Anti-lock brake system valve unit

Abstract

The valve unit (10) for an antilock brake system (ABS) includes a valve body (11), a piston (18) movable within the valve body, and an elastic element (34) acting on the piston. The valve body includes a discharge port (14), a suction port (17), a main chamber (15) communicating with the discharge port, an expansion chamber (16) having a bleed passage (51) providing fluid communication between the main chamber and the expansion chamber, and a bypass passage (50) between the suction port and the discharge port. The piston (18) is movable within the main chamber (15) and has a longitudinal through-hole (31). The piston has a first lateral surface facing away from the discharge port and a second lateral surface opposite the first lateral surface and facing away from the discharge port, the first lateral surface having an area smaller than the area of ​​the second lateral surface. The elastic element (34) exerts an elastic force that moves the piston (18) away from the discharge port (14). The valve unit (10) is operable by the pressure of the brake fluid in the main chamber (15).

Description

[Technical Field]

[0001] The present invention relates to a valve unit for a hydraulic brake system that controls the anti-lock function of the wheels of a vehicle. The valve system is applicable to both motorized and non-motorized vehicles, such as bicycles. [Background technology]

[0002] Antilock braking systems ("ABS") are installed on vehicles with hydraulic brakes to prevent skidding or uncontrolled slippage and reduce the effects of sudden stops. One such system is shown in FIG. 1, in which the four wheels of a vehicle are equipped with brake discs E1-E4 and corresponding sensors S1-S4 that face phono-wheels F1-F4 or equivalent elements and rotate integrally with the brake discs. The sensors S1-S4 detect the rotational speed of the associated wheel according to a known mode and transmit signals indicative of the rotational speed, e.g., via lines N1-N4, to an electronic control unit (ECU) or module that processes the received speed signals. Each brake disc is associated with a brake caliper G1-G4. A master cylinder M, operated by a foot control C, actuates the brake caliper via respective hydraulic lines H1-H4, which are provided with valve units ABS1-ABS4. Each ABS valve unit controls the flow and pressure of brake fluid to its associated brake caliper in response to electrical control signals from the electronic control unit (ECU). When the ECU detects conditions indicative of impending wheel lockup, it activates the respective ABS valve, reducing brake fluid pressure at the affected wheel, thereby reducing braking force at that wheel so that the wheel can rotate while being braked. This process is repeated several times per second in succession during braking to prevent the vehicle from skidding.

[0003] DE10158382A1 discloses an anti-lock braking system for bicycles having master and slave hydraulic cylinders integrated with a hydraulic actuator and a low-pressure hydraulic reservoir forming a hydraulically sealed compact unit with an outlet and a check valve. The system also includes an electronic control, at least one wheel brake, at least one speed sensor, and a hydraulic actuator with a low-pressure hydraulic reservoir connected to a discharge valve and an isolation valve. The check valve is connected in parallel to the discharge valve, and a hydraulic slave cylinder is connected downstream of the isolation valve and the discharge valve.

[0004] EP3392105A2 describes a hydraulic braking system for bicycles that has two motorized valve assemblies. Each valve assembly is independently actuated by its own motorized actuator. The first valve assembly is used to block brake fluid between the master cylinder and the brake caliper, and the second valve assembly is used to open a parallel channel hydraulically connecting the brake caliper to an accumulator. The two motorized actuators are operated independently and in a predetermined sequence by an ECU. The two valve assemblies are located on parallel branches of a hydraulic circuit connecting the brake caliper to a main cylinder operated by a hand lever. During normal braking, the first valve assembly is open, allowing direct fluid communication between the master cylinder and the brake caliper, while the second valve assembly is closed. Under heavy braking conditions, with imminent wheel lockup, the first electric actuator closes the first valve assembly, blocking pressure input from the hand lever and thus blocking pressure from the first valve assembly to the caliper, preventing further buildup of pressure on the caliper. The second electric actuator opens the second valve, allowing pressure to be released into an accumulator located upstream of the second valve in a parallel channel, resulting in a drop in pressure on the caliper and releasing the brake.

