SPRING SUPPLEMENT: Designers guide to spring material selection

What do design engineers need to know before embarking on the design of a spring? DEREK Saynor from the Institute of Spring Technology explains

When specifying a compression or a tension spring, designers will be aware of the forces and degree of accuracy required, together with the operational conditions. From this information, designers must select a material from which a spring can be made. A maximum permissible stress can then be determined, which, together with the load requirements, will enable suitable dimensions to be selected. The formulae and guidance for spring design is included within BS 1726.

Before designing a spring, a commercial spring should be considered. But these should not be used where the function is of vital importance or where a full specification for replacement purposes is required, since the material specifications and characteristics may be difficult to establish.

Selection of materials

The majority of springs are made from carbon or low alloy steels with high working stresses, and are available and economical. Four main forms of these materials, which can be used either without protection in oily atmospheres, or with suitable surface coatings in corrosive environments, are employed in spring design.

Patented cold-drawn spring wire (to BS 5216: 1991) is economical and will allow higher working stresses than other carbon steels up to a wire diameter of approximately 4mm. It should be used on smaller springs wherever possible for economy. Springs made from this material should be stress-relieved at 250-275iC for 20 to 30min.

Hardened and tempered carbon-steel and low alloy steel wire to BS 2803 : 1980 (1986) is more expensive than cold-drawn wire but will allow higher working stresses in larger wire diameters. Springs made from this material should be stress-relieved at 400-425iC for 20 to 30min after forming.

Carbon and low-alloy steels for springs heat treated after forming to BS 1429:1980 (1986) are available as 070A72, 060A96, 735A50 and 685A55. They will allow higher working stresses than cold-drawn wire, above 7mm diameter, and are generally cheaper to use than oil-hardened and tempered wire, although they do not have a high fatigue performance .

Chromium-vanadium steel wire (735A50) has superior hardenability over 070A72 and 060A96 and is recommended for wire of 10mm diameter and greater. The BS 1429 specification is restricted to wires below 16mm diameter, but the materials mentioned may also be used for larger springs.

Silicon-chromium steel wire (685A55) is available as annealed wire for subsequent hardening and tempering. Its fatigue performance is superior to carbon steels, and it allows exceptionally high working stresses under static conditions. It is recommended where space limitations impose these requirements.

Stainless steel wire should be used for springs when resistance either to corrosion, or relaxation at elevated temperatures is required. Three main types of this material are used, but austenitic stainless steels (302S26 and 316S42 to BS 2056:1991) are the most common. Their resistance both to corrosion and to creep is superior to that of other stainless steels. The resistance of stainless steels to attack by chlorides, in particular sea-water, is greatly enhanced by the addition of molybdenum, and for most applications in corrosive conditions, 302S26 should be specified. However, in sea-water or salt solutions, use 316S42.

The tensile strength of materials of this type can be increased only by cold-working, and so they are supplied in the cold-drawn condition. For large springs which cannot be cold-formed (above 12mm wire diameter) other materials must be used. Springs made from these materials should be stress-relieved at 425-475iC for 1 to 2h after forming.

The tensile strength of martensitic stainless steel (420S45 to BS 2056 : 1991) is obtained by hardening and tempering, and it is used for springs above 5mm wire diameter, either in corrosive environments or where resistance to creep or relaxation is required; its performance in this latter respect is very similar to that of the austentic stainless steels, although its corrosion resistance is generally inferior. As this material is hardened and tempered after forming, stress-relieving is not necessary.

17/7 precipitation-hardening stainless steel (301S81 to BS 2056:1991) may be hardened after forming by a precipitation-hardening treatment; resistance to creep is good and resistance to corrosion is better than that of 420S45 but slightly inferior to that of 302S26. The material allows higher working stresses than the latter materials above 4mm wire diameter. As it is precipitation-hardened after forming, stress-relieving is unnecessary.

Copper alloys do not allow the high working stresses permissible with other spring materials, but have certain advantages over carbon-steel springs in that they are non-magnetic, have high electrical and thermal conductivity, and have higher resistance to corrosion.

Spring brass (to BS 2786:1963) contains 65% copper. It has high electrical conductivity, but lower working stresses than other copper alloy spring wires and is susceptible to stress corrosion cracking. Stress-relieving at 100-150iC for 20min is recommended after forming.

Phosphor-bronze (to BS 2873:1969) has spring properties superior to those of spring brass but has a much lower electrical conductivity (15% of that of pure copper). Stress relieving at 150-200iC for 20min is recommended after forming.

