GB2621340A

GB2621340A - Rolling element bearings

Info

Publication number: GB2621340A
Application number: GB2211550.5A
Authority: GB
Inventors: David Lewis Simon
Original assignee: ESR Technology Ltd
Current assignee: ESR Technology Ltd
Priority date: 2022-08-08
Filing date: 2022-08-08
Publication date: 2024-02-14
Also published as: GB202211550D0

Abstract

A rolling element bearing 10 is provided. The rolling element bearing 10 comprises rolling elements 17 and rings 12, 14 having respective raceways 15, 16. The rolling elements 17 are coated with a lamellar lubricant such as molybdenum disulphide or tungsten disulphide, but the raceways 15, 16 of the rolling element bearing 10 are not coated with any lubricant. The lamellar lubricant may comprise at least one dopant and may be deposited by sputtering in a low-pressure chamber. Such a bearing 10 has a lifetime equal to or longer than one in which the raceways 15, 16 are also coated.

Description

Rolling Element Bearings This invention relates to a rolling element bearing, and in particular a rolling element bearing that is suitable for use in space, for example in a satellite.

Rolling element bearings are widely used in engineering where rotation is required, for example in the form of roller bearings or ball bearings. A ball bearing uses a number of balls to maintain the separation of two concentric bearing rings, so one bearing ring can rotate relative to the other. Typically one ring is static and the other is attached to an item that has to rotate.

Similarly, a roller bearing uses a number of short cylindrical rollers to separate two concentric bearing rings. Other types of rolling element bearing include spherical roller bearings, needle roller bearings, and tapered roller bearings. In every case the bearing may include a cage to maintain the separation of the rolling elements as they orbit the bearing.

For prolonged use it is necessary to lubricate a rolling element bearing, to minimise friction and wear of the rolling elements and the running surfaces of the rings (known as races or raceways). It is therefore usual to lubricate them with oil or grease. However lubricants that are suitable for use under normal conditions may be unsuitable for use in a low-pressure or high temperature environment, such as the vacuum experienced by a satellite in Earth orbit, as the lubricant may tend to evaporate and may deteriorate at elevated temperatures. Various different dry or solid lubricants have therefore been developed. Low shear strength metals, such as silver or lead and lamellar solids, based on materials such as molybdenum disulphide, hexagonal boron nitride or tungsten disulphide are also sometimes adopted for this purpose. The lubricity of lamellar materials such as molybdenum disulphide or tungsten disulphide is attributed to their structure, as these solid materials typically have a crystalline structure which features strong, thin layers or lamellae that are stacked together. Whereas the atoms within the layers of the crystal are strongly bonded, the individual layers are very weakly bonded together, and so more easily sheared such that the constituents of the lamellae can slide over each other. The anisotropic nature of this structure means that whilst lamellar materials can support quite a high normal load and separate the surfaces of the bearing, their easy shear means that friction between those surfaces is low despite the high normal load applied, hence they perform effectively as a lubricant. This anisotropy necessitates that when the lubricant is deposited directly onto a bearing surface via sputtering or other PVD means, specific bearing run-in operations may be required which are not usually needed for other classes of solid lubricants applied to rolling element bearings.

Rolling element bearings are commonly "preloaded", for example by employing them in pairs and loading one against another, in order to produce well defined stiffness and torque characteristics. Such preloading is typically achieved in two ways. Firstly, preload may be applied using a compliant element (e.g. a spring or diaphragm) to apply an internal load loop between a pair of bearings and the shaft or housing which accommodates them; this is a so-called "soft" or "compliant" preload. A second and very common preload method may typically employ rigid spacer or structural elements of very accurately controlled size between inner rings and between outer rings of the bearings, or by means of a small dimensional difference, known as "offset", introduced by the manufacturer between the end-faces of the bearing rings thereby to introduce preload (known as "hard" preload). For hard preloaded bearings the offset or difference in size between rigid preloading elements may typically be a few microns and must be tightly controlled. Due to the very high stiffness of the bearings and preloading elements in a hard preloaded bearing setup the application of a lamellar dry lubricant coating to balls and rings may cause a large (e.g. by a factor of 4) increase in the effective preload and a commensurate increase in the torque of the bearings. This is because in effect the application of a lubricant coating to ball and raceway surfaces increases the offset between the end-faces of the inner and outer bearing rings and so the preload. In this situation, controlled initial operation (or "running-in") of the bearings is required to wear away some of the lamellar lubricant applied and so reduce the change in offset (preload) and torque closer to the nominal value intended by the manufacturer or introduced by virtue of the carefully controlled structural elements. Such running-in produces wear debris which itself may contribute to bearing torque noise. Where bearings are compliantly preloaded, the relative displacement of the rings due to the as-deposited coating thickness introduces a negligibly small change in preload and bearing torque.

