This invention relates to leaf springs, and particularly to such springs made of composite (fibre-reinforced plastics) material. More particularly the invention relates to the provision of what is generally called a slipper end on such a spring.

In the most common installation of a leaf spring in a motor vehicle or trailer, a spring generally of part-elliptic or other curved form is straightened with increasing load. The provision of a slipper end is one means by which the change in distance between the ends of the leaf spring can be accommodated as the load on the spring changes. For example, one end of the spring may be pivotally mounted at a fixed position on the vehicle chassis, whilst the other end of the spring bears against a supporting surface and slides thereon as the spring load changes. Sliding movement is also required between individual leaves of a spring assembly comprising two or more leaves, as the spring deflects, and in this case both ends of some of the spring leaves may require to be slipper ended.

In leaf springs made of steel, it is usual to arrange for the sliding movement to occur directly on the surface of the leaf or leaves. The resistance of the steel spring leaves to abrasion is adequate for this purpose. In the case of leaf springs made of composite material, however, the resistance of the spring material to abrasion is not satisfactory, particularly if the spring is operated in a dirty environment as is the case in a motor vehicle. Accordingly it is desirable to

provide a composite leaf spring with an element of an appropriate wear resistant material at any point where abrasive wear of the spring material is likely to occur.

There have been various proposals for means by which structures affording bearing surfaces can be secured to leaf springs of composite material, e.g. as disclosed in our published applications WO-87/06892 or WO-87/07226. The object of such proposals is to avoid the necessity of drilling holes through a composite leaf spring to accept fasteners or the like, because to drill a composite leaf spring undesirably affects the properties of the spring. However, the disclosed structures are relatively complex and involve several separate parts. It is the object of the present invention to provide an improved method of providing a leaf spring of composite material with a slipper end affording a bearing surface.

According to the present invention, we provide a leaf spring, wherein an end portion thereof is provided with a covering of a plastics material which has been applied by a reaction injection moulding process and interfits mechanically with the configuration of the spring end portion to be held fast thereon, the covering of plastics material affording a bearing surface for sliding movement.

Although, as above referred to, the invention relates particularly to leaf springs made of composite material, it has been found in certain circumstances that the provision of a slipper end according to the present invention can be advantageous when the leaf spring is of steel. Although abrasion is not usually a problem with steel springs the reduced frictional resistance to sliding movement resulting from the provision of a

bearing surface of a covering of plastics material can enhance vehicle suspension performance.

As compared with previously proposed slipper end assemblies as above referred to, the invention presents certain significant advantages. Firstly, it is light in weight. Further, the covering of plastics material on the end portion of the spring effectively prevents any penetration of abrasive dirt to the surface of the composite material of the spring end portion. The reaction injection moulding process ensures close contact, and elimination of the possibility of fretting, between the plastics material and spring end portion.

Further features with which the covering of plastics material may be provided will be referred to in the following description. However, two features of potential benefit in preventing wear of the slipper end of the spring are as follows.

The bearing surface of the plastics material may be provided with a number of recesses extending into the material from the surface thereof, to act as retainers for lubricant. Such recesses may comprise small diameter bores extending perpendicular to the bearing surface, which bores may be formed by suitable formations in a mould or die wherein the reaction injection moulding process for applying the plastics material to the spring end portion is carried out.

Further, the plastics material may itself incorporate micro-capsules of lubricant dispersed throughout the material. In use, as the plastics material slowly wears, such micro-capsules of lubricant are crushed to release lubricant onto the bearing

surface. The released lubricant would be retained by the aforementioned recesses, instead of running off the bearing surface.

An important factor in the satisfactory performance of the slipper end is that the plastics material must be firmly held fast on the spring end portion. To this end, the spring end portion may have a keying surface with which the plastics material engages and interfits, the keying surface being adjacent the bearing surface of the plastics material. In use, the load on the bearing surface of the plastics material ensures that the plastics material is pressed into close contact with the keying surface, and interfits therewith so as not to move thereon.

The keying surface may comprise a pattern of depressions or recesses in the surface of the spring end portion, which pattern preferably covers an area extending beneath at least a major portion of, and preferably substantially all, the bearing surface of the covering of plastics material.

The bearing surface of the plastics material may, in cross-section transversely to the direction in which sliding at such surface takes place in use, be of convex curved shape.

The advantage of this is as follows. It will be appreciated that, in order to provide a large area of bearing contact, the surface which is engaged by the bearing surface on the covering of plastics material will usually be flat considered in the direction transversely to the direction of sliding movement, although it may be curved in the nature of a cam surface

in the direction of the sliding movement. If there is any misalignment between the spring end portion and the part it engages, and if the bearing surface on the spring end portion were itself flat, the forces involved in vehicle suspensions would be great enough to cause the spring to twist about its longitudinal centre line, so that the flat surfaces engages face to face. Such misalignment may result from vehicle build tolerances, and from suspension movement in service, e.g. vehicle roll when cornering. Leaf springs of composite material are usually relatively thick in cross-section, so that such twisting introduces high shear forces into the spring. Five degrees of twist in the spring is sufficient to give rise to a potential problem, with two degrees of twist resulting from vehicle build inaccuracy and a further three degrees from vehicle roll when cornering.

