Wheel hub assembly

Abstract

A wheel hub assembly (10) for a motor vehicle, the wheel hub assembly comprising a radial outer ring (31) with at least one raceway (31'), at least one radial inner ring (20, 34) with associated raceways (20', 34'), and at least one ring of rolling elements (32, 33) between the radial outer ring (31) and the radial inner ring (20, 34), the at least one ring of rolling elements (32, 33) being used to slide inside the raceways (31', 20', 34'); at least one raceway (31', 20', 34') being provided with surface micropores (70), the surface micropores having a depth (H) between 0.05 μm and 1.5 μm and defining corresponding microgrooves (70) for accommodating grease.

Description

Technical Field

[0001] The present invention relates to a wheel hub assembly provided with a pair of rolling bearings to rotatably support a wheel of a vehicle on a suspension. In particular, the present invention relates to a raceway of the wheel hub assembly provided with a suitable microporosity.

[0002] Such applications include both the following cases: the case where the outer bearing ring can rotate while the inner bearing ring is stationary, and the reverse case where the inner ring can rotate while the outer ring is stationary. The present invention is also applicable to any type of rolling element (ball, roller, tapered roller, etc.).

[0003] Furthermore, the invention relates to a motor vehicle equipped with such a wheel hub assembly. Background Art

[0004] Wheel hub assemblies equipped with bearing units for rotatably supporting a wheel of a vehicle on a suspension are known and commonly used. The bearing unit usually comprises a pair of rolling bearings.

[0005] In the prior art, a wheel hub assembly comprises a rotating hub equipped with a joint to couple a rotating element of a motor vehicle (e.g., a wheel or a disk of a brake element), and a bearing unit comprising an outer ring, a pair of inner rings (one of which may be the hub itself) and a plurality of rolling elements (e.g., balls, rollers or tapered rollers). All these components have axial symmetry about the axis of rotation of the rotating element (e.g., the hub and the inner ring of the bearing unit).

[0006] Partly due to increasing global competition, the performance required of wheel hub assemblies continues to increase, requiring special attention from manufacturers of these assemblies. In particular, recent years have seen an increased interest in reducing friction in bearing units as a way of reducing CO2 emissions.

[0007] From the perspective of consumers who consider friction reduction to be key in determining the performance of a wheel hub assembly, the technical problem addressed by the present invention is how to reduce friction in the bearing unit and improve the lubrication condition in the raceway without thereby compromising any other equally important requirements (such as the noise level and service life of the bearing unit).

[0008] Document EP2711512B1 describes a wheel hub assembly in which the bearing raceway is provided with a plurality of inner "hollows" containing a liquid lubricant and a plurality of outer hollows containing a solid lubricant. The hollows have a diameter between 0.2 mm and 0.4 mm and a depth of about 5 μm.

[0009] Furthermore, experimental ( / test) evidence indicates that depths exceeding 2-3 μm from the raceway surface in a wheel hub assembly are considered highly disadvantageous as they result in very high noise levels.

[0010] Therefore, it is necessary to define the surface of the raceway (particularly the porosity of the surface of the raceway) with higher accuracy to overcome the above-mentioned disadvantages. Summary of the invention

[0011] The invention aims to form a plurality of micropores in at least one raceway of a bearing unit in a calibrated and controlled manner using laser technology or an alternative technology producing the same technical effect. The "pitting" obtained with such pores promotes the continuous renewal of the oil film on the rolling interface between the raceway and the rolling elements. The size and density of the micropores can be optimized to reduce the friction coefficient and improve the tribological performance without adversely affecting the noise level of the bearing unit.

[0012] The core idea of ​​the present invention is to form micropores on the surface of at least one raceway, which can be one or both of the raceway of the radial outer ring or the raceway of the radial inner ring of the bearing unit. The micropores thus formed act as microgrooves with a depth ranging from 0.05 μm to 1.5 μm to retain grease and release it precisely where it is intended.

