GB2158165A - Linear motion bearing - Google Patents

Abstract

A linear motion bearing comprising an inner cylindrical sleeve 1 and an outer cylindrical sleeve 2 mounted coaxially to define therebetween an annular space 3. The sleeves are maintained in axial separation by a length of wire 5 which is elastically coiled into the form of a multi-turn helix which permits relative axial movement between the sleeves 1 and 2. A method of assembling such a bearing in a state of radial preload is also described. A second coil 11 of thinner wire may separate the turns of wire 5 to reduce friction or two coils 5 may be provided with windings of opposite hands. The wire 5 may be solid or may itself consist of a wire coil (Fig. 13). <IMAGE>

Description

SPECIFICATION Linear motion bearing This invention relates to a linear motion bearing, by which is meant a bearing which controls straight-line relative motion between two members. Such bearings usually further provide the means whereby loads may be transmitted between the two members in a direction transverse to the direction of motion while at the same time resisting relative movement in the direction of loading. Also described is a method of assembling such a bearing.

The classic example of this type of bearing is the cross-head slide on a steam engine, which guides the linear motion of one end of the piston rod while supporting the lateral resultant of the forces transmitted between the piston rod and the connecting rod.

More recently, in other types of application, linear motion bearings have been developed which contain balls or rollers to replace the sliding motion with rolling motion. Such linear bearings are widely used in machine tool slides, where they are often adjusted to a condition in which the rolling elements are at all times preloaded to give increased rigidity.

The bearing of the present invention comprises a pair of bearing members having respective cylindrical or conical bearing surfaces, said bearing members being mounted concentrically to define an annular space between the bearing surfaces, and radially separated by an elongate member which is elastically coiled into the form of a helix coaxial with said bearing surfaces and situated within said space.

In use, the bearing members are able to move relatively in the axial direction but are rigidly constrained in the radial direction by the helical coil. The axial movement is accommodated by a rolling motion of the coil between the two bearing surfaces.

The elongate member may take the form of a solid wire which in its free form is preferably straight. The design is such that when the wire is coiled into the helical form referred to, it is not strained beyond its elastic limit, i.e. if released it would revert to its original straight form. In practice, this requirement defines a maximum diameter of wire of a given material that can be coiled elastically to a given pitch diameter. It means that the diameter of a hardened spring steel wire must be not greater than approximately 0.004 times the pitch diameter of the helix to which it is formed.

Because the wire is strained elastically, it does not absorb energy to overcome inelastic deformation when it rolls between the concentric bearing surfaces. Therefore the major source of resistance to motion is the same as that in other types of rolling element bearing, i.e. friction in the various contacts made by the surfaces of the rolling elements.

Instead of being a solid wire, an alternative embodiment of the invention provides that the elongate member itself takes the form of a helically coiled length of wire.

An advantageous method of assembling the bearing of the invention is also to be described. In this method, the bearing is assembled in a state of radial preload, achieved by inducing an interference fit between the helically wound elongate member and the two bearing surfaces.

in order that the invention may be better understood, several embodiments thereof will now be described by way of example only and with reference to the accompanying drawings in which: Figure 1 is a side view in section of a bearing constructed in accordance with the invention; Figure 2 is an enlarged sectional side view showing the interaction between the elongate member and the bearing surfaces in the bearing of Figure 1; Figure 3 is a view similar to Figure 2 showing an alternative embodiment; Figure 4 is a view similar to Figure 2 showing a further alternative embodiment; Figure 5 is a sectional side view of part of a bearing similar to that of Figure 1, but in which spring means are used to centralise the elongate member; Figure 6 is a view similar to that of Figure 5 showing an alternative embodiment;; Figure 7 is a sectional side view of part of a machine tool showing an application of the bearing of the present invention; Figures 8, 9, 10 and 11 are side sectional views of part of a bearing showing respective ways of mounting the components of the bearing to achieve a radial preload; Figure 1 2 is a side sectional view of part of a bearing similar to that of Figure 1, showing the use of seals to protect the interior of the bearing assembly; Figure 1 3 is a side sectional view of a bearing made in accordance with the invention, showing a further embodiment; Figure 14 is a side sectional view of part of a bearing similar to that of Figure 13, but having conical bearing surfaces; and Figure 1 5 is a side sectional view of part of a machine tool showing the bearing of Figure 14 fitted.

