GB2260385A - Coil spring - Google Patents

Abstract

A coil spring is made from a wire (52) which in transverse cross-section has an oval surface portion (60) nearest to a central axis (18) of the spring. The cross-section has no axis of symmetry and is selected to cause a twist in the wire which counteracts the twist caused when the wire is coiled into a spring. <IMAGE>

Description

COIL SPRING This invention is concerned with a coil spring comprising a length of spring wire coiled about a central longitudinal axis of the spring. The term "wire" is used herein to include material suitable for use in making coil springs whether it is metal or other material.

Conventionally, coil springs are made of wire which has a cross-section which is circular considered transversely of its length. This has the disadvantage that, when such a spring is subjected to a load, the stress experienced by the wire is greatest on the inside of the coil, i.e. in that part of the cross-section which is nearest to the central longitudinal axis of the spring.

It is clearly advantageous to reduce this maximum stress, if this can be done without adversely affecting other properties of the spring, e.g. its natural frequency and its compressed solid length. Even a relatively small reduction in the maximum stress can greatly reduce the risk of cracking, and, hence, greatly increase the service life of the spring.

Attempts to reduce the maximum stress have involved departing from wire having a circular cross-section and using instead wire of a cross-section which is not circular but is symmetrical about a line extending perpendicular to the central longitudinal axis of the spring. The crosssection is selected so that the wire has the same torsional stiffness as the circular cross-section wire so that a spring having similar characteristics is produced. Such cross-sections have inside surfaces designed to spread the stress more uniformly and thereby reduce the maximum stress. One such cross-section has a surface portion in the form of more than one half of an oval with the opposite edges of this oval surface joining two generally-planar surfaces inclined at approximately 100 degrees to one another.The other edges of the planar surfaces are joined by a short convex surface which completes the outline of the cross-section. The oval surface is intended to be centred on a line extending perpendicular to the central longitudinal axis of the spring (the perpendicular line), so as to spread the stress more uniformly, and the crosssection is centred on the perpendicular line, so that the stress is spread symmetrically. However, as springs are coiled by a process which causes the transverse crosssection of the wire to be given a twist relative to the perpendicular line, the use of such non-circular wires has the disadvantage that, during the coiling process the twist results in the line of symmetry of the cross-section not coinciding with the perpendicular line. The oval surface is not then centred on the perpendicular line and does not spread the stress as required.The degree of twist of the cross-section also varies around and along the spring. A non-symmetrical spread of stress is, thus, achieved which may or may not result in a reduction of the maximum stress and varies at different points of the spring. This twisting of the cross-section of circular cross-section wire does not cause problems, because the twisted crosssection is entirely symmetrical and can, thus, be considered to be still centred on the perpendicular line so that maximum stress is not altered.

It is an object of the present invention to provide a coil spring in which the maximum stress is consistently reduced when compared with a spring of similar characteristics made from a circular cross-section wire.

The invention provides a coil spring comprising a length of wire coiled about a central longitudinal axis of the spring by a process which causes the transverse crosssection of the wire to be given a twist relative to a line perpendicular to said axis, wherein the transverse crosssection has a shape which has an oval surface portion nearest to said axis and has axis of symmetry, the shape being selected so that, during the coiling process, it causes a counteracting twist to said twist.

In a spring in accordance with the invention the oval portion of the cross-section can be more closely centred on the perpendicular line giving better and more consistent reduction of the maximum stress. If the twists are made to exactly cancel one another, the oval portion can be exactly centred on the perpendicular line.

Preferably, the oval surface portion of the crosssection adjoins a convex surface portion and the surface of the cross-section is completed by a concave surface portion.

The lack of an axis of symmetry can be achieved by using tubular wire in which a passage extends through the wire. The passage can be non-symmetrical in cross-section or be positioned "off-centre".

There now follows a detailed description, to be read with reference to the accompanying drawings, of a coil spring in accordance with the invention.