[0013] The technical effect is the creation of micro grooves for the lubricant without the risk of damage or noise due to micro holes of very small size.

[0014] Therefore, the present invention discloses a wheel hub assembly, in which at least one raceway is provided with a plurality of micropores on the corresponding surface in contact with the rolling body, wherein the plurality of surface micropores have a depth between 0.05 μm and 1.5 μm and define microgrooves for accommodating grease. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The invention is described below with reference to the accompanying drawings showing non-limiting example embodiments of the invention, in which:

[0016] Figure 1 is a cross section of a wheel hub assembly equipped with a bearing unit according to one aspect of the present invention, and

[0017] Figure 2 yes Figure 1 An enlarged schematic detail view of the wheel hub assembly in , showing a plurality of micropores formed in the raceway of the bearing unit, their characteristic sizes and their distribution density. DETAILED DESCRIPTION

[0018] By way of non-limiting example, the invention is described below with reference to a wheel hub assembly for a motor vehicle provided with a bearing unit.

[0019] Reference Figure 1 A wheel hub assembly according to a preferred embodiment of the present invention is indicated as a whole by reference numeral 10. The figure shows details of an example arrangement.

[0020] The wheel hub assembly 10 has a central axis of rotation X, a hub 20, which is preferably but not necessarily rotatable, and a bearing unit 30, which includes:

[0021] A radial outer ring 31, preferably but not necessarily stationary ( / fixed);

[0022] a radial inner ring 20, defined by the hub 20;

[0023] Another rotating radial inner ring 34, mounted on the hub 20 and rigidly connected to the hub 20; and

[0024] Two rings of rolling bodies ( / rings of rolling bodies) 32 , 33 , in this example balls, are interposed between the radially outer ring 31 and the radially inner rings 20 and 34 .

[0025] Throughout the present specification and claims, terms and expressions indicating position and orientation ( / orientation) such as "radial" and "axial" are to be understood with reference to the central rotation axis X of the bearing unit 30. On the other hand, expressions such as "axially outside ( / axially outward)" and "axially inside ( / axially inward)" relate to the assembled state of the wheel hub assembly and in this case preferably refer to the wheel side and the side opposite to the wheel side, respectively.

[0026] The radial outer ring 31 is provided with two corresponding radial outer raceways 31', and the radial inner rings 20, 34 are provided with corresponding radial inner raceways 20', 34', so that the axial outer ring rolling body 32 between the radial outer ring 31 and the hub 20 and the axial inner ring rolling body 33 between the radial outer ring 31 and the radial inner ring 34 can roll. In order to simplify the graphical representation, the reference numerals 32, 33 are used to represent both a single ball and a ring of balls. Again for the sake of simplicity, the term "ball" shall be used in this specification and in the drawings in an exemplary manner instead of the more general term "rolling element", and the same reference numerals shall be used.

[0027] The axially internal end of the hub 20 defines a rolled edge 22 designed to axially preload the inner race 34 .

[0028] The wheel hub assembly 10 may also be provided with a sealing member 50 to seal the bearing unit from the external environment.

[0029] Reference Figure 2 (as Figure 1 A schematic enlarged detail view of a wheel hub assembly in FIG. 1 shows a plurality of micropores (micropores), their characteristic sizes and their distribution density) formed on any one of the raceways 31', 20', 34' of the bearing unit, the inventive concept involves forming a plurality of very small micropores 70 with a depth of the order of 1.5 μm. In order to create ( / produce) the desired shape and distribution, a laser technique (e.g., a technique used to mark components) can be used to reproduce ( / reproduce / copy) a plurality of micropores 70 on the corresponding surfaces of the raceways 31', 20', 34' in contact with the rolling elements 32, 33. The shape, specific size and distribution of the micropores can be customized to take into account the optimal tribological conditions of the bearing unit and the lubricating grease. The micropores act as microgrooves ( / micro-storage grooves / micro-tanks) with a depth H in the range of 0.05 μm-1.5 μm to hold ( / hold) the grease and release the grease precisely where it is expected.

