TECHNICAL FIELD The present invention relates to a brake member in accordance with the preamble of claim 1. Particularly, the invention relates to a brake member where the occurrence of brake squeal can be reduced by means of the design of a brake lining, particularly by design of the side surfaces of the brake lining.

TECHNICAL BACKGROUND When engaging the brakes of a vehicle, sometimes so called brake squeal is created, something which occurs both where disc and drum brakes are concerned. Typical frequencies of this noise are around a few kHz and within the sensitivity range of the human hearing. The produced noise certainly decreases with the distance from the sound source, but still can be approx. 140 dB close to the braking device. Town buses, which stop at frequent intervals within densely populated areas, are particularly disturbing. One frequent reason for complaint on new private cars, lorries and buses is brake squeal. Even if brake squeal does not influence the braking power, and thereby safety, the problem is still of great importance and needs to be solved. The problem also occurs in vehicles with so-called ABS-brakes.

The problem with brake squeal has been analysed many times, and many different solutions have been tested. For example, it has been tried to dimension the components included in brake systems in order to obtain a changed natural frequency of the system, but still without being able to reduce the occurrence of brake squeal in braking systems.

For example, US 5145037 discloses a disc brake where the occurrence of brake squeal is intended to be reduced by means of bevelling end regions of the brake disc within a region, the extension of which is dependent on the width between claws included in a brake yoke carrying a backing plate where the brake lining is arranged.

Furthermore, from GB 2143919 a disc brake is known where brake squeal is intended to be reduced by means of designing a backing plate included in the disc brake with an inclination in relation to the brake disc included in the disc brake.

Even if the above-mentioned suggestions, in certain predetermined conditions, are instrumental in reducing the occurrence of brake squeal, these solutions do not reduce the occurrence of brake squeal when applying a fluctuating load onto the brake member.

SUMMARY OF THE INVENTION The object of the invention is to provide a brake member where the occurrence of brake squeal when applying a fluctuating load onto the brake member is reduced. This object is achieved by means of a brake member in accordance with the characterising portion of claim 1.

A further object of the invention is to provide a disc brake where the occurrence of brake squeal when applying a fluctuating load onto the brake member is reduced. This object is achieved by means of a brake member in accordance with the characterising portion of claim 16.

Still another object of the invention is to provide a drum brake where the occurrence of brake squeal when applying a fluctuating load onto the brake member is reduced. This object is achieved by means of a brake member in accordance with the characterising portion of claim 17.

Still another object of the invention is to provide a method for manufacturing brake members where the occurrence of brake squeal when applying a fluctuating load onto the brake member is reduced. This object is achieved by means of a brake member in accordance with the characterising portion of claim 18.

Theoretical background of the invention From e. g."An assumed Modes Method Approach to Disc Brake Squeal Analysis", Society of Automotive Engineers, 1999-01-1335 by Husten and Flint, it is evident that, as a result of the rotational symmetry of a brake member such as a brake disc, there are two modes at each natural frequency of the brake member. When a pair of modes exists for a natural frequency, a wave can propagate through the brake member if excitation energy is supplied, wherein noise, so-called brake squeal, may arise. When a brake lining connects brake disc, alternatively brake drum, and brake lining, the connected modes are divided into separate natural frequencies of the system. Non-conservative forces, such as e. g. frictional forces, tend to connect these separate modes and join them into a common natural frequency, wherein brake squeal may arise. Accordingly, in order to prevent the creation of brake squeal, the system has to be designed such that a separation of a set of modes, which primarily exhibit natural frequencies between 1 and 15 kHz, can be maintained. The connection, and thereby the occurrence of brake squeal, can occur for a number of different natural frequencies and is dependent on the interaction between brake lining and brake member, i. e. either disc or drum. In order to enable the design of a brake member where the risk of occurrence of brake squeal is reduced for most natural frequencies, preferably all natural frequencies within the above-mentioned interval, it is of importance that the interaction between brake lining and disc or drum takes place in a predictable way.

Furthermore, in connection with the invention, studies concerning the contact of the brake lining with the brake disc or brake drum have been performed. Fig 1 shows a symbolic representation of a known brake member 1 which interacts with a brake disc 2. The brake member comprises a brake lining 3 and a backing plate 4. The brake lining exhibits a first and a second contact surface 5a, 5b, which are intended to pressed

against said brake disc 2 or, whenever applicable, against a brake drum. Furthermore, the brake lining exhibits a groove 6 which delimits the contact surface 5a from the contact surface 5b.

