Hydraulic brake system

The invention relates to a hydraulic brake system for a vehicle, comprising a cylinder (3) and a piston (4) movably arranged inside the cylinder (3), wherein the piston (4) has a closed end, an open end (6) and a piston wall surrounding a cavity (8) of the piston (4) between the closed end and the open end (5), the piston (4) being movable along a longitudinal axis (5) of the cylinder (3) by hydraulic pressure within the cavity (8) for pressing at least one brake pad (9) against a brake disc (2). An outer diameter of the piston (4) is smaller in a central section (13) between the open end (6) and the closed end than at the open end (6) and at the closed end so that, in the central section (13), a space (14) remains between the piston wall and an inner wall of the cylinder (3).

The invention relates to the field of vehicular brake systems. Specifically, it relates to a hydraulic brake system which comprises a cylinder and a piston movably arranged inside the cylinder, wherein the piston has a closed end, an open end and a piston wall surrounding a cavity of the piston between the closed end and the open end, the piston being movable along a longitudinal axis of the cylinder by hydraulic pressure within the cavity for pressing at least one brake pad against a brake disc.

As reducing weight of vehicles and their sub-systems has been an issue for a long time already, in particular in view of fuel consumption, different measures have been taken in order to make brake systems of this type lighter. Prior art document WO 2007/048532 A1, for example, discloses such a brake system where a lightweight filling body is provided within the cavity of the piston in order to reduce the amount of hydraulic fluid needed in the system, thereby reducing a total weight of the brake system. Manufacturing this brake system is, however, expensive as a comparatively large number of parts need to be produced and assembled.

It is, therefore, an object of the present invention to suggest a low weight brake system which may be manufactured at comparatively low cost.

According to the invention, this object is achieved by a hydraulic brake system for a vehicle according to the independent claim. Different embodiments of a brake system of this kind may additionally show the features of the dependent claims.

The suggested brake system comprises a cylinder and a piston movably arranged inside the cylinder, wherein the piston has a closed end, an open end and a piston wall surrounding a cavity of the piston between the closed end and the open end, the piston being movable along a longitudinal axis of the cylinder by hydraulic pressure within the cavity for pressing at least one brake pad against a brake disc. According to the invention, an outer diameter of the piston is smaller in a central section between the open end and the closed end than at the open end and at the closed end so that, in the central section, a space remains between the piston wall and an inner wall of the cylinder. Thus, the piston has, so to speak, a waisted shape. The cylinder is supposed to be a wheel brake cylinder and may be an integral part of a brake caliper. In addition, the brake system may comprise the brake disc and the at least one brake pad.

Due to the reduced diameter of the piston in its central section, a volume of hydraulic fluid needed for operating the brake system is reduced without any additional parts and without increasing or at least without significantly increasing a weight of any other component of the brake system. This helps minimizing a total weight of the brake system when in use and, thus, a weight of a vehicle equipped with the brake system. At the same time, the measures suggested with the invention do not or at least not significantly increase manufacturing costs of the brake system.

Typically, the cavity of the piston is narrower in the central section of the piston than in at least one of a section between the central section and the open end and a section between the central section and the closed end. Thus, a shape of the cavity may more or less correspond to a shape of an outer surface of the piston so that the piston wall can be kept thin, which, together with a reduced volume of the cavity, may help minimizing the weight of the brake system.

To be ready for use, the hydraulic brake system may further comprise the hydraulic fluid for exerting the hydraulic pressure within the cavity of the piston, the space between the inner wall of the cylinder and the piston wall in the central section of the piston being free from the hydraulic fluid. In order to keep the said space free from the hydraulic fluid, a seal ring surrounding the piston and being in sealing contact with the piston may close a gap between the inner wall of the cylinder and the piston wall between the central section and the open end. An additional seal ring surrounding the piston may be provided between the central section and the closed end to prevent dust from entering the space between the inner wall of the cylinder and the piston wall. Typically, the seal or at least one of the seals is arranged in a groove which may be provided in the inner wall of the cylinder.

In typical embodiments, a smallest value of the outer diameter of the piston in the central section is smaller than 0.8 times or even smaller than 0.7 times the outer diameter of the piston at the open end and at the closed end. Similarly, a smallest value of an inner diameter of the cavity in the central section can be smaller than 0.8 times or even smaller than 0.7 times a largest value of the inner diameter of the cavity at the open end and/or at the closed end of the piston.

The brake system may further comprise a parking brake mechanism, wherein the parking brake mechanism comprises a spindle and a nut, the spindle being arranged along the longitudinal axis and protruding through the open end of the piston into the cavity and wherein the nut is engaged with the spindle to be movable along the longitudinal axis by a rotation of the spindle for applying or releasing a pressure on the piston and thereby activating or deactivating an integrated parking brake. A parking brake mechanism of this type may be the reason that the piston in the wheel brake cylinder is designed as having an open end and a closed end and a cavity to be at least partly filled with hydraulic fluid. Therefore, the suggested measure of reducing the diameter of the piston in its central section is particularly useful in view of weight reduction in a brake system comprising a parking brake mechanism of this type.

In a plane cutting the nut perpendicular to the longitudinal axis, the cavity may have a non circular cross section, e.g. a hexagonal cross section, which may correspond to a shape of the nut. This helps preventing the nut from turning when the spindle is rotated in order to activate or deactivate the parking brake.

