A master cylinder for a motor-vehicle braking system

Abstract: A master cylinder for a motor-vehicle braking system comprises two parallel pistons (18) associated, respectively, with two braking circuits of the motor-vehicle and a common member (58) which is acted on by the brake pedal (22) to operate the pistons (18). The force is transmitted from the pedal (22) to the operating member (58) by means of a rubber ring (62) the deformation of which pressurises a chamber (66) adjacent the operating member (58) so that the latter assists the action of the pedal (22).

Description: The present invention relates to a master cylinder for a motor-vehicle braking system, of the type proposed in Italian patent application T91A000638 filed by the present Applicant on 7.8.1991 and published after the priority date of this application. The object of the present invention is to improve the master cylinder proposed previously by providing it with a particularly simple and functional device for assisting braking. In order to achieve this object, the subject of the invention is a master cylinder for a motor-vehicle braking system comprising: a body, first and second main pistons which are slidable in the body and are operatively connected to a brake pedal in order to operate first and second braking circuits of the motor vehicle, respectively, resilient means operatively interposed between a wall of the body and the main pistons for biasing the latter towards a rest position, first and second pressure chambers which are associated with the first and second main pistons respectively, each of which can also communicate with the respective braking circuit and, when the respective piston is in its rest position, with an exhaust reservoir, in which the two pressure chambers and the respective operating pistons are disposed in parallel, characterized in that the master cylinder also comprises: a rod which is slidable in the body and is connected rigidly to a shank of the brake pedal, a common member for operating the two main pistons, which is slidable in the body and has an annular recess in its face which faces the brake pedal, a rubber ring which is mounted in the annular recess, the shank of the brake pedal having an end surface which is urged against a first portion of the exposed face of the rubber ring, an axially-slidable obturator ring associated with the operating member, auxiliary resilient means for urging the obturator ring against a second portion of the exposed face of the rubber ring, the operating member defining an auxiliary pressure chamber which is in communication both with a compressed-air supply and with the atmosphere by means of a passage controlled by the obturator ring which can move, against the action of the auxiliary resilient means, until it closes the passage, as a result of a deformation of the rubber ring which occurs when the brake pedal is urged against the rubber ring, so as to pressurise the auxiliary pressure chamber so that the operating member acts on the two main pistons, assisting the force transmitted by the brake pedal to the operating member by means of the rubber ring. The member for operating the two main pistons preferably has a plate having two seats with spherical profiles which house two correspondingly-shaped push-rods carried by the respective ends of the two main pistons, the plate in turn having a spherical-profiled bearing surface which engages a corresponding surface on the operating member. By virtue of this characteristic, the thrust produced by the brake pedal is distributed correctly to the two main pistons under all operating conditions. Further characteristics and advantages of the invention will become clear from the description which follows with reference to the appended drawings, provided purely by way of non-limiting example, in which: Figure 1 is a longitudinal sectional view of a master cylinder according to the invention, taken on the line I-I of Figure 2, Figure 2 is section taken on the line II-II of Figure 1, Figure 3 is a view of the detail indicated by the arrow III-III in Figure 2, on an enlarged scale, Figure 4 is a section taken on the line IV-IV of Figure 2, on an enlarged scale, Figure 5 is a section taken on the line V-V of Figure 1, Figure 6 is a section taken on the line VI-VI of Figure 1, and Figure 7 is a section taken on the line VII-VII of Figure 1. With reference to the drawings, the steel or light-alloy body of a master cylinder for a motor-vehicle braking system including first and second braking circuits (not shown), each associated with two of the four wheels of the motor vehicle, is generally indicated 1. As can clearly be seen in Figure 2, the body 1 includes two parallel sections 2a, 2b having cavities 5 which define two pressure chambers 3 for communicating, by means of two outlet connectors 4 (Figures 1, 7), with the two braking circuits, respectively. In the embodiment illustrated, the body 1 includes a first element 