Referring firstly to Figure 1 there is shown a linear motion bearing comprising two bearing members 1, 2 in the form of respective inner and outer cylindrical sleeves of different diameters. The inner sleeve 1 is of smaller diameter and is mounted coaxially within the outer sleeve 2 to define therebetween an annular space 3 which is elongate in the direction of the common axis 4. The sleeves are radially separated by a length of wire 5 which is elastically coiled into the form of a multi-turn helix, coaxial with members 1 and 2, and positioned within the space 3. The wire may for example be made of hardened spring steel. In its free form the wire is preferably straight. The design is such that when the wire is coiled into the helical form, it is not strained beyond its elastic limit -i.e. if released, it would revert to its original straight form.In practice, this requirement defines a maximum diameter of wire of a given material that can be coiled elastically to a given pitch diameter. For example, it means that the diameter of a wire of hardened spring steel must be not greater than approximately 0.004 times the pitch diameter of the helix into which it is formed.

Because the wire is strained elastically, it does not absorb energy to overcome the inelastic deformation when it rolls between the concentric sleeves. Therefore the major source of resistance to motion is the same as that in other types of rolling element bearing, i.e.

friction in the various contacts made by the surfaces of the rolling elements.

In the bearing of Figure 1, friction may arise in the contacts formed between neighbouring turns of the helix since the pairs of contacting surfaces at these positions will move in opposite directions in relation to each other as shown in Figure 2.

In Figure 2, the direction of motion of the various parts, including two adjacent turns of the helix, are shown. Friction occurs at the point A where the adjacent turns contact. Two methods are proposed for the reduction of this friction.

The first of these methods is shown in Figure 3 in which friction between adjacent turns of the helix is reduced by introducing a second helix 6 of wire or some non-metallic material, having radial clearance between the two concentric sleeves. In this case, the section of the second, smaller, wire 6 is able to rotate in the opposite direction to that of the larger wire 5 which supports radial loads transmitted between the two sleeves.

The second method is shown in Figure 4 which is similar to the construction of Figure 3 except that the intervening secondary helix 6 is of substantially rectangular section and is made of a suitable low-friction material. In this case, the helix 6 is not free to rotate but is able to reduce friction in the assembly merely because of the reduced surface friction between itself and the wire 5.

Figure 5 shows another embodiment of the invention in which the annular space 3 between the sleeves 1 and 2 is formed as a substantially closed pocket having annular shoulders 7, 8 at either end. The helix 5 is located in this annular pocket and is maintained at a central position within the pocket by a pair of spring supports 9, 10. These spring supports serve to prevent the wire helix 5 as a whole, or any of its coils individually, from drifting axially out of position during repeated relative axial movement of the sleeves 1, 2.

Figure 6 shows an alternative arrangement to that of Figure 5 in which these same end springs are incorporated as integral extensions of a coil spring 11 which is helically-entwined with the load-carrying coil of wire 5.

When assembled and pre-loaded, the above described bearings allow relatively frictionless axial movement between the two concentric sleeves 1, 2. However, because of the helical disposition of the single rolling element formed by the wire 5, such axial movement will tend to induce a small but finite angular rotation of one sleeve relative to the other, about their common axis 4. In some applications, this rotation may not be acceptable and would therefore either have to be suppressed, or accommodated by the additional provision of a rotary bearing arrangement.

Of these two alternatives, the first may be fulfilled by providing two separate helical coils 5 which are identical in all respects except that they are helically coiled in opposite directions-i.e. one resembles the right-hand screw thread while the other is left-handed.

These helical coils can either be fitted between the same pair of sleeves 1, 2 or may be fitted in separate bearings on the same assembly. In either case, they will tend to induce twist in opposite directions when moved in the same axial direction and therefore each will generate a twisting moment that neutralises that of the other and each will be constrained to move axially without inducing relative rotation between the sleeves 1, 2. In this case, the wire coils will experience a small amount of circumferential sliding relative to the sleeves and the frictional resistance to movement will be increased accordingly.

It is of interest to note, in passing, that the bearing could be designed to induce a given desired amount of relative sleeve rotation, if such a condition was required, for example, for the purpose of translating linear motion into rotary motion.

The second alternative to solving the problem of relative rotation between the sleeves is illustrated by an arrangement in which the described linear bearing is combined with a conventional rotary bearing or bearings. Such an arrangement could advantageously be used in application where one of two rotary bearings requires provision for axial movement to accommodate axial displacements caused, for example, by differential thermal expansion.

Such a problem occurs in machine tools having elongate spindles, and an example of one of these is illustrated in Figure 7. Figure 7 is in two parts A and B, representing respectively the tail and nose of the machine tool. The axis of the machine tool is indicated under reference 4, and the spindle of the machine tool is illustrated under reference 1 2.

The spindle is mounted at the tail and nose positions respectively by tapered roller bearings 13, 14. These latter act between the spindle 1 2 and the frame 1 5 of the machine tool.