Copper-beryllium (to BS 2873:1969) is more expensive than the other copper alloys, but, because it can be hardened by precipitation-hardening, it has higher strength. It has a high electrical conductivity (25-30% of that of pure copper). For solution-treated, or solution-treated and cold-drawn material, a precipitation-hardening treatment is necessary after coiling. Material used in the mill-hardened condition requires stress-relieving after coiling at 200-250iC for 20min.

High nickel alloy springs are used when a non-magnetic material is required at sub-zero temperatures, when high or low electrical or thermal conductivity is needed, where a high resistance to corrosion is important, or where a high resistance to creep or relaxation at high temperatures is necessary.

Monel alloy K-500 is used for its corrosion resistance to many acid and alkali solutions. Compared with other nickel alloys, it is cheaper, has lower resistance to relaxation or creep (slightly lower also than stainless steel), and the working stresses at room temperatures which it can support are lower. The material is hardened by a precipitation treatment after coiling, and therefore does not require a stress-relieving heat-treatment.

Inconel alloy 600, on the other hand, has high resistance to corrosive environments and to relaxation or creep at high temperatures. It comes in the form of cold-drawn wire since it cannot be hardened by heat-treatment, and its spring properties are therefore better at smaller wire diameters. It is used where resistance to relaxation is required at moderately high temperatures in corrosive environments. Stress-relieving after coiling is necessary, but this depends on operating temperature.

Nimonic alloy 90 should be used where high resistance to relaxation or creep is required. For use below 350iC, springs should be coiled from cold-drawn wire, whereas for operation above 350iC, solution-treated wire is recommended. In both cases, after spring coiling, a precipitation-hardening heat-treatment is necessary. A material with slightly superior relaxation resistance is Inconel alloy X750.

Due to its cobalt content (15-21%), Nimonic alloy 90 is unsuitable for use in reactors, but Nimonic alloy 80A or Inconel alloy X750 can be used satisfactorily.

Titanium alloys to BS 2TA11 and BS 2TA12 have good mechanical properties, but because of their high cost, are only used when their exceptional corrosion resistance coupled with low weight is vital. Ni-span alloy C902, on the other hand, is used where a constant spring rate is required over a range of temperature.

Corrosive environments

Carbon-steel springs without protective coating will corrode in all conditions except those where the spring is surrounded by an oily atmosphere. For dry atmospheres, unprotected springs will give a satisfactory service life. However, when a long service life is desired, carbon-steel springs should be manufactured in galvanised wire or coated after forming.

For use in wet atmospheres, such as outside buildings or in tap-water, compression springs with ground ends should be cadmium plated, zinc plated, or coated after manufacture. For tension springs and small compression springs with unground ends which are difficult to electroplate or paint economically because of the closeness of the coils and problems of tangling, galvanised wire should be used.

The selection of cadmium rather than zinc as the protective coating should be made whenever the springs are used in coastal areas, cadmium having better resistance to chlorides than zinc. In inland areas, zinc will give better protection than cadmium, and it is also cheaper. Designers should note, however, that cadmium and zinc coatings are readily attacked by organic acid vapours such as those which occur in wooden packing-cases.

For prolonged use in sea-water, molybdenum-bearing stainless steels such as 316S42 to BS 2056 should be used. For more limited use, springs which are either cadmium plated, plastics coated, or made from some other type of stainless steel may be satisfactory. A general guide to the selection of materials for corrosive environments is given in Table 1. Protection of helical springs by protective coatings is limited by the difficulties in ensuring complete coating on the second and penultimate coils immediately beneath the end of the spring wire and by the fretting which will occur at this point in service.

At elevated temperatures, springs will be subject to creep or relaxation. For most spring applications, relaxation rather than creep will occur. Table 2 gives details of the working temperatures which will give approximately 5% relaxation at two working stress levels after continuous operation for 1000 hours.

The modulus of rigidity of spring materials decreases with increase in temperature, resulting in a directly proportional decrease in the rate. Hence, if the rate (or load to produce unit deflection) at a specific temperature is required, then the modulus for the material at that temperature must be obtained. The modulus of rigidity at room temperature for a number of spring materials is given in Table 3, and the relationship between modulus of rigidity and temperature in Figure 1.

For most spring materials at an elevated temperature, the greatest relaxation (primary relaxation) will occur within the first 70-170 hours, and this is followed by a lesser, steady relaxation (Figure 2). A method of reducing the amount of relaxation or creep which will occur when a spring is used in service is called ’hot-setting’ or hot prestressing. This process is designed to eliminate the primary relaxation and consists of compressing the spring beyond its working stress at a temperature above the expected working temperature.

Carbon and low-alloy steels hardened by heat-treatment show an increased notch-brittleness below -20iC and are not recommended for use at temperatures below this. Cold-drawn carbon and alloy steels such as those to BS 5216 may be used at temperatures down to -80iC. 18/8 austenitic stainless steel (302S26 or 316S42 to BS 2056) increases in tensile strength with no significant change in impact values at cryogenic temperatures.