1a in which the two cylindrical cavities 5 constituting the chambers 3 are formed and a second, cup-shaped element 1c, the mouth of which can be connected to the end portion of the element 1a. The two elements 1a, 1b are fixed together by means of screws which engage holes 1 in juxtaposed lugs 1d (Fig. 6) forming parts of both elements. The element 1a also has a flange 12 with holes 12a for the engagement of screws for fixing the body 1 to the structure of the motor vehicle. The element 1a of the body 1 also includes an integral conical orifice 9 for the snap-engagement of a connector which forms part of a reservoir of plastics material (not visible in the drawings) for topping up the liquid in the braking circuit, of the type illustrated in the Applicant's prior application identified above. With reference to Figure 1, the conical duct 8 in the orifice 9 opens into a transverse duct 8a (Figures 1, 5) which in turn communicates with the two cylindrical cavities 5 defining the chambers 3. Two steel bushes 15 are fixed within the two cylindrical cavities 5 (Figure 4). Sealing rings 16 are interposed between the bushes 15 and the wall of the body 1. Two rods 18, constituting the pistons for operating the two braking circuits of the motor vehicle, are slidable in the two bushes 15 with the interposition of sealing rings 17. The ends of the two rods 18 which face a brake pedal 22 have end push-rods 19 with spherical profiles. The opposite end of each rod 18 carries a resilient ring 23 which is biased towards an end surface 24 of the bush 15 by a helical spring 25 interposed between the rear end of each bush 15 and a collar 26 associated with the push-rod end 19 of each piston 18. Each rod 18 has an axial hole 28 which serves to keep the respective pressure chamber 3 in communication with the orifice 8 connected to the reservoir for the liquid of the braking circuit, through radial and circular passages 29, 30. The front end of each rod 18 is associated with a valve 31 which cuts off the communication between the pressure chamber 3 and the supply reservoir when the respective piston is moved away from its rest position, towards which it is biased by the spring 25. The valve 31 comprises a face-sealing obturator constituted by a rubber disc 32 facing the opening of the axial hole 28 in the rod 18. The rubber disc 32 is guided within a cage 33 (see Figures 4 and Figure 2) associated with the end face of the piston 18. The cage 33 is kept in abutment against the end surface 24 of the bush 15 by a conical helical spring 36 which is interposed between the cage 33 and the end of the chamber 3. In the rest position, the obturator 32 is spaced from the end of the rod 18 so that there is communication between the pressure chamber 3 and the hole 28 in the rod 18 and, consequently, the supply reservoir 11. In this condition, the two braking circuits are at rest. When the pistons 18 are moved towards the left (with reference to the drawings), in the manner which will be described below, the obturators 32 close the openings of the holes 28 so that, when the rods 18 move further, they overcome not only the action of the springs 25 but also that of the springs 36. The structure described above is substantially similar to that of the device which is the subject of the present Applicant's prior application identified above. According to the present invention, the master cylinder also has a device, generally indicated by 50, for assisting the braking action. The brake pedal 22 has a shank 51 with a blind hole 52 into which the end 53 of a rod 54, which is slidable in a cylindrical cavity 55 in the body of the master cylinder, is screwed (in the embodiment illustrated) or snap-engaged. An operating member 58 in the form of a piston, slidable on the rod 54, is also guided for sliding in the cylindrical cavity of the element 1c of the body 1. The operating member 58 includes a first element 58', and a second element 58'', fixed thereto. The element 58' has a bearing surface with a spherical profile which is engaged by a correspondingly profiled surface of a plate 59 which has two seats 60 for the push-rods 19 of the pistons 18, the seats having spherical profiles corresponding to those of the push-rods 19. With reference to Figure 3, the element 58' of the operating member 58 has an annular recess 61 on its face which faces the brake pedal 22, for housing a rubber ring 62. A first portion of the exposed face of the rubber ring 62, that is, the face of the ring which faces the brake pedal 22, is acted on by the end surface of the rod 51 of the pedal, and a second portion thereof is acted on by an obturator ring 63 which is slidable on the rod 51. The obturator ring 63 is interposed axially between the elements 58' and 58'' of the operating member 58. A helical spring 65 interposed between the obturator ring 63 and the element 58'' holds the obturator ring 63 against the rubber ring 62. The element 58'' defines, with