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Considerations in factors of spring design and specification

by Chris Petts, M.D., Lee Spring Ltd

Spring manufacture is of course a well-established technology - Lee Spring for example have been producing springs for nearly 100 years - and we can see that this is very much a case of evolution rather than revolution, however in recent years manufacturing processes and changes in end use criteria have had significant impacts on the specification of these widely used and essential components. For example, medical, aerospace, food and toy applications demand special load characteristics and treatments, which include ultrasonic cleaning, while RoHS, WEEE and REACH have strongly influenced the types of finishes offered.

Consequently it is worth considering the key factors that need to be understood when engineers specify springs today. The first step of course is to match the application to the basic spring configuration, of which there are many types. The most popular of these are compression, extension, torsion, wave and disc springs. Added to these are conical, swivel hook, battery and drawbar springs. More recent additions include continuous-length extension springs, light-pressure and plastic composite springs as featured in Lee Spring's standard and custom ranges.

To start with Compression Springs these offer resistance to a compressive force applied axially. They are manufactured by coiling as constant-diameter cylinders where other common forms include conical, tapered, concave and convex configurations, as well as combinations. Most compression springs are manufactured in round wire which has proven the tests of time for most standard and custom purposes, but square, rectangular, or wire with special sections can be specified if needed.

Generally compression springs are designed to work in a bore or over a rod. For most applications we supply them as standard with end coils closed and ground square for optimum alignment and reduced solid height, in addition they can also be pre-stressed during manufacture to maintain their length at elevated temperatures.

Crucial to accurate design of compression springs is a good understanding of both the practical and the ultimate limitations of available materials, together with some simple formulae. Spring theory is normally developed on the basis of spring rate and the formula for this is the most widely used in spring design:

Operating in the opposite sense to compression springs we can consider Extension Springs which absorb and store energy by offering resistance to a pulling force. Various types of formed ends are used to attach this type of spring to the load component. In this the variety of ends is limited only by the imagination of the designer - popular forms include threaded inserts (for precise control of tension), reduced and expanded eyes on the side or in the centre of the spring, extended loops, hooks or eyes at different positions or distances from the body of the spring, and even rectangular or teardrop-shaped ends. Experience has shown that if possible machine loops and cross-over loop types are the most cost-effective solutions.

It has been found that most failures of extension springs occur in the area of the end, so in order to maximise the life of a spring the curve of the wire should be smooth and gradual as it flows in to the end. A minimum bend radius of 1.5 times the wire diameter is recommended as this will minimise local stresses.

S = ΔF/ΔL = Gd4/8nD3
Where:

Extension Springs are generally wound with initial tension - this produces an internal force that holds the coils together tightly at rest. In practice, this means that, before the spring will extend, a force greater than the initial tension must be applied so as we might expect a spring with high initial tension will exert a high load when subject to a small deflection. If this is combined with a low rate, the spring will exhibit an approximate constant force characteristic which can be of considerable value in specialist situations.

For example, counterbalances, electrical switchgear and tensioning devices all make use of this high initial tension, low-rate concept, whereas a spring balance requires zero initial tension.

Torsion Springs make use of the principle that the ends are rotated in angular deflection to offer resistance to an externally applied torque. However, the wire itself is subjected to bending stresses rather than torsional stresses - springs of this type are usually close-wound. Functionally they reduce in coil diameter while increasing in body length as they are deflected. It is therefore essential to allow for these factors, particularly if they are to be used over a mandrel where clearances must be maintained.

The types of ends for a torsion spring should be subject to careful scrutiny, as should checks of nominal free-angle tolerances relating to application requirements in spring manufacturers' data.

Interestingly, torsion springs are stressed in bending and not torsion, this means that, as a consequence, they can be stressed higher than compression springs; however they can easily be overstressed. It is therefore important that sufficient residual range is always designed into the spring. This normally means allowing a torque of 15 per cent greater than that actually required.

Disc Springs, sometimes also known as Belleville spring washers, are an excellent solution where a compression application requires dealing with high loads in a small space. The conical design of disc springs enables them to support high loads with relatively small deflections and solid heights compared to helical springs. Often they are used to solve vibration, thermal expansion, relaxation and bolt creep problems.

Battery Springs, important in so many modern hand-held devices, instruments and toys are designed to provide efficient and reliable contacts in most situations where portable power is required - often in self-contained battery compartments. Generally they are offered in a variety of mounting configurations so as to accommodate the most popular battery sizes.

Continuous-Length extension springs are designed to be cut to length to meet custom load requirements for unusual applications or maintenance operations. Various loops or hooks can be formed on the ends.

To meet demands for compression springs combining low spring rates with larger diameters, Lee Spring has introduced a Lite Pressure™ range. These are designed to deliver 7-103kPa pressure @ 80 per cent deflection. Manufactured in 316 stainless steel, they are suited to applications such as valves, pistons, syringes, motor brushes, dispensers, contacts and toys. Other specialist ex-stock springs include the High Pressure Range and Constant Force, plus Bantam and Wave Springs.

If a suitable spring cannot be found from stock, designers will find that most springs can be custom-designed and manufactured with support from the experienced engineering team at spring manufacturers such as Lee Spring.

Most stock springs are manufactured in music wire, stainless steel, oil-tempered MB or chrome silicon steel and perhaps obviously material choice is an essential consideration.

Key factors affecting material choice include: meeting stress conditions, either static or dynamic; the capability of functioning at a required operating temperature; compatibility with surroundings - e.g. a corrosive environment; and other special requirements such as conductivity, constant modulus, weight restrictions, magnetic limitations, etc.

Good examples are: music wire springs which are normally supplied with a zinc plating baked for hydrogen embrittlement relief, while die springs are painted different colours to denote duty.

On the other hand battery springs are produced in music wire and nickel coated, because most alkaline batteries use nickel-plated containers. Here the use of similar materials removes the possibility of galvanic corrosion and enhances resistance to wear. Additionally, nickel helps to break down the oxide that forms on the surfaces of batteries.

All of our 316 stainless steel springs are passivated and ultrasonically cleaned which offers medical- and food-grade levels of cleanliness. Other special finishes may be specified.

Lastly, it is often forgotten that spring performance is affected by temperature, which should not exceed 120°C for music wire, 260°C for stainless steel and 245°C for chrome silicon steel.

As a final note regarding the avoidance of failures, please remember that if a spring is used outside its physical capabilities
it will break and the component or product in which it is used will fail. Obviously, getting the load and stress calculations, as well as material choice right will help to avoid this, as will careful allowance for the operating conditions, particularly service temperature and presence of water or solvents.

For further information on Lee Spring products, view website: www.leespring.co.uk or follow them on twitter: https://twitter.com/leespringuk 
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