WO1994001690A1

WO1994001690A1 - Improved bearing assembly

Info

Publication number: WO1994001690A1
Application number: PCT/GB1993/001399
Authority: WO
Inventors: Keith Ian Chester
Original assignee: Keith Ian Chester
Priority date: 1992-07-08
Filing date: 1993-07-05
Publication date: 1994-01-20
Also published as: AU4509293A; GB9214540D0; GB2268549A; GB2268549B; GB9313881D0

Abstract

An improved form of bearing assembly for supporting rotating shafts, of the type which comprises a number of bearing pads (20) supported by a surrounding housing (21), is distinguished from prior assemblies of this general type in that the contact between each pad (20) and the housing (21) is in the form of a curved line. In a preferred form, the surface of each pad (20) engaging the housing (21) is curved in two directions at right angles and the engaging surface of the housing (21) closely matches the pad (20) curvature in only one of those directions and differs from it in the other.

Description

Improved Bearing Assembly

The present invention is concerned with bearing assemblies of both the journal and thrust bearing types and is an improved bearing assembly. Bearing assemblies are used in a very wide range of applications to afford radial and/or axial support for rotating shafts or spindles. While unitary journals or thrust rings are widely used, there are considerable advantages in construct- ing a bearing assembly from several discrete, pads, which individually abut the shaft or thrust collar and are able to adjust their individual orientations to compensate for misalignment of the shaft or other variations in the geometry or loading of the shaft assembly. Effective lubrication of the rotating surface entails forming a thin converging film of lubricant between the moving surface and the bearing and the ability of the bearing pads to adjust their individual positions relative to that surface assists the formation and maintenance of the film. Journal pads and thrust pads are available in many forms, designed to accommodate particular combinations of load, speed and geometry in a wide range of applications. The adjustment of the orientation of each pad is determined by the nature of the contact between the pad and its supporting housing. In general, prior bearing pad assemblies have depended either upon a ridge (that is, straight-line) contact between pad and housing, which allows the pad to tilt about that line as axis, or upon a point contact between pad and housing, which permits tilting of the pad about two or more axes.

Figure 1 of the accompanying drawings illustrates, in sectional view transverse to the axis of the rotating shaft, four conventional forms of bearing pad mounting. In Fig. la, a journal pad 10 is shown supported upon a spherical pivot 11. The pad 12 shown in Fig. lb is supported upon a pivot 13 formed as an insert of hardened metal incorporated in the pad. These two forms of pad bearings allow tilting of the pad in any direction relative to the housing.

Fig. lc illustrates a journal pad 14 formed with an integral ridge 14a which gives straight-line contact between the pad and the housing and allows the pad to tilt only about that ridge as axis. The hardened inserts 15 and 16 of the pad 17 shown in Fig. Id again allow tilting about only one axis, determined by the straight line of contact between the inserts.

Such prior art bearing pad assemblies suffer from various disadvantages. For example, in the case of the pad 10 of Fig. la, the load on the pad in use and the friction between the pivot 11 and the socket in the pad which it engages combine to hinder the tilting movement of the pad necessary to bring the pad into the desired alignment. The force required to tilt the pad when in operation may exceed the safe working limit of the pad.

Better freedom to align in operation is achiev¬ able by providing true point contact between a bearing pad and its housing. However, such point contact leads to heavy wear of the housing, which can only be prevented by the provision of a hardened insert in the housing; bearings incorporating such inserts are relatively expensive and relatively bulky. Against the foregoing background, it is an object of the present invention to provide an improved bearing assembly, whereby some at least of the disadvantages of available prior bearing assemblies are reduced or avoided. The improved bearing assembly according to the present invention, for supporting a rotatable shaft, comprises a plurality of bearing pads supported by a surrounding housing, the contact between each pad and the housing being in the form of a curved line. Thus the assembly of the present invention is distinguished from prior related bearing assemblies wherein the pad/housing contact has been at a point, over a surface or along a straight line. By virtue of the novel curved-line contact, the improved bearing assembly allows the bearing pads the multi-directional freedom of adjustment of point-contact pads or pivot-and- socket pads, while giving the support which is characteristic of pads which contact the housing along a straight line.

