EP2297473A1

EP2297473A1 - A bearing component

Info

Publication number: EP2297473A1
Application number: EP08773591A
Authority: EP
Inventors: Xiao Bo Zhou
Original assignee: SKF AB
Current assignee: SKF AB
Priority date: 2008-06-23
Filing date: 2008-06-23
Publication date: 2011-03-23
Also published as: WO2009155938A1

Abstract

The present invention provides a bearing component for use in a bearing for interference fit applications, the component comprising a rubber layer on a surface thereof, the rubber layer having a modulus of elasticity of 50 MPa or less.

Description

A bearing component

Technical field

The present invention relates to a bearing component having a high elasticity rubber layer on a surface thereof . More particularly, the present invention relates to a bearing for interference fit applications.

Background

An interference fit (sometimes also called a press fit) is a fastening between two parts which is achieved by pushing the two parts together, causing friction. This friction may result simply from the close fit of the shapes of the parts. Additionally, it may result from the compression of one part against the other. In close fit applications, the pressure holding the two parts of an interference fit together, known as the interference pressure, is typically lOMPa or greater, for example 10 MPa to 100 MPa. Applications having a lesser pressure holding the two parts of the fit together are called loose fit applications.

A bearing is an example of one part of an interference fit. A bearing is a device that permits constrained relative motion between two parts. Bearings may be used in many different types of machinery to retain and support rotating components such as, for example, a wheel on a vehicle, a vane on a windmill or a drum in a washing machine. A rolling element bearing comprises inner and outer rings and a plurality of rolling elements (balls or rollers bearings) . The press fitting of a bearing onto a shaft and the fitting of a bearing into its housing are both examples of interference fits.

Due to the friction between the two parts of an interference fit, the two parts are generally held together so that movement of a surface of one of the parts induces the movement of the surface of the other part. In other words, the two surfaces of the interference fit are generally in register and do not slip relative to one another. However, a certain amount of small-amplitude oscillatory movement does occur between the two parts. This happens when the force required for one surface to be moved by the other surface is greater than the frictional force between the surfaces of the two parts. Repeated movements of this type is referred to as fretting. It can be caused by, for example, vibrations being transmitted to one part from the system comprising the interference fit, causing a sudden force on one part of the interference fit to move relative to the other part .

Fretting causes wear. This wear can result in the formation of debris from the surfaces of the parts of the interference fit through, for example, adhesive wear. This debris in turn results in further abrasion of the surfaces of the parts through abrasive wear. Adhesive wear may also result in complete seizure of the contact of the two parts of the interference fit and, in the case of bearings, this seizure results in the failure of the bearing. Since adhesive wear is dependent on the pressure between the two parts of an interference fit, adhesive wear is increased in close fit applications compared with loose fit applications. Fretting also causes corrosion. Fretting results in the exposure of the surfaces of the parts of the interference fit to conditions that can result in the oxidation of the surfaces of the parts. For example, the pressure and heat caused by fretting can result in the oxidation of a surface comprising iron under normal atmospheric conditions to produce iron oxide. This surface oxidation not only changes the surface properties of the parts of the interference fit but can also result in debris being formed, which, in turn, increases the abrasive wear of the fit.

Fretting also causes fatigue. In particular, fretting can result in the uneven build-up of stress across the surface of the parts of the interference fit. This fatigue can result in cracking of the surface of one of the parts of the interference fit. While this cracking does not occur very often in applications involving bearings, when cracking does occur, it can cause total failure of the bearing.

The type of damage caused by fretting is dependent on the displacement of the slipping movement, as described in ASTM STP 1367 , 49 (2000) . At very small slipping displacements, little fretting damage occurs. At slightly greater slipping displacements, under a partial slip regime, fretting fatigue becomes significant, while a small amount of fretting wear also occurs . This magnitude of displacement is typical in a close- fit interference fit. As the displacement of the oscillatory movement further increases, fretting wear starts to increase. Finally, with large displacements under a gross slip regime, wear becomes the dominant mechanism. This is typical for loose fit applications. Corrosion wear also tends to vary with the type of displacement. Specifically, a surface that has been oxidized tends to be harder than the un-oxidized surface, resulting in a reduced amount of wear. However, the oxidized parts of the surface introduce weaknesses into the surface structure, increasing the rate of fretting fatigue.