[0005] Other ABS systems have a valve unit with a piston attached to a hydraulic line controlled by an electric actuator (solenoid) that reciprocates the piston to vary the volume of the hydraulic line and, therefore, regulate the pressure in the braking circuit. Summary of the Invention

[0006] In view of the prior art, the main object of the present invention is to provide an ABS valve unit that can be actuated other than by electrical control of a conventional actuator to intervene in a situation where the braked wheel is locked.

[0007] The present invention provides an ABS valve unit that is actuated by brake fluid pressure present in the hydraulic circuit of a braking system.

[0008] According to one aspect, the present invention discloses a valve unit for a hydraulic braking system for controlling the anti-lock function of a vehicle wheel, as defined in claim 1. Preferred embodiments of the invention are defined in the dependent claims.

[0009] In summary, a valve unit for a vehicle antilock brake system includes a valve body, a movable piston within the valve body, and an elastic element acting on the piston. The valve body includes a discharge port hydraulically connectable to a brake caliper, a suction port hydraulically connectable to a master cylinder, a main chamber fluidly connected to the discharge port, an expansion chamber having an outlet passageway establishing fluid communication between the main chamber and the expansion chamber, and a bypass passageway establishing fluid communication between the suction port and the discharge port. The piston is longitudinally movable within the main chamber and includes a longitudinal cavity extending through the piston between an end face of the piston facing the discharge port and a transverse passageway opening to a side of the piston. The piston collectively includes a first transverse surface facing away from the discharge port and a second transverse surface opposite the first transverse surface and facing away from the discharge port, the first transverse surface having an area smaller than that of the second transverse surface. The elastic element exerts an elastic force that moves the piston away from the discharge port. The piston has two alternative operating positions: in the first position, in normal braking conditions, the elastic force of the elastic body overcomes the longitudinal component of the hydraulic thrust of the brake fluid present in the main chamber, so that the piston is moved away from the discharge port and closes the outflow passage without closing the bypass passage, and in the second position, in the operating condition of the valve unit, the hydraulic thrust of the brake fluid present in the main chamber has a longitudinal component which overcomes the force exerted by the elastic element, so that the piston is moved towards the discharge port and closes the bypass passage while not closing the outflow passage. [Brief explanation of the drawings]

[0010] For a clear understanding of the present invention, certain preferred embodiments will now be described with reference to the accompanying drawings. FIG. 1 is a diagram illustrating the operation of an antilock braking system of a vehicle. FIG. 2 is a longitudinal cross-sectional view of a valve unit according to a first embodiment of the present invention in a first operating state. 3 to 5 show the valve unit of FIG. 2 under different operating conditions. FIG. 6 is a vertical cross-sectional view of a valve unit according to another embodiment. 7 is a longitudinal cross-sectional view of a valve unit according to a further embodiment.

[0011] 2-5, reference numeral 10 generally designates an ABS valve unit for an antilock braking system for a wheel of a vehicle. The valve unit 10 defines a longitudinal axis x and has an elongated shape in a direction defined herein as the longitudinal or axial direction. As understood in this context, terms such as "longitudinal" and "lateral" will be understood with respect to the x-axis.

[0012] Valve unit 10 has a body 11 (or housing) made of a plastic material that defines an actuation direction referred to herein as "longitudinal." Body 11, in this example, has a generally cylindrical tubular shape having a first end 12 and a second end 13 opposite the first end.

[0013] End 12 of body 11 defines a discharge port (or outlet port) 14 that can be hydraulically connected to a brake caliper (not shown) of a brake for a wheel of the vehicle, and a suction port 17 that can be hydraulically connected to a master cylinder (or main cylinder, not shown) that is individually associated with actuation control by a foot pedal or hand lever (not shown) of the vehicle.

[0014] The body 11 has a main hydraulic chamber 15 and an extension or secondary hydraulic chamber 16. The main hydraulic chamber 15 communicates directly with the discharge port 14 and slidingly houses a piston 18 longitudinally.

[0015] The main hydraulic chamber 15 defines a first end region 19 (or end region) having a diameter D1 close to the discharge port 14, a second intermediate region 20 having a diameter D2 larger than diameter D1, and a third region 21 (or base region) further away from the discharge port 14 having a diameter D3 smaller than diameter D2.

