Method for reducing friction in a rolling bearing, in particular in a vehicle wheel hub bearing unit and associated rolling bearing

Abstract

Rolling bearings (2), in particular a wheel hub bearing unit (1) for vehicles, and an associated method for reducing the friction between the rolling elements (5) of the bearing and the respective raceways (6, 7) for the rolling elements formed on the inner and outer rings (4, 3) of the rolling bearing; wherein the rolling elements (5) and the raceways (6, 7) have a surface topography with a predetermined thickness, wherein the rolling elements (5) and / or the raceways (6, 7) are coated with a blackened layer (4) and a lubricating fluid (15) with predetermined physical / chemical properties is inserted between the inner and outer rings (4, 3) and is in contact with the rolling elements (5) and the raceways (6, 7).

Description

Technical Field of the InventionThe present invention relates to a method for surface treatment of surfaces of components of a rolling bearing, such as the outer surfaces of the rolling elements and / or the surface of the raceways for the rolling elements formed on the inner ring and outer ring of a rolling bearing, in order to reduce the friction developing in the components during use. In particular, the method is applicable to the components of a rolling bearing in a vehicle wheel hub bearing unit.The invention also relates to an associated rolling bearing having at least one of its rolling components which have been subjected to the aforementioned surface treatment method.Prior ArtEP2222892B 1 discloses a chemical treatment method which can be applied to surfaces of large bearings, for example for wind turbine blades and / or rolling surfaces, which are subjected to high mechanical loads, as in the case of constant velocity joint components.With the chemical treatment described in EP2222892B1, it is possible to obtain on the treated surfaces a thin layer of metal oxides, in particular iron oxides, which is referred to as "brumming" (abbreviated to "BO"). The layer of BO makes the sliding / rolling surface more resistant to wear, since the brumming is very dense and compact and is therefore able to absorb the mechanical stresses and at the same time protect the underlying metal from polluting substances, such as alkaline or acidic substances, which may cause corrosion.WO2021139973A1 perfects the method described in EP2222892B1 in order to increase the mechanical resistance of the brumming by the deposition of further components such as tungsten oxides in the BO layer.In the case of rolling bearings designed for vehicles, particularly those forming part of a wheel hub bearing unit of motor vehicles, the need to increase the wear resistance of the rolling components of the bearings is less urgent, while there is an increasing urgent need to reduce the friction generated in the rolling bearing during normal operation as much as possible to reduce the energy consumption, which is a critical goal in connection with applications for electric traction vehicles.In fact, in this type of rolling bearings, the rolling elements such as balls or rollers are not only subject to rolling friction but are also subject to combined rolling and sliding conditions; in an automobile wheel bearing forming the wheel hub unit or a part thereof, the relative movement of rolling elements and rolling / sliding tracks tracks tracks the result in a mixed friction behavior subject to rolling and sliding conditions with a sliding / rolling ratio between about 0% and 15% (0% being understood as a pure rolling condition and 100% as a complete sliding condition).There is therefore the need to reduce friction in rolling bearings and in particular in those of vehicle wheel hub bearing units, the more because this aim cannot be achieved simply by using high-performance lubricants, because this measure is not sufficient on the one hand and the chemical additives contained in the high-performance lubricants can lead to corrosion of the rolling components of the bearing on the other hand.SUMMARY OF THE INVENTIONThe object of the present invention is to provide a method for reducing friction in rolling bearings, in particular in rolling bearings of a vehicle wheel hub bearing unit, which is simple and cost-effective to implement and has proven efficiency.It is also an object of the invention to provide an associated rolling bearing with a low friction degree, which is manufactured using the above-mentioned method.The present invention provides a method for reducing friction in rolling bearings, in particular in rolling bearings of vehicle wheel hub bearing units, and an associated rolling bearing having the characteristic features described in the appended claims.In particular, the invention consists in applying a treatment consisting in applying a friction on