During the working life of the brake lining 3, it will be worn down so that the brake lining forms a flat surface when a certain force is applied onto the brake lining via e. g. braking cylinders (not shown). This initial condition is shown in Fig. 1 with a continuous line. Fig. 2 illustrates the pressure distribution across the contact surfaces in a schematic way, wherein the pressure distribution in the run-in condition where the contact surface or surfaces of the brake lining are flat is shown with a continuous line.

The left part of Fig. 1 shows deformation of the brake lining 3, with dashed lines, when it is pressed against the brake disc 2 or, whenever applicable, a brake drum with a force which is smaller than the above-mentioned certain force resulting in the above- mentioned flat equilibrium condition. In this case, the contact surface of the brake disc is displaced from the initial position 7 into a second position 7'. Furthermore, in exaggerated magnification, it is shown how the surface of the brake lining is deformed into a shape which has been indicated with dashed lines when subjected to a smaller load. Since the local deformation stiffness of the brake lining varies across the surface, different contact forces are created across the surface of the brake lining by a force which differs from the above-mentioned certain force which creates said equilibrium condition. Here, local deformation stiffness means the spring constant in a locally delimited area in the direction of the normal to the contact surface. Studies have shown that the spring constant of a homogenous material is lower at an edge region having vertical side surfaces than in regions having inclined side surfaces or in an inner region at a distance from said edge of the homogenous material. This means that the spring constant at the groove 6 located in the brake lining 3 exhibits a lower deformation stiffness than in an inner region of the brake lining, and that the brake lining 3 exhibits a higher deformation stiffness at the bevelled flanks 8 than in the inner region of the brake lining. This results in the brake lining bulging outwards at the region I located closest to said groove 6, and bulging inwards at the region III located closest to the bevelled edge 8, when subjected to the smaller load.

The left part of Fig. 2 shows the pressure distribution across the brake lining 3, with dashed lines, when the brake lining is loaded with a smaller load than the above- mentioned certain force. From the explanation above, it is evident that the pressure within the region I is higher than within the region III when a smaller load than the equilibrium load is applied onto the brake lining. The pressure distribution at the above- mentioned certain force is shown with continuous lines.

The right part of Fig. 1 shows deformation of the brake lining 3, with dashed lines, when the brake lining is pressed against the brake disc 2 or, whenever applicable, a brake drum, with a force exceeding the above-mentioned certain force resulting in said flat equilibrium condition. h1 this case, the contact surface of the brake disc is displaced from the initial position 7 into a third position 7". Furthermore, in exaggerated magnification, it can be seen how the surface of the brake lining is deformed into a shape which is indicated with dashed lines when subjected to a larger load. Since the local deformation stiffness of the brake lining varies across the surface, different contact forces are created across the surface of the brake lining as a result of a force which differs from the above-mentioned certain force resulting in said equilibrium condition.

Also in this case, the spring constant at the groove 6 located in the brake lining 3 exhibits a lower deformation stiffness that in an inner region of the brake lining, and the brake lining 3 exhibits a higher deformation stiffness at the bevelled flanks 8, 9 than in the inner region of the brake lining. This results in the brake lining bulging inwards at the region I located closest to said groove 6, and bulging outwards at the region III located closest to the bevelled edge 8, when subjected to the larger load.

The right part of Fig. 2 shows the pressure distribution across the brake lining 3, with dashed lines, when the brake lining is loaded with a larger load than the above- mentioned certain force. From the explanation above, it is evident that the pressure within the region I is lower than within the region III when a larger load than the equilibrium load is applied onto the brake lining. The pressure distribution with the above-mentioned certain force is shown with continuous lines.

As a result of these shape and pressure changes which occur when applying different pressure forces to the brake lining, it is made considerably more difficult to design brake linings where the risk of connecting a plurality of separated modes can be reduced.

Accordingly, the object of the invention is achieved by means of providing a brake system where a brake lining exhibits a contact surface having an edge area surrounding the contact surface, wherein at least a portion of said edge area exhibits a local deformation stiffness in a direction vertical to a plane through the contact surface which differs 15 % at most from the deformation stiffness in a central area in the interior of the contact surface.

By means of this design, a substantially more constant pressure is created across the brake lining when subjected to different loads.