Typically, the nut fits through the central section of the piston to push against an end wall of the piston at its closed end for activating the parking brake. In other embodiments, the nut does not fit through the central section of the piston and is arranged for pressing against a shoulder of the piston between the central section and the open end for activating the parking brake. This allows reducing the inner diameter of the piston in the central section even further.

The piston may be formed in one piece. Alternatively, the piston may be formed of at least two joint parts, one of them being the end wall at the closed end of the piston. The piston or at least one part of it may be a cast part and/or may me made, e.g., of aluminium or aluminium alloy or of another metal or plastic.

The same or corresponding features are marked by the same reference signs in all Figures. Repeatedly shown identical or corresponding features are not explained again if already explained with respect to another Figure and if the same applies to the different views or embodiments.

The vehicle shown inFIG. 1has four wheels and a wheel brake for each of the wheels.FIG. 1shows a brake caliper1and a brake disc2of each of these wheel brakes, which are of the disc brake type and which are interchangeable. They may be activated not only by hydraulic pressure of a hydraulic fluid but also electrically so that an ordinary hydraulic service brake and an electric parking brake are combined in a brake system of the vehicle.

FIG. 2shows a detail of the brake system, in particular a cylinder3, which is a wheel brake cylinder and which is formed by and, thus, an integral part of the brake caliper1of one of the wheel brakes, and a piston4, which is arranged inside the cylinder3and which is movable along a longitudinal axis5of the cylinder3. As shown inFIG. 2, the piston4has an open end6and a closed end, where an end wall7of the piston delimits a cavity8of the piston4. The piston4can be moved along the longitudinal axis5, in particular, by hydraulic pressure resulting from brake fluid being pressed into the cylinder3and through the open end5into the cavity8of the piston4for pressing at least one brake pad9against the respective brake disc2. The hydraulic pressure of the brake fluid, which is the hydraulic fluid mentioned before, may be generated by an ordinary master cylinder (not shown) and/or by a motor driven pressure supplier (not shown) depending on how far a brake pedal (not shown) of the service brake is pressed down.

The brake system further comprises a parking brake mechanism protruding through the open end6into the cavity8of the piston4. The parking brake mechanism comprises a spindle10, which is arranged along the longitudinal axis5, and a nut11engaged with the spindle10. The spindle10can be driven by an electric motor12so that the nut11is moved along the longitudinal axis5by a rotation of the spindle10for applying or releasing a pressure on the end wall7of the piston4and thereby activating or deactivating the parking brake.

As can be seen inFIG. 2, the piston4has a waisted shape by having a smaller outer diameter in a central section13between the open6end and the closed end than at the open end6and at the closed end. Thus, in the central section13, a ring-shaped space14of significant volume remains between a wall of the piston4and an inner wall of the cylinder3. This space14remains free from the brake fluid filling the cavity8.

Two grooves facing the piston4are provided in the inner wall of the cylinder3, and one of two seal rings15, made of an elastomer, is arranged in each of the grooves so that the seal rings15surround the piston4being in sealing contact with the piston4. One of the seal rings15is arranged between the central section13and the open end6in order to prevent the brake fluid from entering the space14, while the other seal ring15is arranged between the central section13and the closed end of the piston4in order to keep the space14free from dust and dirt.

The cavity8is narrower in the central section13of the piston4than in a section between the central section13and the open end6and in a section between the central section13and the end wall7. To be more precise, due to the particular shape of the piston4, a smallest value of the outer diameter of the piston4in the central section13is smaller than 0.7 times the outer diameter of the piston4at the open end6and at the other end, where the piston is closed by the end wall7, while an inner diameter of the cavity8has a smallest value in the central section13which is smaller than 0.7 times a largest value of the inner diameter of the cavity8, which can be found at the open end6as well as in the section between the central section13and the closed end of the piston4.

FIG. 3shows cross section of same brake system, cut in the central section13in a plane which is perpendicular to the longitudinal axis5and which is designated as A-A inFIG. 2. As can be seen here, the cavity8has, in this plane, a hexagonal cross section, which corresponds to a cross-sectional shape of the nut11. This prevents the nut11from rotating when the spindle10is rotated in order to activate or deactivate the parking brake. Furthermore, the nut11obviously fits through the central section13of the piston4.

FIG. 4shows a similar brake system. In this case, the nut11does not fit through the central section13of the piston4. Instead, the nut11, heaving a larger diameter, is arranged for pressing against a shoulder16of the piston4, the shoulder16connecting the central section13and the aforementioned section between the central section13and the open end6of the piston4. When the electric parking brake is activated in this embodiment, the nut11pushes against the shoulder16of the piston4instead of pushing against the end wall7. A smallest diameter of the cavity8is even smaller in this case.

In the embodiments shown inFIGS. 2 to 4, the piston4is formed in one piece as a cast part which may be made of aluminium or an aluminium alloy or another metal or plastic.

FIG. 5shows a further embodiment, which is similar to and works in the same manner as the embodiment shown inFIGS. 2 and 3. In this case, the piston4is formed of two joint parts, one of them being the end wall7, the other part forming a wall surrounding the cavity8including the central section13of the piston4, the shoulder16and the section between the central section13or the shoulder16and the open end6. One or each of the two parts may be a cast part and may be made of aluminium or an aluminium alloy or another metal or plastic. The two parts may be glued, welded or soldered. A stiffener17reinforces a connection between the end wall7and the other part of the piston4.