[0016] The piston 18 has an end portion 22 that is received in the end region 19 of the main chamber 15, a middle portion 23 that is received in the middle region 20 of the main chamber 15, and a base portion 24 that is received in the base region 21 of the main chamber 15.

[0017] The end 22 of the piston 18 is provided with a pair of longitudinally adjacent, spaced apart sealing end gaskets 26, 27 that are in sliding contact with the end region 19 of the main chamber 15. The middle portion 23 of the piston 18 is provided with a pair of longitudinally adjacent, spaced apart intermediate gaskets 28, 29 that mate with the middle region 20 of the main chamber 15. A base seal gasket 30 is attached to the base 24 of the piston for sealingly mating with the base region 21 of the main chamber 15.

[0018] The piston 18 defines a longitudinal cavity 31 extending through the piston between an end face 32 at the end of the piston facing the discharge port 14 and a transverse passage 33 that discharges to the side of the piston. The transverse passage 33 empties into the intermediate region 20 of the main chamber 15.

[0019] In the portion 13 of the body 11 opposite the end having the suction port 17 and the discharge port 14, a main resilient element 34 is received which urges the piston 18 away from the discharge port 14.

[0020] 2-5, the main elastic element 34 is formed as a single compression spring that is compressed longitudinally between a shoulder 35 of the body 11 and a transverse contrast wall 36 facing the shoulder 35 and spaced longitudinally therefrom. Alternative embodiments to those shown may include more than one elastic element, for example two springs, one of which is already pre-compressed and one of which is only supported, providing different levels of preload along a short longitudinal stroke.

[0021] The lateral contrast wall 36 is received with lateral play and is movable longitudinally within a chamber 38 conveniently formed by the body 11 .

[0022] The stem 37 connects the transverse contrast wall 36 to the piston 18 and, together with it, longitudinally integrates the piston 18 .

[0023] The longitudinal compression of the main elastic element 34 pushes the transverse contrast wall 36 to the left and, in turn, this pulls the piston 18 to the left via the stem 37 .

[0024] The longitudinal distance between the lateral contrast wall 36 and the shoulder 35 can be adjusted to vary the longitudinal elastic force with which the primary elastic element pulls the piston 18 away from the discharge port 14 .

[0025] According to one embodiment, the stem 37 may be formed as a threaded stem that engages through a corresponding threaded through-hole 39 formed through the lateral contrast wall 36 .

[0026] The stem 37 may have an enlarged terminal head 40 having, for example, a spherical circular cross section that engages in a corresponding recess 41 formed in the portion 24 of the piston 18 .

[0027] An adjustment device 42 can be envisioned that adjusts the elastic force pushing the piston 18 away from the discharge port 14. The adjustment device 42 varies the longitudinal position of the lateral contrast wall 36 along the stem 37, thereby adjusting the distance between the lateral contrast wall 36 and the shoulder 35, and consequently the length and compression of the primary elastic element 34. Rotating the stem 37 in a given rotational direction about its longitudinal axis 37A with the adjustment device 42 further compresses and longitudinally shortens the primary elastic element 34, thereby increasing the tractive force moving the piston 18 away from the discharge port 14. Conversely, rotating the stem 37 in the opposite rotational direction decompresses and longitudinally elongates the primary elastic element 34, thereby reducing the tractive force pulling the piston 18 away from the discharge port 14.

[0028] The stem 37 preferably has a central longitudinal axis 37A that is eccentric with respect to the central longitudinal axis 18A of the piston 18. In this way, rotation imparted to the stem 37 about its axis 37A during the step of adjusting the position of the lateral contrast wall 36 does not cause undesired rotation of the piston 18 about its axis 18A, which would result in wear of the sealing gasket attached to the piston 18 and acting on the wall of the main hydraulic chamber 15. Instead of the eccentric arrangement of the axis described above, different embodiments (not shown) may provide detent elements, such as lateral teeth, protruding radially from the piston 18 and / or the lateral contrast wall 36.