DESCRIPTION OF THE FIGURES In the following, the invention will be described in greater detail with reference to the attached drawings, in which Fig. 1 shows the deformation of a known brake lining when subjected to an equilibrium load, a larger and a smaller-load, Fig. 2 shows the pressure distribution of a known brake lining when subjected to an equilibrium load, a larger and a smaller load, Fig. 3 shows a perspective view of a brake lining according to the invention, Fig. 4 shows a diagram of deformation ratio as a function of the inclination of a side surface,

Fig. 5 shows a cross-section of a brake member according to the invention, and Fig. 6 shows a cross-section of an alternative embodiment of a brake member according to the invention.

DESCRIPTION OF EMBODIMENTS Fig. 3 shows a perspective view of a brake member 1 according to the invention. The brake member is arranged, in a way known per se, for interacting with a brake disc or a brake drum by means of elements which are not shown. Examples of interaction and included elements necessary in order to provide a functioning braking device are provided e. g. in US 5145037 and GB 2143 916, which constitute an example of a disc brake where brake members according to the invention can be utilised, and SE 504 272 which constitutes an example of a drum brake where brake members according to the invention can be utilised.

The brake member 1 includes a brake lining 3 and a backing plate 4. Preferably, the brake lining 3 and backing plate 4 are designed in one piece. Alternatively, the brake lining 3 can be attached to the backing plate 4 in any way well known to the skilled person. The brake lining constitutes a wear surface when the brake member is used for braking, and the backing plate distributes the pressure force from braking cylinders (not shown) to the brake lining when the brake member is utilised in a brake device. One example of the design of a brake device having braking cylinders can be found in e. g.

US 5145037.

In a preferred embodiment, the brake lining exhibits a first and a second contact surface Sa, 5b which are intended to be pressed against a brake disc 2 or, whenever applicable, against a brake drum. Furthermore, the brake lining 3 in this embodiment exhibits a groove 6 which delimits the contact surface 5a from the contact surface 5b. The invention also can be utilised with brake linings having a single wear surface, but is

particularly advantageous with brake linings having several wear surfaces separated by grooves, since the edge/area-ratio of the contact surfaces of such brake members usually is larger. Furthermore, the contact surfaces 5a and Sb exhibit an edge area 9 including a first edge area 9a surrounding the first contact surface 5a and a second edge area 9b surrounding the second contact surface 9b. According to what has been described above, the contact surfaces 5a, Sb are worn down during the life of the brake member, so that they are flat and exhibit a surface normal ez. The edge area is defined as the area where the contact surfaces Sa, 5b are connected to side surfaces 10-15. Accordingly, the side surfaces exhibit surface normals eg extending in different directions from the surface normal ez of the contact surfaces. In a preferred embodiment, the side surfaces include a front and a rear flank 11,12 which are bevelled. The flanks are introduced in order to compensate for the edge effects which arise during the force transfer from the braking cylinders to the backing plate. According to what has been described above, the invention can be utilised both with brake linings designed without flanks, as well as with brake linings designed with flanks. According to prior art, the front and rear flanks usually are designed with a surface normal which is inclined approx. 15° in relation to the surface normal ez of the contact surface. In the embodiment where a front and rear flank are arranged on the brake lining 3, said side surfaces include a first set of side surfaces 10,13 extending between said front and rear flanks 11,12. Whenever applicable, said side surfaces further include a second set of side surfaces 14,15 between which said groove 6 extends.

Figure 4 shows a diagram of deformation ratio as a function of the inclination of a side surface. The diagram shows calculations, for somewhat different materials, of the deformation ratin hPtwPPn a rPntral nnint ncatarl nn tha hrnlrP linina anrl a nni t lnr. atar

theoretical background, areas having the same local deformation stiffness will have the same contact pressure when the brake lining is subjected to different loads. Thus, the brake member should be designed such that the local stiffness is the same across the entire contact surface of a brake lining. Accordingly, the diagram shows how the deformation ratio varies with the inclination of a side surface in relation to the contact surface. The calculation has been perfonned for flat side surfaces having a surface normal es which forms an angle between 74° and 90° with the surface normal ez of the contact surface. This means that the side surface forms a flat surface which is slightly bevelled in the portion of the side surface facing the contact surface.

The diagram further shows preferred embodiments of the invention where the brake lining exhibits side surfaces which exhibit an angle between 78 and 86° in relation to the contact surface when the deformation ratio differs by less than approx. 15%, a further preferred embodiment of the invention where the brake lining exhibits side surfaces which exhibit an angle between 80 and 84° in relation to the contact surface when the deformation ratio differs by less than 10%, and still another preferred embodiment of the invention where the brake lining exhibits side surfaces which exhibit an angle between 81 and 83° in relation to the contact surface when the deformation ratio differs by less than approx. 5%.

