Grease composition for bearings of rail vehicles

Abstract

The present invention relates to a grease composition for bearings of rail vehicles, in particular rail wheel bearings and traction motor bearings of rail vehicles. The invention further relates to rail wheel bearings and traction motor bearings comprising said grease composition and the use of the grease composition for grease lubrication of rail wheel bearings and traction motor bearings.

Description

[0001] Grease composition for bearings of rail vehicles

[0002] SUBJECT MATTER OF THE INVENTION

[0003] The present invention relates to a grease composition for bearings of rail vehicles, in particular rail wheel bearings and traction motor bearings of rail vehicles. The invention further relates to rail wheel bearings and traction motor bearings comprising said grease composition and the use of the grease composition for grease lubrication of rail wheel bearings and traction motor bearings.

[0004] BACKGROUND OF THE INVENTION

[0005] In the field of rail mobility, wheel bearings are used millions of times and are one of the critical components, which require a strict maintenance schedule. Axle bearings (axlebox bearings) are an essential design element in the bogies and chassis on all railway vehicles that connect the wheelset to the non-rotating parts of the rail vehicle and must transmit the weight of the vehicle to the wheelsets while providing a smooth rolling movement for the wheelsets. Rolling stock axle bearings are subject to radial impact loads caused e.g., by rail joints and switches, as well as to the static and dynamic radial loads of vehicle weight. Axial loads can be caused by lateral movements e.g., when trains run on curved rails. The combination of significant relative movements and high forces creates the risk for wear. Recent rolling stock are therefore equipped with greased rail wheel bearings, which require monitoring and maintenance. Once a damage process is initiated, it tends to accelerate, as the pre-damaged bearing is source for local vibrations and heat, which both represent additional loads, and subsequently a source for damage increase. Also, the bearings used in the traction motors of e.g., electric locomotives are exposed to high mechanical stresses. Today's traction motor bearings must possess excellent highspeed and heavy-load capabilities. One important factor especially in the development of maintenance free traction motor bearings is to prevent electric pitting.

[0006] An important instrument for reducing the operating costs of rail vehicles is the extension of maintenance intervals. A key influencing factor for increasing the service life of rail wheel bearings is the service life of the grease used for lubrication. The lubricant compositions used to lubricate railway wheel bearings are subjected to severe operating conditions, with the afore-mentioned high mechanical stress on the wheel bearings, as well as high maximum operating temperatures and large temperature differences, high running speeds, long operating times, dust, humidity, and discharges of electric current, etc. The need for regular replacement of lubricants is a major obstacle to maintenance-free operation.

[0007] Axle bearings are in particular selected from cylindrical roller bearings, tapered roller bearings and spherical roller bearings. Actual axle bearings can be classified into the following six types based both on bearing type and sealing device:

[0008] RCT Bearings (Sealed-Clean Rotating End Cap Tapered Roller Bearings), RCC Bearings (Sealed-Clean Rotating End Cap Cylindrical Roller Bearings), Spherical roller bearings, Cylindrical roller bearings combined with ball bearings, Cylindrical roller bearings with ribs, Tapered roller bearings.

[0009] CN 109135886 A describes a lubricating grease composition for rolling bearings of highspeed trains reaching speeds of 250-350 km / h. The lubricating grease composition comprises a base oil, comprising 30 - 38 wt.% mineral oil, 60 - 65 wt.% synthetic oil and 2 - 5 wt.% ester oil with a viscosity of the base oil mixture in the range of 10 - 60 mm2 / s at 40°C. The base oil is thickened with a mixture of lithium soaps, comprising lithium stearate soap, lithium 12-hydroxystearate soap and hydrogenated castor oil lithium soap. Further additives are antioxidants, anti-wearing agents, rust preventive agents and ZnO as structure improving agent.

[0010] CN 106867633 A describes a high-speed railway shaft box double-row tapered roller bearing lubricating grease composition, comprising 50 - 83 wt.% of a base oil mixture, 8 - 25 wt.% thickener and 6 - 25 wt.% further additives. The base oil comprises an 1 : 1 mixture of an ester oil and a synthetic oil with a viscosity of the base oil mixture in the range of 40 - 80 mm2 / s at 40°C and a pour point of -65°C. The thickening agent comprises lithium soaps of 12-hydroxystearic acid, heptanoic acid and myristic acid. Further additives are calcium sulfonate, extreme pressure agents, antirust agents, repairing agents and antioxidants. An example of a repairing agent is a mixture of nano copper oxide and nano zinc oxide.

[0011] JP 2014108999 A (JP 5988376 B2) describes a grease composition for a main electric motor shaft bearing of a railway vehicle, which has excellent heat resistance and a long bearing lubrication life. The grease composition comprises a thickener including a hydroxy fatty acid with a carbon number of 12 to 24 having one or a plurality of hydroxyl groups and a lithium salt of a fatty acid or dicarboxylic acid with a carbon number of 2 to 12. The grease composition further comprises a base oil mixture which has a kinematic viscosity at 40°C of 80 to 200 mm2 / s, comprising an oil with a kinematic viscosity at 40°C of 300 to 500 mm2 / s, an oil with a kinematic viscosity at 40°C of 80 to 300 mm2 / s and an oil with a kinematic viscosity at 40°C of 20 to 80 mm / s. The grease composition further comprises an amine-based antioxidant and a hindered phenolic antioxidant. A use of the grease composition for rail wheel bearings is not described.

[0012] US 2011 136578 A1 describes a grease composition, comprising a base grease, consisting of a base oil and a thickener, and an additive, wherein said base oil contains not less than 50 wt.% of highly refined oil having a viscosity index of 120 to 180. The additive contains at least one poly(meth)acrylate having a kinematic viscosity of not less than 100 mm2 / s and less than 850 mm2 / s at 100°C and zinc dithiophosphate. The thickener contains a lithium soap, lithium complex soap or amide lithium complex soap.

[0013] EP 1719812 A1 describes a grease comprising a base grease and an additive, wherein the base grease comprises a base oil and a thickener, and the additive comprises a substance selected from bismuth and inorganic bismuth compounds. The base oil is selected from polyalphaolefin oils, mineral oils, ester oils, and ether oils and has a kinematic viscosity of 20 to 200 mm2 / s at 40°C. The only ether oil mentioned in the application is phenyl ether oil. Suitable thickeners that can be used for the grease composition are aluminium, lithium, sodium, and further metallic soap-based thickeners such as composite lithium, composite calcium, and composite aluminium and diurea compounds. Further described is a rolling bearing comprising an inner ring, an outer ring, a plurality of rolling elements interposed between said inner ring and said outer ring, wherein a grease according to the invention is applied to a periphery of said rolling elements.

[0014] JP 2010001429 A describes a grease for railway vehicle bearings, capable of effectively suppressing damage by electrolytic corrosion. The grease composition comprises a base oil, a thickening agent, and an additive, containing at least an epoxy compound. Suitable base oils include mineral oils, highly refined mineral oils, liquid paraffin oils, polybutene oils, GTL oils synthesized by the Fischer-Tropsch process, hydrocarbon-based synthetic oils, such as polyalphaolefin oils, alkylnaphthalene oils, alicyclic compounds, natural fats and oils, polyol ester oils, phosphate ester oils, polymer ester oils, aromatic ester oils, carbonate ester oils and further synthetic oils, such as diester oils, polyglycol oils, silicone oils, polyphenyl ether oils, alkyldiphenyl ether oils, alkylbenzene oils, and fluorinated oils. Examples of thickening agents include soaps, such as lithium soap, lithium complex soap, calcium soap, calcium complex soap, aluminium soap, and aluminium complex soap.

[0015] JP 2008111515 A describes a railway vehicle bearing containing a water-resistant grease encased around the rolling elements which can effectively prevent the peeling on a rolling surface attributing to water infiltration and has a good long-term durability. The grease comprises a non-aqueous base oil, a thickening agent, and a water dispersing additive. The base oil is selected from polyalphaolefin oils and mineral oils. Suitable thickening agents are bentone, silica gel, fluorine compounds, soaps such as lithium soap, lithium complex soap, calcium soap, calcium complex soap, aluminium soap, aluminium complex soap, diurea compounds, polyurea compounds, and the like. Urea-based thickeners are preferred. It is mentioned that the grease composition may contain a surfactant as water dispersing additive. Suitable surfactants are disclosed by way of a list that inter alia comprises poly(alkylene) glycols. It is not disclosed that the base oil can be a mixture of at least one polyalphaolefin and at least one poly(alkylene)glycol.

[0016] JP 2007217609 A describes a grease composition with good water resistance, heat resistance and corrosion resistance that keeps lubrication for a long period, comprising a base oil, a thickening agent, and an anticorrosive additive, wherein the base oil contains an ionic liquid. The base oil used with the ionic liquid may be any oil commonly used as a base oil for lubricating compositions, like mineral oils, synthetic oils or natural oils. Among a plethora of suitable oils also polyalphaolefins and polyglycols are mentioned, but not in form of a mixture. Also the thickener may be selected from a great number of different compounds, inter alia composite metal soaps of Li. Preferred as thickener are urea and urethane compounds.

[0017] US 2018 / 0148661 A1 describes an axle oil composition, comprising: 40 to 70 wt.% of polyalphaolefin (PAO) synthetic oil,

[0018] 5 to 35 wt.% of an oil soluble poly alkylene glycol synthetic oil,

[0019] 5 to 20 wt.% of an ester-based viscosity modifier, 0.05 to 0.5 wt.% of calcite, and

[0020] 5 to 20 wt.% of an additive, preferably selected from dithiophosphate-based wearresistant agents, calcium-based detergents, phosphate ester-based friction modifiers, bis-succinimide type ashless dispersants, polysulfide extreme pressure agents, and antioxidants.

