In this arrangement, the piston return spring 9 is initially slightly compressed. The clamping member 11 is released axially in the same proportion so that the valve closure spring 20 displaces the valve body 14 relative to the master cylinder piston 3 until the valve body 14 closes the compensating bore 1 8. From this moment on, pressure will start building up in the pressure chamber located between the master cylinder piston 3 and the intermediate piston 4 in a known manner; this pressure is transmitted to one of the two brake circuits, while the pressure developing in front of the intermediate piston 4 is transmitted to the second brake circuit.

With the master cylinder pin 3 returning to its initial position, the valve body 14 will be lifted clear of the compensating bore 1 8 as soon as the piston return spring 9 is relieved, i.e., as soon as the clamping member serving to anchor the piston return spring 9 transmits a tensile force between the two spring collars 8 and 10 which is greater than the elastic force of the valve closure spring 20.

The embodiment shown in the example of the intermediate piston 4 in Figure 3 left is similar to the embodiment of Figure 2; the valve closure spring 20 is, however, arranged between the head 12 of the clamping member 11 remote from the intermediate piston 4 and the cylinder bottom 2a.

As in the examples of Figures 1 and 2, the front wall 21 of the intermediate piston 4 (or of the master cylinder piston 3 of Figures 1 and 2) close to the valve body 14 is of conical configuration, with the compensating bore 1 8 terminating at its point.

In the master cylinder piston 3 of Figures 3 and 4 and in the intermediate piston 4 of Figure 4, the valve body 14 consists of a cylindrical section 1 4a which is sealed to, and guided in, the piston central bore 15, and of an enlarged plate 1 4b upon which the spring collar 8 takes support. A cross pin 22 secures the cylindrical section 1 4a in the master cylinder piston 3.

In the embodiment of Figure 3, a compression spring 24 urging the valve body 14 into its opening position in the inactive state is disposed between the cylindrical section 1 4a and a blind- end bore 23 in the master cylinder piston 3. In the embodiment of Figure 4, the same purpose is achieved by a compression spring 25 bearing with one end against an area in close proximity to a seal 26 sealing the piston rod 3a of the master cylinder piston 3 in the housing 1, while its other end bears against the end of the piston rod 3a close to the pedal and thereby moves the master cylinder piston 3 into its initial position (to the right when viewing Figure 4).Because the head 12 of the clamping member 11 is held in a bore 27 at the end of the intermediate piston 4 close to the pedal, the intermediate piston forming at the same time the abutment for the piston return spring 9 of the master cylinder piston 3, and because the movement of the intermediate piston 4 to the right is limited by a stop screw 28 screwed down into the housing 1, the spring 25 will continue to urge the master cylinder piston 3 further to the right even when the piston return spring 9 has already reached its end position which is defined by the clamping member's 11 anchoring action.Thereby the end, closed on the cylinder side, of the lateral passageway 16, designed as a longitudinal groove, of the cylindrical section 1 4a of the valve closure member 14 is released in such a manner that its end face projects beyond the central bore 1 5 of the master cylinder piston 3; the valve is open.

In the embodiment of the master cylinder piston of Figure 3, the longitudinal groove forming the passageway 1 6 is designed as a through groove on the cylinder side. in the closing position, the plate 1 4b moves with its end face into sealing engagement with the primary seal 6 of the master cylinder piston 3. The closure force acting on this relatively large valve closure area is overcome by spring 24.

The valve formed at the end face at the intermediate piston 4 may be designed in accordance with the example of the master cylinder piston 3 of Figure 3 or of Figure 4.

Common to the master cylinder tandem designs illustrated is that the anchored piston return spring 9 for the master cylinder piston 3, which spring is arranged between the master cylinder piston 3 and the intermediate piston, is biased by a greater amount than the piston return spring 9 for the intermediate piston 4, which latter spring is located between the intermediate piston 4 and the cylinder bottom 2a, this amount corresponding to the force for displacing the intermediate piston 4.

In the embodiment of Figure 4, the member forming the spring collar 10 and including at the same time the bore 27 accommodating the head 12 of the clamping member 11 is anchored in a bore in the cylinder bottom 2a with a spigot 1 Oa.

This permits transmission of the force exerted by the spring 25 via the master cylinder piston 3, the clamping member 11, the intermediate piston 4, the second clamping member 11 and the member 10. The tensile force thereby occurring between the intermediate piston 4 and the associated valve body 14 also causes opening of the valve disposed in the intermediate piston 4. Only when the intermediate piston 4 is supported upon the screw 28 will the force of the spring 25 act only on the valve in the master cylinder piston 3.

