FR3149656A1 - Rolling bearing - Google Patents

Abstract

The invention relates to a rolling bearing comprising an outer member (1) and an inner member (2) each formed of rolling steel and provided with at least one annular rolling track (1a, 2a) forming a rolling space (3) between said members, at least one row (4) of rolling bodies (4a) being arranged in said space to allow the relative rotation of said members, the rolling bodies (4a) being formed from a ceramic material comprising zirconium dioxide whose electrical resistivity is sufficient to guarantee electrical insulation between the members (1, 2) through the rolling space (3). Figure 1

Description

Rolling bearing

The invention relates to a rolling bearing comprising an outer member and an inner member each provided with at least one annular rolling track forming a rolling space between said members, at least one row of rolling bodies being arranged in the rolling space to allow the relative rotation of the members. The invention also relates to an electrical machine comprising a rotor which is rotatably mounted relative to a stator by means of such a rolling bearing.

In this application, electric currents can pass between the rotor and the stator which, since the components of a bearing are typically made of rolling steel, propagate between the components via the rolling bodies.

Since these currents flow through the rolling bearings are detrimental to their service life, rolling bearings have been proposed in which at least some components provide electrical insulation.

To do this, at least one organ can be made at least partially of electrically insulating material, without this solution being satisfactory in terms of costs and/or reliability of the rolling bearing.

So-called hybrid bearings are also known in which the components are made of conventional rolling steel and the rolling bodies are formed from a material whose electrical resistivity is sufficient to ensure electrical insulation between the components through the rolling space. In particular, the use of ceramic material based on Si ₃ N ₄ has been proposed to solve this electrical problem.

However, particularly in applications with high rotor rotation speeds, the electrically insulating materials used do not have physical properties, in particular thermal and/or mechanical properties, which are relevant, particularly in relation to those of bearing steels, to ensure the reliability of the guidance provided by the rolling bearing.

The invention aims to improve the prior art by proposing in particular a rolling bearing in which the rolling bodies are formed from a material ensuring electrical insulation between the rolling steel members, while ensuring homogeneity of the physical properties between said material and said rolling steel to make the operation of the rolling bearing more reliable.

For this purpose, according to a first aspect, the invention proposes a rolling bearing comprising an outer member and an inner member each formed from rolling steel and provided with at least one annular rolling track forming a rolling space between said members, at least one row of rolling bodies being arranged in said space to allow the relative rotation of said members, the rolling bodies being formed from a ceramic material comprising zirconium dioxide whose electrical resistivity is sufficient to guarantee electrical insulation between the members through the rolling space.

According to a second aspect, the invention proposes an electric machine comprising a rotor which is rotatably mounted relative to a stator by means of such a rolling bearing, said bearing being subjected to passages of electric currents between said rotor and said stator.

Other objects and advantages of the invention will appear in the description which follows, made with reference to the attached,

which represents in partially cut perspective a rolling bearing according to an embodiment of the invention.

In relation to this figure, a rolling bearing is described below comprising an outer member 1 and an inner member 2 each provided with at least one annular rolling track 1a, 2a forming a rolling space 3 between said members.

To allow the relative rotation of the members 1, 2, at least one row 4 of rolling bodies 4a is arranged in the rolling space 3, in particular by being kept spaced apart by a cage 5.

In particular, a lubricant is arranged in the rolling space 3 in order to reduce the friction of the rolling bodies 4a during rotation. Depending on the application considered, the lubrication can be obtained by means of an oil and/or a grease introduced into the rolling space 3. According to one embodiment, a porous polymer matrix can be arranged in the rolling space 3, in particular by being in contact with the rolling bodies 4b, said matrix trapping a lubricant comprising an oil and/or a grease.

In the figure, the members 1, 2 are each formed of a ring between which is arranged a row 4 of rolling bodies 4a of cylindrical geometry. However, the invention is not limited to a particular geometry of the members 1, 2 and/or the rolling bodies 4a.

The organs 1, 2 are each formed from bearing steel, in particular from high-carbon steel to withstand significant loads, both static and dynamic.

In particular, to produce the components 1, 2 of a rolling bearing, a steel called 100Cr6 is conventionally used, the composition of which in weight percentage is in the ranges given below, in particular depending on the intended application: C:
Mn:
S:
Mo:
V:
Cu:
You:
Pb: 0.95 – 1.05
0.2 – 0.4
0 – 0.02
0 – 0.15
0 – 0.1
0 – 0.25
0 – 0.05
0 – 0.15 If :
P:
Cr:
Neither :
Al:
W:
Co:
Fe: 0.17 – 0.37
0 – 0.027
1.3 – 1.65
0 – 0.3
0 – 0.1
0 – 0.1
0 – 0.1
the rest

Alternatively, the members 1, 2 can be made of high carbon chrome steel, in particular according to the designation SUJ2 or AMS52100.

