Manufacturing Materials

The materials used for the manufacture of springs cover a very wide range of metallic and non-metallic substances. However, these notes will be confined mainly to the discussion of metallic types with a small consideration being given to some composite materials.

Steels Used for Cold-Wound Springs

Below a material cross-sectional diameter of approximately 9.53-18.42 mm, certain steels are drawn into wires and cold-wound to form the required shape of the spring. Usually, the wires are then given some form of heat-treatment to relieve the stresses imposed by the winding processes. Typical types of carbon/alloy-steel stock that are used for the manufacture of springs, include:

• Hard-drawn Spring Wire: It is a low-quality and least expensive carbon-steel stock. This wire is liable to possess fine seams in its surface and, as such, would only be used in applications of low stress and not where fatigue loadings could be exerted.
• Oil-tempered Spring Wire: It is a better quality, high-carbon steel stock, though it may also contain surface discontinuities and would be found where long fatigue life is not required.
• Music Wire: It is a carbon-steel stock of high quality and is suitable for small-sized, helical springs in applications involving high fatigue stresses.
• Chrome-Vanadium Steel Wire: It is a stock that has been used for piston-type aero-engine valve springs and is therefore, suitable for high-temperature and high-stress conditions.
• Chrome-Silicon Steel Wire: When used in valve springs, it has a higher fatigue life in the lower cycle ranges (10-100 kHz) than other wires.
• Stainless-Steel Spring Wire: It is obvious from its name and is used in conditions where high corrosion-resistance is the requirement. This grade of wire would also be utilised in applications where resistance to creep at elevated temperatures is desired. Some grades of Stainless-Steel wires can be made to accept magnetism, where this characteristic is needed alongside the other qualities.

Steels Used for Hot-Wound Springs

Above the cross-sectional diameters, it is considered impractical to cold-wind and so the larger diameter metals are hot-wound and then also subjected to various stress-relieving processes. Similar carbon/alloy steels to those already discussed are employed in the manufacture of hot-wound springs with the necessary variations in their contents of carbon, chromium, manganese, molybdenum, nickel, silicon and vanadium.

Steels Used for Cold-Rolled, Flat Springs

These steels vary in composition, depending on their location. But are commonly based on carbon and manganese as the main constituent elements and may be formed from oil-tempered steels or from annealed steels which are subsequently heat-treated.

Non-Ferrous Metals Used for Springs

These are based mainly on copper alloys, where corrosion resistance and good electrical conductivity is required and on nickel alloys where the ability to work at elevated temperatures is desirable. These alloys include:

• Spring Brass: It is comparatively inexpensive, has good electrical conductivity, but is unsuitable for high-stress applications.
• Nickel Silver (also called German Silver): It has better characteristics than brass and is made from different percentages of copper, zinc and nickel.
• Phosphor Bronze: It has a minimum percentage of 90% copper content and has, therefore, excellent electrical conductivity. It is suitable for applications of higher stress levels than those of brass.
• Silicon Bronze: It has similar characteristics to those of phosphor bronze but is less expensive to produce.
• Beryllium Copper: It has similar conductivity and corrosion resistance qualities to those of copper with the addition of beryllium (2-2.5%) imparting greater hardness and other superior mechanical properties.
• High-Nickel Alloys: These are the types more commonly found in aero-engine applications and which fall under various, familiar, trade names such as Monel, ‘K’ Monel (3% aluminium), Permanickel, Inconel and Inconel ‘X’ (2.5% Titanium).

Note: Another high-nickel alloy goes under the name of Ni-Span-C and does contain almost 50% iron. All of these non-ferrous alloys can be found in the cold-rolled or drawn conditions for the manufacture of many types of springs.

Composite Materials Used for Springs

Some composite springs involve the joining of certain metals with elastomers to form the anti-vibration mountings (Metalastic Bushes and Housings) such as those found in aero-engine and auxiliary power unit (APU) installations. Others combine synthetic rubber strands, encased within a sheath of braided cotton, nylon or similar materials. They are usually referred to as Shock Absorbers or Shock Cords rather than Springs and are more familiarly known by the generic name of Bungee Cords. Bungee Cords may be encountered on many light/medium-sized aircraft while their use on heavier aircraft is not unknown.