[0021] WO 2016 / 137880 A1 describes a lubricant formulation comprising: a) at least 50 wt.% of a hydrocarbon base oil, b) 5 to less than 50 wt.% of an oil soluble polyalkylene glycol selected from monol-, diol- and triol-initiated 1 ,2-butylene oxide homopolymers and monol initiated copolymers of 1 ,2-butylene oxide and propylene oxide, and c) 0.1 - 5 wt.% of a sulfurized olefin, where weight-percent is based on total lubricant formulation weight.

[0022] WO 2016 / 137882 A1 describes a lubricant formulation comprising: a) at least 50 wt.% of a hydrocarbon base oil, b) 5 to less than 50 wt.% of one or a combination of more than one oil soluble polyalkylene glycol selected from monol-, diol- and triol-initiated 1 ,2-butylene oxide homopolymers and monol-initiated copolymers of 1 ,2-butylene oxide and propylene oxide, c) 0.1 - 5 wt.% of a sulfurized olefin, and d) 0.1 - 2 wt.% of phosphate ester; wherein the weight-percent of components a)-d) is based on total lubricant formulation weight.

[0023] US 2003087768 A1 describes a lubricating grease composition containing a base oil, at least one thickener and at least one agent for reducing the coefficient of friction, comprising a metal oxide that has a specific surface area of at least 100 m2 / g, selected from the group consisting of titanium oxide, silicon dioxide, aluminium oxide and tin dioxide. The base oil is preferably selected from mineral oils, synthetic oils and mixtures thereof. Examples of suitable synthetic oils are polyalphaolefins, polyesters and polyalkylene glycols. In a preferred embodiment, the base oil is a synthetic oil on the basis of a polyalkylene glycol. In particular, said base oil comprises polypropylene glycol, trimethylol propane ester and alkyl benzene. This combination is also used in the examples. In a preferred embodiment, the thickener is a lithium complex soap of 12- hydroxy stearic acid and azelaic acid. This document does not contain any reference to the use of a mixture as a base oil that comprises a polyalphaolefin and a poly(alkylene)glycol. There is also no reference to the use of an ionic liquid in the lubricating grease composition. US 2010187481 A1 (KR1020120048035, EP2164934) describes the use of ionic liquids to improve the protection against oxidative and thermal degradation of lubricating compositions, consisting of a mixture of

[0024] (a) 5 to 95 wt.% of a base oil or a base oil mixture, based on synthetic, mineral or native oils, which are used individually or in combination,

[0025] (b) 0.05 to 40 wt.% of an ionic liquid and

[0026] (c) 0.1 to 10 wt.% of an additive or additive mixture.

[0027] It is very generally mentioned that the lubricating compositions may be used for railway bearings. The synthetic oils are selected from esters of aromatic or aliphatic di-, tri- or tetracarboxylic acids with one or a mixture of C? to C22 alcohols, polyphenyl ethers or alkylated di- or triphenyl ethers, esters of trimethylolpropane, pentaerythritol or dipentaerythritol with aliphatic C7 to C22 carboxylic acids, C18 dimeric acid esters with C7 to C22 alcohols, complex esters, polyalphaolefins, alkylated naphthalenes, alkylated benzenes, polyglycols, silicone oils and perfluoropolyethers. This document does not contain any reference to the use of a mixture as a base oil that comprises a polyalphaolefin and a poly(alkylene)glycol. There is also no reference to the use of a lithium soap or lithium complex soap in the lubricating composition.

[0028] US 2012202724 A1 (US8697618B2) describes a method of enabling operation of chains, steel belts, wheel bearings, roller bearings, sliding bearings, electric motors, etc. for at least 48 hours by reducing the evaporation loss and the lackification tendency of a lubricant, comprising the steps of: applying a liquid lubricant having a kinematic viscosity at 40°C between 50 mm2 / s and 1000 mm2 / sec and comprising a mixture of

[0029] (a) 99.3 to 30 wt.% of a base oil or a base oil mixture of at least one synthetic oil, group III oils, native oils;

[0030] (b) 0 to 50 wt.% of a polymer or polymer mixture based on polyisobutylene, which can be partly or fully hydrogenated;

[0031] (c) 0.2 to 10 wt.% of an ionic liquid or mixtures of ionic liquids; and

[0032] (d) 0.5 to 10 wt.% of additives or additive mixtures; and operating said one of chains, etc. for at least 48 hours without lackification of the lubricant.

[0033] In one embodiment, the base oil comprises synthetic oils, selected from esters of aromatic or aliphatic di-, tri- or tetracarboxylic acids with at least one C7 to C22 alcohols, polyphenyl ethers or alkylated di- or triphenyl ethers, esters of trimethylolpropane, pentaerythritol or dipentaerythritol with aliphatic C7 to C22 carboxylic acids, C18 dimeric acid esters with C7 to C22 alcohols, complex esters, polyalphaolefins, alkylated naphthalenes, alkylated benzenes, polyglycols, silicone oils and perfluoropolyethers. The examples show inter alia the reduction in the electric resistance of the oils due to the addition of ionic liquids. There is no reference to the use of a mixture as a base oil that comprises a polyalphaolefin and a poly(alkylene)glycol. There is also no reference to the use of a lithium soap or lithium complex soap in the lubricating composition.

[0034] US 2010105586 A1 (US8258088B2) describes a lubricating grease composition consisting of a mixture of (a) 5 to 95 wt.% of a base oil selected from esters of aromatic or aliphatic di-, tri- or tetracarboxylic acids with one or a mixture of C? to C22 alcohols, a polyphenyl ether or alkylated di- or triphenyl ether, an ester of trimethylolpropane, pentaerythritol or dipentaerythritol with aliphatic C7 to C22 carboxylic acids, from C18 dimeric acid esters with C7 to C22 alcohols, from complex esters, poly-a-olefins, alkylated naphthalenes, alkylated benzenes, polyglycols, silicone oils and perfluoropolyethers,

[0035] (b) 1 to 30 wt.% of at least one ionic liquid, containing a cation selected from a phosphonium cations, imidazolium cations, pyridinium cations or pyrrolidinium cations, and whose anion contains fluorine and are selected from bis(trifluoromethylsulfonyl)imide, bis(perfluoroalkylsulfonyl)imide, perfluoroalkylsulfonate, tris(perfluoroalkyl)methidenes, bis(perfluoroalkyl)imidenes, bis(perfluoroaryl)imides, perfluoroarylperfluoroalkyl sulfonylimides and tris(perfluoroalkyl)trifluorophosphate or a halogen-free alkyl sulfate,

[0036] (c) 3 to 50 wt.% of a thickening agent selected from certain urea thickeners or metal soaps, metal sulfonates, metal complex soaps, bentonite, silicate powder, polytetrafluoroethylene (PTFE), polyamide and polyimide, and

[0037] (d) 0.1 to 10 wt.% additives selected from corrosion preventives, oxidation preventives, wear preventives, friction reducing agents, agents to protect against metal effects, UV stabilizers, organic or inorganic solid lubricants, selected from polyimide, polytetrafluoroethylene (PTFE), graphite, metal oxides, boron nitride, molybdenum sulfide and phosphate.

[0038] US 2021 / 0253972 A1 describes a lubricant composition comprising a lubricant, a first ionic liquid which is soluble in polyalphaolefin and a second ionic liquid which is insoluble in polyalphaolefin. This document does not disclose the specific combination of a base oil mixture, comprising at least one polyalphaolefin, and at least one poly(alkylene)glycol, with at least one lithium complex soap, and at least one ionic liquid.

[0039] EP 4239039 A1 is a post-published document and regards a lubricant composition comprising: a) 20 to 99.5% by weight, based on the total weight of the lubricant composition, of a basis oil (Grundbl), wherein the basis oil has a solubility for the ionic liquid methyltrioctylammonium bis(fluorosulfonyl)imide of at least 3% by weight at a temperature of 20°C, and wherein the basis oil comprises a base oil A) (Basisbl) in a proportion of at least 50% by weight, based on the total weight of the basis oil, wherein the base oil A) has a solubility for the ionic liquid methyltrioctylammonium bis(fluorosulfonyl)imide of at least 3 % by weight at a temperature of 20°C, b) 0.5 to 80 % by weight, based on the total weight of the lubricant composition, of an ionic liquid whose anion is bis(fluorosulfonyl)imide.

[0040] It is mentioned that the addition of ionic liquids can lead to an improvement connected with the use of metal soap greases, in particular lithium soap greases and lithium complex soap greases, that have a tendency to excessive release of oil at high temperatures, so that oil losses occur, limiting the lifetime of rolling bearings, despite the use of seals. This document does not contain any reference to the use of a mixture of a polyalphaolefin and a poly(alkylene)glycol as a base oil.

[0041] There is still a need for lubricants for rail wheel bearings with a high thermal stability and in particular a high durability at high operating temperatures.

[0042] There is further a need for long-lasting lubricants for the rail wheel bearings of the rolling stock of high-speed trains, reaching speeds of up to 200 - 450 km / h.

[0043] A particular problem of electric train traffic is the so-called electrolytic corrosion of the rail wheel bearings. When the grounding collector that grounds the current of the electric motor from the wheels to the rails is incomplete, the electric current flows through the inner and outer rings and rolling elements of the rolling bearings and through the axle bearings and wheels to the rail. This may cause sparking between the rolling elements and the surfaces of the bearings of the rail cars, resulting in so-called electrical corrosion, which reduces the life of the bearings. There is a need for lubricants with sufficiently high conductivity to avoid the problem of electrolytic corrosion.