According to one embodiment, the members 1, 2 can be made of high carbon steel as previously mentioned with the additional addition of silicon and/or manganese, in particular according to the name 100CrMnSi6-4 or SUJ3.

The rolling bodies 4a are formed from a ceramic material whose electrical resistivity is sufficient to guarantee electrical insulation between the members 1, 2 through the rolling space 3.

Thus, the bearing can be advantageously used in an electrical machine for rotating a rotor relative to a stator, in which said bearing is subjected to passages of electric currents between said rotor and said stator.

In fact, the electrical insulation provided by the rolling bodies 4a makes it possible to prevent currents from passing through the rolling bearing, and therefore the induced deterioration of said bearing.

The ceramic material of the rolling bodies 4a comprises zirconium dioxide (ZrO ₂ or Zirconia), the specific choice of which makes it possible to combine good electrical insulation with physical properties which, in relation to those of the rolling steel of the members 1, 2, make the operation of the bearing more reliable.

In particular, the electrical resistivity of the material of the rolling bodies 4a may be greater than 10 ¹⁵ Ω*mm ² /m, that of the bearing steel being conventionally between 0.1 and 1 Ω*mm ² /m .

In particular, the rolling bodies 4a can be manufactured by sintering the ceramic material comprising zirconium dioxide, with in particular the possibility of modulating the production performance (sintering quality) or use of the rolling bodies 4a based on ZrO ₂ (toughness) via the composition of the ceramic, in particular the Ytrium content.

Advantageously, the ceramic material of the rolling bodies 4a has thermal performances which guarantee the stability and dimensional precision of the bearing in temperature, in particular the stability of dimensional interference between the members 1, 2 and the rolling bodies 4a.

In particular, in high-speed applications, the rotation of a rotor of an electric machine generates significant temperature variations over time and differentials between the components 1, 2, with differences of up to 40°C, which can significantly degrade the optimized clearances between the components of the rolling bearing, in particular in a manner amplified by the low thermal conductivity of the ceramic materials.

To solve this problem, the ceramic material of the rolling bodies 4a may have a coefficient of thermal expansion whose value is less than 130% of the coefficient of thermal expansion of the rolling steel of the members 1, 2.

Thus, by using a ceramic material whose thermal properties are close to those of bearing steel, a certain dimensional homogeneity of the components of the rolling bearing is ensured as a function of temperature.

In an example relationship, the coefficient of thermal expansion (CTE) of zirconium dioxide material is of the order of 10x10 ^-6 K ^-1 , that of bearing steel being typically of the order of 12x10 ^-6 K ^-1 .

• nuance : Base 100Cr6 avec variantes avec ajouts de silicium et manganèse (0,4 à 1,5% Si / 0,8 à 1,2% Mn) ;
• traitement thermique et/ou thermochimiques (nitruration, carbonitruration, froid, revenus).

According to one embodiment, the thermal stability in service of the members 1, 2 can be improved from the point of view of dimensional stability and/or hardness by a combined solution of grade and/or treatments, for example:

• grade: Base 100Cr6 with variants with additions of silicon and manganese (0.4 to 1.5% Si / 0.8 to 1.2% Mn);
• heat and/or thermochemical treatment (nitriding, carbonitriding, cold, tempering).

Advantageously, the ceramic material of the rolling bodies 4a has mechanical performances which guarantee, in particular for high speed and high rigidity applications, the operational reliability of the bearing, in particular in relation to the resistance to the initiation of cracking/spreading of flaking due to the medium polluted by hard particles and to the resistance to shocks.

According to one embodiment, the ceramic material of the rolling bodies 4a has a Young's modulus of elasticity E whose difference with the Young's modulus of elasticity E of the rolling steel of the members 1, 2 is less than 10%.

In an exemplary embodiment, the Young's modulus of elasticity E of the ceramic material is of the order of 210 GPa, that of the bearing steel being conventionally also of the order of 210 GPa, so as to obtain a homogeneity between the materials which is conducive to the best operation of the rolling bearing.

In particular, the ceramic material of the rolling bodies 4a, despite its high hardness, allows less impact on the steel members 1, 2 thanks to a lower Young's modulus of elasticity E than traditional ceramics, leading to less shear stresses in said members than with rolling bodies 4a for example based on Si ₃ N ₄ , and this without the reduction in rigidity affecting the operation of the rolling bearing.

According to an advantageous embodiment, the ceramic material of the rolling bodies 4a has a Poisson's ratio whose deviation from the Poisson's ratio of the rolling steel of the members 1, 2 is less than 10%.