[0029] Lateral play between the edges of the lateral contrast wall 36 and the chamber 38 is desirable to reduce friction during longitudinal movement of the piston 18 with the wall 36. To ensure that rotation imparted to the stem 37 will cause it to thread or unscrew from the contrast wall 36, at least a portion of the periphery of the wall 36 preferably has a lateral distance P1 from the axis 37A that is greater than the minimum lateral distance P2 between the axis 37A and the inner surface of the chamber 38. In this way, the inner surface of the chamber 38 acts as a contrast wall to block rotation of the lateral wall 36 as the stem 37 rotates.

[0030] According to one embodiment, the adjusting device 42 for rotating the threaded stem 37 has an electric drive that is mounted on the vehicle and arranged thereon and controlled by an electronic control unit (ECU, FIG. 1 ) that is able to adjust the elastic force of the main elastic element 34 as required, as will be described below.

[0031] In the main chamber 15, brake fluid present in a first end region 19 near the discharge port 14 acts on a circular region of the piston 18 having a circumference of diameter D1 defined by the gaskets 26, 27. Between the discharge port 14 and the sealing gasket 27, the brake fluid present in the first region exerts a longitudinal thrust on the piston 18 away from the discharge port 14 towards the left (direction A) in the accompanying drawings.

[0032] The brake fluid contained in the second intermediate portion 20 of the main chamber 15 exerts a longitudinal hydraulic thrust on an area of ​​the piston defined by a circular crown having an outer periphery of diameter D2 corresponding to the diameter of the second intermediate region 20 of the main chamber 15, and an inner periphery of diameter D3 corresponding to the diameter of a third region 21 of the main chamber 15, further from the discharge port 14. The hydraulic pressure of the brake fluid in the second intermediate region 20 of the main chamber is directed to the right in the accompanying drawings and pushes the piston 18 closer to the discharge port 14.

[0033] The diameters D1, D2, and D3 of the three portions 19, 20, and 21 of the main chamber, respectively, and the diameters of the corresponding portions 22, 23, and 24 of the piston 18 are selected so that the area of ​​the circular crown defined by the diameters D2 and D3 is greater than the area of ​​a circle having the diameter D1. As a result, the total hydraulic thrust acting on the piston 18 has a resultant longitudinal force acting on the piston 18 and urging the piston 18 closer to the discharge port 14.

[0034] Thus, the overall hydraulic thrust acting on the piston 18 is directed in a direction opposite to the force generated by the main elastic element 34 and acting on the piston 18 away from the discharge port 14 .

[0035] A first bypass passage 50 is formed in the body 11 of the valve assembly and opens into the end region 19 of the main hydraulic chamber 15, placing this chamber in fluid communication with the suction port 17. A bleed passage 51 is formed in the body 11 of the valve unit and opens into the intermediate portion 20 of the main hydraulic chamber 15, placing this chamber in fluid communication with the expansion chamber 16.

[0036] The expansion chamber 16 receives, in a longitudinally slidable manner, a floating valve element 61 having a sealing gasket 62 that mates with a cylindrical portion 63 of the expansion chamber 16. The floating valve element 61 is movable within the expansion chamber 16 between a shoulder 64 formed near the inlet port 17 and a lateral wall 67 formed away from the inlet port 17.

[0037] The outlet passage 51 flows into the expansion chamber 16 at a location longitudinally disposed near the suction port 17. The outlet passage 51 opens at a first end of the cylindrical portion 63 of the expansion chamber 16, away from the suction port 17.

[0038] A secondary spring element 66, such as a compression spring, is resiliently compressed between the floating valve element 61 and a lateral wall 67 of the valve body 32. The secondary spring element 66 biases the floating valve element 61 in direction B toward the end 12 of the body 11, i.e., toward the inlet port 17. As will be described below, introduction of pressurized brake fluid from the primary chamber 15 to the expansion chamber 16, in contrast to the force of the secondary spring element 66, causes the floating valve element 61 to move in direction A, longitudinally away from the discharge port 14 and the suction port 17, resulting in an immediate reduction in pressure in the primary chamber 15 and in the branch of the hydraulic circuit extending from the discharge port 14 to the brake caliper.