[0030] The technical result is a reduction in the losses caused by friction in the bearing unit, without causing any adverse effects in terms of damage or noise levels due to micropores of very small size: as indicated above, surface imperfections with a depth exceeding 2 μm-3 μm are considered defects in the wheel hub assembly due to the noise levels that they cause that are unacceptable to the end customer. In addition, the presence of micropores provides another advantage, in particular a reduction in vibrations due to the improved damping values ​​associated with the contact between smooth surfaces. Finally, the data provided in the technical literature show that in the presence of such grease-lubricated surfaces, the amplitude of the vibrations is reduced compared to the corresponding case with oil lubrication. Therefore, if grease is used in combination with a raceway provided with micropores according to the invention, it is much more effective than lubricating oil in reducing noise.

[0031] Combining the effects related to the size of the micropores and the effects related to the distribution density of the micropores, and depending on different flow rates and load conditions, the reference grease should have a high ability to reduce friction and wear and should be widely used in industrial applications.

[0032] In fact, grease is a high viscosity lubricant suitable for lubricating surfaces subjected to high pressure. Considering the relationship between grease and micropores, the size (width and depth) of the micropores should enable the base oil of the grease to enter the micropores. The type of grease suitable for use in wheel hub assemblies should be a mixture containing the following materials:

[0033] A thickener (e.g., lithium soap or urea soap), which acts as a carrier for any additives and base oil required for use under high pressures,

[0034] Base oil has a specific function of lubricating the contact area. In particular, the viscosity range of the base oil should be 70 mm at 40 ° C. 2 / s and 150mm 2 / s.

[0035] Grease must have a certain level of consistency to prevent the thickener from entering and plugging the micropores, thereby preventing the base oil from working and reducing the effectiveness of the system and potentially adversely affecting friction in the bearing. The consistency level should be equal to or greater than 2.

[0036] A non-limiting example is a high quality lithium-based multi-purpose grease. This grease has, for example, the following properties: NLGI grade: 2, thickener type: lithium, drop point: >180°C, base oil viscosity: >100 mm at 40°C 2 / s.

[0037] The reduction in friction is mainly attributed to the formation of a film of grease lubricant, which is stable and includes an oil film, a transfer film, and a deposited film. The effect of the hydrodynamic pressure of the superficial micropores increases the coupling gap and reduces the probability of contact between the rough areas of the contact surface (raceway / rolling element). In the case of mixed lubrication conditions and hydrodynamic lubrication conditions, the micropores act as microfluidic bearings, so that the hydrodynamic pressure increases due to the asymmetric distribution of pressure, which leads to greater hydrodynamic lift or greater film thickness. Therefore, the number of rough areas of contact is reduced, and friction and wear are also reduced. In boundary lubrication conditions and dry contact lubrication conditions, these micropores act as microgrooves of lubricants to continuously retain and supply lubricants, or act as micro-containers to capture wear waste and contaminant particles, thereby reducing the resulting surface ridges.

[0038] Advantageously and from the point of view of optimizing the friction reduction / noiselessness trade-off, the diameter D of the micropores (or the equivalent diameter if the micropore shape is not circular) should be within the range of 1 μm to 5 μm.

[0039] Preferably, the depth / diameter ratio (H / D) must fall within the range of 0.1-0.4 to minimize the friction coefficient.

[0040] In the above trade-off, the distribution density of the micropores is also important. For example, the micropores on the raceway surface can be uniformly arranged according to a rectangular matrix with a density Td, where the density Td is calculated using the following formula:

[0041] Td=(πD2) / (4×K×L)×100

[0042] in:

[0043] D: Micropore diameter

[0044] K, L: The distances between the centers of consecutive micropores in two orthogonal directions.

[0045] Typically, the density of the micropores is between 10% and 20%, which together with the size range of the micropores themselves (equivalent diameter D and depth H) ensures that the micropores provide good friction properties for the raceways under all load and speed operating ( / running) conditions of the bearing unit.

