WO1997000146A1 - Spring coiling - Google Patents

Abstract

A coil spring is manufactured by first bending wire (10) to a first curvature which has a smaller radius of curvature than the required curvature for the spring. Subsequently, said wire is bent to the required curvature.

Description

SPRING COILING

This invention is concerned with the manufacture of coil springs, ie springs which consist of a length of wire coiled into a helical shape. Such springs have many uses but one example where the invention may be utilised is in the manufacture of springs for operating poppet valves in internal combustion engines.

The conventional method of manufacturing coil springs is to force wire into contact with one or more bending tools, the shape of the tools and the angle of contact being such that the wire is given the curvature required in the finished spring, ie the wire is bent to its final form in a one stage bending operation. The bending operation induces stresses in the wire which can cause it not to be thermally stable in service.

It is an object of the present invention to provide a method of manufacturing a coil spring which reduces the likelihood of the spring being thermally unstable in service.

The invention provides a method of manufacturing a coil spring, characterised in that the method comprises bending wire to a first curvature which has a smaller radius of curvature than the required curvature for the spring, and subsequently bending said wire to the required curvature.

In a method in accordance with the invention, the wire is first bent more than is required, imparting greater stress thereto than is conventional, and is then bent back to the required curvature, relaxing the stress partially. This is found to result in springs with a reduced tendency to be thermally unstable in service. Such springs also have a reduced tendency to crack in service. Preferably, said wire is bent to the first curvature by a first bending means and is bent to the required curvature by a second bending means positioned downstream of the first bending means by a distance which is less than the circumference of one coil of the wire, ie the circumference of the spring. This arrangement is convenient for manufacture. More preferably, the distance is less than one half of the circumference of one coil of the wire.

It is found that good results are obtained if the first curvature has a radius of curvature which is less than that of the required curvature by between 5% and 35%.

There now follows a detailed description, to be read with reference to the accompanying drawing, of a method of manufacturing a coil spring which is illustrative of the invention.

The drawing is a diagrammatic view of an apparatus for carrying out the illustrative method.

The illustrative method is a method of manufacturing a coil spring of conventional helical form from a wire 10. The wire is passed between two driven pinch rollers 12 which feed the wire 10 through a guide 14. After leaving the guide 14, the wire 10 comes into engagement with two bending tools 16 and 18 which together form first bending means. The tools 16 and 18 are of conventional form and are shaped so that the wire 10 is bent to a first curvature which has a smaller radius of curvature than the required curvature for the spring. Specifically, the wire 10 comes firstly into engagement with a curved surface 16a of the tool 16 and comes secondly into engagement with a curved surface 18a of the tool 18. The tools 16 and 18 both engage an outer surface of a coil formed by the wire 10 and act to decrease the radius of curvature of the portion of wire which they engage. Thus, the wire 10 begins to form a helix with said smaller radius of curvature.

In the illustrative method, the wire 10, which has been bent by the tools 16 and 18, is subsequently bent to the required curvature by engagement with a bending tool 20 which forms second bending means. The tool 20 is in the form of a cylindrical peg which enters the first coil formed by the wire 10. The outer cylindrical surface of the tool 20 engages an inner surface of the coil formed by the wire just downstream of the tool 18. The bending tool 20 is, thus, positioned downstream of the bending tools 16 and 18 by a distance which is less than the circumference of one coil of the wire. The tool 20 increases the radius of curvature of the portion of the wire 10 which it engages.

After the tool 20, the outer surface of the wire 10 engages a stabilising tool 22 which engages the outside of the first coil. This tool 22 acts to keep the wire 10 in engagement with the tools 16, 18 and 20. After the tool 22, the wire 10 passes over a finger 24 which, in conventional manner, acts to determine the pitch of the spring formed by the wire 10.

In the illustrative method, the wire 10 is bent by the tools 16 and 18 to a radius of curvature which is approximately 70% of the required radius of curvature.

Springs made by the illustrative method were found in tests to be more thermally stable than conventionally-made springs.

Claims

A method of manufacturing a coil spring, characterised in that the method comprises bending wire (10) to a first curvature which has a smaller radius of curvature than the required curvature for the spring, and subsequently bending said wire to the required curvature.

A method according to claim 1, characterised in that said wire (10) is bent to the first curvature by a first bending means (16, 18) and is bent to the required curvature by a second bending means (20) positioned downstream of the first bending means by a distance which is less than the circumference of one coil of the wire.

A method according to either one of claims 1 and 2, characterised in that the first curvature has a radius of curvature which is less than that of the required curvature by between 5% and 35%.

A spring manufactured by a method according to any one of claims 1 to 3.