Copper alloys show some increase in strength and conductivity at low temperatures, but are unaffected in other respects. The nickel alloys are satisfactory but do not have advantages over stainless steel, unless non-magnetic properties are required. Inconel alloy 600 retains this down to -40iC and Monel alloy K-500 down to -100iC. Ni-span alloy C902 is only used where its constant elastic properties are required, which it retains down to -45iC. Titanium alloys have no outstanding properties at low temperature.

Springs operating under repeated loading conditions are subject to fatigue, and all springs which are subjected to more than 10,000 cyclic operations in service must be designed to safe working stresses based on existing fatigue data. All spring steels have a fatigue limit between 106 and 107 cycles; thus the working stress which can be applied to a steel spring for operation without failure up to 106 and 107 cycles may also be applied for a spring operating for any greater number of cycles. Non-ferrous springs have no clearly defined fatigue limit. Therefore, no matter how low the design stresses, fatigue failure will eventually occur. Non-ferrous springs are not recommended for unlimited-life fatigue application.

The safe design stresses which will allow infinite spring-life under corrosion-free working conditions are shown in Figure 3. From this graph, designers must take account of both initial stress and maximum working stress when designing for fatigue applications.

It is always better to use compression springs rather than tension springs for fatigue applications because of the fretting which occurs between the end-loops of a tension spring and the pins over which these end-loops are fitted. Pre-stressing and shot-peening are not possible with tension springs. If it is necessary to use these springs the working stresses must be lower. In addition, the stress used for calculation purposes must be the highest stress in the spring, which will often occur in the end-hook rather than in the body of the spring. Working stresses for tension springs should be not greater than 75% of those for unpeened compression springs.

An important influence on the fatigue life of springs is the surface condition of the wire. In the case of carbon steels, which are particularly susceptible to decarburization during heat-treatment, the following are not recommended for fatigue conditions: BS 5216, 2803, 1429 NS and HS qualities.

The working stresses which may be used under fatigue conditions can be increased by introducing residual compressive stresses in the surface of the material. Shot-peening is the most commonly used method, but should not be confused with shot-blasting which usually has detrimental effects. Figure 3 shows the allowable working stresses for a shot-peened spring to give infinite life under corrosion-free conditions. Shot-peening is recommended for all compression springs subjected to fatigue conditions. It is unsuitable for small wire diameters and cannot be applied to tension springs.

The allowable working stress of a spring depends upon whether or not it was pre-stressed during manufacture. It is possible to increase the apparent elastic limit of all spring materials by straining the material beyond the natural elastic limit and then releasing the load. Pre-stressing, which results in improved performance under both static and fatigue conditions but with the corresponding disadvantage of increased costs, is carried out by manufacturing a spring longer than the desired finished length and compressing it to solid a number of times, so the desired free length is achieved.

The pre-stressing of tension springs is limited by the amount of initial tension which can be achieved when coiling the spring and by the desired initial tension in the free length of the spring after pre-stressing. Because of these limitations, tension springs are not normally pre-stressed. The maximum working stress permissible for pre-stressed and unprestressed springs are given in Table 4. Here, the maximum working stress is given as a percentage of the tensile strength Rm.

Fourier analysis of the deflection-time curve may show harmonics which coincide with the natural frequency of the spring for springs subjected to loading by cams. Harmonics above the 13th do not normally have to be considered, nor do those whose amplitudes are less than 1/2000 of the total spring movement under static conditions. As a guide, the frequency of the loading curve of the spring wherever possible should be less than 1/13 of the natural frequency of the spring. Where this is not possible, the natural frequency of the spring must not be an integral multiple 1 and 13 of the frequency of the loading curve. The spring index-the ratio of the mean coil diameter to the wire diameter-should not in any circumstances be less than 2.5 for any spring and should preferably lie between 4 and 10.

{{Institute of Spring TechnologyTel: Sheffield (01142) 760771Enter 530}}

DESIGNING TORSION SPRINGS

Fortress Interlocks commissioned IST to design a torsion spring for use in two of its safety interlock products-the AutoLok4 and the AutoStop4. These control interlock devices are used for the protection of people from dangerous machinery and can be operated in any orientation. The shutters contained in the head of the devices must be in the correct position to enable continued operation, whilst preventing tampering to the product. The devices are used in various applications, such as process lines, packaging lines, and machine centres The required life of the springs in the devices is in excess of 1 million cycles.

The IST brief was to replace a stock spring which was being used in prototyping trials. The stock spring required some modification prior to usage and failed to meet the high life requirement. The long term operational capabilities of the stock spring were also in question. The design was a challenge: the lifetime requirement for the spring was very high, and the working length of the spring could not exceed 1.88mm.

