SERVO-ASSISTED VACUUM BRAKE DEVICE FOR VEHICLES

Abstract: The brake device comprises a master hydraulic cylinder (6) which pumps pressurised fluid into the braking system (3) following the translation of a control rod (2) operated mechanically by the user, and which is disposed, with respect to the control rod, downstream of the vacuum actuator (10) and upstream of the vacuum control valve (25) which is independent of the actuator and is connected thereto by pipes; the control rod (2) is rigid with the piston of the vacuum actuator (10), and the control valve is composed of a hydraulic actuator (28) controlled by the pressure in the braking system and a second pneumatic actuator (26), the chambers of which are connected to those of the vacuum actuator to counterbalance the thrust exerted by the hydraulic actuator on a spool (38) arranged to selectively connect the vacuum actuator (10) to a vacuum source or to atmosphere.

Description: This invention relates to a servo-assisted brake device for vehicles, in particular motor vehicles, of the so-called vacuum type, ie in which the mechanical action performed by the user during braking, for example by pressing his foot on the brake pedal, is assisted beyond a certain force threshold by a pneumatic actuator, one chamber of which is gradually connected to atmosphere while the other chamber remains permanently connected to a vacuum source, for example the intake manifold and/or a suitable vacuum pump driven by the vehicle engine. Two main types of servo-assisted vacuum devices (servo brakes) are available, known to experts of the art by the names Master-vac and hydro-vac. In the first type, the vacuum actuator is mounted directly rigid with the master hydraulic cylinder downstream of this latter with respect to the control rod operated by the brake pedal. A control valve is provided integral with the vacuum actuator and rigid with the control rod, to control access of atmospheric pressure into one of the two actuator chambers by a mechanical device sensitive to the force exerted on the control rod. When this exceeds a given threshold value, determined by the compressibility of an elastic reaction disc, the air flows into one chamber of the actuator to thus exert on its piston, which is rigid with the control rod, a thrust concordant with that imparted by the user and which assists the operation of the master cylinder. In the second type, the actual master cylinder is controlled directly, by the control rod alone, and the pneumatic actuator with the control valve is disposed downstream of the master cylinder and controls the movement of a piston, the task of which is to directly increase in the braking system the hydraulic pressure produced mechanically by the user acting on the master cylinder. The threshold force for the operation of the servo brake is a pressure threshold, which is sensed by the control valve, this being also formed integrally with the pneumatic actuator. The aforesaid devices have the drawback of a certain constructional complexity and in particular a relatively large amount of idle travel which makes operation of the brake less prompt than in a non servo-assisted system. This results in a certain insensitivity of the brake pedal during the initial part of its travel, and this can be annoying for the user. Finally, the presence of the control valve formed integrally with the vacuum actuator further increases the overall size and complexity of this latter. The object of the invention is to provide a servo-assisted vacuum brake device which is free of the aforesaid drawbacks, and in particular is of simple construction and small overall size, in addition to possessing minimum idle travel. This object is attained according to the invention by a servo-assisted vacuum brake device for a vehicle, comprising a master hydraulic cylinder arranged to pump fluid under pressure into the vehicle braking circuit following the translation of a control rod mechanically operable by the user against the action of elastic means, a pneumatic vacuum actuator a first chamber of which is permanently connected to a vacuum source by way of a unidirectional valve, and the second chamber of which, separated from the first by a sliding piston, is connected to a vacuum control valve arranged to selectively connect said second chamber to atmosphere or to said vacuum source, and means for operating the control valve in accordance with the force exerted by the user on the control rod, characterised in that said vacuum actuator is disposed upstream of said master cylinder with respect to said control rod, and is traversed in a fluid-tight manner by this latter, which is rigid with said sliding piston, and in that, in combination, said control valve is disposed downstream of said master cylinder with respect to said cont rol rod, said control valve operating means comprising a hydraulic actuator connected hydraulically into said vehicle braking circuit downstream of said master cylinder, and a second pneumatic actuator disposed mechanically in series with said hydraulic actuator, between this latter and said control valve, and comprising a respective third and fourth chamber which are connected permanently to said first and said second chamber of the pneumatic vacuum actuator respectively. The invention will be more apparent from the non-limiting description given hereinafter of one embodiment thereof with reference to the accompanying drawings, in which: Figure 1 shows a servo-assisted vacuum brake device constructed in accordance with the invention; Figure 2 is a longitudinal section through a first detail of the device according to the invention; and Figure 3 is a longitudinal section through a second detail of the device according to the invention. In Figures 1 to 3, the reference numeral 1 indicates overall a servo-assisted vacuum brake device forming part of a braking system of known type, not shown for simplicity, of a vehicle of any known