GB2175353A - Plain bearings - Google Patents

Abstract

A plain bearing (10) is formed by helically winding a wire to form a bearing surface (10B) for supporting a rotating member (20). During the winding operation the bearing surface of the wire becomes convex. This convexity, in conjunction with the helical winding, provide some degree of hydrodynamic lubricating force even in slowly rotating, highly loaded bearings such as rotary drill bits. The convex bearing surface also accepts misalignment of the bearing surfaces with generally evenly distributed loading among all coil segments. <IMAGE>

Description

SPECIFICATION Bearing This invention relates to a bearing, and more particularly to a plain bearing formed of a helically wound wire such as shown for example in U.S. Patent 4,514,098.

Friction bearings such as cylindrical or journal bearings formed of a coiled wire or spring or the like are known in the art for providing a bearing surface for a rotating or oscillating shaft. Such friction bearings normally are defined by adjacent convolutions or coils of a continuous wire. Although, for the most part, friction bearings (otherwise known as plain bearings), have significantly greater frictional forces between relatively moving surfaces than roller or ball bearings, such bearings do have the distinct advantage over roller or ball bearings by, in the same bearing envelope or dimensional space, providing significantly greater bearing surface in virtual bearing contact over which the load can be distributed to thereby reduce stress.

Therefore, in order to take advantage of such load spreading capabilities, it was heretofore believed that the surfaces of the friction bearing should be in generally complemental face-to-face engagement with the paired relatively moving surface to generally maximize the load bearing surface within the available bearing envelope or housing, except for certain considerations for distributing lubricant between the engaging surfaces to reduce or minimize the frictional forces or to dissipate heat caused by the friction.Typical of such lubricant distributing structure in a friction journal bearing is an axially extending helical groove purposely provided on the bearing surface, and providing a volume of lubricant in the groove such that the relative movement between the opposed surfaces caused one surface to wipe the lubricant from the groove and distribute it into the load bearing engaged surfaces.

The above configuration is particularly effective in a continuously rotating shaft and journal bearing. Such helical groove has the advantage of, upon relative rotation between the journal bearing and shaft, pumping the lubricant along the bearing surfaces for continuous replenishment from some source outside the bearing, which also provides a cooling effect to the bearing surfaces.

In the U.S. Patent above-identified, a wound wire journal bearing is described formed of a wound wire, which in the final configuration, forms a sleeve-like journal of a helix-wound wire. The edges of the wire, on the bearing surface side, are machined so that a groove is provided between the adjacent convolutions that provide a helix configured lubricant reservoir.

In addition to having the lubricant spreading and pumping features, such wire journal bearing was found to have load distributing characteristics, through the inherent flexure of the adjacent convolutions, that provided superior bearing life, especially in the environment of a highly loaded journal bearing for a rock bit.

However, with such journal bearing it was still assumed that the bearing surface of the wire between the helix configured groove should be maximized by having a bearing surface substantially complemental to the shaft surface to minimize the stresses for such bearing. In accordance therewith, it was considered necessary to, subsequent to forming the helixwound wire bearing, subject the bearing surface (i.e. the i.d.) to a finishing process to obtain a substantially flat surface.

According to one aspect of the present invention there is provided a bearing comprising a helically coiled wire disposed in load transmitting engagement between relatively rotating members such that one load transmitting surface of said wire engages one member along an apparent line of contact for relative motion therebetween and an opposite load transmitting surface of said wire engages the other member and wherein the cross section of the wire in a radially projected plane along the axis of rotation provides said one of said load transmitting surfaces having a generally convex profile with respect to the surface of said one member engaged thereby, and wherein said apparent line of contact is along a portion of said convex profile.