In the past, there have been three approaches to combating fretting in interference fits. The first approach has been to try to control the load on and the motion of the interference fit. This approach aims to reduce the small- amplitude oscillatory movement to which the interference fit is subjected by managing the loads around the fit. In this way, this approach aims to reduce amount of energy entering into the fit and to reduce the oscillatory movement induced by this energy. However, this method requires very careful design and control of its system, which is not practical for many applications.

The second approach has been to try to avoid the potentially extremely adverse effects of fretting fatigue. This has been achieved by making structural alterations to the interference fit itself. An example of this approach is described in Wear 261, 1114-1120 (2006) and Wear 258, 898- 905 (2005) . The authors of these papers link all of the underlying causes of fretting fatigue to a single parameter, namely friction coefficient. In particular, they suggest that, by minimizing the friction coefficient, the maximum tensile stress and the maximum shear stress can be also reduced, thereby reducing fretting. These papers then recommend using a low friction material as a coating, such as a fluoropolymer . A similar system is described in US 6334713. This document describes the use of a low abrasion, low friction material between a wear ring and a shaft. The use of a low abrasion material cushions the wear ring from abrasive contact with the shaft, thereby reducing the fretting of the shaft by reducing abrasion.

The use of a low friction coefficient material is described as advantageous in Wear 258, 898 (2005) because it reduces the friction between the two parts of an interference fit. A reduction in friction results in a reduction of load at the surfaces of the interference fit, the load being at its maximum when the slipping movement of one surface relative to the other is small. Since fretting fatigue is dependent on the load at the surfaces, in particular the maximum load, the fretting fatigue is reduced. However, while this approach of universally reducing the coefficient of friction may be satisfactory in some loose fit applications in which slip between the surfaces of the parts of the fit is acceptable, this approach is not satisfactory in close fit applications, in which an increase in the relative movement between surfaces caused by a reduced fiction coefficient is not desirable. Furthermore, because the coefficient of friction is reduced throughout the slip displacement with this approach, the amount of slip and, as a result, the amount of wear on the surface of the parts also increases. In addition, a fluoropolymer coating such as that described in the Wear article referred to above is difficult and expensive to prepare, requiring several processing steps to form it on the parts of the interference fit. A third approach has been to try to reduce the wear of the interference fit by using, for example, a hard coating. However, these hard coatings have tended to be ceramic, which are brittle in nature and therefore subject to cracking and breaking up, thereby forming debris. This debris in turn results in an increase in wear. Therefore, these coatings, instead of alleviating the effects of fretting, can actually increase the wear of the interference fit under some circumstances.

Currently, the most successful approach to countering the effects of fretting is to use a grease or paste to reduce the friction in the interference fit, thereby reducing the maximum load experienced by the parts of the interference fit by reducing the frictional force between the parts.

However, in addition to the disadvantages mentioned above, this approach also only leads to a temporary effect because the grease or paste eventually escapes from the area of contact in the interference fit.

Summary

The present invention aims to provide a new approach to combat the effects of fretting in an interference fit. In particular, the present invention provides a bearing component for use in a bearing for interference fit applications, the component comprising a rubber layer on a surface thereof, the rubber layer having a modulus of elasticity of 50 MPa or less. Brief description of the drawings

Figures 1 to 3 are provided to illustrate the frictional properties of certain rubbers that can be used in the present invention. The figures show a comparison of the displacement amplitude at which the friction coefficient was measured (x-axis, μm) with the measured coefficient of friction (on the y-axis) . Figure 1 is for NBR rubber, Figure 2 is for ACM rubber and Figure 3 is for FKM rubber.

Detailed Description

The present invention will now be further described. In the following passages different aspects of the invention are defined in more detail. Each aspect so defined may be combined with any other aspect or aspects unless clearly indicated to the contrary. In particular, any feature indicated as being preferred or advantageous may be combined with any other feature or features indicated as being preferred or advantageous.

The inventor of the present invention recognises that previous approaches to reducing fretting fatigue in an interference fit have relied on allowing a limited movement between the two parts of an interference fit to reduce the load experienced by the parts of the fit. This has been achieved by reducing the friction coefficient between the two parts of the fit. The inventor further recognises that this limited movement between the parts means that the parts do not maintain a constant register between each other, making this approach not ideal when the interference fit is used in applications where a constant register between the parts is required, for example in close fit applications.

Therefore, the inventor has found a type of material that addresses the problems of fretting fatigue, fretting wear and maintains a constant register between the parts of an interference fit when coated on one part of the interference fit. This material is a type of rubber.