the end wall of the element 1c, an auxiliary pressure chamber 66 which communicates with a connector 68 through passages 67 in the element 1c (Figure 1). The connector 68 is intended to be connected, by means of a flexible tube (not shown), to a compressed-air supply, for example, a compressed-air accumulator supplied by a compressor driven by an electric motor or by the engine of the motor vehicle. When the obturator ring 63 is in the rest position shown in Figure 3, the compressed air which reaches the interior of the auxiliary pressure chamber 66 through the connector 68 and the passage 67 passes from the chamber 66 through axial passages 64a and 58a into the chambers 69 (Figure 3) of the two pistons 18, from which it can escape to the atmosphere through holes 70 in the wall of the body of the device. When the brake pedal 22 is depressed, it exerts a force on the end push-rods 19 of the two pistons 18 by means of the rubber ring 62, the operating member 58 and the plate 59. The first movement of the pedal 22, however, presses the shank 51 against the rubber ring 62. The ring is deformed but its volume remains constant, thus causing a rearward movement of the obturator ring 63, against the action of the helical spring 65, to a position in which a circular end edge 63a of the obturator ring 63 contacts a sealing ring 64 of the element 58'' so as to break the communication between the pressure chamber 66 and the atmopshere. The chamber 66 is thus pressurised and causes the member 58 to move towards the left (with reference to Figure 3 ), thus assisting the braking action. During the braking, the spherical contact of the push-rods 19 against the plate 59 enables the thrust to be distributed correctly to both the pistons 18 in all operating conditions.

Claims: A master cylinder for a motor-vehicle braking system comprising: a body (1) , first and second main pistons (18) which are slidable in the body (1) in order to operate first and second braking circuits of the motor vehicle, resilient means (25) operatively interposed between a wall of the body (1) and the main pistons (18) for biasing the latter towards a rest position, first and second pressure chambers (3) which are associated with the first and second main pistons (18), respectively, each of which can also communicate with the respective braking circuit and, when the respective piston (18) is in its rest position, with an exhaust reservoir, in which the two pressure chambers (3) and the respective operating pistons (18) are disposed in parallel, characterized in that the master cylinder also comprises: a rod (54) which is slidable in the body (1) and is connected rigidly to a shank (51) of the brake pedal (22), a common member (58) for operating the two main pistons (18), which is slidable in the body (1) and has an annular recess (61) in its face which faces the brake pedal (22), a rubber ring (62) which is mounted in the annular recess (61), the shank (51) of the brake pedal having an end surface which is urged against a first portion of the exposed face of the rubber ring (62), an axially-slidable obturator ring (63) associated with the operating member (58), auxiliary resilient means (65) for urging the obturator ring (63) against a second portion of the exposed face of the rubber ring (62), the operating member (58) defining an auxiliary pressure chamber (66) which is in communication both with a compressed-air supply (68) and with the atmosphere by means of a passage controlled by the obturator ring (63) which can move, against the action of the auxiliary resilient means (65), until it closes the passage, as a result of a deformation of the rubber ring (62) which occurs when the brake pedal (22) is urged against the rubber ring, so as to pressurise the auxiliary pressure chamber (66) so that the operating member (58) acts on the two main pistons (18), assisting the force transmitted by the brake pedal (22) to the operating member (58) by means of the rubber ring (62). A master cylinder according to Claim 1, characterized in that the operating member (58) has a plate (59) having two seats (60) with spherical profiles which house two correspondingly-shaped end push-rods (19) carried by the respective ends of the two main pistons (18), the plate (59) in turn having a spherical-profiled bearing surface which engages a corresponding surface on the operating member (58). A master cylinder according to Claim 2, characterized in that the operating member (58) is in the form of a piston which is slidable on the rod (54) of the brake pedal (22) and is also slidable in the body (1). A master cylinder according to Claim 3, characterized in that the obturator ring (63) has a circular end edge (63a) which cooperates with a sealing ring (64) carried by the operating member (58).

Inventors: GANDIGLIO ROMOLO, GANDIGLIO, ROMOLO
Applicants: FIAT AUTO SPA, FIAT AUTO S.P.A.
Application year: 1993
Main classification: B60T13
IPC codes: B60T11, B60T13