It has been found that a preferred embodiment of bearing gives additional stress relief and compensation for misalignment as compared with prior art wire bearings in which the bearing surface is complemental to the surface of the supported part. In addition to further accommodating misalignment between the shaft and the journal without inducing point loading or high stresses in the wire bearing, the convex surface of the preferred embodiments provides a wedge-shaped volume between the mating bearing surfaces having a helix lead with respect to the axis of rotation of the shaft, which, even under highly loaded conditions and upon relatively slow rotation, tends to force lubricant between the load bearing surfaces of the bearing pair.The lubricant film formed thereby may not be continuous as in a hydrodynamically lubricated bearing; however, it does provide greater lubrication capabilities and some bearing surface separation not normally achievable in boundary lubricated friction bearings. Such configuration further results in a bearing having an improved life, particularly in the environment of a journal bearing of a rotary rock bit.

The invention will be better understood from the following description of a preferred embodiment thereof, given by way of example only, reference being had to the accompanying drawings, wherein: Figure 1 is an isometric schematic representation of coil-forming from a wire to provide a helical coil such as can be used as a journal bearing element in accordance with the present invention; Figure 2 is a cross-sectional view along line 2-2 of Fig. 1 showing a typical wire configuration having generally planar surfaces prior to forming it into a helical coil; Figure 2A is a cross-sectional view along line 2-2 of Fig. 1 of an alternative wire configuration to provide a helical groove in the bearing surface of the coil; Figure 3 is a cross-sectional view of the wire of Fig. 2 configuration subsequent to coiling, illustrating in exaggeration, the resulting form;; Figure 3A is an axial cross-sectional view such as along line 3-3 of Fig. 1 and showing the helical coil bearing disposed in load transmitting engagement between a rotating member and an opposed journal member; Figure 3B is a view similar to Fig. 3A, showing the wire configuration of Fig. 2A in the same environment; Figure 4 is a view similar to Fig. 3A, showing a realignment of adjacent coils under conditions when forces cause the surfaces of the respective members to become misaligned; and, Figure 5 is an enlarged view of a portion of Fig. 4 which illustrates, in exaggeration, the ability of the wire configuration of the bearing of the present invention to respond to misaligned surfaces.

Description of the Invention It has been found that a wire bearing configuration of the present invention is particularly suitable for a journal bearing of a rotary rock bit generally as shown and described in the commonly owned U.S. Patent 4,514,098 to which reference may be had for further details of such relationship. Thus, although other applications are anticipated for the use of a helical coil journal bearing of the present configuration, throughout this description the bore defined herein represents the bore or bearing cavity of a rotary cutter, and the journal represents the journal pin of the rock bit, all as illustrated in the above referenced patent.

The art of coil forming is well known and the following discussion is intended to be a simplified overview of such art as it relates to making helical coils from wire. Thus, in order to wind a wire to form a helical coil, the wire can be wrapped around a lathe-mounted mandrel by rotating the mandrei which holds one end of the wire fixed thereon, and feeding the wire from the other end at a desired orientation to impart the required spring characteristics to the helical coil being wound, such as pitch, compression or extension-type coil, preload, etc. Alternatively, and more likely used where large quantities are involved, the coiling is performed automatically with an automatic coiler. In this process the wire is forced and guided by rollers through coiling points which produce the coil form (eliminating any mandrel) with the required characteristics.In both instances, the straight portion of the coiled wire is subsequently trimmed after the desired number of coils have been formed.

The wire bearing described in the present invention can be made using any of the well known wire coiling procedures. Referring to Fig. 1, such a helical wire coil is shown which would be appropriate for use as a journal bearing in accordance with the present invention. Such a wire bearing would have an outer diameter D, an inner diameter d, and an axial length L after the straight end has been appropriately trimmed.

With reference to Figs. 2 and 2A, appropriate cross sections of the feed wire are shown having substantially planar surfaces along a width W and a height H, with the surface along the width providing a generally planar surface for bearing engagement with a journal or a pin. In Fig. 2A, such corresponding surface has been notched at each internal corner a certain dimension N, such that when a helical coil is formed of a wire of such configuration, each notch cooperates with the corresponding notch on the adjacent coil to form a groove which extends helically throughout the axial length of the coil, and provides a lubricant reservoir groove or volume also functioning as a debris trap, as is well known in sleeve journal bearings having axial grooves formed therein.

