TECHNICAL FIELD

The invention relates to a brake disc for arrangement in a vehicle to allow reduction of a speed of the vehicle by pressing first and second brake pads against the brake disc resulting in frictional interaction between first and second brake pad surfaces and corresponding first and second annular braking surfaces. The invention further relates to a brake pad for use with the brake disc according to the invention, a braking system and a vehicle. The invention can be applied in heavy-duty vehicles, such as trucks, buses and construction equipment. Although the invention will be described with respect to a truck, the invention is not restricted to this particular vehicle, but may also be used in other vehicles such as buses and construction equipment. BACKGROUND

Disc brakes are commonly used in various vehicles to provide for efficient and reliable braking.

In an effort to improve the braking performance of a disc brake system, US 3 750 788 discloses a brake disc with a serrated cross-sectional shape forming teeth having a relatively low height, to increase friction for a given braking force used for pressing brake pads against the brake disc.

There appears to be room for improvement of the brake disc and braking system according to US 3 750 788, in particular concerning the management of the heat generated during braking.

SUMMARY

An object of the invention is to provide for improved braking, in particular concerning management of the heat generated during braking.

According to a first aspect of the invention, this object is achieved by the brake disc according to claim 1 . The present invention is based on the realization that a disc brake system can be improved by providing for more efficient heat dissipation from the brake disc, and that this can be achieved by increasing the heat dissipating area where the temperature during braking is particularly high, i.e. at the braking surfaces of the brake disc. The present inventor has further realized that an advantageous way of increasing this heat dissipating area is to macroscopically vary the thickness of the brake disc across a braking region of the brake disc. Embodiments of the present invention thus provide for improved heat dissipation where the temperature is the highest. In addition, the increased contact area between brake disc and brake pad provides for a lower maximum temperature for a given braking power.

The protrusion distance - the axial distance from the base plane to the braking surface - may advantageously vary continuously with radial distance from the rotational axis, which reduces the risk of excessive wear on the brake disc and/or brake pads, and reduces the requirement on the mounting tolerance of the brake pads in relation to the surface structure of the brake disc. According to embodiments, the protrusion distance may vary between a minimum protrusion distance and a maximum protrusion distance, a difference between the minimum protrusion distance and the maximum protrusion distance being at least 5 percent of a width of the braking surface. According to embodiments, the difference between the minimum protrusion distance and the maximum protrusion distance may be at least 6 mm. In embodiments where there is enough space to accommodate a somewhat thicker brake disc, the above-mentioned difference may be considerably greater, such as at least 20 mm. The larger this difference can be made, the more the size of the interfacing surfaces between brake pad and brake disc is increased. This in turn provides for a more efficient heat dissipation.

According to various embodiments, the central portion average protrusion distance may be greater than the distal portion average protrusion distance. Such a configuration may facilitate exchange of brake pads. In these embodiments, the central portion average protrusion distance may be greater than each of the proximal portion average protrusion distance and the distal portion average protrusion distance, such that the surface profile cross-section has a generally convex shape.

According to other embodiments, the central portion average protrusion distance may be less than each of the proximal portion average protrusion distance and the distal portion average protrusion distance, such that the surface profile cross-section has a generally concave shape.

According to various embodiments, the protrusion distance as a function of radial distance from the rotational axis may exhibit a plurality of local extrema. For instance, the surface profile cross-section may exhibit a generally sinusoidal variation with radial distance from the rotational axis of the brake disc. Hereby, the surface area of the braking surfaces of the brake disc can be increased even further, providing for even more efficient heat dissipation from the brake disc.

According to various embodiments, a central portion distance along the surface profile cross-section may be longer than each of a proximal portion distance along the surface profile cross-section and a distal portion distance along the surface profile cross-section. For instance, the above-mentioned central portion distance may be at least five percent longer than the proximal and distal portion distances. The difference between the central portion distance and the proximal and distal portion distances can be achieved in various ways. For example, the average amplitude of protrusion distance variations may be greater in the central portion than in the proximal and distal portions and/or the wavelength of protrusion distance variations may be shorter in the central portion than in the proximal and distal portions.