Figure S shows a cross-section of a brake member 1 according to the invention. The brake member 1 exhibits a brake lining 3 and a backing plate 4. The brake lining 3 exhibits a contact surface 5a having a surface normal Cz. Furthermore, the brake lining exhibits a side surface 16, which preferably is chosen optionally from said first and second set of side surfaces 10,13-15. The side surface 16 exhibits a surface normal es.

The surface normal of the side surface and the surface normal of the contact surface form an angle of 90°-a° with each other when the brake member is in an unloaded condition. The side surface 16 and the contact surface 5a are connected in an edge area 9. Furthermore, Figure 5 shows the brake member in a condition when subjected to a heavy load, indicated with dashed lines, wherein it can be noticed that the entire contact surface 5a has been subjected to a parallel displacement into a new position 5a'. A parallel displacement under constant pressure across the surface is achieved by means

of giving the edge area the same local deformation stiffness as a central area 17 in the interior of the contact surface. The central area 17 in the interior of the contact surface can be defined as all points on the contact surface which are at a distance from an edge 9 which exceeds the thickness H, preferably three times the thickness of the brake lining 3, as measured in the direction of the surface normal ez of the contact surface.

As a result of the side surfaces being designed in an inclined manner, a brake member according to the invention, having a limited variation of the local deformation stiffness between edge and interior of the contact surface 5a of the brake lining 3, is achieved. In the example shown in Figure 5, the side surface 16 is designed flat, which means that the edge will be stiffened to the same degree independent of how much the lining is worn down. Figure 6 shows the invention according to another embodiment, where the surface normal es of the side surface 16 varies with the distance from the backing plate 4. This embodiment can be utilised when the influence of the backing plate 4 on the local deformation stiffness of the brake member 1 makes a separate compensation by means of the edge design necessary in order to obtain an edge area having a small deviation in deformation stiffness depending on the thickness and thereby the wear of the brake lining. In this case, according to a preferred embodiment of the invention, the side surface 16 is designed with an arbitrary surface normal es within the main portion 18 of a wear area 19 within the side surface 16, which surface normal eg makes an angle between 78-86° with an arbitrary surface normal ez within the main portion of the contact surface when the brake member is in an unloaded condition. The wear area is defined as the portion of the side surface which is intended to be worn down during the working life of the brake member 1. The main portion 18 of the wear area can be constituted by the entire thickness of the brake lining or a local part of this, however, preferably at least 50% of the thickness of the brake lining when the brake lining is in an original condition. Preferably, the side surface is designed with a surface normal es which makes a larger angle with the surface normal ez of the contact surface at portions of the side surface which are located close to the backing plate 4 than at portions which are located close to the contact surface 5a.

The brake lining and the backing plate are made of materials which are well known to the skilled person.

The invention also relates to a method for manufacturing a brake member including a backing plate carrying a brake lining which exhibits at least one contact surface and also side surfaces surrounding said at least one contact surface, wherein the side surfaces are connected partly to said contact surface and partly to said backing plate, wherein said contact surface is designed with an edge area surrounding the contact surface, wherein at least a portion of said edge area exhibits a local deformation stiffness in a direction vertical to a plane through the contact surface which differs 15% at most from the deformation stiffness in a central area in the interior of the contact surface.

In a preferred embodiment of said method, a local deformation stiffness which differs 10 % at most is obtained.

In a further preferred embodiment of said method, a local deformation stiffness which differs 5 % at most is obtained.

In a preferred embodiment of said method, said surfaces are designed for allowing said edge area to exhibit said deformation stiffness. hi a further preferred embodiment of said method, the side surfaces are designed with an arbitrary surface normal within the main portion of a wear area within said second set of side surfaces which makes an angle to an arbitrary surface normal within the main portion of the contact surface which is between 78-86°, preferably beween 80-84°, and even more preferably 81- 83°, when the brake member is in an unloaded condition.

According to the invention, a method where said brake member is formed from a material having a varying deformation stiffness, and the variation is arranged to allow a portion of said edge area to exhibit a local deformation stiffness in a direction vertical to a plane through the contact surface which differs 1S % at most from the deformation stiffness in a central area in the interior of the contact surface, can also be utilised. In a preferred embodiment, the deformation stiffness differs 10% at most, preferably 5%.

The invention can be utilised for different types of disc and drum brakes which are well known to the skilled person.