[0046] For density values ​​less than 10%, a small amount of micropores may result in a lack of grease on the contact surface sufficient to cause hydrostatic lift. In the case of micropore density values ​​exceeding 20%, although the grease retained on the contact surface is present more, the space between each micropore is reduced and acts as a rough area, thereby affecting the distribution of the lubricant film and promoting the detachment of the lubricant film from the contact surface, and causing an increase in friction resistance. Second, since the grease is mainly inside the micropores, the grease may be less effective because it is not completely absorbed into the contact surface to form a support film.

[0047] The above ranges related to the size and distribution density of the micropores are also determined by considering different potential situations related to the wheel hub assembly. Figure 1 In the case of the wheel hub assembly in , similar to the forced coupling of the hub 20 to the constant velocity joint (if present), the rolling process of axially preloading the radial inner ring 34 changes the load on the raceway.

[0048] Preferably, a fine polishing process should be used to remove the burrs ( / burrs) formed around the edges of the micro-holes during the laser process to obtain ( / achieve) a roughness of less than 0.1 μm.

[0049] The flow direction should be substantially perpendicular to the axis of the micropores. In practice, it is acceptable for the axis of the micropores to be tilted up to 15° from the normal to the raceway surface. This tilt is useful for fluid dynamic effects under mixed lubrication conditions.

[0050] As indicated, the invention is advantageous under all operating conditions of the bearing unit. Taking into account the range of loads and speeds for which the presence of micropores of the above-mentioned size and distribution is advantageous, it should be noted that in typical wheel hub assembly applications, the loads come from contact pressures between 0.5 GPa and 4 GPa, and the speed range is between 100 rpm and 1000 rpm. Of course, the density of the micropores can vary depending on the operating conditions. In fact, in the presence of low contact pressures, the surface roughness has a certain influence on the overall friction behavior, since a larger micropore density leads to a higher surface roughness, making it easier to obtain a higher friction coefficient. On the other hand, when the load increases, the friction coefficient decreases due to the large amount of oil continuously released through the micropores and grease along the contact surface. Larger loads and speeds increase the shearing stress, apparent viscosity and thickener concentration of the grease, which favors the formation of a protective film and surface smoothing phenomena, both of which contribute to reducing friction.

[0051] Another important parameter in the dimensioning of the solution is the location of the micropores. Ideally, the micropores should be located between the raceway and the rolling element at a contact angle α (e.g. Figure 1 Of course, the actual contact surface varies due to different load conditions, so the micro-holes are not located only on the theoretical contact area. Advantageously, according to the invention, the micro-holes are located along the contact surface corresponding to a lateral acceleration that is substantially zero. Depending on the type of tire used, the contact surface corresponding to zero lateral acceleration is the contact surface corresponding to a nominal contact angle α with a margin of ±10°.

[0052] The micropores can be formed by ablating the material by means of rapid melting and evaporation ( / vaporization) using high-power laser pulses. The laser technology used must be able to provide a low thermal effect on the surface of the metal material, so that the generation of a re-melt layer ( / re-melt layer) is avoided as much as possible. This aspect is very important for wheel hub assemblies, because the bearing unit (as known) has a heat-treated area corresponding to the raceway for improving the fatigue resistance associated with rolling.

[0053] Furthermore, with regard to the technique for forming micro-holes along a flat surface, it should be noted that the surface of the raceway is not flat but essentially toroidal. Therefore, since the distance between the laser pointer and the surface is variable, at least the power and orientation of the laser must be adjustable.

[0054] To obtain such features, an example application is a laser with an output wavelength of <600 nm, a pulse width of ≤85 nm and a power of about 25 W.

[0055] The shape of the micropores does not have to be circular. For example, the shape of the micropores can be elliptical. In particular, the elliptical shape can have an eccentricity in the range of 1 to 5, wherein the main axis is arranged in the rolling direction of the rolling bodies 32, 33 relative to the raceways (31', 20', 34').