[0044] It is an object of the present invention to provide a lubricant composition which has an advantageous application profile and has as many of the afore-mentioned desired properties as possible. It was now surprisingly found that this object is achieved by a grease composition, comprising a mixture of at least one polyalphaolefin and at least one poly(alkylene)glycol as base oil, at least one lithium complex soap as thickener and at least one ionic liquid.

[0045] SUMMARY OF THE INVENTION

[0046] A first object of the invention is the use of a grease composition, comprising a) a base oil mixture, comprising a1 ) at least one polyalphaolefin, and a2) at least one poly(alkylene)glycol, b) at least one lithium complex soap, and c) at least one ionic liquid, for grease lubrication of rail wheel bearings and traction motor bearings of rail vehicles.

[0047] A special embodiment of the invention is the use of a grease composition, comprising a) a base oil mixture, comprising a1 ) at least one polyalphaolefin, and a2) at least one poly(alkylene)glycol, wherein components a1) and a2) are immiscible or only partly miscible, b) at least one lithium complex soap, and c) at least one ionic liquid, for grease lubrication of rail wheel bearings and traction motor bearings of rail vehicles.

[0048] In a special embodiment, the grease composition comprises: a1 ) 20 to 77.9 % by weight, based on the total weight of the composition, of at least one polyalphaolefin, a2) 20 to 77.9 % by weight, based on the total weight of the composition, of at least one poly(alkylene)glycol, a3) 0 to 50.0 % by weight, based on the total weight of the composition, of at least one base oil different from a1) and a2), b) 2.0 to 25.0 % by weight, based on the total weight of the composition, of at least one lithium complex soap, c) 0.1 % to 10.0 % by weight, based on the total weight of the composition, of at least one ionic liquid, d) 0 % to 20.0 % by weight, based on the total weight of the composition, of at least one additive, especially selected from thickeners different from component b), corrosion inhibitors, antioxidants, metal deactivators, organic or inorganic solid lubricants, viscosity index improvers, anti-wear additives, radical scavengers, UV stabilisers, pour point depressing additives, friction modifiers, extreme pressure additives, dispersing active compounds, and mixtures thereof.

[0049] In a further special embodiment, the grease composition is used for improving at least one property selected from: increasing the service life of rail wheel bearings and traction motor bearings of rail vehicles, increasing the range of possible operating temperatures in the rail wheel bearings, in particular increasing the upper operating temperature, increasing the speed parameter range of the rail wheel bearings, in particular increasing the upper speed limit of the bearings, reducing the noise emission of rail wheel bearings, increasing the electrical conductivity.

[0050] A further object of the invention is a rail wheel bearing, contained in a wheel arrangement for a rail vehicle, comprising a grease composition as defined above and in the following.

[0051] A further object of the invention is a traction motor bearing for rail vehicles, comprising a grease composition as defined above and in the following.

[0052] DETAILED DESCRIPTION OF THE INVENTION

[0053] The grease composition according to the invention is particularly advantageous for grease lubrication of rail wheel bearings and meets the complex property profile connected with this application. The grease composition used according to the invention has at least one, preferably two, three or more of the following advantages:

[0054] The used grease composition has a high thermal stability. Thus, it is possible to increase the possible operating temperatures in the rail wheel bearings, and in particular to increase the upper operating temperature. The used grease composition is suitable for a heat management that allows to balance heat generation of the bearings and heat removal of lubrication grease.

[0055] Actual electric drive units are required to achieve low friction, downsizing, and weight reduction towards energy and electricity savings. At the same time, the rotational speed increases and can be 20.000 rpm or more. Rail wheel bearings lubricated under use of the grease composition of the invention have excellent high speed rotational performance that suppresses heat generation and damage when rotating at high speed.

[0056] The service life of rail wheel bearings can be remarkably increased. With the grease lubrication according to the invention a limiting rotational speed of rolling bearings, expressed as rotation supporting d x mn value of at least 750 000, especially at least 1 000 000 or more can be achieved. The d x mn value can be calculated with the following equation: d x mN = speed x (OD + ID) / 2, wherein speed = Maximum Operating Speed (rpm)

[0057] OD = Bearing Outside Diameter (mm) (also denoted as "D") ID = Bearing Inside Diameter (mm) (also denoted as "d")

[0058] The grease lubrication according to the invention allows a calculated operating life of rail wheel bearings and traction motors bearings of more than 1 .5 million kilometers and often more than 2 - 3 million kilometers.

[0059] The grease composition exhibits a good electrical conductivity. Thus, shortening of the life of the traction motor bearings and wheel bearings due to electrolytic corrosion can be avoided. Rolling bearings used in railcars have the problem that if the grounding current collector that grounds the current of the electric motor(s) to the rail is incomplete, the electric current of the motor(s) passes through the inner and outer rings and rolling elements of the rolling bearing and reaches the axle. Electric current flows between bearings, wheels and rails. For this reason, in traction motor bearings and axle bearings, sparks may occur between the rolling elements and the rolling surface of the outer ring, or between the rolling elements and the rolling surface of the inner ring, causing so-called electrolytic corrosion and shortening the bearing life.

[0060] The grease compositions fulfil the requirements of DIN EN 12082:2021 -09 (Railway applications - Axleboxes - Performance testing) and DIN EN 12081 :2017-11 (Railway applications - Axleboxes - Lubricating greases).

[0061] Component a) (base oil mixture)

[0062] The grease composition for rail wheel bearings used according to the invention comprises a base oil mixture a), comprising a1 ) at least one polyalphaolefin, a2) at least one poly(alkylene)glycol and a3) optionally at least one base oil different from a1 ) and a2).

[0063] Preferably, the at least one polyalphaolefin a1 ) has a kinematic viscosity at 40°C according to DIN EN 16896 - 2017-02 in the range of 40 to 1500 mm2 / s, more preferably 60 to 680 mm2 / s, in particular 68 to 400 mm2 / s.

[0064] Preferably, the at least one poly(alkylene)glycol a2) has a kinematic viscosity at 40°C according to DIN EN 16896 - 2017-02 in the range of 32 to 1500 mm2 / s, more preferably 68 to 680 mm2 / s, in particular 100 to 400 mm2 / s.

[0065] In an especially preferred embodiment, a1 ) has a kinematic viscosity according to DIN EN 16896 - 2017-02 of 46 to 80 mm2 / s and a2) has a kinematic viscosity according to DIN EN 16896 - 2017-02 of 220 to 460 mm2 / s.

[0066] In a further especially preferred embodiment, a1 ) has a kinematic viscosity according to DIN EN 16896 - 2017-02 of 320 to 680 mm2 / s and a2) has a kinematic viscosity according to DIN EN 16896 - 2017-02 of 46 to 100 mm2 / s.

[0067] Preferably, the grease composition used according to the invention comprises from 40.0 to 95.0 % by weight, preferably from 50.0 to 90.0 % by weight, in particular from 60.0 to 85.0 % by weight, based on the total weight of the composition, of at least one base oil mixture a).

[0068] Suitable polyalphaolefins a1 ) (PAO) are commercially available and can be produced catalytically from ethylene according to known methods, wherein initially alphaolefins with longer chain length are obtained as an intermediate. From this, polyalphaolefins are synthesized substantially by oligomerisation, wherein usually isoparaffins with a varying number of side chains of equal length are obtained. Synthesis of polyalphaolefins a1) can be performed by acid-catalysed (conventional) or metallocene-catalysed olefin polymerisation. Conventional PAO and metallocene-catalysed PAO differ in their structure and the resulting product properties. Conventional PAO products show a high degree of isomerization resulting from ionic oligomerization under formation of charged intermediates that readily undergo carbocation rearrangement. In the metallocene oligomerization the olefin is inserted into a metal-carbon-bond without the formation of charged intermediates and the resulting products are free from isomerization. Suitable polyalphaolefins are e.g., the oligomers, preferably the dimers, trimers, tetramers, pentamers and higher oligomers with more than 5 repeating units of alphaolefins, and mixtures of these oligomers.

[0069] Preferably, the alphaolefins used for the preparation of polyalphaolefins a1) are selected from C6-C20 alphaolefins, more preferably, Cs-C alphaolefins, in particular Cs-Cu alphaolefins. In a preferred embodiment, the alphaolefins used for the preparation of polyalphaolefins a1 ) are selected from 1 -octane, 1 -nonen, 1 -decene, 1 -dodecene, 1- tridecene, 1 -tetradecene, 1 -pentadecene, 1 -hexadecene, 1 -heptadecene, 1 -octadecene, 1 -nonadecene, 1 -eicosene, 1-docosene and mixtures thereof. In particular, the alphaolefins are selected from Cs-Cu alphaolefins, in particular 1 -octene, 1 -decene, 1- dodecene and mixtures thereof. In a preferred embodiment, 1 -decene and 1 -decene- containing alphaolefin mixtures are used for the production of polyalphaolefins.

[0070] Technically available polyalphaolefins are usually present in the form of a mixture. By way of example, a typical decene dimer can contain 80 to 99.8% by weight of decene dimer, 0.1 to 19.8% by weight of decene monomer and 0.1 to 19.8% by weight of decene trimer. It is also possible to use polyalphaolefin copolymers and polylalphaolef in mixtures from alphaolefins with different numbers of carbon atoms, e.g., decene / dodecene copolymers or mixtures of decene homopolymers and dodecene homopolymers. By selecting suitable compositions of these copolymers and mixtures, the properties of the polyalphaolefins can be adjusted over a wide range depending on the respective requirements.

[0071] Preferably, the grease composition used according to the invention comprises at least 20.0 % by weight, preferably at least 25.0 % by weight, based on the total weight of the composition, of at least one polyalphaolefin a1 ).

