FIELD OF THE INVENTION

The present invention relates to a flanged hub of a wheel bearing unit having a flange part and a hub part which are made from different materials. More specifically, the hub part comprises a raceway made of bearing steel and the flange part predominantly comprises a second material that is joined to the hub part in a moulding process. BACKGROUND

A flanged hub of the above kind is known from US6485188. Specifically, this document concerns a wheel mounting with a bearing race embedded in a cast component. In one embodiment, the cast component is a bearing outer ring with a flange for receiving a vehicle wheel and brake disc. The flanged ring can be made of aluminium which is cast around an insert made of bearing steel that comprises hardened outer raceways. The resulting flanged hub is lighter than a flanged hub made entirely of bearing steel, which benefits the car owner in terms of fuel economy. One drawback of using aluminium instead of bearing steel is that aluminium has a higher thermal conductivity. This can be disadvantageous when an excessive amount of heat is generated due to friction between e.g. brake pads and a brake disc. Since part of the brake disc is mounted against the aluminium flange, heat is transferred through conduction from the brake disc to the bearing steel insert via the flange. The aluminium flange allows more heat to be conducted than a bearing steel flange. Thus, under severe and prolonged braking conditions, there is a greater risk that an amount of heat will be transferred to the raceways that causes a temperature increase which can adversely affect raceway hardness. Consequently, there is room for improvement. DISCLOSURE OF THE INVENTION

The present invention resides in a flanged hub of a wheel bearing unit comprising a hub part and a flange part, the hub part having a raceway made of a first material, being a bearing steel material, and the flange part having a mounting surface for the attachment of a brake rotor and vehicle wheel rim. The flange part predominantly comprises a second material that is joined to the hub part in a moulding process. According to the invention, the flange part further comprises an integrally moulded heat shield made of a third material which, in comparison with the second material, has a lower coefficient of thermal conductivity.

Thus, a flanged hub according to the invention inhibits heat transfer from the brake rotor to the raceway via the flange part, meaning that the raceway is protected from reaching a temperature that could reduce the hardness of the bearing steel.

In a further development of the invention, the third material of the heat shield has a higher modulus of elasticity in axial direction than the second material. As a result, flange stiffness is enhanced because the heat shield is integrally moulded in the flange part. The overmoulded flange is therefore capable of withstanding higher axial bending loads than a flange of the same thickness that is made entirely from the second material. Consequently, it is also possible to reduce the thickness of the overmoulded flange while retaining a required level of strength and stiffness.

The heat shield is positioned at an outboard side of the flange part, such that the mounting surface is at least partly defined by an outer surface of the heat shield. Thereby, the area of surface contact between the brake rotor and the second material of the flange part is reduced, which in turn reduces the amount of heat transfer. In a further development, the outboard surface of the heat shield has a surface profile, to reduce the area of contact between the brake rotor and the mounting surface and thereby reduce heat transfer. For example, the outboard surface can be grooved, or a have certain surface roughness. Advantageously, any parts of the mounting surface which are formed by the second material can also be grooved or roughened.

The second material of the flange part which is moulded to the hub part preferably comprises a lightweight metal such as aluminium, magnesium, or alloys thereof. Moulding should be understood as a process in which the second material is joined to the hub part in a moulten state or a semi-moulten state. In a particularly preferred embodiment, the second material is aluminium and is joined to the hub part in a semi-solid metal process, being one of a rheocasting process, a rheoforming process, a rheomoulding process, a thixocasting process, a thixoforming process or a thixomoulding process. In a conventional molten metal process such as casting, the microstructure of the cast metal or metal alloy contains interlocking dendrites that result in material brittleness. The advantage of a semi-solid process is that spherical grains are formed, and the metal has fine, uniform microstructures which give enhanced mechanical properties. Semi-solid metal processing is also less susceptible to air entrapment and a component produced in this way has fewer defects and lower porosity than e.g. a cast component. The second material may also be a fibre composite material comprising a polymer matrix and e.g. carbon fibres. In such examples of a flanged hub according to the invention, the second material will additionally insulate the raceway of bearing steel against potentially harmful temperature rises. A heat shield remains advantageous, however, as it will protect the polymer matrix from temperature rises that could cause melting. Also, as mentioned above, by using a heat shield material with a higher elastic modulus in axial direction, flange stiffness can be enhanced.