In contrast thereto, in the embodiment of Figure 3 the force required for opening the valves is not transmitted via the housing 1. The spring 24 acts to open only the valve in the master cylinder piston 3; for the valve in the intermediate piston 4 no separate opening spring is required because the valve closure area which is determined by the area of cross-section of the compensating bore 1 8 is so small that the force of the piston return spring 9 suffices for opening the valve. This combination of the two valve types shown in Figure 3 enables the two pistons 3 and 4 to be pre-assembled as a constructional unit, with the need to provide a special fastening at the cylinder bottom 2a, as required in the embodiment of Figure 4, being eliminated.

In the valve designs of Figure 3, right-hand half, and of Figure 4, the anchoring device 10, 11 is fastened to the intermediate piston 4 and, respectively, to the cylinder bottom 2a. In these embodiments, the area of cross-section of the compensating valve is so large that the force of compression springs 24 and 25 which should be as low as possible in order not to degrade the efficiency of the master cylinder, is not sufficient to overcome in the operating state the force of the hydraulic pressure and to open reliably the compensating valve. By fastening the anchoring device 10, 11 to the cylinder bottom 2a or the intermediate piston 4, the valve body 14 is retained against the force of the hydraulic pressure, whereas the hydraulic pressure acting on the annular end surface, projecting over the head 1 3 of the valve body 14, of the respective master cylinder piston 3 continues to move the respective piston in the direction of brake release until the compensating valve is open. The function of compression springs 24 and 25 is to hold the pistons in their opening positions or to move them into their opening positions when the hydraulic pressure in the cylinder chambers has already decreased prior to the compensating valve being opened.

Claims

1. A brake master cylinder for the hydraulically actuated brakes of an automotive vehicle of the kind including at least one master cylinder piston with a piston return spring anchored by a central clamping member and a valve body which is slidable relative to the master cylinder piston and arranged to open a compensating port at the master cylinder piston when the piston return spring is relieved, characterised in that the clamping member 1) for the piston return spring (9) carries at its end close to the master cylinder piston (3) the valve body (14) which in the inactive position is at a small distance from the compensating port designed as a compensating bore (18) in the front wall of the master cylinder piston (3), and in that the clamping member (11) is arranged to pull the valve body (14) into its opening position in the master cylinder piston's (3) inactive position and constitutes a stop for the valve body (14).

2. A master cylinder as claimed in claim 1, characterised in that the clamping member (1 1 ) is acted upon by a force of a valve closure spring (20) directed in opposition to the compensating bore (18).

3. A master cylinder as claimed in claim 2, characterised in that the valve closure spring (20) is a compression spring inserted between the valve body (14) and a spring collar (8) close to the piston to seat the piston return spring (9).

4. A master cylinder as claimed in claim 1, characterised in that the valve closure spring (20) is arranged as a compression spring between the end of the clamping member (11) remote from the valve body (14) and the cylinder bottom (2a) or the intermediate piston (4).

5. A master cylinder as claimed in claim 1, characterised in that the valve body (14) is guided in an endwise central bore (15) of the master cylinder piston (3), with clearance being left for passageways (16).

6. A master cylinder as claimed in claim 5, characterised in that the valve body (14) guided in the endwise central bore (15) of the master cylinder piston (3) includes at least one lateral passageway (1 6) in the form of a longitudinal groove.

7. A master cylinder as claimed in claim 1, characterised in that the front wall (21 ) of the master cylinder piston (3) is of conical configuration, with the compensating bore (18) terminating at its point.

8. A master cylinder as claimed in claim 1, characterised in that the end surface of the valve body (14) close to the compensating bore (18) accommodates a seal (17) the outer diameter of which is greater than the diameter of the compensating bore (18) and which ends flush with the end surface of the valve body (14).

9. A brake master cylinder in tandem design with an anchored master cylinder piston return spring arranged between the master cylinder piston and an intermediate piston as claimed in claim 1, characterised in that the piston return spring (9) is biased by a greater amount than a piston return spring (9) for the intermediate piston (4), which latter spring is located between the intermediate piston (4) and the cylinder bottom (2a), the said amount corresponding to the force for displacing the intermediate piston (4).

10. A master cylinder as claimed in claim 1, characterised in that the anchoring device (10, 1 1 ) has its end remote from the valve body (14) secured to the cylinder bottom (2a) or to the intermediate piston (4).

11. A master cylinder as claimed in claim 1, characterised in that the valve body (14) is provided with a plate ( 1 4b) moving in the closing position into sealing engagement with the primary seal (6) of the master cylinder piston (3).

12. A brake master cylinder in tandem design as claimed in claim 1, characterised in that the valve body (14) of the master cylinder piston (3) includes a plate (14b) which in the closing position is in sealing engagement with the primary seal (6) of the master cylinder piston (3), in that a valve opening spring (24) acting between the master cylinder piston (3) and its valve body (14) is provided, and in that the intermediate piston (4) has a thin central compensating bore (18) in front of which the valve body (14) is situated.

1 3. A brake master cylinder for the hydraulically actuated brakes of an automotive vehicle substantially as described with reference to the accompanying drawings.