the entire surface of rolling elements and / or on the raceways for the rolling elements in a rolling bearing, in particular of the type intended for / forming a wheel hub bearing unit, which can be used synergistically in combination with specific chemical / physical properties of the lubricant used in the rolling bearing and also in combination with selected surface topography parameters of the rolling elements, i.e. both the rolling elements and the raceways for the rolling elements, such as the roughness, in order to obtain an improved low friction performance and long life of the rolling bearing.In fact, it has been found in experimental experiments that the improvement obtained with respect to a reduction of friction seems to relate to an improved adhesion of the lubricating oil in connection with the improved wettability of the surfaces treated with brake layers, thanks to the appropriate selection of the other parameters (properties of the lubricating oil and geometry of the treated surfaces).Brief description of the FiguresThe invention will now be described with reference to the accompanying drawings which show a non-limiting example of an embodiment of the invention in which:FIG. 1 shows in schematic form a radial sectional view of a wheel hub bearing unit which is formed by a rolling bearing whose outer and inner rings are provided with fastening flanges; andFIG. 2 also shows, only in schematic form, a view on a larger scale of a section of FIG. 1, in which one of the components of the invention is shown in dashed lines.Detailed DescriptionWith reference to FIGS. 1 and 2, reference numeral 1 globally denotes a wheel hub bearing unit for vehicles, which, in the non-limiting example shown, consists of a rolling bearing 2, of the so-called "double flanged" type.The rolling bearing 1 comprises a radially outer ring 3, a radially inner ring 4 and a plurality of rolling elements 5 consisting of balls in the non-limiting example shown, but the following description is also applicable to a rolling bearing with rolling elements consisting of all the types of conical or cylindrical rollers.The rolling elements 5 are interposed between the outer ring 3 and the inner ring 4 so as to be relatively rotatable about a common axis of symmetry A of the inner ring 4 and the outer ring 3.The rolling elements 5 (see FIG. 2 ) engage in respective annular raceways 6, 7 facing each other and formed respectively on a radially inner side surface 8 of the outer ring 3 and on a radially outer side surface 9 of the inner ring 4.In the non-limiting example shown, the rolling bearing 2 consists of two rows of rolling elements 5 arranged side by side, in the present case two rows of balls, and therefore comprises a pair of annular raceways 6 arranged in tandem and facing a corresponding pair of annular raceways 7 also arranged side by side in tandem.In the non-limiting example shown, the inner ring 4 is composed of two different elements coupled together, in the present case a ring 4b provided with a radially outer mounting flange 10 for a vehicle wheel (not shown) formed as one piece with the ring 4b at an end 11 of the ring 4b opposite the outer ring 4, and a small inner ring (SIR) 4c embedded at an end 12 of the ring 4b opposite the end 11 and inserted radially into the outer ring 3. The small inner ring 4 cis axially locked at the end 12 by upsetting in a known manner.The outer ring 3 also has a radial outer flange 13 for fixing to a suspension pillar of a vehicle, which is known and not shown for the sake of simplicity of illustration.According to other possible embodiments, which are not shown for the sake of simplicity of illustration, the rolling bearing 2 could be a normal rolling bearing with single cylindrical rings 3 and 4, the latter being embedded on an axle provided with a flange 10 and the former being inserted in a support which is in turn integral with or forms the suspension pillar of the vehicle, or always provided with the flange 13 for attachment to a suspension pillar (not shown) of the vehicle.According to a first aspect of the invention, at least some or all contact surfaces of the so-called "ball set", i.e. all outer surfaces of the rolling bodies 5 and / or the entire surface of the raceway(s) 6 and 7 for the rolling bodies 5, are coated on the inside with a brake layer 14 of predefined thickness, which is shown in dashed lines on the balls 5 and, for the sake of simplicity, on a single raceway 6.Moreover, the entire set of balls must be in contact with a lubricating fluid 15 schematically represented in the form of a chunk arranged between the balls 5, but in reality forming a substantially uniform layer (not shown for simplicity of illustration) arranged on all contact surfaces, in other words those of the rolling bodies 5 and those of the raceways 6 and 7.Brumming is a thin layer of