Suitable materials for the heat shield include mild steel, stainless steel and ceramics, which are stiff materials and are poorer thermal conductors than most lightweight metals. Stainless steel and ceramic materials have the additional advantage of being corrosion-resistant and will therefore prevent corrosion between e.g. a steel brake disc and an aluminium flange. When the heat shield is made of mild steel, its outboard surface may be treated with a chemically passive coating. Alternatively, the outboard surface may be given a sacrificial coating of e.g. ZnAI15, which will preferentially corrode and thereby protect the brake disc mounting surface from corrosion.

In some embodiments, the heat shield consists of a disc with a circular outer periphery. Alternatively, the heat shield can be shaped such that its outer periphery follows the geometry of the outboard face of the flange. In other embodiments, the heat shield comprises a plurality of separate, thermally insulating elements that are arranged around the outboard side face of the flange.

In a further development, the heat shield comprises one or more axial extensions. This increases the second moment of area of the heat shield in axial direction, thereby enhancing flange stiffness. Furthermore, the one or more axial extensions provide additional surface area against which the overmoulded second material can contract as it cools, thereby improving radial locking. The disc part may also comprise openings into which the overmoulded second material flows. This provides additional radial locking and also rotationally locks the heat shield with the flange.

In a still further development of the invention, the axial extensions are formed by threaded inserts for receiving mounting bolts or stud bolts. One advantage of this further development is that heat transfer via the brake rotor mounting bolts is inhibited, providing the flanged hub with even better thermal protection.

As demonstrated, a flanged hub according to the invention has several advantages. Other benefits will be come clear from the following detailed description and accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 shows a cross-section of an example of a wheel bearing unit comprising a first embodiment of a flanged hub according to the invention;

Fig. 2a shows a partial cross-section of a further example of a wheel bearing unit comprising a second embodiment of a flanged hub according to the invention;

Fig. 2b is a perspective view of a heat shield used in the flanged hub of Fig.

2a.

Fig. 3a and 3b show a partial cross-section of further embodiments of a flanged hub according to the invention.

DETAILED DESCRIPTION

An example of a wheel bearing unit 100 comprising a flanged hub 120 according to the invention is shown in Figure 1. The flanged hub 120 in this example is adapted for inner ring rotation and comprises a hub part 122 made of bearing steel. The hub part 122 has a radially outer surface that serves as an inner raceway 125 for an outboard row of rolling elements and further comprises a seat for a separate inner ring that forms the raceway for an in board row of rolling elements. To reduce the weight of the flanged hub 120 in comparison with conventional hubs that are made entirely from bearing steel, a flange part 127 of the flanged hub is made from aluminium. The aluminium of the flange part has been joined around the hub part 122 in a rheocasting process. The flange part 127, generally known as the wheel mounting flange, has a mounting surface 130 to which a mounting part 142 of a brake rotor 140 and a wheel rim (not shown) are bolted. A brake disc is shown in this example, but the brake rotor could also be a drum brake. Heat that is generated during braking is therefore transferred to the wheel mounting flange 127 through the mounting surface 130. The generated heat is then conduced through the flange 127 to the hub 122 made of bearing steel. Aluminium has a higher coefficient of thermal conductivity than bearing steel, meaning that a faster rate of heat transfer occurs through the aluminium. Therefore, when an excessive amount of heat is generated in the brake rotor 140, e.g. due to prolonged braking when driving down a long and steep slope, the heat that is lost through conduction and through dissipation to the surroundings may not be sufficient to prevent a harmful rise in temperature at the inboard raceway 125. For example, if the inboard raceway experienced a temperature of e.g. 140 °C for 15 seconds, raceway hardness would be lost, dramatically reducing the service life of the bearing unit 100.

To prevent such an occurrence, the flange 127 of a flanged hub 120 according to the invention comprises an integrally moulded heat shield 150. In the depicted example, the heat shield is made of stainless steel, which has a coefficient of thermal conductivity k of 16 W/mK. Aluminium has a k-value of around 250 W/mK; bearing steel has k-value of around 30 - 45 W/mK [the given values are for a reference temperature of 25 °C]. The heat shield 150 in this example is a stainless steel disc, which is positioned within the flange 127 such that the mounting surface 130 is formed by an outboard surface of the heat shield. Consequently, heat transfer from the brake disc mounting part 142 to the aluminium of the flange 127 is slowed down and a damaging increase of temperature at the raceway 125 becomes practically impossible. A further advantage of a stainless steel mounting surface 130 is that corrosion between the flange and the mounting part 142 of the brake rotor is prevented.