With a convexly curved cross-section shape of the bearing surface, however, in a newly built vehicle contact between the bearing surface of the covering of plastics material and the part it engages will occur over a small area. The spring is not caused to twist. Relative rapid wear will ensue until the bearing surface of the covering of plastics material develops a flattened region at an attitude which will compensate for any initial misalignment between the spring and vehicle. After the initial rapid wear, a substantially increased spring life can be expected to result as the spring is not subject to such high loads resulting from its twisting.

The invention will now be described by way of example with reference to the accompanying drawings, of which:-

Figure 1 is a side elevation of one embodiment of slipper end spring according to the invention;

Figure 2 is a section on the line A-A of Figure 1;

Figure 3 is an elevation as Figure 1, of a further embodiment of the invention;

Figure 4 is a plan of the spring end of Figure 3;

Figure 5 is a section on the line A-A of Figure 3;

Figure 6 is a section through yet a further embodiment of the invention, on line B-B of Figure 7;

Figure 7 is a plan view of the spring end of Figure 6;

Figure 8 is a section on the line A-A of Figure 6.

Referring firstly to Figures 1 and 2 of the drawings, these show an end portion 10 of a leaf spring made of composite (fibre-reinforced plastics) material.

The portion 10 is completely enclosed by a covering 11 of plastics material which presents an upwardly facing bearing surface 12 for sliding engagement with, e.g., an appropriate part of a vehicle structure or, in the case of spring assembly made of a number of individual leaf springs, another leaf spring.

The covering 11 of plastics material is applied to the spring end portion 10 by a reaction injection moulding process. This involves positioning the spring end portion in a suitable mould with separable parts, and whose internal configuration is such as to give the

plastics material the required external shape. Where the spring end portion enters the mould, the mould has a formation to prevent escape of the plastics material during the reaction injection moulding process by which it is introduced. Such formation of the mould causes the plastics material to have a portion 14 of reduced thickness, followed by a thicker band 13 embracing the spring so as to prevent any possibility that dirt might penetrate between the plastics material and the spring.

It will be further noted that, adjacent its free end, the spring end portion has a recess 15 which is penetrated by the plastics material to provide a firm mechanical interlock to hold the plastics material to the spring. The plastics material itself is provided with a hooked end portion 16, which is engagable with a rebound stop in use on a vehicle. The plastics material could be provided with other formations to co-operate with other components, e.g. to hold spring leaves in alignment with one another in a multi-leaf spring assembly.

Referring now to Figures 3, 4 and 5 these show a spring slipper end which basically is the same as that of Figures 1 and 2, and therefore will not again be described in detail. In this case, however, the plastics material is provided in its bearing surface (20) with a pattern of recesses which are in the form of short bores extending into the plastics material, perpendicular to the surface 20. Such bores would be formed by providing the mould with an array of pins or the like. Further, the plastics material may contain, dispersed throughout it, micro-capsules 22 of lubricant.

In use, any wear of the plastics material on its bearing surface 20 causes fracture of the micro-capsules

of lubricant, and such lubricant becomes dispersed over the surface 20 and enters the recesses 21. The ensures lubrication of the surface 20 where it slides against another component, e.g. a vehicle chassis part.

Referring now to Figures 6, 7 and 8, these show a further embodiment of spring slipper end which again is generally the same as those of the embodiments above described. Thus there is shown a spring end portion 30 with a covering 31 of plastics material, the latter having a bearing surface 32. The covering of plastics material is applied to the spring end portion by a reaction injection moulding process, and includes a sealing band 33 as above described.

In this embodiment, however, the spring end portion has, in its surface 34 beneath the bearing surface 32, a pattern of recesses or depressions 35 so that the surface 34 acts as a keying surface. The recesses 35 actually comprise a grid of intersecting grooves, as shown superimposed on Figure 7, and extend beneath substantially all the bearing surface 32. The opposite side of the spring end portion is provided with a further keying surface comprising a like grid of intersecting grooves 36.

During the application of the plastics material to the spring end portion by the reaction injection moulding process, the plastics material enters the grooves 35, 36 so as to ensure an effective mechanical interfitting between the spring end portion and plastics material, with the result that the plastics material is held fast on the spring end portion despite forces arising in use. Particularly, loads on the bearing surface 32 ensure that the plastics material is pressed into the grooves 35, so

that there is no tendency for the plastics material to attempt to move on the spring end portion.

The bearing surface 32 of the plastics material may have, as for the embodiment of Figures 3 to 5, a pattern of recesses for lubricant retention.

Also visible in Figures 6 and 8, shown therein in broken lines at 37, is the possibility that the bearing surface of the plastics material may initially have a convexly curved shape in cross-section transversely to the direction at which sliding occurs at the bearing surface in use. When first put into service, assuming the part with which the bearing surface engages is flat in such transverse cross-section, initial rapid wear will take place until a flat surface is worn in the plastics material covering the spring. If there is any misalignment between the spring such flat surface will develop as shown somewhat exaggerated at 37a or 37b in Figure 8A of the drawings. In either event, the spring will not have any twisting forces applied to it.

A suitable plastics material to provide a wear-resistant bearing surface and, when applied by a reaction injection moulding process, to remain securely on the end portion of the spring, is an appropriate grade of a nylon-type copolymer material. A suitable filler material would be provided to give the plastics material suitable bearing properties.