metal oxides, particularly iron oxides, which can be obtained on each steel surface according to the method taught in EP2222892B1, the contents of which are incorporated herein for the necessary parts by simple references. Layer 14 may also comprise other hard metal oxides, such as tungsten, according to the method taught in WO2021139973A1, the contents of which are incorporated herein for the necessary parts by simple reference.In particular, the bruning layer 14 can contain at least 99% by weight iron oxide in the form of a composite structure with the molecular formula Fe 3 O 4 and a maximum of 0.5% by weight free iron oxide with the molecular formula Fe 2 O 3 which is however not a component of the composite structure. The bruning layer 14 is also made to be substantially impermeable.Brüner layers are normally used in the art only for rolling bearing parts having large dimensions and / or subjected to high mechanical loads in order to extend the life of the bearing thanks to the absorption by the brüner layer of a part of the mechanical loads.According to the present invention, it has surprisingly been shown that the presence of very thin Brunelian layers on the contact surfaces of the ball set, when combined with a specific surface topology of these sliding surfaces and with contact lubrication obtained with selected kinds of lubricating fluid, is able to greatly reduce the friction that arises during use in rolling bearings of any kind, so that they have a low degree of friction, and in particular in such rolling bearings that are subjected to typical mechanical and tribological stresses of vehicle bearings, such as in wheel hub bearing units for vehicles, such as the wheel hub bearing unit 1, wherein the friction conditions to which the rolling bodies 5 are subjected are mixed, i.e. rolling and sliding friction.Experimental tests have been carried out by the applicant on ball bearings with a double row of balls with balls with a diameter between 8 and 22 mm, in a first sample of ten ball bearings with balls which according to the invention are completely coated on their outer surface with a bruning layer, and in a second sample of ten identical bearings with balls free of a bruning layer, using identical lubricant conditions and mechanical load conditions. The experiments have shown that there is a surprising reduction in friction equal to 50% for the bearings with balls coated with a brick in zero load conditions and 25% in conditions in which there is a load compared to bearings which are identical but have balls free of a bricking layer.To obtain this result, it has been found that the rolling bodies 5 must have a controlled surface roughness, preferably of between 0.02 and 0.08 micrometer, and in combination the raceways 6 and 7 must also have a controlled surface roughness, preferably of between 0.05 and 0.15 micrometer.In principle, the roughness of the raceways 6, 7 must preferably be somewhat greater than that of the rolling bodies, in the present test case that of the balls 5.In the case of rolling elements 5 consisting of balls, it has also been found that other dimensional parameters which can define the parameters of the surface topography must be observed, for example the circularity parameter of the balls must remain within 0.4 and 1 micrometer for ball diameters of between 8 and 22 mm.Conversely, in the case of other types of rolling elements 5, such as tapered rollers, having a variable diameter of between 1 and 16 mm and a length of between 10 and 25 mm, the conicity (angle formed by the side surface of the roller with the axis of symmetry of the roller) must preferably be within 1.5 and 6 degrees.Moreover, a suitable choice of the lubricating fluid 15 to be arranged between the inner ring 4 and the outer ring 3 and in contact with the rolling bodies 5 and the raceways 6, 7 has also proved to be essential for obtaining the surprising result of a marked reduction in friction.In particular, the lubricating fluid 15 present in the rolling bearing 2 is a synthetic-based lubricating fluid treated by hydrocracking and obtained by chemical synthesis from petroleum, or with a base formed with PAO (polyalphaolefins) which must have the following characteristic parameters:• Viscosity range of lubricant base between: 25-120 mm 2 / s at 40° C. according to DIN 51562 and 5-20 mm 2 / s at 100° C. according to DIN 51562;• Type of thickener: lithium complex, calcium complex or polyurea• Working penetration (60 shots, according to ISO 2137): maximum 305• Oil separation (at 40°C for 1 week, according to DIN 51817): less than 4%• Surface tension of the oil base: between 25 and 35 mN / m (millinewtons per meter).The bruising layer on the ready-to-use bearing 2 must have a very small radial thickness of between 0.35 and 0.75 microns, i.e. much thinner than in the known uses of bruising