[0072] Suitable poly(alkylene)glycols a2) (PAG) are homopolymers, copolymers and mixtures (blends) thereof. In the sense of the invention, the term copolymer also denotes polymers derived from three, four or more different monomers (terpolymers, quaterpolymers, etc.). Suitable poly(alkylene)glycols a2) are polyethylene glycols, polypropylene glycols, polybutylene glycols, polytetrahydrofurans and copolymers of two or more different alkylene oxide copolymers.

[0073] Suitable alkylene oxides for preparation of poly(alkylene)glycols a2) are, for example, ethylene oxide, propylene oxide, epichlorohydrin, 1 ,2- and 2,3-butylene oxide. Suitable examples of copolymers are copolymers of ethylene oxide and propylene oxide, copolymers of ethylene oxide and butylene oxide, and copolymers of ethylene oxide, propylene oxide and at least one butylene oxide. The alkylene oxide copolymers may comprise the copolymerized alkylene oxide units in randomly distributed form or in the form of blocks.

[0074] The poly(alkylene)glycols a2) may exhibit hydrogen atoms as end groups (terminal groups) or may be partly or completely end-capped (i.e. exhibit groups different from hydrogen atoms as end groups). Suitable end groups of the polyalkylene glycols a2) are hydrogen, alkyl, aryl, alkylaryl, arylalkyl, alkyloxy, aryloxy, alkylaryloxy, arylalkyloxy, acyl and hydroxy end groups. The alkyl groups in alkyl, alkylaryl, arylalkyl, alkyloxy, alkylaryloxy and arylalkyloxy, preferably have 1 to 6 carbon atoms, particularly preferably 1 to 2 carbon atoms. The aryl groups in aryl, alkylaryl, arylalkyl, aryloxy, alkylaryloxy and arylalkyloxy preferably have 6 to 14 carbon atoms, particularly preferably 6 to 10 carbon atoms.

[0075] When (poly)alcohols are used as starters for the formation of the poly(alkylene)glycols a2), the starting compound is incorporated into the polymer chain or forms an end group thereof. Suitable starting groups consist of compounds comprising active hydrogen such as e.g., water, n-butanol, ethylene glycol, propylene glycol, neopentyl glycol, pentaerythritol, ethylene diamine, phenol, cresol, other (Ci-Ci6-mono-, di- or tri-alkyl) aromatics, (hydroxyalkyl) aromatics, hydroquinone, aminoethanolamines, triethylenetetramines, polyamines, sorbitol or other sugars. Other C-H acidic compounds such as carboxylic acids or carboxylic anhydrides, can also be used as starting compounds. Other suitable starting compounds include longer chain alcohols, such as C10-C18 alcohols. Cyclic ether alcohols such as hydroxyfurfuryl or hydroxytetrahydrofuran, nitrogen heterocyclics or sulphur heterocyclics can also be used as starting groups. Such poly(alkylene)glycols are disclosed in WO 01 / 57164.

[0076] The poly(alkylene)glycols a2) used according to the invention can be produced by reacting alcohols, including polyalcohols, as starting compounds with oxiranes such as ethylene oxide, propylene oxide and / or butylene oxide. Following the reaction, these possess only one free hydroxy group as end group. Polyalkylene glycols with only one hydroxy group are preferred over those with two free hydroxy groups. Polyalkylene glycols which e.g., after a further etherification step, comprise no free hydroxy groups any longer are particularly preferred regarding the stability, hygroscopicity and compatibility. The alkylation of terminal hydroxyl groups leads to an increase in the thermal stability. Thus, in an especially preferred embodiment according to the present invention, the PAG base oil comprises end-capped PAG, i.e. where no free hydroxyl groups are present.

[0077] Preferably, the proportion of propylene repeat units is at least 30% by weight, preferably at least 50 wt.%, more preferably at least 70 wt.%, based on the total weight of the poly(alkylene)glycols a2).

[0078] Preferably, the poly(alkylene)glycol a2) is selected from ethylene oxide / propylene oxide copolymers, propylene oxide homopolymers and mixtures thereof. Preferably, the poly(alkylene)glycols a2) have a density of from 900 to 1400 kg / m3, more preferably from 950 to 1100 kg / m3, in particular from 960 to 1070 kg / m3. The density can be determined according to ASTM D7042-21 a with a Stabinger Viscometer.

[0079] Preferably, the grease composition used according to the invention comprises at least 20.0 % by weight, preferably at least 25.0 % by weight, based on the total weight of the composition, of at least one poly(alkylene)glycol a2).

[0080] Components a1 ) and a2) used according to the invention are immiscible or only partly miscible.

[0081] In the sense of the invention, miscible liquids are totally soluble in each other at all proportions, irrespective of the temperature, forming a homogeneous single-phase system. Accordingly, immiscible liquids form a multi-phase system at all proportions, irrespective of the temperature. A pair of liquids is considered partially miscible if there is a certain temperature or temperature range, where the liquids will form a multi-phase liquid system, and another temperature or temperature range, where the liquids are miscible in all compositions. In other words, in mixtures of partly miscible components a1 ) and a2) the solubility of the components is dependent on the temperature. In one embodiment, the solubility increases with increasing temperature. A pair of liquids is considered partially miscible if there is a temperature (known as the upper critical temperature), where the liquids become miscible in all compositions. In another embodiment, the solubility increases with decreasing temperature. A pair of liquids is also considered partially miscible if there is a temperature (known as the lower critical temperature) below which the liquids become miscible in all compositions. Some mixtures of liquids may show both, an upper and lower critical temperature, forming two-phase liquid systems at temperatures between these two temperatures.

[0082] Miscibility is the ability of one substance to mix with another substance, forming a homogeneous mixture. Solubility is the ability and extent of a solute to dissolve in a particular solvent. The result may either be a homogeneous or a heterogeneous mixture. In the sense of the invention, in a multi-phase system of immiscible or partly miscible components a1 ) and a2), each phase may contain a certain amount of dissolved component from the other phase.

[0083] The solubility experiments are generally conducted at room temperature (20°C) and ambient pressure (1013 hPa). A clear glass test tube is charged with polyalphaolefin a1 ) or poly(alkylene)glycols a2) and then the other species is added in the desired mixing ratio. Usually the weight ratio is tested in a range of 90 : 10 to 10 : 90 in steps of 10 wt.%. A hand magnifier with 10x magnification is used for observation. Upon gentle mixing, streaks caused by changes in density and the refraction of light are briefly observed for those compounds that are miscible. Soluble mixtures form a single phase upon complete dissolution. Insoluble mixtures appear as an opaque emulsion during mixture returning to two phases shortly afterwards. In a preferred embodiment, components a1 ) and a2) are immiscible at 20°C. Pressure generally has no relevant effect on mixtures of components a1 ) and a2). Miscibility tests are generally performed at ambient pressure (1013°hPa).

[0084] The miscibility of the polyalphaolefins a1 ) and the poly(alkylene)glycols a2) can be expressed by the Hansen solubility parameters (HSP) of the two components. The system of the HSP is known from the literature and based on a set of three parameters for each solvent (dispersing component d, polar component p, and hydrogen-binding component h). Methods for the determination of HSP are described in detail in HANSEN SOLUBILITY PARAMETERS, A User's Handbook, second edition, CRC Press (2007). HSP values regarding a plethora of solvents are also described in Wesley L. Archer, Industrial Solvents Handbook, first edition, Marcel Dekker Inc. (1996). HSP values can also be calculated using a standard software, like HSPiP of Charles Hansen Consulting, Inc. (Horsholm, Denmark, hansen-solubility.com).

[0085] Miscibility of two solvents 1 and 2 can be expressed e.g., by the distance Ra between the Hansen solubility parameters:

[0086] Ra = {4(6di- 5d2)2+(5pi- bp2)2+(bhi - 5h2)2}05wherein

[0087] 5di - bd2 is the difference of the dispersing component of the HSP between the two solvents 1 and 2,

[0088] 6pi - 6p2 is the difference of the polar component of the HSP between the two solvents, and

[0089] 6hi - bh2 is the difference of the hydrogen-binding component of the HSP between the two solvents

[0090] As an illustration, the Hansen Solubility Parameters for individual poly(alkylene)glycols a2) (solvent 1 ) and polyalphaolefins a1 ) (solvent 2), used as base oils, are calculated based on 10 monomer units (for the polyalphaolefin based on decene, i.e. 8 alkylene / alkyl units in the side chains). This approach allows to reflect the different co-polymerization ratio of EO / PO polymers. Table A shows the separate parameters for the single combinations and the corresponding Ra value.

[0091] Table A The b2values (b2= bd2+ bp2+ bh2) in table A were determined on the basis of the calculated bd, bpand bhvalues with 8 decimal places. The values refer to a temperature of 25°C.

[0092] The afore-mentioned calculation of solubility parameters of lubricant base oils is based on a method developed by Van Krevelen et al. (D. W. van Krevelen, K. te Nijenhuis, Properties of Polymers, 4thedition, Elsevier Amsterdam, 2009, Chapter 7, ISBN 978-0-08- 054819-7) to determine the solubility parameter by incremental contributions of the individual functional groups of the molecule. Those incremental contributions can be calculated by the following equations (1 ), (2) and (3) for the dispersive (Eq 1 ), polar (Eq 2) and hydrogen-bonding (Eq 3) part. An overview for the calculated values of various functional groups is given in table B. The molar volume is also summarized by the individual functional groups of the molecule (Eq 4). The approach of van Krevelen et al. is developed from the cohesion energy. The values are standardized on 25°C.