According to the present invention there is provided a rolling element bearing comprising rolling elements and rings defining raceway surfaces, wherein the rolling elements are coated with a lamellar lubricant, but other components of the rolling element bearing and associated surfaces are not coated with any lamellar lubricant.

The lamellar lubricant may comprise molybdenum disulphide or tungsten disulphide. The lamellar lubricant may also comprise an interlayer between substrate and lubricant coating, one or more dopants, for example a metal or metallic oxide or a gaseous element or even be itself structured into multiple nano-layers. The lamellar lubricant may be deposited by sputtering in a low-pressure or evacuated chamber. The thickness of the lamellar lubricant may be less than 2 microns, for example 1 micron or less.

Such a rolling element bearing may be suitable for use in a vacuum or in space.

The invention will now be further described by way of example only and with reference to the accompanying drawings, in which: Figure 1 shows diagrammatic sectional views of a ball bearing; and Figure 2 shows graphically the expected variation of ball bearing lifetime with peak Hertzian contact stress between balls and raceways for bearings with both the balls and the raceways coated; and also shows the corresponding lifetimes for bearings of the present invention, marked as Test.

Referring to figure 1, a ball bearing 10 comprises two concentric rings 12 and 14 that are spaced apart by a number of balls 17, eight being shown in this drawing. There may also be a cage 13 to hold all the balls 17 in their relative positions around the bearing 10. The bearing rings 12 and 14 have curved surfaces known as raceways 15 and 16 that are contacted by the balls 17. Such a ball bearing 10 is well known.

In accordance with the present invention, prior to assembly of the ball bearing 10, all the balls 17 are coated with molybdenum disulphide (Mo52) by sputtering in a low pressure or evacuated chamber, using energetic ions within an inert gas plasma (e.g. argon) to sputter MoS2 from a block of that material. The coating is substantially uniform and of thickness no more than 1 micron. It may for example be of thickness 0.5 pm or of thickness 1.0 p.m. No such coating is provided to the raceways 15 and 16, or to the cage 13 if there is one.

The ball bearing 10 that incorporates these coated balls 17 may be used in equipment for use in a vacuum, such as a space satellite.

Referring to figure 2, this shows graphically how the expected lifetime, measured either in numbers of revolutions or number of ball passes, varies with the peak Hertzian contact stress between ball and raceway (related to the total load, i.e. preload plus any externally applied loads, applied to the bearing), when the bearing is operating in a vacuum. As is evident, the expected lifetime decreases the larger the Hertzian contact stress (and load) applied; the axis representing the lifetime is logarithmic. This graphical data is for ball bearings in which both the balls 17 and also the raceways 15 and 16 are coated with Mo52. The corresponding test data, at a single load, and with only the balls 17 coated with Mo52 of thickness 0.5 pm, and the raceways 15 and 16 without any such coating -in accordance with the present invention -are also shown, marked Test. It will be seen that the lifetime of a bearing of the present invention is as long or even longer than the lifetime of a bearing in which the raceways are coated as well as the balls.

It will be appreciated that the present invention has the following benefits: 1) it introduces a reduced total thickness of lubricant into each bearing and for hard-preloaded bearings this results in a smaller or even negligible change in effective bearing preload, meaning reduced or zero run-in may be required to recover the original bearing preload (so greatly shortening processing time and reducing cost); 2) it generates less free lubricant during initial bearing operation so reducing the need for run-in and associated processing operations aimed at removing local excesses of lubricant (again shortening processing time and reducing cost); 3) it enables lubricated bearings to be produced at less cost and with fewer process steps; 4) it enables a de-coupling of the lubricant application from the production of the bearing rings so reducing overall manufacturing lead time for dry lubricated bearings.

Hence, dry lubricated bearings of the present invention can be produced more rapidly and at lower unit cost than bearings produced according to the prior production routes.

Claims

Claims 1. A rolling element bearing comprising rolling elements and rings defining raceway surfaces, wherein the rolling elements are coated with a lamellar lubricant, but other components of the rolling element bearing and associated surfaces are not coated with any lubricant.
2. A rolling element bearing as claimed in claim 1 wherein the lamellar lubricant comprises molybdenum disulphide or tungsten disulphide.
3. A rolling element bearing as claimed in claim 1 or claim 2 wherein the lamellar lubricant also comprises at least one dopant.
4. A rolling element bearing as claimed in claim 3 wherein the dopant is a gaseous element.
5. A rolling element bearing as claimed in any one of the preceding claims wherein the lamellar lubricant has been deposited by sputtering in a low-pressure or evacuated chamber so building up a lubricant structure wherein the lamellae are substantially parallel to the surface upon which they are deposited.
6. A rolling element bearing as claimed in any one of the preceding claims wherein the thickness of the lamellar lubricant is less than 2 microns, for example 1 micron or less.