type, also not shown for simplicity, for example a motor vehicle, which is arranged to transform the mechanical work performed by the user on a control rod 2, for example by means of a known pedal, not shown, into pressure energy to be fed in the form of pressurized oil into a braking circuit 3 of known type, shown only partly for simplicity, which is connected to suitable braking mechanisms (shoes or discs) of the vehicle wheels, also of known type and therefore not shown for simplicity. The control rod 2 is connected to said pedal, not shown, by a connecting rod 4 secured to one end of the rod 2 in an adjustable manner by means of a threaded coupling 5, and controls by virtue of its axial translation a master hydraulic cylinder 6, of known type and therefore not further described, which is arranged to pump pressurised fluid, for example oil, into the circuit 3 following translation of the rod 2 towards the cylinder 6 resulting from mechanical action by the user, for example a thrust, exerted by said pedal on the connecting rod 4. The circuit 3 comprises a pair of delivery pipes 7 and 8 emerging from the master cylinder 6, and this latter cooperates directly with an end 9 of the rod 2 (Figure 3), which is also adjustable. Directly secured to the master cylinder 6, upstream of this latter with respect to the position of the rod 2, there is a pneumatic vacuum actuator 10, which is traversed in a fluid-tight manner by the rod 2 and is composed of a simple rigid outer casing 11 comprising two half-casings 12 and 13 between which a deformable diaphragm 14 is clamped in a fluid-tight manner, a sliding piston 15 rigidly carried by the diaphragm 14 and rigid with the rod 2, and a counteracting spring 16 inserted between the cylinder 6 and piston 15 in such a manner as to oppose the translational movement of the rod 2 towards the cylinder, and housed within the casing 11. In particular, the diaphragm 14 and relative piston 15 divide the interior of the casing 11 into a first chamber 18 and a second chamber 19, which communicate with the outside through respective connectors 20 and 21 and are bounded by the half-casings 13 and 12. The spring 16 is housed in the chamber 19 and acts directly on the piston 15, the chamber 18 being delimited at the base wall of the half-casing 13 by a connection plate 22 provided with screws 23 to allow the assembly comprising the actuator 10 and cylinder 6 to be projectingly fixed on a body element of said vehicle, not shown for simplicity, for example against the separation bulkhead between the passenger compartment and engine compartment. The device 1 also comprises a unidirectional valve 25 of known type, connected in known manner, not shown for simplicity, to a vacuum source, such as the vehicle intake manifold, and a control valve 26 for the actuator 10, which according to the invention is not incorporated in this latter but instead is disposed independently downstream of the master cylinder 6 with respect to the position of the rod 2 and incorporates respective operating means comprising a hydraulic actuator 28 connected hydraulically in series with or branching into the circuit 3 downstream of the cylinder 6 (ie downstream of this latter with respect to the flow direction of the fluid pumped by it), and a second pneumatic actuator 29 disposed mechanically in series with the actuator 28 between this latter and the external part of the valve 26. The pneumatic actuator 29 comprises a pair of fluid-tight chambers 30 and 31 separated from each other in a fluid-tight manner by a deformable diaphragm 32 and provided with respective connectors 33 and 34. By way of the connector 34, the chamber 31 is branched into a vacuum pipe 35 connecting the chamber 19 to the valve 25, whereas the chamber 30 is permanently connected in series with the chamber 18, for example by a hose 36. The valve 26 comprises a spool 38 disposed in the actuator 29 and rigid with the diaphragm 32, an air intake 39 communicating with the environment at atmospheric pressure, for example with the interior of the vehicle engine compartment, and a valving element 40 disposed coaxially to the spool 38 directly on the inside of the air intake 39 and arranged to selectively open and close this latter towards the spool 38 against the action of elastic means. The spool 38 comprises an internal T-shaped duct 41 having respective mouths 42 and 43, frontal and lateral respectively, provided in correspondence with respective opposite ends 44 and 45 of the spool 38 to lie in correspondence with the chambers 30 and 31 respectively, so as to connect these latter together. Specifically, the duct 41 connects the chamber 31 to a valve seat 46 for the valving element 40. This seat communicates with the interior of the air intake 39 on the side facing the valving element 40, and with the chamber 30 on the opposite side by way of respective holes 47 provided through a guide element 48 for the spool 38, this element disposed in the chamber 30 in correspondence with the seat 46 and slidingly engaged by the end 44. The element 48 forms part of the air intake 39, which also comprises a housing 49 mounted rigidly on the seat 46 on the outside of the actuator 29, and a filter 50 disposed about the valving element 40 in front of respective through apertures 51 in the housing 49. The valving element 40 is slidingly housed in this latter, together with a spring 52 arranged to urge it with predetermined force against the seat 46 so as to close in a fluid-tight manner any communication between the external environment and chamber 30 through the air intake 39. the valving element 40, which is formed for example from a pair of discs, is also arranged to cooperate in a fluid-tight manner with the end 44 of the spool 38 in order to close the mouth 42 disposed in front of the seat 46, so as to interrupt communication between the chambers 30 and 31 through the duct 41. The end 45 of the spool 38 cooperates with the mobile part