One of the inherent effects resulting from the operation of forming a wire into a helical coil and affecting the dimensions of the wire cross-section section, is the "keystone" effect, which is shown in exaggerated form in Fig. 3. Assuming the width of the wire is a constant W before coiling about the coil axis A, during coiling, the wire attains radially inner and outer widths W1 and W2 respectively, having the relationship that W1 is greater than W, which in turn is greater than W2, resulting in the generally keystone configuration shown.

This keystone form must be taken into consideration if the length of the helically coiled wire, in the axial direction, is critical. Otherwise, such keystoning is so negligible that it can normally be disregarded. Furthermore, although this type of distortion can be mathematically approximated, it is usually dependent upon the manufacturing technique, and a closer estimate can only be achieved through physical experimentation. However, in the description of the present invention, this keystone effect on the axial sidewalls of the wire is not taken into consideration in that the axial length of the bearing can be trimmed to the exact dimension subsequent to it being formed for a proper axial space. Furthermore, the form of the sidewalls, it is felt, does not contribute to the improved bearing characteristics of the coiled wire in forming a journal bearing.

The improved bearing characteristics of such coiled wire bearing configuration relate to a more subtle effect of deformation of the wire cross-section during the helical coiling operation which heretofore has been unappreciated or neglected in the design of a helically coiled wire for use as a bearing and, at least in the inventor's experience, has been specifically subsequently machined away when the bearing was to be used in a low speed highly loaded application which was previously felt to require maximizing the area of the bearing surface within the parameters of a good bearing design.

Referring to Fig. 3, a cross-sectional view of the helically coiled wire of Fig. 2 is shown, and it is therein seen, in exaggerated detail, that the coiling operation produces a concavity on the surface of the wire which is further away from the coil axis A (the O.D. of the coil) and a convexity on the surface of the wire which is closer to the coil axis A (the l.D. of the coil). As a consequence, the height H of the cross-section changes to a new value H' which, in general, is greater than the original height H.

The amount of concavity X and the amount of convexity Y produced during the forming of the helical coil, are usually very small and depend upon the dimensions of the wire crosssection, the diameter of the helically coiled wire (d in Fig. 1) and, to some extent, the properties of the material being used and the manufacturing technique employed.

It has been found that if a steel wire of width W equal to 0.350 inches and height H equal to 0.200 inches, is coiled on a lathe to form a coil of outside diameter, D equal to 3.000 inches, the concavity X produced measures 0.002 inches and the convexity Y measures 0.002 inches to give a midified wire height H' equal to 0.202 inches. The convexity Y resulting in such coiled wire is very desirable when the coil is used to transmit load such as in a bearing application, and in particular, in the rock bit bearing application such as disclosed in the above-identified U.S. Patent 4,514,098.

Referring to Figs. 3A and 3B, the coiled wire having a cross-sectional configuration of Figs. 2 and 2A respectively, are shown as a bearing 10 disposed within a bearing cavity 12 defined by a bore 14 of a rotating member 16 such as a cone cutter, and the surface 18 of a stationary axle or journal pin 20, where the coil is used as a plain bearing to transmit load between at least two of its contacting surfaces. The bore surface 14 contacts the wire 10 at the extremities of the concave surface 10A, while the journal surface contacts the wire 10 at the center portion of the convex surface 10B.

Referring now to Fig. 4, the surfaces defining the bearing cavity 12 are shown as placed in misalignment by a force F which causes the bore 14 of the cone 16 to be misaligned (its aligned position shown in phantom) with the surface 18 of the pin 20. (Under the normal operating conditions of a rock bit, this would be the assumed relationship.) As can be seen, the wire section of the end coil 10' of the side closer to the applied load F will rotate or rock (i.e. assume a somewhat cocked position), by an amount equal to the misalignment and will elastically deflect radially so that adjacent sections of the wire 10" will also support a part of the load F. Edge loading typical of a rigid sleeve bearing is thus avoided and the applied load F is more evenly distributed across the entire axial length of bearing 10.