[0093] Table B:

[0094] The increment values for the individual functional groups are taken from the literature (e.g.: HANSEN SOLUBILITY PARAMETERS, A User's Handbook, second edition, CRC Press (2007)). These calculation methods that have been specifically developed to estimate macromolecules are suitable to approximate the solubility parameters for base oils in order to have an understanding for the polarity and miscibility of these lubricant components. The values in table A) for polyalphaolefins and polyglycols were made based on the increment values by V. Krevelen et al. (table B). The method is suitable for the calculation of the solubility parameters of lubrication base oils with any number of repeat units on the basis of the individual functional groups of the molecule.

[0095] Preferably, in the grease composition of the present invention, the Hansen Solubility Parameter distance Ra between the polyalphaolefin a1 ) (PAO) and the poly(alkylene)glycol a2) (PAG), calculated by the formula Ra = {4(5d(PAG) - 6d(PAO))2+(6p(PAG) - 6p(PAO))2+(6h(PAG) - 6h(PAO))2}0'5is at least 9.5 (J / cm3)05, more preferably at least 10.0 (J / cm3)05.

[0096] Preferably, the weight ratio of component a1 ) to component a2) is in a range of 99.0 : 1.0 to 1.0 : 99.0, preferably 10 : 90 to 90 : 10, in particular 70 : 30 to 30 : 70 .

[0097] The base oil mixture a) contained in the grease composition used according to the invention optionally comprises at least one base oil (component a3) different from a1 ) and a2).

[0098] Component a3) is preferably selected from: esters, preferably selected from dipentaerythritol esters, trimellitates, hemimellitates, pyromellitates, estolides, pentaerythritol esters, dimer fatty acid esters, further dimeric and trimeric acid esters, trimethylolpropane esters (TMP ester), neopentylglycol esters, ethers, preferably selected from polyphenyl ethers, diaryl ethers, triaryl ethers, linear or branched perfluoro polyether oils (PFPE oils), polyglycols, in particular homo- and / or copolymers of ethylene oxide, propylene oxide, 1 ,2-butylene oxide and / or tetrahydrofuran (THF), preferably ethers derived from monoalcohols, dialcohols and trialcohols as starters, synthetic hydrocarbons, paraffinic mineral oils, naphthenic oils, alkylated benzenes, further copolymers of alkenes, preferably LAO (linear alphaolefins) with unsaturated esters, polyisobutenes, silicon oils, and mixtures thereof.

[0099] In a preferred embodiment, base oil a3) is selected from esters of aromatic polycarboxylic acids, polyol ester oils, estolides, dimer fatty acid esters further dimeric and trimeric acid esters, carbonate ester oils and mixtures thereof, preferred aromatic polycarboxylic acids a3) are trimellitates, hemimellitates, pyromellitates. Preferred polyol ester oils a3) are pentaerythritol esters, dipentaerythritol esters, trimethylolpropane esters and neopentylglycol esters.

[0100] In a further preferred embodiment, base oil a3) is selected from ether oils, in particular diaryl ethers, triaryl ethers and polyphenyl ethers.

[0101] In a further preferred embodiment, base oil a3) is selected from synthetic hydrocarbons different from a1 ), preferably polybutenes, polyisobutenes and copolymers of alphaolefins with at least one comonomer, alkylnaphtalene oils and alkylbenzene oils

[0102] In a further preferred embodiment, base oil a3) is selected from natural fats and oils.

[0103] Preferably, the base oils a3) have a kinematic viscosity at 40°C according to DIN EN 16896 - 2017-02 of in the range of 50 to 400 mm2 / s, more preferably 60 to 300 mm2 / s, in particular 70 to 220 mm2 / s. If the grease composition used according to the invention comprises at least one base oil a3), the amount is at least 0.1 % by weight, especially at least 0.5 % by weight, more especially at least 1 .0 % by weight, based on the total weight of the composition. In a special embodiment, the grease composition used according to the invention does not comprise any base oil apart from components a1 ) and a2).

[0104] Preferably, the grease composition used according to the invention comprises 0 to 50.0 % by weight, especially 3,0 to 25.0 % by weight, more especially 10.0 to 20.0 % by weight, based on the total weight of the composition, of at least one base oil a3).

[0105] Component b) (lithium complex soap)

[0106] The grease composition used for rail wheel bearings according to the invention, comprises as component b) at least one lithium complex soap.

[0107] Suitable lithium complex soaps can be obtained by reaction of a lithium base (like lithium hydroxide, lithium oxide or lithium carbonate) with b1 ) at least one carboxylic acid component having 10 to 32 carbon atoms, selected from unsaturated or saturated C10-C32 monocarboxylic acids or unsaturated or saturated C10-C32 hydroxy monocarboxylic acids and esters of said C10-C32 monocarboxylic acids and C10-C32 hydroxy monocarboxylic acids and b2) at least one complexing agent.

[0108] Component b1 ) is preferably selected from saturated or unsaturated C12-C22 monocarboxylic acids, saturated or unsaturated C12-C22 hydroxy monocarboxylic acids and derivatives and mixtures thereof. Suitable derivatives are esters. In particular, the monocarboxylic acids and hydroxy monocarboxylic acids b1 ) are selected from lauric acid, myristic acid, palmitic acid, stearic acid, arachidic acid, myristoleic acid, palmitoleic acid, oleic acid, linoleic acid, arachidonic acid, behenic acid, 10-hydroxystearic acid, 12- hydroxystearic acid, 17-hydroxystearic acid, 2-hydroxytetradecanoic acid, 3- hydroxytetradecanoic acid, 2-hydroxyhexadecanoic acid, 3-hydroxyhexadecanoic acid and mixtures thereof. Preferred as 10-hydroxystearic acid is (R)-10-hydroxystearic acid.

[0109] Component b2) is preferably selected from saturated or unsaturated C2-C16 dicarboxylic acids, saturated or unsaturated C2-C8 monocarboxylic acids, saturated or unsaturated C2- Cs hydroxy monocarboxylic acids, derivatives of the C2-C16 dicarboxylic acids, C2-C8 monocarboxylic acids and C2-C8 hydroxy monocarboxylic acids, boric acid, phosphoric acid and mixtures thereof. Particularly suitable as dicarboxylic acids b2) are adipic acid, sebacic acid, azelaic acid, 3-tert.-butyl-adipic acid and esters and mixtures thereof. Particularly suitable as monocarboxylic acids b2) are acetic acid and propionic acid. Particularly suitable as component b2) are also hydroxybenzoic acids e.g., salicylic acid, 3-hydroxybenzoic acid, 4-hydroxybenzoic acid, 2-hydroxy-4 hexylbenzoic acid, 2,5- dihydroxybenzoic acid, 2,6-dihydroxybenzoic, 4-hydroxy-4-methoxybenzoic acid and mixtures thereof. Suitable borates are metaborate, diborate, tetraborate or orthoborate, such as for example, monolithium orthoborate. Suitable phosphates are the dihydrogen phosphate, hydrogen phosphate or pyrophosphate.

[0110] In a preferred embodiment, component b2) is selected from saturated or unsaturated C2- C dicarboxylic acids and esters and mixtures thereof.

[0111] For saponification reaction with a lithium base, carboxylic acids per se or suitable derivatives, in particular esters, may be used. The carboxylic acid component of the monocarboxylic acids, hydroxy carboxylic acids and / or dicarboxylic acids of components b1) and b2) can be the acids per se (with free acid groups) or partial or full esterification products with lower monohydric, dihydric or trihydric alcohols. Suitable derivatives of the monocarboxylic acids, hydroxy monocarboxylic acids and dicarboxylic acids are the esters with Ci-Ce alcohols, preferably C1-C4 alcohols. The derivatives are preferably selected from methyl, ethyl, n-propyl, isopropyl, n-butyl, sec.-butyl esters and mono-, di- and triglycerides.

[0112] In a preferred embodiment, the lithium complex soaps employed as component b) consist essentially of lithium salts of the afore-mentioned monocarboxylic acids, hydroxy carboxylic acids and / or dicarboxylic acids. In particular, at least 95 wt.-% of the acids and esters employed for the formation of component b2) have been converted into lithium salts.

[0113] Particularly preferred as lithium complex soaps are the lithium salts of 12-hydroxystearic acid combined with azelaic acid and / or sebacic acid, especially 12-hydroxystearic acid combined with azelaic acid.

[0114] Preferably the grease composition comprises 4.0 to 30.0 % by weight, especially 6.0 to 25.0 % by weight, more especially 10.0 to 20.0 % by weight, based on the total weight of the composition, of at least one lithium complex soap b).

[0115] Component c) (ionic liquid)

[0116] The grease composition for rail wheel bearings according to the invention, comprises as component c) at least one ionic liquid (IL).

[0117] Preferably, the ionic liquid has anions selected from bis(perfluoroalkylsulfonyl)imides, bis(fluorosulfonyl)imide, tris(perfluoroalkylsulfonyl)methides, tris(perfluoralkyl)trifluoro- phosphates, and mixtures thereof.

[0118] The ionic liquid preferably has anions selected from bis(trifluoromethylsulfonyl)imide, bis(fluorosulfonyl)imide and mixtures thereof.

[0119] Particularly preferred, the anion is bis(trifluoromethylsulfonyl)imide (bta) or bis(fluorosulfonyl)imide (fsi). Preferably, the ionic liquids contain cations selected from the group of symmetrical and asymmetrical ammonium ions NR1R2R3FV and phosphonium ions PRI R2RSR4+. The radicals R1 to R4 are independently of each other branched or unbranched, substituted or unsubstituted Ci to C24 alkyl groups, preferably Ci to C18 alkyl groups, particularly preferably C& to C18 alkyl groups or substituted or unsubstituted C& to C30 aryl groups. Preferred substituents are alkoxy, carboxy, amido, amino, thiocarboxy, carbamoyl, oxo, thioxo and / or hydroxy.