Studies have shown that the temperature resulting from braking is not the same across the contact area between the brake disc and the brake pad, but that the temperature may be higher in the central portion than in the proximal portion and the distal portion, in particular during severe operating conditions. The expansion-contraction cycling resulting from this temperature gradient has been found to increase the risk of brake disc cracks and also lead to an increased risk of noise and vibrations from the braking system. By increasing the surface area more in the central portion than in the proximal and distal regions in accordance with these embodiments of the present invention, the stress on the brake disc can be reduced, whereby the risk of formation of brake disc cracks can consequently be reduced. This in turn provides for an increased utilization of a vehicle equipped with a braking system including the brake disc.

According to a second aspect of the invention, the above-mentioned object is achieved by the brake pad according to claim 14. According to embodiments, the brake pad may comprise a friction member and a structured coating on the friction member. The structured coating may be applied to the friction member to achieve a surface profile cross-section of the brake pad surface that may be a scaled inverse of the surface profile cross-section of the brake disc surface of the brake disc according to embodiments of the present invention. In particular, the surface profile cross-section of the brake pad surface may be scaled to exhibit smaller amplitudes than the surface profile cross-section of the brake disc. After an initial wear-in period, the surface profile cross-sections of the brake pad and the brake disc may fit perfectly with each other. According to one example, the structured coating may be provided in the form of a so- called "green coat". It should, however, be noted that any other suitable material or substance may be used. It should also be noted that the surface profile of the friction member may be different from the surface profile provided by the structured coating. According to embodiments of the present invention, there is provided a braking system for a vehicle, comprising a brake disc according to embodiments of the first aspect of the present invention; first and second brake pads according to embodiments of the second aspect of the present invention; and an actuator for reducing the distance between the first and second brake pads to press the brake disc between the first and second brake pads.

Each of the first and second brake pads may be made of a brake pad material being less wear resistant than a brake disc material of the brake disc. Further, each of the first and second braking surfaces may lack a coating being harder than the brake disc material. Moreover, the braking system according to embodiments of the present invention may advantageously be comprised in a vehicle for allowing controlled reduction of the speed of the vehicle. In summary, the present invention thus relates to a brake disc for arrangement in a vehicle to rotate around a rotational axis. First and second annular braking surfaces extend radially between an inner braking surface radius and an outer braking surface radius. Each of the first and second braking surfaces has an annular proximal portion, an annular distal portion, and an annular central portion. At least one of the first and second braking surfaces is structured to protrude from a base plane perpendicular to the rotational axis to exhibit a surface profile cross-section with a plane including the rotational axis. A protrusion distance from the base plane to the surface profile cross- section varies with radial distance from the rotational axis in such a way that a central portion average protrusion distance in the central portion is different from at least one of a proximal average protrusion distance in the proximal portion and a distal average protrusion distance in the distal portion.

Further advantages and advantageous features of the invention are disclosed in the following description and in the dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

With reference to the appended drawings, below follows a more detailed description of embodiments of the invention cited as examples. In the drawings:

Fig. 1 a is a partly cut-out perspective view of a braking system according to a first embodiment of the present invention; Fig. 1 b is a cross-section view of the braking system in fig 1 a;

Fig. 1 c is a partly cut-out perspective view of the braking system according to a second embodiment of the present invention; Figs 2a-b schematically illustrate an example temperature distribution during braking and a possible resulting failure mode for a conventional brake disc;

Fig 3 is a partial cross-section view of a braking system according to a third embodiment of the present invention;

Fig 4a is a partial cross-section view of a braking system according to a fourth embodiment of the present invention; Fig 4b is a partial cross-section view of a braking system according to a fifth embodiment of the present invention; and

Fig 4c is a diagram schematically showing the distances along the proximal, central and distal portions of the braking surface cross-section for the brake disc configurations in fig 4a and fig 4b.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS OF THE INVENTION

Fig 1 a is a partly cut-out perspective view of a braking system 1 according to a first embodiment of the present invention, comprising a brake disc 10, first 20a and second 20b brake pads, and a braking actuator arrangement 21 . In the braking system 1 according to the present first embodiment, the brake disc 10 is attachable to a rotatable member of a vehicle by means of mounting holes 12 (only one of the mounting holes is indicated by a reference numeral for ease of drawing) so that the brake disc 10 rotates around a rotational axis 1 1 of the brake disc 10 when the vehicle is moving.