Referring now to Fig. 5, a detailed view of a single wire cross-section of the wire bearing of Fig. 4 is shown. Thus, when the load F is applied to the wire bearing, radial forces F1 and F2 act on the wire section 10' of the end coil, with generally F1 being greater than F2, and a reaction force F3 reacts on the wire 10' at the point of contact with the journal surface 18. As can be seen, the imbalance in the forces F1 and F2, causes a rocking or cocking of the wire section 10' from the original position by an angle a (measured from a line perpendicular to the coil axis). This rotation is enough to avoid point contact at the end of the misaligned wire bearing 10.Thus, it is readily apparent that the convexity and concavity of the coil bearing provides a resilient structure able to deflect and angularly rotate to accommodate the misalignment caused by the applied bearing loads.

Furthermore, it is apparent from viewing Fig,. 5, that the cross-section of each winding of the bearing 10 identifies a simply supported beam with an intermediate load and, therefore, it will elastically deflect in the radial direction as indicated by the amount T. This elastic deflection T of the cross-section will increase with increasing radial ioad. An increase in deflection means that more load can be carried by an adjacent wire section which will in turn deflect (although to a lesser extent) and share some load with its adjacent section, with this reaction continuing to occur until the load is more evenly distributed across the entire axial length of the bearing. It is noted that the elastic deflection of the wire sections will occur even if misalignment is not present, but it will not be as pronounced since the bearing load is not concentrated on a single wire section.

As an example of the above and still referring to Fig. 5, it is seen that if the concentrated radial load on a steel wire section of a helically coiled wire of outer diameter D=3.000 inches, is (Fl +F2)=F3=8000 Ibs.

and the wire section dimensions are W=.350 inches, H=.200 inches, then the radial elastic deflection T will be expected to be approximately 0.001 inches while accommodating a rotation angle a of the order of 210 or less.

Also, as previously indicated, such helically coiled wire with this cross-sectional wire dimensions will have a convexity Y in Fig. 5 of approximately 0.002 inches. However, if the convexity Y is below a value of 0.0005 inches, then the wire section tends to act more like a rigid section of wire and the desirable characteristics described in this invention become marginal.

It has been found that convexity Y becomes significantly desirable when certain wire crosssection dimensions are used and the wire is coiled to form a helical coil of specific diameters. Referring to the previous Figures, along with Fig. 5, convexity Y becomes desirable for wire cross-sections having an envelope of dimensional height H equal to or greater than .050 inches when the ratio d over W() is less than or equal to 20 with d and WO measured in the same dimensional unit, and d being the internal diameter of the formed helical coil, and W" being the dimension of the portion of wire cross-section, which during operation may come into mating contact with the journal surface 18, and transmit either part or all of the load applied to the wire section. W( is shown in Fig. 2A and W is equal to W in Figs. 2, and 5.

Referring to Fig. 3A, it is seen that when the convex surfaces 10B of the wire sections of wire bearing 10, first come into contact with the journal surface 18 before any bearing load is applied, and in the absence of misalignment as shown therein, each wire section contacts the journal 18 at the center of its convex portion, defining an apparent line of contact of width A. Thus, point contact, in general, does not occur because the convex surface of the wire retains some residual flatness, assuming that the now convex surface was originally flat, although the width of contact line A is relatively small. Furthermore, after a load F is applied to the wire bearing as shown in Fig. 5, and the load transmitting wire sections of wire bearing 10 rotate and deflect, the apparent line of bearing contact A increases to a new value A'.Due to this increase in the apparent line of contact, each affected wire section can transmit a higher unit load than normally possible and without overstressing the bearing. Therefore, a wire bearing having characteristics as described herein, provides the substantial advantage of adjusting to increases in unit bearing load by increasing the apparent line of contact of the individual load supporting wire sections of wire bearing 10.