A still further advantage of integrating a heat shield of stainless steel is improved flange stiffness. Stainless steel and aluminium respectively have a modulus of elasticity of 200 and 70 GPa. Consequently, the flange 127 of the flanged hub 120 according to the invention has higher axial bending strength and stiffness than a flange of the same thickness that is entirely made from aluminium.

Depending on the application requirements, the heat shield 150 can be designed with a second moment of area in axial direction that optimises flange stiffness. For example, the heat shield may have a minimum thickness in axial direction. Alternatively, the second moment of area of the heat shield can be increased, without increasing the weight of the shield, by effectively making the shield longer in axial direction. In other words, the heat shield may comprise a disc portion, which at least partly serves as the mounting surface, and further comprise one or more axial extensions. In a further development of the invention, the axial extensions are formed by threaded inserts. An example of a wheel bearing unit comprising a flanged hub according to the further development is shown in Fig. 2a, while Fig. 2b is a perspective view of the heat shield that is integrally moulded in the flange. In this example, the bearing unit 200 is adapted for outer ring rotation and the hub part 222 of the flanged hub 220 comprises outer raceways for first and second rows of tapered rollers. The hub part 222 is made from bearing steel and the flange 227 in this example is predominantly made of aluminium, which has been joined to the hub part in a rheocasting process. The integrally moulded heat shield 250 is made of mild steel in this example.

The heat shield 250 comprises a disc part 255, against which the brake rotor mounting part 242 is mounted. The heat shield further comprises a plurality of axial extensions 257 having an inner thread 260 for receiving e.g. stud bolts. The stud bolts connect the brake rotor mounting part 242 to the flange 227, and consequently form an additional path for heat conduction from the brake rotor 240 to the overmoulded material of the flange. The internally threaded axial extensions 257 of the heat shield 250 therefore retard heat transfer, providing the flanged hub with additional thermal protection. A further advantage of incorporating internally threaded extensions in the heat shield is that the necessary flange mounting holes can be accurately positioned and toleranced.

In addition to enhancing stiffness, the axial extensions 257 of the heat shield rotationally lock the heat shield 250 within the flange 227 and provide additional surface area against which the overmoulded flange material can contract as it cools. Contraction takes place in a radially inward direction, meaning that radially outer surfaces of the heat shield effectively serve as attachment interfaces. In the example of Fig. 1 , the flange 127 has a larger outer circumference than the heat shield 150. Consequently, a portion of the overmoulded aluminium is able to contract against the outer circumferential surface of the shield 150. Needless to say, increasing the surface area against which the cooling aluminium can contract leads to better retention of the heat shield within the flange.

In the example of Figure 2b, the disc part 255 of the heat shield comprises a plurality of openings 256. During the rheocasting process, the semi-solid aluminium also flows into these openings and provides additional retention. A further example of a heat shield which comprises such openings is shown in Fig. 3a, which depicts part of a further embodiment of a flanged hub 320a according to the invention. The heat shield 350a is a disc with openings 356. Consequently, the outer circumference of the shield forms a first attachment interface 365 and the radially outer surface 367 of the heat shield at the openings 356 form a second attachment interface.

A still further embodiment of a flanged hub according to the invention is shown, in part, in Fig. 3b. The flanged hub 320b comprises a still further example of a heat shield 350. In this example, the heat shield is formed by a disc, whereby an outer periphery of the disc has been bent over, thereby creating an axial extension in the form of a lip 357. The lip 357 not only provides an attachment interface of increased surface area, but also increases the second moment of area of the heat shield 350b. A number of aspects/embodiments of the invention have been described. It is to be understood that each aspect/embodiment may be combined with any other aspect/embodiment. The invention may thus be varied within the scope of the accompanying patent claims. Reference numerals

100 wheel bearing unit

120 flanged hub

122 hub part of flanged hub

125 inner raceway on hub part

127 flange part of flanged hub

130 mounting surface of flange part

140 brake rotor

142 mounting part of brake rotor

150 heat shield

200 wheel bearing unit

220 flanged hub

222 hub part of flanged hub

227 flange part of flanged hub

240 brake rotor

242 mounting part of brake rotor

250 heat shield

255 disc part of heat shield

256 openings in disc part

257 axial extensions (threaded inserts)

260 inner thread

320a, 320b flanged hub

350a, 350b heat shield

356 openings in heat shield

357 axial extension of heat shield (lip) 365, 367 first and second attachment interface