In an exemplary embodiment, the Poisson's ratio of the ceramic material is of the order of 0.3, that of the bearing steel being conventionally also of the order of 0.3, so as to obtain a homogeneity between the materials which is conducive to the stable and predictive operation of the rolling bearing.

According to an advantageous embodiment, the rolling steel of the members 1, 2 has, at least at the level of at least one of the rolling tracks 1a, 2a, an HV hardness which is greater than 60% of the HV hardness of the ceramic material of the rolling bodies 4a.

To do this, the rolling steel of the members 1, 2 may have undergone a specific hardening treatment of at least one of the rolling tracks 1a, 2a to have an HV hardness greater than 750, in particular being at least 850. According to one embodiment, at least the rolling tracks 1a, 2a and in particular the members 1, 2 may have the HV hardnesses defined above.

In particular, the members 1, 2 may have undergone heat treatment in the mass, superficially and/or by surface enrichment of surface hardening, for example of the cementation, carbonitriding, nitriding, induction type.

Thus, particularly in relation to an HV hardness of the ceramic material of the rolling bodies 4a which is less than 1500, the hardness difference is reduced in order to make the operation of the rolling bearing more reliable.

• un renforcement mécanique par :
  • un faible écart de module d’élasticité d’Young E entre l’acier à roulement et le matériau céramique des corps roulants 4a, notamment pour diminuer les contraintes dans les organes 1, 2 ;
  • un renforcement de la résistance mécanique en milieu pollué des organes 1, 2, notamment par carbonitruration de l’acier à roulement ;
  • une bonne tenue en température, notamment en utilisant des nuances d’acier à roulement au Si/Mn avec traitement masse et/ou thermochimique pour les organes 1, 2 ;
• un renforcement dimensionnel par :
  • des CTE proches entre ledit matériau céramique et l'acier à roulement ;
  • utilisation de nuances d'acier à roulement avec Mn/Si et traitements thermiques adaptés des organes 1, 2.

Advantageously, a rolling bearing according to the invention can have:

• mechanical reinforcement by:
  • a small difference in Young's modulus of elasticity E between the rolling steel and the ceramic material of the rolling bodies 4a, in particular to reduce the stresses in the components 1, 2;
  • strengthening of the mechanical resistance in polluted environments of components 1, 2, in particular by carbonitriding of the bearing steel;
  • good temperature resistance, in particular by using Si/Mn bearing steel grades with mass and/or thermochemical treatment for components 1, 2;
• dimensional reinforcement by:
  • close CTEs between said ceramic material and the bearing steel;
  • use of rolling steel grades with Mn/Si and suitable heat treatments of components 1, 2.

Claims (9)

Rolling bearing comprising an outer member (1) and an inner member (2) each formed of rolling steel and provided with at least one annular rolling track (1a, 2a) forming a rolling space (3) between said members, at least one row (4) of rolling bodies (4a) being arranged in said space to allow the relative rotation of said members, said bearing being characterized in that the rolling bodies (4a) are formed from a ceramic material comprising zirconium dioxide whose electrical resistivity is sufficient to guarantee electrical insulation between the members (1, 2) through the rolling space (3). Rolling bearing according to claim 1, characterized in that the ceramic material of the rolling bodies (4a) has a coefficient of thermal expansion whose value is less than 130% of the coefficient of thermal expansion of the rolling steel of the members (1, 2). Rolling bearing according to one of claims 1 or 2, characterized in that the ceramic material of the rolling bodies (4a) has a Young's modulus of elasticity E whose deviation from the Young's modulus of elasticity E of the rolling steel of the members (1, 2) is less than 10%. Rolling bearing according to any one of claims 1 to 3, characterized in that the ceramic material of the rolling bodies (4a) has a Poisson's ratio whose deviation from the Poisson's ratio of the rolling steel of the members (1, 2) is less than 10%. Rolling bearing according to any one of claims 1 to 4, characterized in that the rolling steel of the members (1, 2) has, at least at the level of at least one of the rolling tracks (1a, 2a), an HV hardness which is greater than 60% of the HV hardness of the ceramic material of the rolling bodies (4a). Rolling bearing according to claim 5, characterized in that the rolling steel of the members (1, 2) has undergone a specific hardening treatment of at least one of the rolling tracks (1a, 2a) to have an HV hardness greater than 750. Rolling bearing according to claim 6, characterized in that the HV hardness of at least one of the rolling tracks (1a, 2a) is at least 850. Rolling bearing according to any one of claims 5 to 7, characterized in that the HV hardness of the ceramic material of the rolling bodies (4a) is less than 1500. Electric machine comprising a rotor which is rotatably mounted relative to a stator by means of a rolling bearing according to any one of claims 1 to 8, said bearing being subjected to passages of electric currents between said rotor and said stator.