layers.Moreover, it has been found in experimental experiments that, again rather surprisingly, preferably only the outer surfaces of all rolling bodies 5 are coated with a brake layer 14, while the raceways 6, 7 in combination have a bare surface of hardened steel with the above-mentioned surface roughness of between 0.05 and 0.15 micrometers.Finally, further experimental experiments carried out on bearings with two rows of rolling elements, one of which is formed from a row of balls and the other of which is formed from a row of tapered rollers, have shown that it is possible to obtain a reduction in friction of about 10-15%, even by coating only the rolling elements of one of the two rows of rolling elements with the brake layer 14.In a preferred embodiment of the invention, which is not shown for the sake of simplicity of illustration, as it is obvious to the skilled person, the rolling bearing 2 has a series of balls 5 on the side of the end 11 and a series of tapered rollers on the side of the end 12, i.e. the relevant variant of the wheel hub bearing unit 1 has tapered rollers instead of the balls arranged on the right side of FIG. 2; moreover, according to this embodiment, only the tapered rollers are coated with the bruning layer 14 while the balls 5 are bare, it being understood that the other design parameters (roughness) and lubricant parameters described above are observed.In this embodiment it has surprisingly been noted that the reduction of friction is equal to that which can be obtained in the embodiment of Fig. 2, but wherein the ball bearing has two rows of balls and wherein all balls 5 of both rows of rolling elements are coated with a braking layer 14.In other words, in the particular embodiment with a row of balls and a row of rollers, savings can be made in the handling costs of the balls 5, only the tapered rollers being handled and the same end result being obtained.From what has been said above, it is clear that the invention also extends to a method for reducing friction in rolling bearings, in particular in rolling bearings 2 of a wheel hub bearing unit 1 of vehicles, the rolling bearing 2 comprising a radially outer ring 3, a radially inner ring 4 and a plurality of rolling elements 5 interposed between the outer ring 3 and the inner ring 4 so as to be relatively rotatable about a common axis of symmetry A of the inner ring 4 and the outer ring 3, the rolling elements 5 being engaged with respective annular raceways 6, 7 facing each other and respectively formed on a radially inner side surface 8 of the outer ring 3 and on a radially outer side surface 9 of the inner ring 4.According to the main aspect of the invention, the friction reduction method comprises the following steps: a) obtaining the rolling bodies 5 and the raceways 6, 7 with predetermined geometric surface topography parameters by suitable machining, for example lapping, polishing or barrel polishing; b) subjecting all the rolling bodies 5 to at least one row of rolling bodies, in the case of rolling bearings having two rows of rolling bodies, and / or at least one or both of the raceways 6 and 7 after step a) and, prior to the insertion of the rolling bodies 5 between the inner ring 4 and the outer ring 3 for coupling with the raceways 6, 7, a brazing deposition step until the rolling bodies 5 and / or one or both of the raceways 6, 7 are completely coated with a brazing layer 14 of predetermined thickness; and c) introducing a lubricating fluid 15 having predetermined physical / chemical properties between the inner ring 4 and the outer ring 3 and in contact with the rolling bodies 5 and with the raceways 6, 7.Step b) is carried out according to the method of EP2222892B1, optionally improved according to WO2021139973A1, until a substantially uniform layer 15 of brumming having a radial thickness of between 0.7 and 1.5 microns is obtained.Moreover, step a) is carried out to obtain a surface roughness of the rolling bodies 5 (whether they are balls or rollers) of between 0.02 and 0.08 micrometer and, in combination, a surface roughness of the raceways 6, 7 of between 0.05 micrometer and 0.15 micrometer.The rolling elements 5 and the raceways 6, 7 for the rolling elements must be made of hardened steel and must be heat treated to have in combination: the rolling elements 5 a surface hardness of between 65 and 70 HRC (Rockwell C grade) and a core hardness of between 62 and 68 HRC; and the raceways 6, 7 a surface hardness of between 59 and 62 HRC in the case where the inner ring 4 and the outer ring 3 have been completely hardened and of between 680 and 800 HV (Vickers grade), the inner ring 4 and the outer ring 3 having been hardened only along the raceways 6, 7, preferably by induction hardening.Moreover, the lubricating fluid 15 introduced in step c) between the inner ring 4 and the outer ring 3 