of the actuator 28. This latter comprises a chamber 55, for example cylindrical, traversed transversely by the conduits 7 and 8 of the circuit 3, and a pair of respective plungers 56 and 57 mobile axially in the chamber 55 and disposed in tandem, ie coaxial and adjacent one behind the other, so that the plunger 56 separates in a fluid-tight manner that part of the chamber 55 traversed by the conduit 8 from that part traversed by the conduit 7, and the plunger 57 closes the chamber 55 towards the spool 38 in a fluid-tight manner. The end 45 of this latter cooperates by contact with the plunger 57 and this latter cooperates by contact at its opposite end with the plunger 56. In this manner a pressure increase in either one or in both the conduits 7 and 8 produces direct or indirect movement (this latter resulting from movement of the plunger 56) of the piston 57 against the spo ol 38 to consequently cause this latter to translate in the element 48 towards the seat 46 and valving element 40, with deformation of the diaphragm 32. In use, when the user is not pressing on the brake pedal, the rod 2 is in the position shown in Figure 3 and the spring 16 retains the piston 15 against the plate 22. Under these conditions, the cylinder 6 is deactivated, there is no pressure in the circuit 3 and the valve 26 with the actuator assembly 28 and 29 rigid with it is in the configuration shown in Figure 2. Consequently the vacuum produced by the vacuum source extends through the pipe 35 and connectors 34 and 21 into the chambers 19 and 31, and from this latter it is transmitted through the duct 41 and holes 47 of the element 48 to the chamber 30, and from here through the pipe 36 and into the chamber 18. The external air cannot enter the chamber 30 as the seat 36 is closed by the valving element 40. When the user brakes by operating the brake pedal, he causes the rod 2 to translate towards the cylinder 6, with consequent operation of this latter and pressurisation of the circuit 3. This pressure operates the actuator 28 to cause the plunger 57 to move towards the left (Figure 2) together with the spool 38, until the end 44 of this latter is brought against the valving element 40 with consequent closure of the mouth 44. Under these conditions the vacuum in the chambers 30, 31, 18 and 19 remains unaltered, but the closure of the mouth of the duct 41 towards the chamber 30 interrupts communication between this latter and the chamber 31. Furthermore, the movement of the spool 38 and plunger 57 ceases, as this is opposed by the force which keeps the valving element 40 closed, deriving from the action of the spring 52 plus the action of atmospheric pressure. If the user continues to press the pedal in order to continue the braking action, he exerts on the rod 2 a progressively increasing force which corresponds to a progressively increasing pressure produced by the master cylinder 6. On attaining a predetermined threshold corresponding to the setting of the spring 52, the thrust exerted by the oil pressure on the valving element 40 by way of the plunger 57 and spool 38 causes the valving element 40 to move towards the left with compression of the spring 52 and opening of the seat 46. Under these conditions, communication between the chambers 30 and 31 remains interrupted because the end 44 of the spool 38 still rests in a fluid-tight manner against the valving element 40, but the air intake 38 is opened because the interior of the housing 49 comes into communication with the seat 46 and consequently into communication with the chamber 30 by way of the holes 47. The air at atmospheric pressure thus enters the chamber 30 and from here it moves into the chamber 18 to exert a thrust against the piston 15 which acts on the rod 2 in a direction concordant with the thrust exerted by the user and proportional to it. Finally, during breaking, the thrust exerted by the air on the rod 2 is automatically controlled on the basis of the thrust exerted by the user on the pedal, because the air entering the chamber 30 also operates the actuator 29 in the reverse direction to that produced by the pressurised oil on the actuator 28. As the two actuators are in series, ie are disposed one adjacent to the other in the operating direction, the spool 38 moves towards the right with partial or total closure of the seat 46, to adjust the air flow towards the chamber 18 proportionally to the hydraulic pressure obtained from the force exerted by the user on the pedal. If this tends to decrease, there is a pressure decrease in the circuit 3, which enables the air pressure to further move the spool 38 towards the right, possibly with closure of the seat 46 (and thus cessation of air entry) and possible reopening of the mouth 42 (and this reduction in the pressure against the piston 18 by consequent draw-off of air towards the vacuum source), which if protracted in time leads to cessation of the braking action. In contrast, if the force tends to increase, the oil pressure rises and produces a new leftward movement of the valving element 38 with consequent intake of a larger air quantity to compensate the volume increase of the chamber 18, thus ensuring that the supplementary thrust on the rod 2 is constant. The advantages of the invention are apparent from the aforegoing description. Separating the control valve from the vacuum actuator reduces the overall size of this latter, and allows easier location of the entire braking system within the engine compartment. It also enormously simplifies the construction both of the actuator and of the control valve, to allow an increase in productivity and reduction in production costs. Finally, it enables the advantages of both known vacuum systems to be combined in a single device while at the same time obviating their drawbacks, in that the idle travel is reduced to a minimum as the mechanical devices which would cause it (reaction discs, hydraulic valves etc.) have been dispensed with.