[0056] In summary, the solution according to the invention has a number of advantages related to:

[0057] Improved lubrication conditions, thus improving the service life ( / service life) and anti-friction performance of the bearing unit,

[0058] Improved lubrication and thus reliability of rotating contact elements,

[0059] Again due to better lubrication conditions, it is possible to reduce the grease filling level,

[0060] In terms of noise levels, there was no adverse effect.

[0061] In addition to the above-mentioned embodiments of the present invention, there are many other variations. In addition, the embodiments are merely examples, and the examples shown neither limit the scope of the present invention, nor the application of the present invention, nor the feasible arrangements of the present invention. In fact, although the above description enables those skilled in the art to perform the present invention according to at least one example embodiment of the present invention, many variations of the described components may also be used without therefore exceeding the scope of the present invention as defined in the appended claims, which should be understood literally and / or according to its legal equivalents.

Claims

1. A wheel bearing assembly (10) for a motor vehicle, comprising a wheel hub (20) and a bearing unit (30), wherein the bearing unit (30) further comprises: - a radial outer ring (31) comprising a raceway (31'); - at least one radially inner ring (20, 34) comprising at least one raceway (20', 34'); - at least one row of rolling elements (32, 33) between the radial outer ring (31) and the radial inner ring (20, 34) for sliding inside the raceways (31', 20', 34'), The wheel bearing assembly (10) is characterized in that at least one raceway (31', 20', 34') is provided with a plurality of surface micropores (70) having a depth (H) between 0.05 μm and 1.5 μm and defining microgrooves for accommodating grease.

2. The wheel bearing assembly (10) according to claim 1, characterized in that The equivalent diameter (D) of the micropores (70) is in the range between 1 μm and 5 μm.

3. The wheel bearing assembly (10) according to claim 1 or 2, characterized in that: The ratio between the depth (H) and the equivalent diameter (D) of the micropores (70) is in the range between 0.1 and 0.

4.

4. The wheel bearing assembly (10) according to claim 1 or 2, characterized in that: The density (Td) of the micropores (70) is comprised between 10% and 20% and is calculated according to the following formula: Td=(πD2) / (4×K×L)×100 in: D: equivalent diameter of micropore (70), K, L: the distance between the centers of two consecutive microholes (70) in two orthogonal directions.

5. The wheel bearing assembly (10) according to claim 1 or 2, characterized in that: The grease is a mixture comprising a base oil and a thickener, wherein the base oil has a viscosity of 70 mm at 40°C. 2 / s and 150mm 2 In the range between 300 Å and 100 Å, the thickener acts as a carrier for the base oil.

6. The wheel bearing assembly (10) according to claim 5, characterized in that The consistency grade of the grease is greater than or equal to 2.

7. The wheel bearing assembly (10) according to claim 1 or 2, characterized in that: The surface roughness around the edge of the microhole is less than 0.1 μm.

8. The wheel bearing assembly (10) according to claim 1 or 2, characterized in that: The inclination of the axis of the micro hole (70) relative to the orthogonal direction of the raceway (31', 20', 34') is between 0° and 15°.

9. The wheel bearing assembly (10) according to claim 1 or 2, characterized in that: The micropores (70) are positioned along the contact surface between the raceways (31', 20', 34') and the rolling elements (32, 33), said surface being angularly comprised in the range between -10° and +10° relative to a nominal contact angle (α) between the raceways (31', 20', 34') and the rolling elements (32, 33), said nominal contact angle (α) being defined relative to a radial plane of the bearing unit (30).

10. The wheel bearing assembly (10) according to claim 1 or 2, characterized in that: The micropore (70) is elliptical in shape, the eccentricity of the ellipse is between 1 and 5, and the major axis of the ellipse is arranged along the rolling direction of the rolling body (32, 33) relative to the raceway (31', 20', 34').

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