The curved line of contact of the bearing pad and housing in the assembly according to the present invention may be achieved by so designing the surface of the pad which engages the housing that it is curved in two directions at right angles to each other and so designing the surface of the housing which engages the pad that it closely matches the curvatures of the pad in one of those directions and differs from it in the second of those directions. The extent to which the shape of the housing surface departs from a close match to the curvature of the pad surface in this latter direction is a matter of choice. For example, the housing surface may be flat in the second direction. However it is preferred that the differ¬ ence between these surfaces be fairly small, as certain advantages may thereby be achievable. In particular, as described below, it may thus be possible to dispose a further layer of lubricant between the pad and the housing.

Thus, in a particularly preferred form of the bearing assembly according to the present invention, the interengaging surfaces of the pads and housing are each curved in two directions at right angles and the interengaging surfaces closely match each other in one said direction and differ by a small amount in the other said direction. The difference in angular terms may be about the same size as, or slightly greater than, the operating angle between the shaft and the working face of the bearing; typically this is less than one degree of arc. For example, in the case of two inter¬ engaging part-spherical surfaces, the radii of curvature of the surfaces in the second-mentioned direction may differ by less than one per cent or even less than a half of one per cent.

In the case of either a journal bearing pad or a thrust bearing pad, the second direction, that is, the direction in which the interengaging surfaces differ, is preferably circumferential relative to the axis of the rotatable shaft. The pads may advantageously be held in position within the housing by locating pins, for example projecting from the housing into sockets in the pads. The extent to which the pins project should be such that the pads are prevented from movement around the shaft but are not supported by the pins. In a particularly preferred form of the invention, provision is made for the introduction of lubricant into the space between the interengaging surfaces of the pad and the housing. The lubricant may simply be intended to reduce frictional forces impeding the adjustment of the pad orientation or may be used to displace the pad by a small amount, either to adjust its orientation or to move the pad by a small amount in a direction radial to the shaft. In this latter context, a hollow may be provided in the surface of the pad in which to retain the desired small quantity of lubricant. The retention of the lubricant under pressure may be aided by encircling the hollow with an 0-ring or similar seal. The invention will now be further described with reference to the accompanying drawings, which illustrate, in Figs. 2 to 7, various preferred embodiments of the improved bearing assembly according to the present invention. Fig. 2a_ is a transverse sectional view of a first embodiment of journal bearing assembly and Fig. 2b is a corresponding sectional view in a plane parallel to the axis of the shaft. The bearing is assembled from four bearing pads 20, only one of which is shown, each secured in a housing 21 by a locating pin 22, which prevents the pad from being displaced relative to the housing but does not support the pad. In the direction seen in Fig. 2a, the radius A of the radially outer surface of the bearing is less than the radius B of the inner surface of the housing by about 0.04 per cent. However in a direction parallel to the axis of the shaft, as seen in Fig. 2b, these two surfaces are a close match. As a result, the surfaces contact each other along a curved line parallel to the -shaft axis, about which line the pad 20 is able to tilt to adjust to irregularities in the relative alignment of the shaft and housing. The bearing assembly illustrated in Fig. 3 resembles that shown in Figs. 2a and 2b, but two bores 33 are provided in the housing 31 to enable oil to be introduced to the space between the housing and the bearing pad 30. Similar bores (not shown) are provided for the other three bearing pads. The introduction of oil through the bores

30 assists self-adjustment of the pads but also affords the possibility of adjustment of the pads being controlled by the pressure of the oil injected.

Figs. 4a and 4b illustrate a further embodiment of the bearing assembly. In that embodiment, a hollow or trough 44 is provided in the curved outer face of each of the bearing pads 40, only one of which is shown. By means of a bore 43, oil may be introduced into the trough 44 to enable pressure to be applied to the back of the pad, to control the attitude of the pad or to move the pad in a radial direction relative to the shaft which is journalled by the bearing.

Figs. 5a and 5b show a variant form of the bearing of Figs. 4^ and 4b, in which the trough 54 (only one of which is shown) in the outer face of each bearing pad 50 is encircled by an O-ring 55. By means of the O-ring, an enhanced oil seal is formed around the trough 54 and higher oil pressures may be established in the trough. By such higher oil pressures it becomes possible to move the pad 50 by a significant distance, for example of the order of 0.5 mm. Such movement may be effected to adjust either the clearance or the damping of the bearing and thus to affect secondary bearing performance characteristics such as power loss or oil flow requirement.