Since the wire bearing 10 can adjust to changing bearing load conditions, it always provides the minimum necessary amount of load supporting area (A'Xthe circumferential distance of contact in Fig. 5), and therefore, provides the best balance in the utilization of available bearing space between load supporting area and lubricant room. This advantage cannot be provided by a rigid bearing structure, or bearing structures which cannot change their apparent bearing contact area under load.

As previously mentioned, each individual wire section in the unloaded and aligned conditions, presents an apparent line of contact A. Referring to Fig. 3A, an angle 6 is identified as the contact angle or the angle formed by the tangent to the convex portion of the wire section at the point of contact with the journal surface 18. This angle 6 becomes important when relative motion exists between the convex portion of the wire section and its mating bearing surface 18. As shown in Fig.

3A, if the wire bearing 10 (coiled left handed as in Fig. 1), rotates relative to the journal with angular velocity w (this can be envisioned by assuming the wire sections are moving out of the plane of the paper), then each coil of the wire bearing will have an axial component of velocity V with respect to the journal surface 18. Velocity V increases with increasing wire bearing angular velocity co and with increasing pitch P of the wire bearing coils (i.e.

the distance in the axial direction between two adjacent wire sections center lines). The angle o provides an entrance angle for the lubricant which is wedged under the area of width A (A' in Fig. 5) of bearing contact as the coils traverse the journal surface 18 with axial velocity V. This wedge effect is well known and it is the basis for the theory of hydrodynamic lubrication. However, in a typical rock bit bearing application, the loads are inherently high and the bearing rotational velocities are relatively low, and the formation of a thick (hydrodynamic) lubricant film under the contacting bearing surfaces is unlikely.Since this application is not conducive to hydrodynamic lubrication, it is believed that a mixed mode of lubrication with partial contact of the asperities of the bearing surfaces exists, with the wedge effect provided by the wire bearing of this invention promoting the formation of a thicker bearing lubricant film which will reduce friction and, therefore, wear of the bearing surfaces.

It should be noted that in a typical plain journal bearing, the above wedge effect is produced in a circumferential direction while the wedge effect described herein produced by the wire bearing is in the axial direction and complements the wedge effect in the circumferential direction (i.e. in the direction of the helix of the wire bearing). The axial wedge affect becomes more beneficial with increasing axial velocity V. Therefore, the wire bearing described herein can benefit from and operate safely at higher than normal bearing rotational velocities. It is theorized that any angle 6 of 5 or smaller will be very beneficial to the operation of the wire bearing.It must be noted that, even with the application of heavy bearing loads, the change in the angle 6 due to radial wire section elastic deflection is negligible.

The dimensions and configurations of the bearing section, which give the wire bearing of this invention the advantages herein described, can be obtained either through the coiling effect of a properly cross-sectioned wire, or as a result of a secondary operation subsequent to coiling, such as grinding, to provide the arcuate convexity to the bearing surface of each coil.

Also, it should be noted that the concavity X illustrated in Figs. 3, 3A and 3B, is not critical to any specific dimension but assists in concentrating the load at the top corners of the wire section, thus helping to produce the radial elastic deflection of the wire section.

Further, it should be noted that the wire bearing having a cross-sectional configuration as described herein, can operate under less than normal bearing clearances without seizing or locking-up due to the ability of the wire sections to elastically deflect and accept misalignment. This is of significant importance since a reduced bearing clearance translates into improved reliability of any bearing sealing eiement. In fact, the reduced bearing clearance reduces the magnitude of bearing vibration and, therefore, improves the dynamics of any sealing element associated with sealing the bearing cavity. This sealing element can also be designed with less deflection, so that its life and the life of the tool are extended.