and in contact with the rolling elements 5 and the raceways 6, 7 must be selected from the group consisting of: lubricant having a hydrocracking-treated synthetic base obtained by chemical synthesis from petroleum; lubricant having a base formed with PAO (polyalphaolefins).In particular, the lubricating fluid 15 is chosen to have the following parameters, all in combination with each other:• Viscosity range of lubricant base between: 25-120 mm2 / s at 40°C according to DIN 51562 and 5-20 mm2 / s at 100°C according to DIN 51562;• Type of thickener: lithium complex, calcium complex or polyurea• Working penetration (60 shots, according to ISO 2137): maximum 305• Oil separation (at 40°C for 1 week, according to DIN 51817): less than 4%• Surface tension of the oil base: between 25 and 35 mN / m (millinewtons per meter).According to a further characteristic feature of the invention, once the inner ring 4 and the outer ring 3 have been assembled with the rolling elements 5, the rolling bearing 2 is not yet fully finished to achieve the surprising reduction in thickness that occurred during the experimental experiments, but must first be subjected to a run-in phase of preferably 2 to 3 hours until a reduction in the radial thickness of the brake layer 14 of at least about 50% is obtained in order to achieve the greatly reduced thicknesses of the layer 14.The advantage of low friction between the elements in sliding conditions is ensured, thanks to the improved and stable lubricating film, also during sliding movements and not just during rolling movements. Examples of sliding conditions in a rolling bearing for a wheel hub unit 1 according to the invention are:relative movement of the rolling elements and the pockets of the cage containing these rolling elements;relative movement of the edges of the conical rolling elements and of the guide flange surface on the rings in the case of bearings with conical rollers.In the case of rolling bearings 2 with two rows of rolling elements, one of the two rows being formed of balls and the other being formed of tapered rollers, step b) is preferably carried out only on all rolling elements 5 consisting of tapered rollers, while the balls remain untreated.The improvement in low friction (rolling and sliding friction) is enhanced after an inlet phase (for example 3 hours at 900 U / min under load, as in the case of a car wheel bearing), wherein the layer of the friction may lose about 50% of its thickness, reducing the manufacturing roughness of the metal components. However, the brumen conversion layer 14 remains present and functional for the entire life of a vehicle.The frictional wear performance of the raceways under poor lubricant conditions is improved by up to 25%; consequently, the depth of the "stall mark" tracks on the vehicle applications is reduced, which is particularly important during long distance transportation of the vehicles and / or in cold conditions, especially in heavy vehicles such as SUVs.The friction layer also acts as a chemical barrier to protect against deleterious lubricant additives, thereby increasing the potential for the use of high performance lubricants which are currently not suitable for applications in bearings for vehicle wheel hub bearing units.One of the advantages of brumming over other types of coatings is the fact that it forms a conversion layer firmly bonded to the base material and only insignificantly affects the dimensions of the metallic component, so that additional or preparatory machining operations that could negatively affect the final result are not required.Moreover, even if a brake layer is worn out, it is not detrimental to the performance of the bearing, because it is not a hard layer that is foreign to the material of the bearing itself.An advantage of brumming when applied to rolling elements is that the rolling elements can be surface treated elsewhere than the final assembly, can be shipped as bulk material in packages and handled without special care without the risk of the coating becoming flaked off, removed or degraded in performance. A minimum amount of oiling in the surfaces is sufficient for the proper shipping and preservation of the metallic components.The brumming treatment also has slight anti-corrosion properties useful during storage and shipping of the final product and to improve the overall life of the bearing (e.g., when the inner rollers are exposed to moisture).All the objects of the invention are thus achieved.References included in the specificationThis list of documents cited by the applicant has been produced in an automated manner and is only included for the better information of the reader. The list is not part of the German patent application or utility model application. The DPMA does not take any adhesion for any faults or omissions.Patent Literature citedEP 2222892B