[0120] In a preferred embodiment of the invention, the radicals R1 to R4 are selected such that they have a total of at least 10 carbon atoms, preferably at least 20 carbon atoms, even more preferably at least 25 carbon atoms.

[0121] Preferably, the ionic liquid c) is selected from

[0122] (trihexyl(tetradecyl)phosphonium) bis(trifluoromethylsulfonyl)imide, (tributyl(tetradecyl)phosphonium) bis(trifluoromethylsulfonyl)imide, (tetraoctyphosphonium) bis(trifluoromethylsulfonyl)imide, (methyltrioctylammonium) bis(trifluoromethylsulfonyl)imide, trihexyl(tetradecyl)phosphonium) bis(fluorosulfonyl)imide, (tributyl(tetradecyl)phosphonium) bis(fluorosulfonyl)imide, (tetraoctylphosphonium) bis(fluorosulfonyl)imide, (methyltrioctylammonium) bis(fluorosulfonyl)imide and mixtures thereof.

[0123] Preferably, the grease composition comprises 0.1 to 10.0 % by weight, preferably 0.2 to 5.0 % by weight, in particular 0.5 to 2.0 % by weight, based on the total weight of the composition, of ionic liquids c).

[0124] Component d) (additives)

[0125] The grease composition for rail wheel bearings according to the invention optionally comprises at least one additive as component d). Various conventional grease additives may be incorporated into the lubricating greases in amounts normally used in this field of application, to impart certain desirable characteristics to the grease.

[0126] Preferably, additive d) is selected from thickeners different from component b) (additional thickeners), corrosion inhibitors, antioxidants, metal deactivators, organic or inorganic solid lubricants, viscosity index improvers, anti-wear additives, radical scavengers, UV stabilisers, pour point depressing additives, friction modifiers, extreme pressure additives, dispersing active compounds, and mixtures thereof.

[0127] Suitable thickeners different from component b) (additional thickeners) are generally solid substances which are almost or completely insoluble in the base oils (insoluble solids) and have a thickening effect. For the purposes of this invention, all solids insoluble in the base oil are considered thickeners, regardless of whether they have an effect on friction and wear. A special embodiment are inorganic and organic solid lubricants which are mentioned as separate component. Preferably, the additional thickener is selected from simple metal soaps, complex metal soaps from metals different from lithium, urea thickeners, metal sulfonate thickeners, waxes, boron nitride, carbon black, graphite, graphene, metal chalcogenides, highly fluorinated polymeric substances and fluorine-free substances. The fluorine-free substances are preferably selected from fluorine-free polymers which have aromatic, heteroaromatic and / or heterocyclic groups and a melting or decomposition point, measured according to DIN EN ISO 11357-1 , 2008.04, of at least 200°C, fluorine-free phthalocyanines, silicone resins, lignins, inorganic layer silicates which may be functionalized with organic groups, phosphorus compounds, melamine derivatives and mixtures of two or more of these thickeners.

[0128] Simple metal soap thickeners are preferably metal soaps of the elements of the first and second main groups of the periodic table, in particular lithium soaps. Preferably, the metal soaps are derived from at least one saturated or unsaturated Cs-22 fatty acid or a derivative thereof, more preferably at least one saturated or unsaturated C14-20 fatty acid or a derivative thereof. One particular derivative is hydrogenated castor oil, which is the glyceride of 12-hydroxystearic acid. 12-hydroxystearic acid is a particularly preferred fatty acid.

[0129] Metal complex soaps from metals different from lithium are preferably metal complex soaps of the elements of the first and second main groups of the periodic table, such as sodium complex soaps, calcium complex soaps and aluminum complex soaps.

[0130] Preferred urea thickeners are the reaction products of at least one diisocyanate with at least one amine, selected from monoamines, polyamines and mixtures thereof.

[0131] Preferably, the diisocyanate is selected from 2,4-diisocyanatotoluolene, 2,6- diisocyanatotoluolene, 4,4'-diisocyanatodiphenylmethane, 2,4’-diisocyanto- diphenylmethane, 4,4'-diisocyanatodiphenyl, 4,4'-diisocyanato-3,3'-dimethyldiphenyl, 4,4'- diisocyanato-3,3'-dimethylphenylmethane and mixtures thereof. Preferably, the amine is selected from monoamines of the formula Ra2N-Rb, diamines of the formula Ra2N-Rc-NRa2 and mixtures thereof, wherein

[0132] Rais independently selected from hydrogen, linear or branched C1-C22 alkyl and Ce-Cu aryl,

[0133] Rbis independently selected from linear or branched C1-C22 alkyl and Ce-Cu aryl, Rcis a divalent bridging group, preferably selected from C1-C22 alkylene and Ce-Cu arylene.

[0134] Metal sulfonate thickeners are in particular calcium sulfonate thickeners.

[0135] Waxes suitable as additional thickeners b1 ) are in particular polyethylene (PE) waxes, polypropylene (PP) waxes and polyamide (PA) waxes.

[0136] Metal chalcogenides suitable as additional thickeners are in particular molybdenum disulfide, tungsten disulfide and metal selenides. Highly fluorinated polymeric substances suitable as additional thickeners are in particular PTFE.

[0137] Preferred additional fluorine-free thickeners are boron nitride, carbon black, graphite, graphene, tin (IV) sulfide, zinc (II) sulfide and tungsten sulfide. Preferred further fluorine- free components are talc, bentonite, mica and fumed silica, which may be functionalized with organic groups. A special type of bentonite is montmorillonite, the sodium ions of which may be partially or completely exchanged for ammonium ions. Aluminosilicates, aluminum oxide and silica (e.g. Aerosil) are also suitable as further fluorine-free thickeners. Other suitable fluorine-free thickeners are nanoparticulate silicon dioxide functionalized with organic groups, zinc pyrophosphate, calcium (pyro)phosphate and zirconium hydrogen phosphate. Further suitable fluorine-free thickeners are melamine derivatives selected from melamincyanurate, melamine phosphate and 1 ,3,5-triazine-2,4,6 (1 H,3H,5H)-trithione.

[0138] The additional thickener may be used alone or as a combination of two or more of the afore mentioned further thickeners.

[0139] The grease composition preferably comprises from 0 to 10 % by weight, more preferably from 0 to 8 % by weight, in particular from 0 to 5 % by weight, based on the total weight of the composition, of at least one thickener different from the lithium complex soaps b). If the grease composition comprises at least one thickener different from the lithium complex soaps b), then the amount is at least from 0.5 % by weight, more preferably at least 1 .0 % by weight, based on the total weight of the composition. In a special embodiment, the grease composition used according to the invention does not comprise any added thickener apart from the lithium complex soaps b).

[0140] Suitable corrosion inhibitors are salts of various acids, such as sulfonates, naphthenates, carboxylates, amine phosphates, succinic acid half esters or partial polyol esters. Suitable corrosion inhibitors are also esters, nitrogen compounds and heterocyclic compounds.

[0141] Suitable antioxidants are aromatic aminic antioxidants, such as alkylated phenyl-alpha- naphthylamines, dialkyldiphenylamines, aralkylated diphenylamines, sterically hindered phenols, such as butylhydroxytoluene (BHT), bis-2,6-di-t-butylphenol derivatives, sulfur- containing hindered phenols, sulfur-containing hindered bisphenol, and mixtures thereof.

[0142] Suitable metal deactivators are chelating agents that passivate the metal surface. Nonlimiting examples of metal deactivators are triazoles or thiadiazoles, especially aryl triazoles, such as benzotriazole and tolyltriazole, alkyl derivatives of such triazoles and benzothiadiazoles such as R(C6H3)N2S, where R is H or Ci- to C -alkyl.

[0143] Anti-wear additives are preferably selected from amine neutralized phosphates, alkylated and non-alkylated triaryl phosphates, alkylated and non-alkylated triaryl thiophosphates, Zn-, Mo- or W-dialkyldithiophosphates, Zn-, Mo- or W-diaryldithiophosphates, carbamates, thiocarbamates, Zn-, Mo- or W-dithiocarbamates, dimercaptothiadiazoles, organoborates, organophosphites and mixtures thereof.

[0144] Examples of VI improvers comprise olefin copolymers, polyalkyl methacrylates and olefin copolymers with dispersing activity.

[0145] High-pressure additives act as topcoat formers and / or surface-active substances. Preferred high-pressure additives are selected from thiophosphates, such as zinc dithiophosphate, molybdenum oxide sulfide dithiophosphate, molybdenum amine compounds, sulfur compounds, such as sulfurized oils and fats, sulfurized fatty acids, sulfurized fatty acid esters, alkylated polysulfides and mixtures thereof.

[0146] Preferred additives for improving the pour point are selected from linear or branched, alkylated, acrylated and / or aliphatic homopolymers and copolymers, which can be used individually or in combination. A specific example of a pour point depressant is polyalkyl methacrylate.

[0147] Rail wheel bearings and traction motor bearings

[0148] A further object of the invention is a rail wheel bearing, contained in a wheel arrangement for a rail vehicle, comprising a grease composition as defined above. Reference is made to all suitable and preferred embodiments of the grease composition as defined above.

[0149] Suitable as rail wheel bearings are generally the different types of rolling-element bearings (also denoted as roller bearings). Roller bearings, in contrast to ball bearings, use cylinder rolling elements, rather than balls, to maintain the separation between moving parts of the bearing. However, special types of ball bearings, like twin tandem bearings, are also suitable as rail wheel bearings.