The bearing 1 3 at the tail position is modified by the provision of a linear bearing of the type described herein. The inner race member 1 6 of the bearing is rigidly attached to the spindle; the outer race member 17, however, is attached to the housing 1 5 by way of a linear bearing and a sleeve 1 8. The sleeve 18 is attached to the frame 1 5. A pocket 19, such as is described above in relation to Figure 5, is formed in the outer surface of the outer race member 1 7 and this defines, with the sleeve member 18, the required annular space into which the helical coil 5 may be placed.A coil spring 20 acting between the bottom of a blind bore in the sleeve 1 8 and the wide end of the outer race member 1 7 acts to bias the outer race member in a leftwards direction in Figure 7. A plurality of such compression springs 20 are provided, circumferentially spaced about the axis 4.

These springs act to induce an axial preload between the two tapered roller bearings 1 3 and 14.

In operation, the temperature differentials that arise in the spindle 1 2 and the frame 1 5 cause small but finite relative axial movements due to thermal expansions. The provision of the linear bearing allows these expansions to develop without significantly affecting the preload since the outer race of the tapered roller bearing 1 3 at the tail position is able to move relative to the rear end of the frame. It is known from past experience that if the outer race of the rear tapered roller bearing is simply made a sliding fit in the frame, then satisfactory performance cannot be guaranteed. The vibrations and oscillatory movements of the outer race relative to the frame tend to promote fretting corrosion damage and the bearing would eventually lose its freedom of axial movement.

There will now be described a method of assembling the described linear bearing. The object of the exercise is to assemble the bearing in a state of radial preload, this being achieved by inducing an interference fit between the coil of wire and the two sleeves 1, 2. Various methods are possible: 1. The outer sleeve 2 may be heated to cause sufficient radial expansion to enlarge the radial gap between the sleeves to allow the wire helix 5 to be inserted.

2. The inner sleeve may be cooled sufficiently to cause thermal contraction, again to allow the wire helix to be inserted.

The above two methods may be used either alone, or in combination. Whichever method is used, the assembly operation must be speedily executed due to the rapidity with which heat is transferred from one sleeve to the other. Therefore, the wire helix must be prepared in advance and moved quickly into its location between the sleeves.

A further method of assembly utilises the elastic movement of one or other of the sleeves and thereby avoids problems that may be caused due to the rapid transfer of heat when assembling using the heating or cooling methods described above. In this case, the two sleeves 1 and 2 are machined to dimensions that provide a slight radial clearance for the wire helix 5 which can then be wound into position either by manual manipulation or by a suitable wire winding device. When assembled, the bearing can then be preloaded by causing radial displacement of one or other, or both, of the sleeves by means of elastic strain. This will be described further with reference to Figures 8 to 11.

Referring firstly to Figure 8, the inner sleeve 1 is provided with a tapered internal bore which matches a tapered seating on a shaft 21 to which the bearing is to be fitted.

During final assembly, the inner sleeve 1 is expanded elastically by being forced onto the tapered seating by means of a nut 22 which is threadedly engaged on a thread 23 formed on the shaft 21. This acts to induce a radial preload in the bearing.

Figure 9 shows a similar arrangement to that of Figure 8, but in which the outer sleeve 2 is provided with a tapered outer surface which matches the tapered bore of an external sleeve 24. During final assembly, the outer sleeve 2 is forced onto the external sleeve 24 by means of an end cover 25, thus causing radial elastic contraction of the outer sleeve 2.

In the embodiment shown in Figure 10, the outer sleeve 2 is made in a form which induces radial contraction of the bore of the outer sleeve when axially clamped in its housing. To achieve this, the middle portion of the outer surface of the outer sleeve is relieved by a groove 26. Consequently, when an axial force is applied through the outer sleeve by an annular member 27, the middle portion is subjected to stresses which induce a radially inwards deflection, resulting in a condition which preloads the wire helix 5.

Figure 11 shows a further embodiment of the concept described above in relation to Figure 10 in which the end faces of the outer sleeve 2, and the mating abutment faces between which it is clamped are inclined to the axis 4, as shown under references 28 and 29, to induce radially inwards deformations at the ends of the outer sleeve, as well as at the middle.

Figure 1 2 illustrates an embodiment of the described bearing in which the assembly is protected by self-contained seals 30, 31 fitted in grooves in the inner sleeve 1. The seal may alternatively be fitted in a groove in the outer sleeve 2.

Figures 13, 14 and 1 5 show variations of the bearing described above in which the multi-turn helix 5 is formed not of solid wire but itself takes the form of a helical coil spring.

Figure 1 3 shows the basic concept, with inner and outer sleeves 1 and 2, as before and the coil spring, shown under reference 32, wound helically between the inner and outer sleeves.