Normally, rubber would not be considered as a material to reduce fretting. This is because most rubbers have a high coefficient of friction in sliding contact with other materials. This means that that an interference fit having one part coated in rubber would experience a high maximum tensile stress and high maximum shear stress both at small and large displacement amplitudes, necessarily leading to a considerable degree of wear and abrasion. Conversely, while a rubber with a low coefficient of friction results in a lesser maximum tensile stress and shear stress, its low coefficient of friction makes it unsuitable for applications requiring the two parts of the interference fit to maintain a constant register.

However, the inventor has found that a particular type of rubber results in a reduced amount of wear and abrasion in use. This type of rubber has a high elasticity.

Without wishing to be bound by theory, the inventor suggests that, when load is applied to a bearing component coated with a rubber having a high elasticity, two different mechanisms operate to dissipate the load. The first mechanism is friction generated when two parts of the component slide relative to one another. This mechanism also occurs when using rubbers having low elasticity. The second mechanism is the elastic deformation of the rubber. This mechanism is much less prominent in rubbers having low elasticity.

The inventor has found that these two mechanisms operate to varying extents dependent on the size of load applied to the bearing component. In particular, at low applied loads, elastic deformation is prominent. This elastic deformation is accompanied by a small amount of relative movement of the two bearing components relative to one another. In contrast, at higher applied loads, significant slipping of the two parts of the bearing component occurs, which results in friction being the most important mechanism.

One consequence of these two mechanisms operating to varying degrees at different amplitudes of displacement is that, when the friction coefficient of a rubber having a high elasticity is measured, it is found to be dependent on the amplitude of displacement of the movement used to measure the coefficient. In particular, the measurement of the friction coefficient does not distinguish between movement resulting from the elastic deformation of the rubber coating and movement resulting from actual slipping of the two parts relative to one another. Since proportionally more movement results from the elastic deformation of the rubber at small amplitudes of displacement, rubbers having a high elasticity are in practice measured as having a lower coefficient of friction at small amplitudes of displacement compared to at larger amplitudes of displacement. Therefore, by using the rubber of the present invention, the maximumum tensile stress and shear stress at low amplitudes of displacement is reduced because friction at these amplitudes is reduced. Since elasticity is prominent at the amplitude of the oscillations experienced by rotating parts in a typical interference fit, i.e. amplitudes of less than 0.1 mm, the use of the bearing component of the present invention results in a reduced amount of wear and abrasion in an interference fit.

Accordingly, it is thought that the use of a rubber of high elasticity allows the dissipation of energy around the interference fit so that slipping of the surfaces of the fit is minimized. In other words, instead of energy being dissipated by the two parts of the fit slipping relative to one another, energy is absorbed by the rubber itself because of the rubber's elasticity. According to the invention, excessive slip of the interference fit is avoided because the majority of the relative movement takes place through elastic deformation of the rubber matrix rather than through slip at the contact interface of the interference fit.

At the same time, the rubber layer on the surface of the bearing has the advantage that, on the occasion that excessive force is placed on the interference fit and slipping does occur at the contact interface, the rubber exhibits a high friction coefficient. As a result, when slipping does occur, excessive slipping is prevented by the increase in friction coefficient with increased displacement amplitude. The rubber of the present invention is preferably chosen so that it has a high coefficient of friction at high displacement amplitudes, as this material property facilitates the maintenance of a constant register between the two parts of an interference fit.

As described above, the frictional properties of a rubber having a high elasticity on one surface of an interference fit varies according to the amplitude of relative displacement. Therefore, the frictional properties of a rubber in an interference fit are best characterized by two different coefficients of friction, namely a sliding friction coefficient and a fretting friction coefficient. The sliding friction coefficient reflects the friction coefficient of the material at a high displacement amplitude, which is to be understood as an amplitude greater than 0.1 mm. The fretting friction coefficient reflects the friction coefficient of the material at a low displacement amplitude, which is to be understood as an amplitude less than or equal to 0.1 mm.

As used herein, the sliding coefficient of friction refers to the maximum coefficient of friction measured during reciprocal sliding of a steel ball over a rubber surface. The reciprocal sliding has an amplitude of 4 mm, a reciprocal frequency of 0.5 Hz. The sliding movement of the steel ball is sinusoidal. A load of 2 N is applied. For measurements made herein, the steel ball was made of AISI 52100 ball bearing steel, hardened HRC 61 to 63, with the ball diameter being 12.7 mm. The sliding is carried out in an ambient environment, i.e. at room temperature (2O⁰C) and at a humidity controlled to be 60%.