[0150] The rail wheel bearings are in particular selected from cylindrical roller bearings, tapered roller bearings and spherical roller bearings. In a special embodiment, the rail wheel bearings are selected from:

[0151] RCT Bearings (Sealed-Clean Rotating End Cap Tapered Roller Bearings) RCC Bearings (Sealed-Clean Rotating End Cap Cylindrical Roller Bearings) Spherical roller bearings

[0152] Cylindrical roller bearings combined with ball bearings

[0153] Cylindrical roller bearings with ribs

[0154] Tapered roller bearings.

[0155] The grease composition according to the invention can also be advantageously used for traction motor bearings. Traction motor bearings can be employed in form of sealed and pre-lubricated units. The traction motor bearings are in particular selected from deep groove ball bearings, four point contact bearings and cylindrical roller bearings.

[0156] In the following, the invention is explained in more detail by means of several examples. EXAMPLES

[0157] Example 1 (according to the invention) I) Preparation of a grease composition according to the invention in the form of grease lubrication for rail wheel bearings

[0158] Table 1) Grease composition according to the invention II) Application properties

[0159] Table 2)

[0160] Example 2 (comparative)

[0161] The grease composition of comparative example 2 is identical with the grease of example 1 with the difference that no ionic liquid was used and the amount of base oil was increased by 1 %. The application properties with regard to the FE 9 long-term bearing test according to DIN 51821 and the specific electrical resistance according to DIN EN 62631 -3-1 are summarized in table 3).

[0162] Table 3)

[0163] The result of the FE 9 lifetime test is significantly inferior compared to the example 1 according to the invention.

[0164] Example 3 (according to the invention)

[0165] I) Preparation of a grease composition according to the invention in the form of grease lubrication for rail wheel bearings Table 4) Grease composition according to the invention

[0166] II) Application properties Table 5)

[0167] Example 3 shows a significant improvement regarding lifetime (FE 9 test result) and electrical resistance compared with comparative example 2. Example 4 according to the invention

[0168] Table 6) Grease composition according to the invention

[0169] II) Application properties

[0170] Table 7)

[0171] Example 4 with only 0.5 % ionic liquid still shows significant improvement regarding electrical resistance compared with comparative example 2.

[0172] Example 5 according to the invention

[0173] Table 8) Grease composition according to the invention

[0174] II) Application properties

[0175] Table 9)

[0176] Example 5 with 2 % ionic liquid still shows further improvement regarding electrical resistance compared with comparative example 2 and inventive examples 1 and 3 with lower concentration of ionic liquid. Example 6 according to the invention

[0177] Table 10) Grease composition according to the invention

[0178] II) Application properties

[0179] Table 11 )

[0180] Example 6 with 5.0 % ionic liquid shows further improvement regarding electrical resistance compared with comparative example 2 and inventive examples 1 , 3 and 4.

[0181] Example 7 according to the invention

[0182] Table 12) Grease composition according to the invention

[0183] II) Application properties

[0184] Table 13)

[0185] Example 7 uses an IL with a tetraalkylammonium cation instead of a phosphonium cation. The anion is the same as in examples 3, 4, 5 and 6. The concentration of the IL is comparable to Example 2. Surprisingly, the tetraalkylammonium IL leads to an improvement of the electrical resistance both in comparison to the comparative example 2 and the inventive example 3.

[0186] Example 8 (according to the invention)

[0187] Table 14) Grease composition 8 according to the invention

[0188] II) Application properties

[0189] Table 15)

[0190] Example 8 shows significant improvement regarding the electrical resistance compared with the comparison example 2 and the with example 3 (inventive), which comprises the same amount of IL but no trimellitic ester. Example 9 (according to the invention)

[0191] Table 16) Grease composition 9 according to the invention

[0192] II) Application properties

[0193] Table 17)

[0194] The example 9 shows significant improvement regarding the electrical resistance compared with the comparison grease 2 and the with grease 2 (inventive), which comprises the same amount of IL but no ester. Example 10 (according to the invention)

[0195] Table 18) Grease composition 10 according to the invention

[0196] II) Application properties

[0197] Table 17)

[0198] Example 10 shows significant improvement regarding the electrical resistance compared with the comparison example 2 and example 3 (according to the invention), which comprises the same amount of IL but another type of polyglycol a2). The flow pressure is significantly improved.

[0199] Example 11 : Solubility investigations

[0200] Components in weight %

[0201] The polyalphaolefin and the polypropylene glycol homopolymer are not miscible.

[0202] Example 12: Solubility investigations

[0203] Even by adding an trimellitic ester, no homogenous mixture from poly propyleneglcol and polyalphaolefin can be obtained. Example 13: solubility investigations

[0204] Polyalphaolefin and EO / PO copolymer are not miscible. Example 14 solubility investigations

[0205] The ionic liquid is insoluble both in polyalphaolefin and in polypropylene glycol homopolymer.

[0206] Testing methods

[0207] In the following, some of the afore mentioned procedures used to characterize the grease compositions and / or its components are described. Unless otherwise specified, the described standards refer to the version valid at the time of the application.

[0208] Kinematic viscosity at 40°C and 100°C, viscosity index (VI), density of base oils Measurement of kinematic viscosity can be carried out according to according to DIN EN 16896 - 2017-02 with a Stabinger viscometer e.g., SVM 3000 (Anton Paar).

[0209] Cone penetration according to ISO 2137:2007 (= DIN ISO 2137:2016-12)

[0210] The consistency of greases can be determined by means of a standardized measuring procedure in the form of cone penetration in 0.1 mm according to DIN ISO 2137 (in German also denoted as Walkpenetration). The cone penetration is measured after fulling of the grease in a grease kneader or grease whaler with the aid of a penetrometer as the penetration depth of a standard cone under defined conditions. The measured cone penetration can then be assigned to a defined NLGI class (NLGI = National Lubricating Grease Institute) in accordance with DIN 51818:1981 -12.

[0211] Testing of corrosiveness to copper of greases according to DIN 51811 :2017-05 The "copper stripe test" is used to characterize the extent to which lubricating greases have a corrosive effect on copper. For testing, the copper strip is immersed in the grease, which is placed in a cup, leaving the upper 10 mm of the strip above the surface of the grease. The cup is then place in an air convection oven which has already reached the test temperature for 24 hours At the end of the test period, the copper strip is removed, cleaned with solvent and carefully dried and the discoloration of the copper strip is determined by comparison with a colour scale and given as the degree of corrosion (freshly polished (= 1a), slightly tarnished (=1 b), moderately tarnished (= 2a to 2e), dark tarnish (= 3a to 3b), and corrosion (= 4a to 4c)).

[0212] Dropping point according to DIN ISO 2176:1997 The dropping point of a lubricating grease is the temperature at which the grease passes from a semisolid to a liquid state under the conditions of the test. This change in state is typical of greases containing conventional types of thickeners, in particular soaps.

[0213] Determination of flow pressure according to Kesternich (DIN 51805-2:2016-09)

[0214] The Kesternich method tests the flow of greases at low temperatures. The test nozzle is filled with the grease sample and after the device has been cooled to the selected temperature [-40°C], pressure is applied to the grease sample and increased every 30 seconds until the grease sample has been forced out of the nozzle.

[0215] Oil separation according to DIN 51817:2014-08

[0216] The oil separation test monitors the tendency of base oils to separate from a lubricating grease at elevated temperatures for a defined period of time.

[0217] Water resistance test (statical) according to DIN 51807-1 :2020-03 part 1

[0218] Static test of the behaviour of lubricating greases in the presence of water. For the test, a thin strip of grease is placed on a glass strip using a template. The glass strip is dipped up to 80% in a test tube with water. The sealed test tube is then placed in a heating cabinet for 3 hours at 90 °C. Once this time has passed, the glass strip is removed and checked for changes with a visual inspection. The corresponding evaluation level is determined according to the following stages: 0 = no change to 3 = major change.

[0219] Low temperature torque test according to IP 186 / 93 (2015)

[0220] This test method developed by the Energy Institute (El) is used to determines the resistance caused by the grease at sub-zero temperatures down to -73 °C in an axially loaded ball-bearing rotating at 1 rpm.

[0221] Roll stability according to ASTM D 1831-2021

[0222] Roll stability is a measure of the stability of a grease when subjected to mechanical shearing. In the roll stability test according to ASTM D 1831 , grease is placed in a cylinder containing a steel roller and the cylinder is rotated. The penetration of the grease is measured before and after the test, and the difference in penetration values indicates the roll stability.

[0223] Long-term bearing test according to DIN 51821-1 :2016-07

[0224] The upper service temperature limit of lubricating greases for roller bearings is determined on an FE 9 test apparatus at a temperature of 140°C, an axial load of 1500 N and 6000 rpm, 2 cm3fill quantity, 1500 n axial load. Installation type is B (with lids). The test can be also run at higher temperatures.

[0225] Determination of Corrosion Prevention Properties according to DIN 51802:2017-10 - EMCOR Test

[0226] Dynamic resistance test for determining the corrosion-preventing properties of greases using the SKF EMCOR method with distilled water or any concentration of salt water. After the test the corrosion on the bearings is rated with a rating of 0 corresponding to no corrosion and a rating of 5 being the worst rating. Wear protection test according to DIN 51819-1 :2016-12

[0227] The wear protection of the grease composition is determined in the FE-8 test with a tapered roller bearing; 3000 1 / min; 20°C; 10 kN; steel cage, test duration 500h. In the wear test, the test duration and the weight loss of the bearing components are used to classify the lubricant's suitability for the application.

