The feasibility of replacing a metal helical compression spring with a composite one having the same characteristics has been studied. The envisioned benefits of a composite spring are the reduction of weight and, more important, better chemical resistance and electrical insulation. A conceptual design based on a mechanical analysis has been made. The required formulas have been derived from those known for isotropic springs. Resin Transfer Molding (RTM) has been chosen as production technique for this scope and special glass fiber braids as material/preform. An optimization has been performed to obtain a product with the best suitable characteristics for the given requirements. Finite elements calculations have been done to verify the results obtained from the analytical study and to obtain detailed information on the stress distribution inside the spring. The production of several prototypes has shown the feasibility of such a product.

Composite springs are sometimes necessary when the particular requests of the customer is not only a given spring constant but also other characteristics which a metal spring could not give. In particular composites are preferred to metals when weight is a fundamental requirement in the choice of the material. In the meantime, composites have also other characteristics that could make a composite spring attractive, in particular corrosion resistance and electrical insulation, when glass or aramide are used.

The geometrical characteristics necessary to describe the problem are given:

According to the classical theory of springs, the spring rate R of a helical spring is given by:

(1)

where:

The bending and shear stresses arising on the spring are defined as:

Being the torsion moment of inertia of a solid circular section given by:

(2)

It is evident that the main contribution to the stiffnes of the spring is given by the outer layer of the coil, whereas the contribution of the core is negligible. This means that better stiffness vs. weight ratio (i.e. the weight effectiveness) can be achieved using hollow coil section.
The classical theory of springs can be easily adapted to a circular hollow sections with inner diameter d_i changing the moments of inertia:

(3)

Therefore the stresses can be re-written as:

(4)

(5)

Where  is the inclination angle of the coils.
Since  is in general very small, the stress distribution is dominated by the shear stress, which is maximum at full compression.

From the theory shown in the previous section, it is evident that shear modulus is the main material property affecting the spring performance. In composite materials, changing fibre orientation can control shear modulus, whose maximum is found at ±45°. In our case, the small bending stress contribution can be accounted for by placing fibres at ± anglewhere  is properly chosen in order to minimise strain.

Being braiding the simplest technique to place fibre at a custom ± angle, glass fibres in the form of braided socks were chosen as reinforcement. Braided socks are also very easy to handle and fit very well to hollow circular sections.

The choice of glass fibres is mainly due to the lower cost in comparison to other reinforcing fibres. Moreover, aramide fibres have poor properties in compression whereas carbon fibre, notwithstanding their superior mechanical properties, cannot be used because of the electrical conductivity.

Given both the geometrical constraints and the required spring stiffness, only two design parameters are left free: the coil wall-thickness (t=(d_o-d_i)/2) and the fibre orientation angle controlling the shear modulus. Those parameters must be chosen so as to minimise the maximum stress and strain levels within the coil. An optimisation program based on eqs. (4) and (5) and the general lamination theory was used to evaluate the best parameters choice for a spring with the following requirements:

The lower strain and strain levels were found at  = 55° and t=1.9 mm.

This result was confirmed by means of Finite Element Analysis. Although Finite Elements show higher stresses than the analytical method, the same trend was found.

The method used for the production of the springs is a variation of the RTM (Resin Transfer Moulding) process. Through this method, the dry braids are positioned in the mould before being impregnated with the resin, making production very clean and simple. In this case, an open mould consisting of a helically grooved mandrel was used, and the braids are impregnated by plunging in a bowl filled with resin. The production process can be described in few steps:

The feasibility of producing composite helical springs with optimal fibre orientation has been demonstrated. The manufacturing process used is suitable for prototyping, because the low cost of the open mould, but not for series production, because it is quite a long process and labour-intensive. In particular the "bath" impregnation is very long and difficult, for non-expert workman. To avoid that, a closed mould could be needed, that allows applying the injection pressure required impregnating the reinforcement throughout the spring. The cost of such closed mould, however, was too high for this feasibility study and needed further developments. Metal springs have several advantages: they are very cheap to produce and can be produced in almost all kind of measures and in a very broad range of stiffness. On the other hand, they could become very heavy and have corrosion and insulation problems. In the cases where lightness and/or insulation and corrosion characteristics are more important than the price, a composite spring can be successfully used.

CLS wishes to thank Antonello Antoniazzi of ABB Ricerca SpA in Milano who supported this research.