WO2013005032A1

WO2013005032A1 - Profiled bearing

Info

Publication number: WO2013005032A1
Application number: PCT/GB2012/051560
Authority: WO
Inventors: Gary GOSSAGE; Peter Michaelides; Andraz VATOVEC
Original assignee: Messier-Dowty Limited
Priority date: 2011-07-06
Filing date: 2012-07-04
Publication date: 2013-01-10
Also published as: GB2492174A; GB201111507D0

Abstract

A bearing (10) arranged to receive a shaft (8) therein, the bearing including a contact surface (12) having a curved surface profile, the surface profile being arranged to conform to the curvature of the shaft (8) when the shaft is subject to a predetermined load.

Description

PROFILED BEARING

Background Art

Bearings are typically provided in mechanical systems to allow constrained relative motion of two or more parts with minimal resistance, often when the parts are under considerable loads. In cases where high load forces exist in a system, any bending moment applied to a shaft can cause the shaft to bend in line with the loading arrangement of the system. When a shaft is supported by a bearing and is bent under such loads forces will act between the bearing and the shaft. These forces will also cause the bearing to bend, but due to the bearing typically being stiffer than the shaft or having hardened material properties, the bearing tends not to deform to the same extent as the loaded shaft and therefore tends not to have the same profile as the loaded shaft. Localised areas of high load pressure between the bearing and the shaft at the edges and centre point of the bearing are therefore created, which increases resistance between the bearing and the shaft. In the short term this will cause diminished performance due to an increased coefficient of friction. In the long term this can cause fretting, leading to an uneven bearing surface and decreased lifetime, and in the worst case scenario serious damage to the system due to bearing failure.

Initial efforts to reduce the localised forces associated with shaft bending have focused on creating an inner profile which changes shape in response to the bend profile of the shaft. Examples of this approach can be found in US3933396A and JP56164217A. These systems incorporate multiple moving elements which increase weight and complexity of the bearing, as well as inducing high power loss, thereby reducing efficiency at a high cost. Another approach has been to create a permanent profile on the inside surface the bearing. Examples of this approach can be found in JP9177758 A, where chamfers have been incorporated into to the edges of the bearing, and US4995735 where a linear profile has been introduced to part of the inside profile to increase the surface contact area between the bearing and shaft. These approaches have shown promise but localised areas of high load are still present in these systems.

Summary of the Invention

According to a first aspect of the present invention there is provided a bearing arranged to receive a shaft therein including a contact surface having a curved surface profile, the surface profile being arranged to conform to the curvature of the shaft when the shaft is subject to a predetermined load.

The curved surface profile preferably extends across the entirety of the contact surface.

The contact surface may additionally or alternatively be arranged to enclose a majority of the shaft length.

The bearing may be arranged to support a central portion of the shaft. The bearing may comprise a plain bearing.

The shaft and bearing may be independently static or rotating with the same or different speeds relative to a fixed datum.

According to a further aspect of the present invention there is provided a roller bearing comprising a central bearing according to the first aspect, an outer bearing shell, and a plurality of further bearings radially located between the outer bearing shell and the central bearing.

One or more radial keys may be located between the contact surface of the central bearing and a shaft received therein.

According to a further aspect of the present invention there is provided a method of manufacturing a bearing arranged to receive a shaft comprising determining the shape of the shaft when the shaft is subjected to a predetermined load, and forming a contact surface of the shaft bearing having a curved surface profile conforming to the determined shape of the loaded shaft.

Brief Description of the Drawings

Embodiments of the present invention are described below, by way if illustrative example only, with reference to the accompanying figures, of which;

Figure 1 is a schematic illustration of a bearing and shaft under no external load according to the prior art;

Figure 2 is a schematic illustration of a bearing and shaft under external load according to the prior art;

Figure 3 is a schematic illustration of a shaft supported by a bearing according to an embodiment of the present invention under no external load;

Figure 4 is a schematic illustration of a shaft supported by a bearing under external load according to an embodiment of the present invention; and

Figure 5 is a schematic illustration of a roller type bearing according to an embodiment of the present invention.

Description of the Embodiments

Figure 1 shows a shaft 2 supported by a bearing 4 according to the prior art. The bearing has an inner surface 6 that has a profile which is linear and axially parallel to the centre line of the bearing i.e. a straight bore. When a load is applied to either of the bearing or shaft it may cause the shaft to bend as illustrated in figure 2. The bearing may deform as well, but the bearing is usually more rigid than the shaft and so will not bend to the extent of the shaft. When sufficient load has been applied, the shaft will therefore begin to exert localised areas of high pressure on the inner surface of the bearing due to the shaft no longer conforming to the profile of the bearing inner surface. This will result in a nonuniform distribution of bearing pressure along the length of the bearing, where much of the load will be transmitted through the edges and centre of the bearing (labelled X, Y and Z respectively in figure 2). These localised areas of high pressure increase friction between the bearing and the shaft which can lead to poor performance, and can cause extensive and uneven fretting along the inner walls of the bearing and the surface of the shaft reducing the lifetime of the components.

Embodiments of the present invention mitigates these problems by more uniformly distributing the load between a shaft and bearing along the length of the bearing and by reducing instances of localised high pressure within the system.