[0039] The expansion chamber 16 is in fluid communication with the inlet port 17 through a passageway 68 formed in the body 11, and a one-way valve 69 is provided between the expansion chamber 16 and the inlet port 17. The one-way valve 69 has a ball 70 and a spring 71 that urges the ball 70 away from the inlet port 17, thereby closing the passageway 68. The one-way valve 69 allows brake fluid to flow in only one direction, from the expansion chamber 16 to the inlet port 17.

[0040] In the illustrated embodiment, a lateral passage 72 is cut into the body 32 for structural reasons to facilitate construction of the bypass passage 50. The lateral passage 72 is permanently closed by a plug, represented generally at 73.

[0041] For structural reasons, the body 11 may have two or more complementary parts, in this example a main part 11A and a connecting part 11B. The main part 11A forms the main hydraulic chamber 15, the expansion chamber 16, the inlet port 17 and the discharge port 14. The connecting part 11B is tightly connected to the main part 11A by a gasket 74.

[0042] 2 shows the valve unit 10 in a normal braking condition, i.e., when the vehicle is braked but the wheels receiving brake fluid from the discharge ports 14 are not locked and therefore not slipping. Brake fluid is filled in both the main chamber 15, the first end region 19, and the second intermediate region 20 by means of the longitudinal through-cavity 31. The main elastic element 34 acts to pull the piston 18 to the left (direction A), overcoming the hydraulic thrust forces which, as a whole, produce a resultant force tending to push the piston 18 to the right (direction B).

[0043] For moderate hydraulic pressures that occur under normal braking conditions, i.e., without reaching a locking state of the braked wheel, the hydraulic pressure given by the difference between the area of ​​the circular crown with diameters D2 and D3 and the area of ​​the circle with diameter D1 will have a resultant force with a longitudinal component of lesser magnitude than the longitudinal force exerted by the main elastic element 34.

[0044] Under normal braking conditions, the elastic force of the main elastic element 34 exceeds the hydraulic thrust and, in the rest position (or retracted position), causes the piston 18 to be offset to the left, away from the discharge port 14. In the rest position, the piston 18 may abut against a lateral shoulder 75 formed by the body 11.

[0045] When the piston 18 is in the rest position (FIG. 2), it does not close the bypass passage 50, allowing direct transfer of brake fluid from the suction port 17 to the discharge port 14. The antilock brake system is not activated. In the rest position of the piston, the outflow passage 51 between the main chamber 15 and the expansion chamber 16 is instead closed between the two gaskets 28, 29 of the second part 23 of the piston 18.

[0046] In a locked wheel braking condition, the pressure of the brake fluid present in the main hydraulic chamber 15 increases until the resultant left-to-right force of all hydraulic thrust forces acting on the piston 18 reaches and exceeds a value that has a longitudinal component of greater magnitude and opposite direction to the elastic force exerted by the main elastic element 34. Thus, the brake fluid pressure in the main hydraulic chamber 15 urges the piston 18 to the right (direction B, FIG. 3), further compressing the main elastic element 34 and moving the piston 18 away from the discharge port 14.

[0047] By moving toward the discharge port 14, the piston 18 closes the bypass passage 50 between the gaskets 26 and 27 (FIG. 3), blocking the flow of brake fluid from the master cylinder to the brake caliper via the valve unit. At the same time, the piston 18 opens the outflow passage 51 between the main chamber 15 and the expansion chamber 16, partially opening the outlet for brake fluid from the main hydraulic chamber 15 to the expansion chamber 16.

[0048] The pressure of the brake fluid entering the expansion chamber 16 pushes the floating valve element 61 away from the suction port 17 (to the left, in direction A), overcoming the elastic force of the secondary spring 66. The volume of the expansion chamber 16 then increases, and as a result, the brake fluid pressure in the primary chamber 15 momentarily decreases.

[0049] The longitudinal through-bore 31 of the piston 18 simultaneously reduces the pressure in the main hydraulic chamber 15, reducing the brake fluid pressure in the branch of the hydraulic circuit extending from the discharge port 14 to the brake caliper, thereby reducing the braking force exerted by the brake caliper and unlocking the wheel.