To reduce the speed of the vehicle, the braking actuator arrangement 21 can be controlled to move the first 20a and second 20b brake pads towards each other to press the brake disc 10 between the first 20a and second 20b brake pads. This results in frictional interaction between the first 25a and second 25b brake pad surfaces and corresponding first 15a and second 15b annular braking surfaces on opposite sides of the brake disc 10.

As is shown in fig 1 a, each of the first 15a and second 15b annular braking surfaces extends radially between an inner braking surface radius R, and an outer braking surface radius R ₀ in relation to the rotational axis 1 1 . As is also schematically indicated for the first braking surface 15a in fig 1 a, each of the braking surfaces 15a-b has an annular proximal portion 17, an annular distal portion 18, and an annular central portion 19 between the proximal portion 17 and the distal portion 18. The proximal portion 17, the distal portion 18, and the central portion 19 have the same widths r _p as indicated in fig 1 a.

5

In the braking system of fig 1 a, the surface profile cross-sections 14a-b, with a plane 24 including the rotational axis 1 1 , of the braking surfaces 15a-b are generally convex and the corresponding braking surfaces 25a-b of the brake pads 20a-b conform to the shapes of the braking surfaces 15a-b. This provides for an increased heat dissipating area of the 10 brake disc 10 where the temperature is the highest during braking, and further helps to reduce the maximum temperature due to the increased contact area between the braking surfaces 15a-b of the brake disc 10 and the corresponding braking surface 25a-b of the respective brake pads 20a-b.

15 The configuration of the braking surfaces 15a-b of the brake disc 10 will now be described in greater detail with reference to the cross-section view in fig 1 b. The cross-section view is in the plane 24 (see fig 1 a) including the rotational axis 1 1 . Referring to fig 1 b, each of the first 15a and the second 15b braking surfaces of the brake disc 10 is structured to protrude from a respective base plane 16a-b (perpendicular to the rotational axis 1 1 ) to

20 exhibit a surface profile cross-section 14a-b with the plane 24 (see fig 1 a) including the rotational axis 1 1 . As can be seen in fig 1 b, each of the surface profile cross-sections 14a-b of the first 15a and second 15b braking surfaces exhibits a respective protrusion distance h _a (r) and h _b (r) along a normal to the respective base planes 16a-b. Each of the protrusion distances h _a (r), h _b (r) varies with radial distance r from the rotational axis 1 1 in

25 such a way that the central portion average protrusion distance in the central portion 19 is greater than the proximal average protrusion distance in the proximal portion 17 and the distal average protrusion distance in the distal portion 18.

As is also indicated in fig 1 b, each of the brake pads 20a-b comprises a friction member 30 22a-b and a backplate 23a-b for attachment of the brake pads 20a-b to the braking actuator arrangement 21 . Each of the backplates 23a-b has a generally planar mounting side 26a-b for attachment to the braking actuator arrangement 21 , and a friction member side 27a-b to which the friction member 22a-b is fixed. To provide for an increased service life of the brake pads 20a-b, the friction member side 27a-b of the backplate 22a-b is 35 curved to substantially follow the braking surface profile 25a-b of the brake pad 20a-b. To illustrate that embodiments of the present invention are equally useful regardless of the overall configuration of the brake disc, fig 1 c schematically shows a second embodiment of the braking system 1 according to the present invention, which differs from 5 the braking system in figs 1 a-b in that the brake disc 10 is attachable to a rotating member of a vehicle by splines 29 rather than by the mounting holes 12 shown in figs 1 a-b.