An embodiment of the present invention is shown in figures 3 and 4. In this embodiment a bearing 10 supports a shaft 8, the bearing including a curved, axially non-linear surface 12. When an expected or predetermined load is applied to the shaft 8, as shown in Figure 4, the bend profile of the shaft 8 will match the curved profile of the inner surface 12 of the bearing. Because of this, the load distribution between the shaft 8 and the bearing 10 is uniform along the length of the inner surface 12. Consequently, the load applied to the shaft 8 is transferred uniformly from the shaft 8 to the bearing 10, thereby substantially reducing localised areas of high load pressure and as a result reducing fretting and uneven wear. Bearing and shaft lifetime is therefore increased and the risk of damage to components in the system is reduced.

As illustrated in Figures 3 and 4, the contact surface of the bearing 10 may enclose a majority of the shaft length. This is likely to be the case where the shaft 8 is only supported by a single bearing or when the shaft 8 is a fixed pin. Equally, the bearing 10 may support a central portion of the shaft 8, particularly when one or both ends of the shaft 8 may receive a bending load.

A further embodiment of the present invention is schematically illustrated in Figure 5. As in the embodiment illustrated in Figures 3 and 4, a shaft 8 is supported by a bearing 10 that includes curved contact surfaces 12, the curvature of the contact surfaces being matched to the expected curvature of the shaft 8 when the shaft is loaded to a predetermined extent. The bearing 10 is itself supported within an outer bearing shell 14 by a number of additional non-contoured bearings 16 in an analogous fashion to a conventional roller or race bearing. The additional non-contoured bearings 16 are preferably roller or needle bearings or other bearings of an analogous type, although they may be any suitable bearing, such as plain bearings. Depending on the type of bearing selected, it may be desirable to provide a mechanical key between the shaft 8 and the inner, contoured bearing surfaces 12 to ensure adequate mechanical drive between the shaft and the bearing surfaces. For example, one or more radial projections may be formed on either the shaft or the contoured surfaces that are arranged to be movably received in corresponding slots formed in either the contoured surfaces or shaft respectively, particularly at the outer edges of the bearing when the deflection of the shaft from the (opposing) contoured surface is greatest.

The shaft is shown in Figure 5 in the loaded, curved, state, although it will be appreciated that the extent of curvature shown in Figure 5 (as well as Figures 3 and 4) is greatly exaggerated. In reality the degree of deflection of the shaft from the outer edge of the contact surface when the shaft is fully loaded is likely to be of the order of less than 1mm.

In accordance with embodiments of the present invention, the curvature of the shaft or pin when loaded is predetermined, for example by using existing stress analysis methods or computational finite analysis methods, or a combination of stress analysis and computational finite analysis methods. The contact surface of the bearing 10 is substantially manufactured to exhibit the predetermined profile, for example by machining, casting, or other suitable engineering method. Preferably the load to be applied to the shaft is fixed. However, it may be that a range of shaft loads are possible. In such circumstances it is preferable to determine or estimate the maximum expected load and determine the shaft, and thus bearing, curvature based on that maximum value, because even if the shaft experiences a load less than the maximum expected value such that the curvature of the shaft is less than the curvature of the bearing surfaces, and therefore the shaft does not contact the bearing surface over the full length of the surface, the continuously curved bearing profile avoids any step changes in profile where the shaft is in contact with the bearing surface that would cause fretting and wear. It is for this reason that the profiled bearing of the present invention is advantageous over prior art bearings having chamfered edges; where the chamfered profile joins the non-profiled section of such bearings a step change occurs that still gives rise to a peak in shaft pressure and therefore causes wear.

In embodiments of the present invention the shaft 8 may be either a rotatable shaft or a fixed pin. In the latter case the bearing 10 is preferably a bushing arranged to receive the pin and maintain the pin in a desired location. Within the context of the present invention the term bearing is intended to encompass such bushes. The bearing may be a plain bearing or any other bearing suited to the particular implementation known to the skilled person.

Claims

1. A bearing arranged to receive a shaft therein, the bearing including a contact surface having a curved surface profile, the surface profile being arranged to conform to the curvature of the shaft when the shaft is subject to a predetermined load.

2. A bearing according to claim 1, wherein the curved surface profile extends across the entirety of the contact surface.

3. A bearing according to claim 1 or 2, wherein the contact surface is arranged to enclose a majority of the shaft length.

4. A bearing according to any preceding claim, wherein the bearing is arranged to support a central portion of the shaft.

5. A bearing according to any preceding claim, wherein the bearing comprises a plain bearing.

6. A bearing according to claim 1, whereby the shaft and bearing are independently static or rotating with the same or different speeds relative to a fixed datum.

7. A roller bearing comprising a central bearing according to any preceding claim, an outer bearing shell, and a plurality of further bearings radially located between the outer bearing shell and the central bearing.

8. A roller bearing according to claim 7, wherein one or more radial keys are located between the contact surface of the central bearing and a shaft received therein.

9. A method of manufacturing a bearing arranged to receive a shaft comprising:

determining the shape of the shaft when the shaft is subjected to a predetermined load; and

forming a contact surface of the shaft bearing having a curved surface profile conforming to the determined shape of the loaded shaft.