[0050] The decompression of the main hydraulic chamber 15 causes the elastic force of the main elastic element 34 to again overcome the resultant longitudinal force of the hydraulic thrust, thereby causing the main spring element 34 to relax and again pull the piston 18 to the left, away from the discharge port 14, reopening the bypass passage 50 and closing the bleed passage 51. As a result, the master cylinder is again in fluid communication with the brake caliper.

[0051] When the bypass passage 50 is reopened, the expansion chamber 16 (FIG. 2) still contains some brake fluid because the floating valve element 61 has moved in direction A. The volume of brake fluid contained in the expansion chamber 16 must be returned to the hydraulic circuit so that the brake control (foot or hand lever) can return to its initial stop position. Releasing the brake control reduces the pressure in the hydraulic circuit, causing the secondary spring 66 to expand and move the floating valve element 61 toward the suction port 17 (direction B), essentially emptying the expansion chamber 16 (FIG. 2) and reintroducing brake fluid to the hydraulic circuit. Emptying the expansion chamber 16 is made possible by the one-way valve 69, which automatically closes under the action of a spring 71 associated with a ball 70.

[0052] In the embodiment shown in FIGS. 2-5, the minimum force required to move the piston 18 and thus trigger the antilock function is adjustable. This adjustment is achieved by applying a greater or lesser elastic preload to the primary elastic element 34 as needed. When the primary elastic element 34 has a low elastic preload, a relatively low level of brake fluid pressure in the primary chamber 15 is sufficient to overcome the elastic resistance that keeps the piston 18 away from the discharge port 14 (to the left). A low preload level can be achieved in this example by rotating the adjustment device 42 on the stem 37 in a direction that moves the lateral contrast wall 36 away from the lateral shoulder 35 (FIG. 4). When the longitudinal length of the primary elastic element 34 is large, the compressive force with which this elastic element holds the piston when stopped is low. Therefore, a low maximum hydraulic pressure is not sufficient to trigger the ABS system. Such a low pressure adjustment is preferable when the vehicle is driven on slippery, icy, or unpaved roads, whereby wheels that are more easily braked with moderate hydraulic pressure may lock.

[0053] Conversely, for driving on paved surfaces with a higher grip coefficient, a wheel slip condition occurs at a higher hydraulic pressure level. Therefore, for driving on non-skid asphalt roads, the preload of the primary elastic element 34 can be increased by rotating the stem 37 with the adjustment device 42 in a manner that brings the lateral contrast wall 36 closer to the lateral shoulder 35, thereby shortening and further pre-compressing the primary elastic element 34. With a shorter longitudinal length of the primary elastic element 34, the compressive force with which this elastic element holds the piston 18 in a stopped position increases. As a result, a higher maximum hydraulic pressure will be required to overcome the elastic force and trigger the ABS system.

[0054] In embodiments where the adjuster 42 is electrically controllable, it may be activated by an electrical signal from an on-board electronic processing unit (ECU) when the ECU receives a speed signal from a wheel sensor indicating a lock or slip condition.

[0055] The stem 37 and the lateral contrast wall 36 are longitudinally integral with the piston 18. Preferably, the adjustment device 42 is rotatably coupled to the stem 37 but is longitudinally disengaged therefrom, for example by a splined coupling 44, so as not to increase the inertial mass longitudinally integral with the piston 18.

[0056] As will be appreciated, the ABS system can be operated without electrical control and power supply, as operation of the ABS system is dependent on the brake fluid pressure in the main hydraulic chamber 15 intervening the piston 18. Some embodiments, such as those shown in Figures 2 to 5, provide the ability to set the intervention level of the ABS valve unit depending on road conditions.

[0057] In a simplified embodiment, it is envisaged that the ABS valve unit is implemented in a vehicle without an electric power supply. According to an alternative embodiment, the adjusting device 42 may comprise a manually rotatable knob 42 (FIG. 6) that allows the user to select the degree of pre-compression of the main elastic element 34 and therefore the elastic force acting on the piston 18. An embodiment may provide the possibility to rotate the knob 42 among a number of predetermined angular positions, each corresponding to a level of pre-compression of the main elastic element.