A further development of this idea utilises the radial flexibility of the coil spring 32 to provide the means of generating an axial force between the sleeves 1 and 2. Figure 14 shows such an assembly in which the coil spring 32 is located between tapered sleeves.

In this case, the annular space 3 between the sleeves 1 and 2 is not cylindrical, as described in previous embodiments, but takes a conical form in the direction of the axis 4. It will be seen that when the sleeves experience relative axial movement which radially compresses the coil spring 32, an axial resisting force is generated in proportion to the radial compression of the coil spring. This enables the axial preload on the bearing to be controlled. It will be clear, by the way, that the tapered form of sleeves shown in this embodiment could be applied to any of the embodiments described above although it is used where the helix 5 takes the form of a coil spring 32 to particular advantage.

Figure 1 5 shows how the arrangement of Figure 14 can be incorporated into the mounting of a tapered roller bearing, shown under the reference 33, in a machine tool spindle as an alternative to the design shown in Figure 7. In this case, the axial preload on the bearings is controlled by the degree of radial compression applied to the coil spring 32 by the adjustment accomplished during assembly by rotation of a nut 34 acting on a threaded part 35 of the spindle 1 2. As before, the outer race member of the bearing 33 doubles as the inner sleeve 1, the inner race member of the bearing being slidably mounted on the shaft 12.

Claims (20)

1. A linear motion bearing comprising a pair of bearing members having respective cylindrical or conical bearing surfaces, said bearing members being mounted concentrically to define an annular space between the bearing surfaces, and radially separated by an elongate member which is elastically coiled into the form of a helix coaxial with said bearing surfaces and situated within said space.

2. A bearing as claimed in claim 1 wherein the elongate member, whilst being strained elastically, is not strained beyond its elastic limit.

3. A bearing as claimed in either one of claims 1 or 2 wherein the elongate member takes the form of a solid wire having a round cross section.

4. A bearing as claimed in either one of claims 1 or 2 wherein the elongate member itself takes the form of helically coiled wire.

5. A bearing as claimed in any one of the preceding claims further including spacer means for separating adjacent turns of the elongate member.

6. A bearing as claimed in claim 5 wnerein said spacer means comprises a secondary helix interwound with the helically wound elongate member within said annular space.

7. A bearing as claimed in claim 6 wherein said secondary helix is formed from wire having a circular section and whose diameter is such as to define a radial clearance within the space.

8. A bearing as claimed in claim 6 wherein said secondary helix is formed from wire having a rectangular cross section.

9. A bearing as claimed in any one of the preceding claims wherein said annular space is bounded in the axial direction by a pair of axially spaced shoulders formed on the outer surface of the inner bearing member or the inner surface of the outer bearing member or both.

10. A bearing as claimed in claim 9 wherein the helix is maintained at an axially central position within the space by means of a pair of helical spring supports, each biased between a respective end of the helix and the adjacent one of said shoulders.

11. A bearing as claimed in claim 9 when appendant to any one of claims 6, 7 or 8 wherein the turns of the helix are interwound with the turns of a coil spring acting between said shoulders in order, on the one hand, to keep the helix positioned axially centrally within the space and, on the other hand, to separate adjacent turns of the helix.

1 2. A bearing as claimed in any one of the preceding claims wherein the facing surfaces of the bearing members are axially tapered in such a way that said annular space is axially tapered.

1 3. A bearing as claimed in any one of the preceding claims, said bearing comprising a further helix coiled in the opposite direction to the first-mentioned helix.

14. A bearing as claimed in claim 1 3 wherein the second-mentioned helix is mounted between the same bearing members as the first-mentioned helix.

1 5. A bearing as claimed in any one of the preceding claims wherein either the outer surface of the outer bearing member or the inner surface of the inner bearing member is axially tapered in order to enable the member con cerned to be radially expanded or contracted during assembly or fitting of the bearing to provide a radial preload in the helically coiled elongate member or members.

1 6. A bearing as claimed in any one of claims 1 to 14 wherein the outer surface of the outer bearing member is relieved by a groove in order to allow the outer member to be deformed to provide a radial preload in the helically coiled elongate member or members.

1 7. A method of assembling the bearing claimed in any one of claims 1 to 14, in a state of radial preload, said method comprising inducing an interference fit between the helically wound elongate member and the bearing surfaces.

1 8. A method as claimed in claim 1 7 comprising heating the outer bearing member to cause sufficient thermal expansion to allow the helix to be inserted into the annular space.

19. A method as claimed in claim 17 comprising cooling the inner bearing member to cause sufficient thermal contraction to allow the helix to be inserted into the annular space.

20. A linear motion bearing substantially as hereinbefore described with reference to the accompanying drawings.