Claims: 1. A servo-assisted vacuum brake device for a vehicle, comprising a master hydraulic cylinder arranged to pump fluid under pressure into the vehicle braking circuit following the translation of a control rod mechanically operable by the user against the action of elastic means, a pneumatic vacuum actuator a first chamber of which is permanently connected to a vacuum source by way of a unidirectional valve, and the second chamber of which, separated from the first by a sliding piston, is connected to a vacuum control valve arranged to selectively connect said second chamber to atmosphere or to said vacuum source, and means for operating the control valve in accordance with the force exerted by the user on the control rod, characterised in that said vacuum actuator (10) is disposed upstream of said master cylinder (6) with respect to said control rod (2), and is traversed in a fluid-tight manner by this latter, which is rigid with said sliding piston (15), and in that, in combination, said control valve (26) is disposed downstream of said master cylinder (6) with respect to said control rod (2), said control valve operating means (26) comprising a hydraulic actuator (28) connected hydraulically into said vehicle braking circuit (3) downstream of said master cylinder (6), and a second pneumatic actuator (29) disposed mechanically in series with said hydraulic actuator (28) between this latter and said control valve (26), and comprising a respective third (31) and fourth (30) chamber which are connected permanently to said first (19) and said second (18) chamber of the pneumatic vacuum actuator (10) respectively. 2. A device as claimed in claim 1, characterised in that said control valve (26) comprises a spool (38) disposed within said second pneumatic actuator (29) and provided internally with a duct (41) arranged to put said third (31) and said fourth (30) chamber into mutual communication, an air intake (39) communicating with the environment at atmospheric pressure, and a valving element (40) for said air intake (39) arranged to cooperate with a first end (44) of said spool (38) in order to close a first mouth (42) of said internal duct of the spool and to be moved by this latter against the action of elastic means (52) in such a direction as to open said air intake. 3. A device as claimed in claim 2, characterised in that said spool (38) is carried rigidly on said pneumatic actuator (29) by a mobile element (32) thereof which separates said third from said fourth chamber in a fluid-tight manner, and by way of its second end (45), which is distant from the first and is provided laterally with a second mouth (43) of said internal duct (41), it cooperates with said hydraulic actuator, said first mouth (42) of the inner duct being disposed in correspondence with said fourth chamber (30) in front of said air intake (39), and said second mouth (43) of the internal duct bei ng disposed in correspondence with said third chamber (31). 4. A device as claimed in claim 3, characterised in that said air intake (39) comprises a guide element (48) for said first end of said spool and disposed in said fourth chamber (30) in correspondence with a valve seat (46) for said valving element, and provided with a plurality of communication holes (47) between said seat and said fourth chamber, a housing (49) mounted rigidly on said valve seat and internally containing said valving element (40) and a spring (52) arranged to hold this latter against said valve seat, and a filter (50) disposed about said valving element in front of respective lateral through apertures (51) in said housing (49). 5. A device as claimed in claim 4, characterised in that said hydraulic actuator (28) comprises a chamber (55) traversed transversely by respective conduits (7, 8) of said braking circuit (3) and respective mobile plungers (56, 57) disposed adjacent one behind the other and cooperating with said spool (38), they closing in a fluid-tight manner said chamber (55) towards said spool, such that become moved by the pressure produced by said master cylinder (6) to consequently move said spool (38) against said valving element (40), thus distancing this latter from said valve seat (46). 6. A device as claimed in one of the preceding claims, characterised in that said fourth chamber (30) is connected to said second chamber (18) by a hose (36), and said third chamber (31) is connected into a vacuum pipe (35) which connects said first chamber (19) to said unidirectional valve (25). 7. A device as claimed in one of the preceding claims, characterised in that said elastic counteracting means (16) for said control rod (2) are housed in said second chamber (18) and act directly on said sliding piston (15).

Inventors: MATTIS, ENZO
Applicants: FIAT AUTO S.P.A.
Application year: 1987
Main classification: B60T13
IPC codes: B60T13