[0058] Instead of a threaded coupling 39 between the stem 37 and the lateral contrast wall 36, an adjustment by means of a member having a cam surface may be used.

[0059] According to another embodiment (FIG. 7) that does not require a power supply, the main elastic element 34 that stresses the piston 18 away from the ejection port 14 may be formed as a traction elastic element having a first end that is restrained by the piston 18 and a second end that is restrained by a lateral contrast wall 36. The lateral contrast wall may be fixed relative to the body 11 as required, or may be adjustable in longitudinal position to adjust the pre-tension of the main elastic element 34, and therefore the traction force acting on the piston to move away from the ejection port.

[0060] According to another embodiment (not shown), an adjustment device 42 may be associated with the main elastic element 34 implemented as a traction spring to adjust its pretension.

[0061] While specific embodiments of the present invention have been described, it should be understood that the disclosure is provided purely for purposes of illustration and not limitation, and that the present invention should not be limited thereby. Various modifications will become apparent to those skilled in the art in light of the above examples. The scope of the present invention is limited only by the appended claims.

Claims

1. 1. A valve unit for an antilock braking system of a vehicle, comprising: The valve unit includes: a valve body (11) having a discharge port (14) hydraulically connectable to a brake caliper (G), a suction port (17) hydraulically connectable to a master cylinder (M), a main chamber (15) in fluid communication with the discharge port (14), an expansion chamber (16) having an outlet passage (51) in fluid communication between the main chamber (15) and the expansion chamber (16), and a bypass passage (50) in fluid communication between the suction port (17) and the discharge port (14); a piston (18) movable longitudinally within the main chamber (15), the piston having a longitudinal cavity (31) extending through the piston between an end face (32) of the piston facing the discharge port (14) and a transverse passage (33) opening into a side face of the piston, the piston (18) generally having a first transverse surface facing the discharge port (14) and a second transverse surface opposite the first transverse surface and facing away from the discharge port (14), the first transverse surface having an area smaller than an area of ​​the second transverse surface; at least one elastic element (34) exerting an elastic force to move the piston (18) away from the discharge port (14); and The piston (18) a first position under normal braking conditions, in which the force of the elastic element overcomes the longitudinal component of the hydraulic pressure of the brake fluid present in the main chamber (15), causing the piston (18) to move away from the discharge port (14) and close the outflow passage (51) without closing the bypass passage (50); and a second position in an operating state of the valve unit, in which the hydraulic thrust of the brake fluid present in the main chamber (15) has a longitudinal component that overcomes the force exerted by the elastic element (34), causing the piston (18) to displace towards the discharge port (14) and close the bypass passage (50) but not close the outflow passage (51). Valve unit.

2. 2. A valve unit according to claim 1, The main chamber (15) a first region (19) disposed near the discharge port (14) and having a first diameter (D1); a third region (21) located away from the discharge port (14) and having a third diameter (D3) smaller than the first diameter (D1); a second region (20) located intermediate the first region (19) and the third region (21) and having a second diameter (D2) greater than the first diameter (D1); Forming The piston (18) a first part (22) that slides sealingly in a first region (19) of the main chamber (15); a sealingly sliding intermediate portion (23) in the second intermediate region (20) of the main chamber; a third part (24) that slides sealingly in a third region (21) of the main chamber; and the first lateral surface has an area of ​​a circle having the first diameter (D1); and the second lateral surface has an annular crown area having an outer periphery with a diameter corresponding to the second diameter (D2) and an inner periphery with a diameter corresponding to the third diameter (D3); Valve unit.

3. The valve unit according to claim 1 or claim 2, further comprising: an adjusting device (42) for adjusting the elastic force exerted by the elastic element (34) on the piston (18); Valve unit.

4. A valve unit according to claim 1, claim 2, or claim 3, The valve unit includes: a lateral shoulder (35) facing away from the discharge port (14) and integral with the valve body (11); a transverse contrast wall (36) facing said transverse shoulder (35) and spaced longitudinally therefrom; a longitudinal stem (37) connecting the transverse contrast wall (36) to the piston (18) and longitudinally integrating the piston with the transverse contrast wall (36); and the elastic element (34) is a compression spring compressed longitudinally between the lateral contrast wall (36) and the lateral shoulder (35); Valve unit.