To further illustrate advantages of the braking system according to the present invention, the concept of "thermal localization" and an important related failure mode will be briefly 10 explained below with reference to figs 2a-b.

Fig 2a shows the result of a simulation of the temperature on a brake disc surface of a prior art brake disc 31 resulting from the application of the brake ("Simulation of Thermal Stresses in a Brake Disc" by Asim Rashid, Licentiate Thesis No. 1603, LIU-TEK-LIC- 15 2013:37). As is schematically shown in fig 2a, application of the brake results in a hot band 32 in the central portion of the braking surface where the temperature is as high as in excess of 600°C. In the proximal and distal portions, the temperature is well below 400°C.

20 The repeated engagement and disengagement of the braking system may eventually result in the formation of radial cracks 34 in the brake disc 31 as is schematically shown in fig 2b. By reducing the maximum temperature in the hot band 32 through embodiments of the present invention, the formation or radial cracks may be prevented, or at least considerably delayed, resulting in an improved utilization of the vehicle comprising the

25 braking system.

So far, a brake disc 10 having a relatively simple macroscopically convex surface profile cross-section 14a-b has been described with reference to figs 1 a-c. Several other surface profile cross-sections are possible and may be advantageous as can readily be realized 30 by one of ordinary skill in the relevant art, based on the present disclosure. However, some examples of advantageous braking surface profile cross-sections 14a-b, as well as brake pad configurations will be described below with reference to fig 3 and figs 4a-c.

Referring first to fig 3, schematically showing a partial cross-section of a third embodiment 35 of the braking system 1 according to the invention, the heat-dissipating surface area of the braking surfaces 15a-b of the brake disc 10 has been further increased by superimposing a smaller scale, here sinusoidal, variation on the large scale generally convex variation in protrusion distance h _a (r), h _b (r) from the respective base planes 16a, 16b. In this third embodiment, the small scale variation of the braking surface cross-section 14a-b of the brake pads 20a-b has been achieved by providing a structured coating 36a-b on the generally concave friction members 22a-b.

To even more efficiently address the hot band issue explained above with reference to figs 2a-b, the increase in braking surface area may be made the greatest in the central portion 19 of the braking surfaces 15a-b, where the temperature is the highest. This will contribute to reducing the radial temperature differential across the braking surfaces 15a-b, and thereby reduce the problem of radial cracks 34 discussed further above with reference to figs 2a-b. Fig 4a schematically shows a partial cross-section of a fourth embodiment of the braking system according to the invention. As can be seen in fig 4a, a smaller scale protrusion distance variation with substantially constant wavelength and varying amplitude has been superimposed on the large scale generally convex variation in protrusion distance h _a (r), h _b (r) from the respective base planes 16a, 16b. The amplitude is greater in the central portion 19 than in the proximal portion 17 and the distal portion 18 of the braking surface 15a-b.

Alternatively, as in the fifth embodiment schematically shown in fig 4b, a smaller scale protrusion distance variation with substantially constant amplitude and varying wavelength may be superimposed on the large scale generally convex variation in protrusion distance h _a (r) _> h _b (r) from the respective base planes 16a, 16b. The wavelength is here shorter in the central portion 19 than in the proximal portion 17 and the distal portion 18 of the braking surface 15a-b. In the fourth and fifth embodiments described above, the central portion distance d _cen trai along the surface profile cross-section 14a-b in the central portion 19 is longer than the proximal portion distance d _proX imai along the surface profile cross-section in the proximal portion 17 and the distal portion distance d _dista i along the surface profile cross-section in the distal portion 18. This relation is schematically illustrated in the diagram of fig 4c. It is to be understood that the present invention is not limited to the embodiments described above and illustrated in the drawings; rather, the skilled person will recognize that many, combinations, changes and modifications may be made within the scope of the appended claims.