Claims (19)

1. A bearing comprising a helically coiled wire disposed in load transmitting engagement between relatively rotating members such that one load transmitting surface of said wire engages one member along an apparent line of contact for relative motion therebetween and an opposite load transmitting surface of said wire engages the other member and wherein the cross section of the wire in a radially projected plane along the axis of rotation provides said one of said load transmitting surfaces having a generally convex profile with respect to the surface of said one member engaged thereby, and wherein said apparent line of contact is along a portion of said convex profile.

2. A bearing according to claim 1 wherein the said convex profile has a depth of convexity defined as the perpendicular distance from a chord joining the two extremities of the convex profile to a point on the convex surface furtherest from said chord and wherein said depth is at least 0.0005 inches (0.0127mm).

3. A bearing according to claim 2 wherein the axial length of the chord of said convex profile is at least 0.05 inches (1.27mm) and not greater than 0.75 inches (19.05mm).

4. A bearing according to any preceding claim wherein said coiled wire is helically wound to form: a generally cylindrical sleeve defining an inner diameter; a substantially uniform axial width of each coil; and a substantially uniform radial height of each coil, and wherein said radial height is greater than 0.05 inches (1.27mm) and the ratio of said inner diameter to said axial width is not greater than 20.

5. A bearing according to claim 4 wherein the ratio of said axial width to said radial height is at least 0.25 and not greater than 7.5.

6. A bearing structure according to any preceding claim wherein said opposite load transmitting surface of said wire forms a concave profile with respect to the engaged surface of the other member.

7. A bearing structure according to claim 4 wherein said convex profile and said concave profile are formed through the winding operation of forming said helically wound wire.

8. A bearing according to any preceding claim wherein an angle of contact is defined as the angle between a tangent to the convex profile at the point of contact with the surface of said one member and said one surface, and wherein said angle is on the order of 5O or less.

9. A bearing substantially as herein described with reference to the accompanying drawings.

10. A support, and a bearing according to any preceding claim rotatably mounting a rotatable member on the support.

11. A drilling tool comprising: a first member; a second member mounted for rotation relative to the first member; and a plain bearing disposed in load transmitting engagement therebetween, said bearing being mounted to be rotatable with respect to said first member and stationary on said second member and wherein said plain bearing comprises a helically coiled wire forming a continuous series of adjacent helical wire segments with each segment having opposed surfaces in bearing engagement with said first and second members respectively and wherein said opposed surface thereof in bearing engagement with said first member defines a convex profile with respect to the surface of said first member, said bearing engagement therebetween being along an apparent line of contact forming a portion of said convex profile.

12. A drilling tool according to claim 11 wherein the opposed surface in bearing engagement with said second member defines a concave profile with respect to the surface of said second member.

13. A drilling tool according to claim 11 or claim 12 wherein said convex profile has a depth of convexity defined as the perpendicu lar distance from a chord joining the two extremities of the convex profile to a point on the convex surface furthest from said chord and wherein said depth is at least 0.0005 inches (0.0127mm).

14. A drilling tool according to claim 13 wherein the axial length of the chord of said convex profile is at least 0.05 inches (1.27mm) and not greater than 0.75 inches (19.05mum).

15. A drilling tool according to any of claims 11 to 14 wherein said coiled wire is helically wound to form: a generally cylindrical sleeve defining an inner diameter; a substantially uniform axial width of each segment; and a substantially uniform radial height of each segment, and wherein said radial height is greater than 0.05 inches (1.27mm) and the ratio of said inner diameter to said axial width is not greater than 20.

16. A drilling tool according to claim 15 wherein the ratio of said axial width to said radial height is at least 0.25 and not greater than 7.5.

17. A drilling tool according to any of claims 11-16 wherein said convex profile and said concave profile are formed through the winding operation of forming said helically wound wire.

18. A drilling tool according to any of claims 11-17 wherein said drilling tool is a rotary rock bit having a rotatable cutter mounted on an axle with said axle defining said first member and said cutter defining said second member.

19. A drilling tool substantially as herein described with reference to the accompanying drawings.