The design study considered a number of factors including the specified angular working positions, the maximisation of the operational torque at the initial position, the space constraints, and the environmental considerations. It also examined the long term relaxation (loss in operational torque with time and temperature), the increase in operation stresses due to the bend in one of the tangential legs, and the spring manufacturing dimensional tolerances and subsequent stress tolerances.

In addition, the increase in body length at the final working position was examined as was the reduction in the inside diameter of the spring at the final working position. The minimum clearance on the operating mandrel was also considered.

Design torques and loads were calculated using IST’s Windows based Version 5 torsion spring checking program. However, the evaluation of the fatigue performance was made based on IST’s long experience in evaluating the fatigue performance of springs. The manufacturing tolerances were from BS 1726: Part 3, 1988. Both these and the operational tolerances were calculated within the checking program.

IST designed a spring with less than five coils with a wire diameter of 0.25mm manufactured from BS 2056: 1991, 302S26 Grade II, stainless steel wire. The final design considered the increase in the operational stress caused by the bend in one of the tangential legs. The spring design was a complete success and met every criteria imposed upon IST. Fatigue trials of the product have shown that springs are still fully operational after over 1 million cycles.

Fibre optic connector design

Use of fibre optics in the telecommunications industry has increased dramatically over the years. But one of the major barriers to its employment has been the necessity for either a continuous length of fibre, or the use of complex connection methods. Due to the method in which information is passed using the medium of light, it is critical that any joins in the cable are accurately aligned. A range of connectors have recently been developed by Hewlett Packard specifically to meet this need for telecommunications and networking systems manufacturers.

Whilst the connector itself had been designed for the necessary alignment, what was required was a spring clip to hold the mating connectors together. HP approached IST to design the spring. The critical nature of the connector meant that the retaining force had to be a balanced compressive force holding the connector together without exerting any lateral force which would cause a degradation in the connector performance. The spring load had also to operate up to 80iC and function consistently for more than 1,000 cycles. A schematic of the spring is shown in Figure 1.

The end geometry of the clip was specified to enable the spring to be easily removed and fitted with the use of special tooling. The forked end shape ensures that there is a consistent axial connection force either side of the fibre optic cable.

Because springs change shape, the change must be accounted for in the design. For this application, as the ends are deflected apart to go over the connector, the centre portion of the spring will move closer to the line of force through the connector and hence the body of the connector. As lateral forces are not permitted upon the clip the spring geometry was designed to account for this deflection by the curved portion in the main body of the spring.

{{Table 1: Guide to the corrosion resistance of spring materials.

Carbon Carbon and and alloy alloy steels Stainless Copper Nickel steels with steel alloys alloys protective coating

Normal internal atmosphere fair excellent excellent excellent excellentInland external atmosphere poor good excellent excellent excellentCoastal external atmosphere n/s+ fair good good excellentTap-water n/s+ fair excellent excellent excellentSea-water n/s+ fair good fair excellent

Dilute acidsResistance varies according to the particular acid involved, its concentration, and its temperature. For detailed information seeRabald (1968).

Strong acids

n/s+ = not satisfactory}}

{{Table 2: Relaxation of spring materials (after continuous operationfor 1000 hours)

Temperature (iC) giving approximately 5 % relaxation at workingstresses below

Material Specification 275N/mm2 400N/mm2

Patented cold-drawn BS 5216 150 120Hardened and tempered carbon steel BS 2803 200 170Carbon steels – heat- treated after forming BS 1429 200 170Austenitic stainless steel Monel alloy K-500 BS 2056 (302S26) 320 300Inconel alloy 600 – 260 230Nimonic alloy 90 – 500 450Beryllium-copper – 120 100}}

{{Table 3: Modulus of rigidity G for some spring materials at roomtemperature

Materials Modulus of rigidity (Kn/mm2)

Hard-drawn carbon steel 79.3Carbon steel 79.3Silicon-manganese steel 79.3Chromium-vanadium steel 79.3Martensitic stainless steel 79.3Precipitation hardened stainless steel 75.8Austenitic stainless steel 65.5 – 75.8Phosphor-bronze 43.0Hard-drawn brass 36.0Copper-beryllium 41.3Monel alloy K-500 65.5Inconel alloy 600 76.0Nimonic alloy 90 82.7Titanium alloys 34.5 – 41.4Ni-span alloy C902 63.0 – 70.0}}

{{Table 4: Maximum permissible working stresses for various materials

Maximum working and solid stresses

Prestressed Unprestressed compression compression and springs tension springs

Material Specific- Working % Solid % Working % Solid % ation Rm Rm Rm Rm