Report on Coil Springs with Wires of Noncircular Cross Section

Abstract

This report refers to the development of the coil springs with wires of non circular cross section which has been applied more and more to the automotive engine valve springs and to the other springs of motor cars.
Investigation on the subject had been made by the cooperation of 9 automobile manufacturers, 13 spring makers and 6 spring wire makers in Japan from October 1988 to September 1989.
The report makes clear the recent development of the noncircular wire springs (N. C. W. S.) and the problems to be solved which are summarized as follows.
Development of N. C. W. S. in Japan:
1) The springs have been mass produced for automotive engine valve springs by some Japanese spring makers since about 1984.
The total production of N. C. W. S. in Japan is more than 3, 000, 000 springs per month in 1989.
2) The N. C. W. S. has been applied not only to the valve springs but also to the clutch springs and the automatic transmission springs.
Now some new applications of N. C. W. S. including suspension springs of cars are tried.
3) More than 18 types of N. C. W. S. with wire sections of oval, elliptic, and their modifications have been applied for patent. Some of them have been applied to the mass production.
4) The investigation shows that the wire size, the spring indices and the aspect ratio of the wire cross sections are in some range respectively for practical uses.
5) The cost of N. C. W. S. in mass production is 20-30% higher than the one for circular wire springs.
Merits of N. C. W. S. in Designing and Problems to be solved:
1) Small solid height of N. C. W. S. makes possible the compact designing of automotive parts.
2) Mass reduction of the springs are possible by the optimum designing which varies both of the shape of wire cross section and the effective turns of coils.
3) The stress variation around the wire cross section is smallest for ellipse comparing with circle and Fuchs' oval section.
4) The stress of N. C. W. S. can be analyzed with sufficient accuracy by the FEM using solid elements or by the analytical methods with two dimensional FEM or BEM, or the Fourier expansion collocation method developed by Nagaya.
But the conventional designing formulae must be derived for the practical use.
5) The effect of the inclination of wire cross section on the stress distribution must be made clear.
Problems in Production and Inspection of N. C. W. S.:
1) The torsion of the wire of coils about it's axis varies the stress distribution which is especially important for the N. C. W. S. Therefore the practical methods of measuring the torsion must be settled for each type of the sections.
2) A practical method must be established for measuring the area of the breaking section of the N. C. W. tensile specimen.
3) The detecting accuracy must be improved for the detection of the surface flaws of the non circular wires.