[0228] Determination of the specific resistance of lubricating greases in accordance with DIN 53482 (May 1983 version)

[0229] For this purpose, a circular plate electrode according to 5.3, Figure 2 of the mentioned DIN standard is modified with a 1 mm thick PTFE ring between electrode 2 and the guide piece 4 whereby a cylindrical cavity is created between electrode 1 (measuring electrode) and electrode 2) (counter electrode) with a height of 1 mm and an area of 20 mm2. The cell constant is thus 200. The measured value [Ohm] must be multiplied by this to obtain the specific resistance. With a spatula the lubricating grease is inserted into the cavity described above and placed on the electrode 1 . The measuring cell composed in this way is introduced into a shielding chamber (TOM 300-2, Fischer Elektronik, 15749 Mittenwalde, Germany). A Milli-TO 3 (Fischer Elektronik, 15749 Mittenwalde, Germany) is used as a voltage source. A DC voltage of 10 V is applied and the measured value [in Ohm] is read after 1 minute. The measurement is repeated three times, wherein new grease is applied to the cavity each time. The average value over the three individual measurements is multiplied by the cell constant and the specific resistance is thus obtained. If necessary, the measuring voltage can be increased as described above.

[0230] Solubility investigation

[0231] For the solubility tests (examples 11 - 14) the components of the mixture are heated under mild stirring to 60°C. After 1 hour the heating and stirring is stopped. The samples are allowed to cool to 20°C. The miscibility is evaluated after the samples have rested for 4 hours at 20°C.

Claims

CLAIMS1 . The use of a grease composition, comprising a) a base oil mixture, comprising a1) at least one polyalphaolefin, and a2) at least one poly(alkylene)glycol, b) at least one lithium complex soap, and c) at least one ionic liquid, for grease lubrication of rail wheel bearings and traction motor bearings of rail vehicles.

2. The use of a grease composition, comprising a) a base oil mixture, comprising a1) at least one polyalphaolefin, and a2) at least one poly(alkylene)glycol, wherein components a1) and a2) are immiscible or only partly miscible, b) at least one lithium complex soap, and c) at least one ionic liquid, for grease lubrication of rail wheel bearings and traction motor bearings of rail vehicles.

3. The use according to claim 1 or 2, wherein components a1) and a2) are immiscible at 20°C.

4. The use according to any of the preceding claims, wherein the poly(alkylene)glycol a2) is selected from ethylene oxide / propylene oxide copolymers, propylene oxide homopolymers and mixtures thereof, preferably the poly(alkylene)glycol a2) is selected from propylene oxide homopolymers.

5. The use according to any of the preceding claims , wherein the polyalphaolefin a1) (PAO) and the poly(alkylene)glycol a2) (PAG) have a distance Ra between the Hansen Solubility Parameters, calculated by the formulaRa = {4(5d(PAG) - 6d(PAO))2+(6p(PAG) - 6p(PAO))2+(6h(PAG) - 6h(PAO))2}0'5of at least 9.5 (J / cm3)05, more preferably of at least 10.0 (J / cm3)05.

6. The use according to any of the preceding claims, wherein the grease composition additionally comprises at least one additive (component d), especially selected from thickeners different from component b), corrosion inhibitors, antioxidants, metal deactivators, organic or inorganic solid lubricants, viscosity index improvers, anti-wear additives, radical scavengers, UV stabilisers, pour point depressing additives, friction modifiers, dispersing active compounds, and mixtures thereof.

7. The use according to any of the preceding claims, wherein the grease composition additionally comprises at least one base oil (component a3) different from a1 ) and a2), preferably selected from: ester oils, preferably selected from aromatic ester oils, like trimellitates, hemimellitates, pyromellitates; polyol ester oils, preferably pentaerythritol esters, dipentaerythritol esters, trimethylolpropane esters and neopentylglycol esters; estolides; dimer fatty acid esters; further dimeric and trimeric acid esters; carbonate ester oils; ether oils, preferably selected from diaryl ethers, triaryl ethers, alkyldiphenyl ether, polyphenyl ethers, linear or branched perfluoro polyether oils (PFPE oils); mineral oils; synthetic hydrocarbons different from a1 ), preferably polybutenes, polyisobutenes and copolymers of alphaolefins with at least one comonomer; alkylnaphthalene oils; alkylbenzene oils; natural fats and oils; phosphate ester oils; silicon oils; fluorinated oils; and mixtures thereof.

8. The use according to any of the preceding claims, wherein the grease composition comprises from 40.0 to 95.0 % by weight, preferably from 50.0 to 90.0 % by weight, in particular from 60.0 to 85.0 % by weight, based on the total weight of the composition, of at least one base oil mixture a).

9. The use according to any of the preceding claims, wherein the grease composition comprises at least 20.0 % by weight, preferably at least 25.0 % by weight, based on the total weight of the composition, of at least one polyalphaolefin a1 ).

10. The use according to any of the preceding claims, wherein the grease composition comprises at least 20.0 % by weight, preferably at least 25.0 % by weight, based on the total weight of the composition, of at least one poly(alkylene)glycol a2).1 1 . The use according to any of the preceding claims, wherein the weight ratio of component a1 ) to component a2) is in a range of 99.0 : 1 .0 to 1 .0 : 99.0, preferably 10 : 90 to 90 : 10, in particular 70 : 30 to 30 : 70 .

12. The use according to any of the preceding claims, wherein the grease composition comprises 0 to 50.0 % by weight, especially 3.0 to 25.0 % by weight, more especially 10.0 to 20.0 % by weight, based on the total weight of the composition, of at least one base oil a3).

13. The use according to any of the preceding claims, wherein the grease composition comprises 4.0 to 25.0 % by weight, especially 6.0 to 23.0 % by weight, more especially 10.0 to 20.0 % by weight, based on the total weight of the composition, of at least one lithium complex soap b).

14. The use according to any of the preceding claims, wherein the ionic liquid c) comprises at least one anion, selected from bis(perfluoroalkylsulfonyl)imides, bis(fluorosulfonyl)imide, tris(perfluoroalkylsulfonyl)methides, tris(perfluoroalkyl)trifluorophosphates and mixtures thereof, preferably selected from bis(trifluoromethylsulfonyl)imide, bis(fluorosulfonyl)imide and mixtures thereof.

15. The use according to any of the preceding claims, wherein the ionic liquid c) comprises at least one cation, selected from symmetrical and asymmetrical ammonium ions, NR1R2R3RT and phosphonium ions PRI R2RS 4+, wherein the radicals R1 to R4 are each independently of one another branched or unbranched, unsubstituted or substituted Ci to C24 alkyl groups or unsubstituted or substituted C& to C30 aryl groups.

16. The use according to any of the preceding claims, wherein the ionic liquid c) is selected from (trihexyl(tetradecyl)phosphonium) bis(trifluoromethylsulfonyl)imide, (tributyl(tetradecyl)phosphonium) bis(trifluoromethylsulfonyl)imide, (tetraoctylphosphonium) bis(trifluoromethylsulfonyl)imide, (methyltrioctylammonium) bis(trifluoromethylsulfonyl)imide, trihexyl(tetradecyl)phosphonium) bis(fluorosulfonyl)imide, (tributyl(tetradecyl)phosphonium) bis(fluorosulfonyl)imide, (tetraoctylphosphonium) bis(fluorosulfonyl)imide, (methyltrioctylammonium) bis(fluorosulfonyl)imide and mixtures thereof.

17. The use according to any of the preceding claims, wherein the grease composition comprises 0.1 to 10.0 % by weight, preferably 0.2 to 5.0 % by weight, in particular 0.5 to 2.0 % by weight, based on the total weight of the composition, of at least one ionic liquid c).

18. The use according to any of the preceding claims, wherein the grease composition comprises: a1 ) 20 to 77.9 % by weight, based on the total weight of the composition, of at least one polyalphaolefin,a2) 20 to 77.9 % by weight, based on the total weight of the composition, of at least one poly(alkylene)glycol, a3) 0 to 50.0 % by weight, based on the total weight of the composition, of at least one base oil different from a1) and a2), b) 2.0 to 25.0 % by weight, based on the total weight of the composition, of at least one lithium complex soap, c) 0.1 % to 10.0 % by weight, based on the total weight of the composition, of at least one ionic liquid, d) 0 % to 20.0 % by weight, based on the total weight of the composition, of at least one additive, especially selected from thickeners different from component b), corrosion inhibitors, antioxidants, metal deactivators, organic or inorganic solid lubricants, viscosity index improvers, anti-wear additives, radical scavengers, UV stabilisers, pour point depressing additives, friction modifiers, extreme pressure additives, dispersing active compounds, and mixtures thereof.

19. The use according to any of the preceding claims for improving at least one property selected from increasing the service life of rail wheel bearings and traction motor bearings, increasing the range of possible operating temperatures in the rail wheel bearings, in particular increasing the upper operating temperature, increasing the speed parameter range of the rail wheel bearings, in particular increasing the upper speed limit of the bearings, reducing the noise emission of rail wheel bearings, increasing the electrical conductivity.

20. A rail wheel bearing, contained in a wheel arrangement for a rail vehicle, comprising a grease composition as defined in any of claims 1 to 18.21 . A rail wheel bearing according to claim 20, in the form of a cylindrical roller bearing, tapered roller bearing or spherical roller bearing, preferably selected from:RCT Bearings (Sealed-Clean Rotating End Cap Tapered Roller Bearings), RCC Bearings (Sealed-Clean Rotating End Cap Cylindrical Roller Bearings),Spherical roller bearings,Cylindrical roller bearings combined with ball bearings, Cylindrical roller bearings with ribs, andTapered roller bearings.

22. A traction motor bearing for rail vehicles, comprising a grease composition as defined in any of claims 1 to 18, wherein the bearing is preferably selected fromDeep groove ball bearings, Four point contact bearings, and - Cylindrical roller bearings.

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