As used herein, the fretting coefficient of friction refers to the maximum coefficient of friction measured during reciprocal sliding of a steel ball over the rubber surface. The reciprocal sliding has an amplitude of 0.025 mm and a reciprocal frequency of 20 Hz. The sliding movement of the steel ball is sinusoidal. A load of 16.3 N is applied. For measurements made herein, the steel ball was made of AISI 52100 ball bearing steel, hardened HRC 61 to 63, with the ball diameter being 12.7 mm. The sliding is carried out in an ambient environment, i.e. at room temperature (2O⁰C) and at a humidity controlled to be 60%.

To enable the relative movement between the two parts of the interference fit to take place through elastic deformation of the rubber matrix, the modulus of elasticity of the rubber of the present invention is 50 MPa or less. (A low modulus of elasticity relates to a high elasticity) . More preferably, the modulus of elasticity is 30 MPa or less, for example 15 MPa or less, most preferably 11 MPa or less. Suitably, a rubber according to the invention can have a modulus of elasticity of 7-11 MPa. By way of comparison, the modulus of elasticity of rubbers in general is as much as 100 MPa or even greater.

The modulus of elasticity, also known as the Young's Modulus, is measured using a nano- indention method according to ISO 14577 or ASTM 2806. It is to be noted that the selection of a rubber having a lesser modulus of elasticity is advantageous because it reduces the amount of slipping movement at small amplitudes of displacement, thereby reducing the overall wear and abrasion of an interference fit. Preferably, the sliding friction coefficient of the rubber for use in the present invention is at least about 0.5. This is comparable to the friction coefficient of an uncoated metal surface. Preferably, the sliding friction coefficient is greater than this, preferably at least about 1.0. In further preferred embodiments, the friction coefficient of the rubber is at least about 1.25, more preferably at least about 1.3.

The fretting friction coefficient of the rubber of the present invention is less than the sliding friction coefficient, for example at least 50% less, preferably at least 75% less, such as at least 90% less. Preferably, the fretting friction coefficient is 0.2 or less. More preferably, it is 0.1 or less. More preferably, it is 0.05 or less.

The Poisson's ratio of the rubber is preferably about 0.25 to 0.75, more preferably about 0.5. It is calculated from the shear modulus of the rubber. The Poisson's ratio can be measured according to ASTM D575.

The term "rubber" as used in the present application refers to both natural and synthetic rubbers, including composite rubbers . Rubbers that are especially suited for use in the present invention include nitrile butadiene rubbers, FMK rubbers, silicone rubbers and polyacrylic-base rubbers (e.g. ACM rubbers) . Examples of suitable silicone rubbers include Silicon White and Silicon Red, supplied by Weicon GmbH. Silicone White is a general purpose rubber, while Silicon Red is designed for applications up to 230° C. Types of these rubbers are chosen that have a modulus of elasticity of less than 50 MPa, preferably less than 11 MPa.

The minimum thickness of the rubber layer is preferably at least about 10 μm in thickness. Below this thickness, the inventors have found that sliding on the rubber surface increases and the effects of fretting also increase. More preferably, the thickness of the rubber layer is at least about 25 μm, more preferably at least about 100 μm.

Alternatively, the preferred minimum thickness of the rubber can be related to the properties of the rubber layer and the maximum displacement amplitude during the working of the fit according to the following relationship (d(P,f) is the maximum displacement amplitude divided by the thickness of the rubber) :

d{PJ)=f-

where f is the sliding friction coefficient, P is the applied load (MPa) and G is the shear modulus of the rubber, which is related to the Poisson ratio (a) as follows:

G=

2x(l+α)

Accordingly, to determine the preferred minimum thickness, 1 / d(P,f) is calculated and then multiplied by the maximum displacement amplitude during the working of the fit. The rubber coating of the present invention can be applied to a bearing component of the present invention by a conventional method. For example, the bearing component may^¬ be dipped into melted rubber. Alternatively, the rubber coating may be sprayed onto the bearing component .

The bearing component may be, for example, an inner ring, whereby the rubber coating is provided on a radially inner surface (bore) of the inner ring. The bearing component can also be an outer ring, whereby the coating is provided on a radially outer surface of the outer ring.