[0003] EP 2222892B1 [0004, 0005, 0025, 0044]WO 2021139973A1 [0005, 0025, 0044]Cited Non-Patent LiteratureDIN 51562

[0048] ISO 2137

[0048] DIN 51817

[0048]

Claims

A method for reducing friction in rolling bearings (2), in particular in rolling bearings of wheel hub bearing units (1) of vehicles, said rolling bearing comprising a radially outer ring (3), a radially inner ring (4) and a plurality of rolling elements (5) interposed between said outer ring and said inner ring, for making them relatively rotatable about a common axis of symmetry (A) of said inner and outer rings (4,3), said rolling elements engaging respective annular raceways (6,7) facing each other and respectively formed on a radially inner side surface (8) of said outer ring and a radially outer side surface (9) of said inner ring; characterized in that the method comprises the steps of: (a) obtaining said rolling elements (5) and said raceways (6,7) having predetermined geometric surface topography parameters by suitable machining; (b) subjecting all rolling elements (5) of at least one row of rolling elements, in the case of rolling elements (2) having two rows of rolling elements, or / and at least one or both raceways (6, 7), after step (a) and prior to the insertion of the rolling elements between the inner and outer rings (4, 3) for coupling with the raceways (6, 7), to a friction deposition step until the rolling elements (5) and / or one or both of the raceways (6, 7) are completely coated with a friction layer (4) of predetermined thickness; and (c) introducing a lubrication fluid (15) having predetermined physical / chemical properties between the inner and outer rings (4, 3) and in contact with the rolling elements (5) and the raceways (6, 7).Method according to claim 1, characterized in that in step (b) a brumming layer (14) having a radial thickness between 0.7 and 1.5 microns is deposited.Method according to claim 1 or 2, characterized in that step (a) is carried out to obtain a surface roughness of the rolling bodies (5) between 0.02 and 0.08 microns and, in combination, a surface roughness of the raceways (6, 7) between 0.05 and 0.15 microns.Method according to any of the preceding claims, characterized in that both the rolling bodies (5) and the raceways (6, 7) for the rolling bodies are made of a hardened steel and have been heat treated so as to have in combination: the rolling bodies (5) a surface hardness between 65 and 70 HRC (Rockwell C-grades) and a core hardness between 62 and 68 HRC; and the raceways (6, 7) a surface hardness between 59 and 62 HRC, wherein the inner and outer rings (4, 3) have been completely hardened, and between 680 and 800 HV (Vickers-grades), wherein the inner and outer rings (4, 3) have been hardened only along the raceways (6, 7), preferably by induction hardening.Method according to any one of the preceding claims, characterized in that the lubricating fluid (15) introduced in step (c) between the inner and outer rings (4, 3) and in contact with the rolling elements (5) and with the raceways (6, 7) is selected from the group consisting of: lubricant with a hydrocracking-treated synthetic base obtained by chemical synthesis from petroleum; lubricant with a base formed with PAO (polyalphaolefins).Method according to claim 5, characterized in that the lubricating fluid (15) is selected to have the following parameters in combination with each other: - viscosity range of the lubricating base between: 25-120 mm 2 / s at 40°C according to DIN 51562 and 5-20 mm 2 / s at 100°C according to DIN 51562; - type of thickener: lithium complex, calcium complex or polyurea; - working penetration (60 strokes according to ISO 2137): maximum 305; - oil separation (at 40°C for 1 week according to DIN 51817): less than 4 percent; - base oil surface tension: between 25 and 35 mN / m (millinewtons per meter).Method according to any one of the preceding claims, characterized in that once the inner and outer rings (4, 3) are assembled with the rolling elements (5), the rolling bearing (2) is subjected to an inlet phase, preferably for a period of between 2 and 3 hours at 900 U / min, until a reduction in the radial thickness of the brake layer of about 50 percent is achieved.Low friction rolling bearing (2), in particular wheel hub bearing units (1) of vehicles, comprising a radially outer ring (3), a radially inner ring (4) and a plurality of rolling elements (5) interposed between the outer ring and the inner ring (3, 4) to make them relatively rotatable about a common axis of symmetry (A) of the inner and outer rings, said rolling elements (5) engaging respective annular raceways (6, 7) facing each other and respectively formed on a radially inner side surface (8) of the outer ring (3) and a radially outer side surface (9) of the inner ring (4); characterized in that in combination: (i) the rolling elements (5) of at least one row of rolling elements, in the case of rolling bearings having two rows of rolling elements, and / or at least one or both of the corresponding raceways (6, 7), completely coated with a brake layer (14) of predetermined thickness; (ii) the rolling elements (5) have a surface roughness of between 0.02 and 0.08 micrometers and, in combination, the raceways (6, 7) have a surface roughness of between 0.05 and 0.15 micrometers; (iii) there is a lubricating fluid (15) between the inner ring (4) and the outer ring (3) and in contact with the rolling elements (5) and the raceways (6, 7) with a hydrocracking-treated synthetic base obtained by chemical synthesis from petroleum or with a formed PAO (poly-alpha-olefin) base having the following characteristic parameters: - viscosity range of the lubricating base between: 25-120 mm 2 / s at 40°C according to DIN 51562 and 5-20 mm 2 / s at 100°C according to DIN 51562; - type of thickener: lithium complex, calcium complex or polyurea; - working penetration (60 strokes according to ISO 2137): maximum 305; Oil separation (at 40°C for 1 week, according to DIN 51817): less than 4%; - Base oil surface tension: between 25 and 35 mN / m (millinewtons per meter).Low friction rolling bearing according to claim 8, characterized in that the brake layer (4) has a radial thickness between 0.35 and 0.75 microns.Low friction rolling bearing according to claim 8 or 9, characterized in that only all rolling elements (5) of at least one row of rolling elements, in the case of rolling elements having two rows of rolling elements, are coated with the friction layer (4), while the corresponding raceways (6, 7) have a bare surface of hardened steel having the surface roughness of between 0.05 and 0.15 microns; preferably the rolling bearing (2) comprises a first row of rolling elements (5) consisting of balls and a second row of rolling elements (5) consisting of conical rollers, only the rolling elements (5) of the second row of rolling elements being completely coated with the friction layer (14).

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