In the drawings: Figure 1 is a diagrammatic view of the coiling of a prior art spring, Figure 2 is a view, corresponding to Figure 1, of the coiling of a spring in accordance with the invention; Figure 3 is a cross-sectioned view of the wire of the spring shown in Figure 2; and Figure 4 is a cross-sectional view of the wire of another spring according to the invention.

Figure 1 shows diagrammatically a coiling machine 10 to which wire 12 is fed in the direction of the arrow 14.

Figure 1 also shows the first half coil 16 of the spring coiled from the wire 12 by the machine 10. The machine 10 coils the wire 12 about a central longitudinal axis 18 of the spring by a well-known process involving tools which bend the wire into a coil.

The wire 12 has a transverse cross-section which has a shape which has an oval surface portion 20 nearest to the axis 18, i.e. the portion 20, when viewed in cross-section has the shape of a portion of an oval, the surface portion 20 abuts, at its two longitudinally-extending edges, two planar surface portions 22 and 24 and the surface of the wire is completed by a short convex portion 26 which joins the surface portions 22 and 24. The shape of the crosssection is symmetrical about a line 27 bisecting the nonplanar surface portions 20 and 26.

The coiling process performed by the machine 10 causes the wire 12 to twist so that the transverse cross-section of the wire is given a twist in the direction of the arrow 28 relative to a line 30 perpendicular to the axis 18.

Thus, after one half turn of the coil, the line 27, which initially coincided with the line 30, is at an angle 32 thereto. This twist causes the oval surface portion 20 not to be directed centrally towards the axis 18 so that this surface portion 20 does not act, when the spring is in use, to spread stress uniformally. Furthermore, the twist increases the compressed solid length of the spring.

Figure 2 shows the machine 10 being used to coil a spring according to the invention from a wire 52. The coil is formed about the axis 18 and Figure 2 also shows half a coil of the spring. The spring is, thus, a length of wire 52 coiled about the central longitudinal axis 18 by a process which causes the transverse cross-section of the wire to be given a twist, in the direction of an arrow 54, relative to a line 56 perpendicular to the axis 18.

However, in this case, the transverse cross-section of the wire 52 has a shape which is selected so that, during the coiling process, it causes a counteracting twist, in the direction of the arrow 58, to the twist afore-mentioned.

The transverse cross-section of the wire 52 has a shape which has an oval surface portion 60 which is nearest to the axis 18. The portion 60 adjoins a convex surface portion 62 and the surface of the cross-section is completed by a concave surface portion 64. It will be observed that the cross-section of the wire 52 has no axis of symmetry.

Since the twist in the direction of the arrow 54 is substantially counteracted by the twist in the direction of the arrow 58, the cross-section of the wire 52 is not twisted substantially relative to the line 56. The oval surface portion 60, therefore, remains centred on the line 56 and can spread, when the spring is in use, the stress consistently to reduce the maximum stress. Furthermore, because the cross-section is not twisted, the compressed solid length is reduced.

Figure 4 shows an alternative cross-section of a wire of a spring according to the invention, having an oval surface portion 70, a convex surface portion 72 and a concave surface portion 74.

Claims (4)

1A coil spring comprising a length of wire coiled about a central longitudinal axis of the spring by a process which causes the transverse cross-section of the wire to be given a twist relative to a line perpendicular to said axis, wherein the transverse cross-section has a shape which has an oval surface portion nearest to said axis and has no axis of symmetry, the shape being selected so that, during the coiling process, it causes a counteracting twist to said twist.

2 A coil spring according to Claim 1, wherein the oval surface portion of the cross-section adjoins a convex surface portion and the surface of the cross-section is completed by a concave surface portion.

3 A coil spring according to either one of Claims 1 and 2, wherein the lack of an axis of symmetry is caused by a passage extending through the wire.

4 A coil spring substantially as hereinbefore described with reference to and as shown in Figures 2 and 3, or Figure 4, of the accompanying drawings.