The application of the present invention to thrust bearing assemblies is illustrated by the embodiments shown in Figs. 6 and 7 respectively.

Fig. 6a is a plan view in the direction of the axis of the shaft and Figs. 6b and 6£ are sectional views of individual bearing pads 60 in the directions of the arrows B-B and C-C respect- ively. The bearing assembly comprises eight thrust pads 60 supported by a housing 61 and retained therein by locating pins 62. The interengaging surfaces of the pads and housing are both curved but whereas, in the direction of the section C-C, the curvatures of the interengaging surfaces are a close mutual match, in the direction circumferential to the shaft as shown in Fig. 6b, there is a small difference in curvature of these surfaces. Thus these two surfaces meet along the curved line of contact seen in Fig. 6£ and the pads 60 are able to tilt about that line as axis.

A variant form of thrust pad bearing is illus¬ trated in Fig. 7. In the case of the Fig. 6 embodi¬ ment, each pad is mounted in an individual generally spherical pocket in the housing, while in the form of bearing shown in Fig. 7, the pads are all located in a uniform circular track 73, of part-toroidal form, as can be seen in Figs. 7b and 7c, which are cross-sectional views in the directions of the arrows B-B and C-C respectively of Fig. 7a_. Thus in the radial direction seen in Fig. 7c_, the pads 70 are a close fit in the track 73, whereas in the circumferential direction shown in Fig. 7b, each pad is free to tilt and may be allowed by the pin 72 to slide a small distance. The base of the track 73 is flat as shown but the curvature of the pads 70 has been exaggerated in the interest of clarity.

As will be understood, the bearing assembly according to the present invention, in particular in its illustrated preferred forms, represents a significant improvement over prior available bearing assemblies such as those discussed above. Many of the disadvantages inherent in such prior assemblies are reduced or eliminated. In addition, in particular in those embodiments wherein oil is introduced to the outer or rear face of the pad, the possibility of controlling pad attitude and position to reflect alignment and balance changes occurring in operation arises. The facility for adjusting the clearance and/or restrain the movement of the shaft may be of considerable benefit in critical rotating equipment, for example in heavily loaded gears where changes in shaft position can seriously affect gear performance. The provision of hydrostatic effects can also affect the performance of the system as a whole. The choice between constant pressure and constant volumetric supply to a particular design and the use of valves with differing characteristics could provide machine operators with a range of controls to improve the running performance of their machines.

Claims

1. An improved bearing assembly comprising a plurality of bearing pads supported by a surrounding housing, the contact between each pad and the housing being in the form of a curved line.

2. A bearing assembly as claimed in claim

1, wherein the surface of each pad which engages the housing is curved in two directions at right angles to each other and the surface of the housing which engages the pad closely matches the curvature of the pad in one of those directions and differs from it in the second of those directions.

3. A bearing assembly as claimed in claim

2, wherein said housing surface is flat in said second direction.

4. A bearing assembly as claimed in claim 2, wherein the difference between said surfaces in said second direction is small.

5. A bearing assembly as claimed in claim 4, wherein said two surfaces are each part-spherical and the radii of curvature of said surfaces in said second direction differ by less than one per cent.

6. A bearing assembly as claimed in any of the preceding claims, wherein said second direction is circumferential relative to the axis of a rotatable shaft supported by said bearing assembly.

7. A bearing assembly as claimed in any of the preceding claims, wherein said pads are held in position within said housing by locating pins.

8. A bearing assembly as claimed in any of the preceding claims, having means for the intro- duction of lubricant into the space between each pad and the housing.

9. A bearing assembly as claimed in claim

8, having a hollow in the surface of said pad which engages the housing, to retain lubricant so introduced.

10. A bearing assembly as claimed in claim

9, wherein said hollow is encircled by an O-ring or a similar seal.

11. A bearing assembly as claimed in any of the preceding claims, which is a journal bearing or a thrust bearing.

12. An improved bearing assembly substantially as hereinbefore described with reference to, and as illustrated in, any of Figs. 2 to 7 of the accompanying drawings.