5. A valve unit according to claim 4, the valve unit comprises an adjusting device (42) for adjusting the elastic force exerted by the elastic element (34) on the piston (18), the adjusting device being configured to adjust the longitudinal distance between the lateral contrast wall (36) and the lateral shoulder (35). Valve unit.

6. A valve unit according to claim 5, The stem (37) is a threaded stem portion that engages a corresponding threaded through-hole (39) formed through said lateral contrast wall (36), whereby rotation imparted to said stem about its longitudinal axis (37A) results in a change in the longitudinal distance between said lateral contrast wall (36) and said piston (18); Valve unit.

7. A valve unit according to claim 6, The stem (37) is an enlarged end head (40) of circular cross section that engages in a recess (41) formed in the piston (18); Valve unit.

8. A valve unit according to claim 6 or claim 7, The stem (37) is having a central longitudinal axis (37A) that is eccentric with respect to the central longitudinal axis (18A) of the piston (18); Valve unit.

9. A valve unit according to any one of claims 6 to 8, The stem (37) has a central longitudinal axis (37A), and the transverse contrast wall (36) has a transverse distance (P1) from the central longitudinal axis that is greater than a minimum transverse distance (P2) between the central longitudinal axis (37A) and a surface (38) of the body (11) transversely opposite the stem (37), and has a periphery that acts as a stop to prevent rotation of the transverse contrast wall (36) when the stem (37) is rotated. Valve unit.

10. A valve unit according to claim 6, The adjusting device (42) rotatably coupled to the stem (37) by a splined longitudinal joint (44) that allows relative longitudinal movement between the adjustment device (42) and the stem (37); Valve unit.

11. A valve unit according to any one of claims 4 to 10, The lateral contrast wall (36) a valve body (11) that is received with lateral play and is longitudinally movable within a chamber (38) formed by the valve body (11); Valve unit.

12. 2. A valve unit according to claim 1, The elastic element (34) a resilient traction element having a first end restrained by the piston (18) and a second end restrained by a lateral contrast wall (36) integral with said valve body (11); Valve unit.

13. A valve unit according to claim 12, The lateral contrast wall (36) It is fixed relative to the valve body (11) or its longitudinal position can be adjusted to adjust the pre-tension of the elastic element (34) and therefore the traction force acting on the piston away from the discharge port (14). Valve unit.

14. A valve unit according to claim 3, The adjusting device (42) an electric drive unit electrically connectable to an electronic control unit (ECU) disposed on a board of the vehicle; Valve unit.

15. A valve unit according to claim 3, The adjusting device (42) Has a manually adjustable mechanism, Valve unit.

16. 16. A valve unit according to any one of claims 1 to 15, The expansion chamber (16) a vertically slidable floating valve element (61); and a secondary spring element (66) that pushes the floating valve element (61) toward the suction port (17); Accommodates The expansion chamber (16) is in fluid communication with the suction port (17) via a passage (68), and one-way valves (69, 70, 71) are positioned to allow brake fluid to flow from the expansion chamber (16) towards the suction port (17) in only one direction (B). Valve unit.

17. A valve unit according to claim 16, The floating valve element (61) a shoulder (64) of the valve body (11) disposed in the expansion chamber closest to the suction port (17) and a lateral wall (67) disposed in the expansion chamber farthest from the suction port; The outflow passage (51) open into the expansion chamber (16) at a location disposed longitudinally adjacent to the shoulder (64); Valve unit.

18. A valve unit according to claim 4, The elastic element (34) at least two compression springs between said lateral contrast wall (36) and said lateral shoulder (35); The first spring is always longitudinally pre-compressed between said lateral contrast wall (36) and said lateral shoulder (35); and The second spring is has a shorter free length than the first spring and is not longitudinally pre-compressed between the lateral contrast wall (36) and the lateral shoulder (35); Valve unit.

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