The present invention also provides a bearing component for use in a bearing for interference fit applications, the component comprising a rubber layer on a surface thereof, the rubber layer having a sliding friction coefficient of 0.5 or greater. The use of a rubber layer having a sliding friction coefficient in this range (and preferably 1.0 or greater, more preferably 1.25 or greater, such as 1.3 or greater) is advantageous because it contributes to maintaining a constant register between the two parts of an interference fit. Further, the rubber layer preferably has a modulus of elasticity of less than 50 MPa, preferably less than 11 MPa, which reduces fretting fatigue by minimising slip at the contact interface, by enabling relative movement to take place through elastic deformation of the rubber matrix. Preferably, the fretting friction coefficient is less than the sliding friction coefficient, for example at least 50% less. The preferred embodiments of this aspect of the invention (e.g. fretting friction coefficient and preferred sliding friction coefficient and elasticities) are the same as the previous aspect . The present invention also provides a bearing comprising a bearing component as herein described.

The present invention also provides a system comprising a bearing as herein described, and a member forming an interference fit with the bearing. The bearing may be mounted on a shaft with an interference fit by, for example, heating it to about 100° C and allowing it to shrink onto the shaft.

The present invention also provides for the use of a rubber layer having a modulus of elasticity of 50 MPa or less in an interference fit to reduce or prevent fretting.

Examples

The effect of the present invention is demonstrated by the following examples. Three plates were prepared with the following rubber layers:

(1) a nitrile butadiene rubber coating (NBR) having a modulus of elasticity of about 8 MPa,

(2) a fluorocarbon-base rubber coating (FKM) having a modulus of elasticity of about 10 MPa, and

(3) a polyacrylic-base rubber coating (ACM) having a modulus of elasticity of less than 11 MPa.

The rubber layers had a thickness of 1.9 mm. The variation in the coefficient of friction was then measured during reciprocal sliding of a steel ball over the rubber surface. For each of the three rubber layers, the maximum coefficient of friction was measured under the following test conditions:

- a sliding amplitude of 0.025 mm, a reciprocal frequency of 20 Hz and an applied load of 16.3 N; - a sliding amplitude of 0.1 mm, a reciprocal frequency of 20 Hz and an applied load of 16.3 N;

- a sliding amplitude of 4 mm, a reciprocal frequency of 0.5 Hz and an applied load of 2 N.

The sliding and fretting friction of rubber layers (1) to (3) is shown in Figures 1 to 3 respectively.

The temperatures at which these rubbers may be applied and used were also measured. The results were as follows:

These results demonstrate that these rubbers have a low fretting friction coefficient and a high sliding friction coefficient. These results further demonstrate that the rubbers of the present invention effectively dissipate energy at low amplitude displacements so as to reduce fretting fatigue, while at the same time damp the displacement at larger amplitudes so as to resist further displacement. This results in a reduction in wear. These results also show that the rubbers of the examples of the present invention are suitable for use at high temperatures, for example at 100° C or greater, for example 140°C or greater.

Claims

CLAIMS :

1. A bearing component for use in a bearing for interference fit applications, the component comprising a rubber layer on a surface thereof, the rubber layer having a modulus of elasticity of 50 MPa or less.

2. A bearing component as claimed in claim 1, wherein the rubber layer has a modulus of elasticity of 11 Mpa or less.

3. A bearing component as claimed in claim 1 or 2, wherein the rubber layer has a sliding friction coefficient of 0.5 or greater.

4. A bearing component as claimed in claim 3, wherein the sliding friction coefficient of the rubber layer is 1.0 or greater.

5. A bearing component as claimed in any one of the previous claims, wherein the modulus of elasticity of the rubber layer is from 7 MPa to 11 MPa.

6. A bearing component as claimed in any one of the preceding claims, wherein the rubber layer is at least 10 μm in thickness.

7. A bearing component as claimed in any one of the preceding claims which is an inner and/or outer ring.

8. A bearing comprising a bearing component as defined in any one of the preceding claims.

9. A system comprising: a bearing as defined in claim 7, and a member forming an interference fit with the bearing.

10. The use of a rubber layer having a modulus of elasticity of 50 MPa or less in an interference fit to reduce or prevent fretting.

11. The use as claimed in claim 10, wherein the sliding friction coefficient of the rubber is 0.5 or greater.