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: S = ΔF/ΔL = Gd4 Where:

/8nD3 Srate in

Fspring force ΔF ΔL D d G n

change in spring force deflection

mean coil diameter wire diameter

modulus of rigidity number of active coils

EXTENSION SPRINGS Operating in the opposite sense to compression springs, Extension Springs 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. 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 extension

spring failures 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

N/mm N N

mm mm mm

N/mm2 -

end. A minimum bend radius of 1.5 times the wire diameter is recommended as this will minimise local stresses. 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. Counterbalances, electrical switchgear and tensioning devices, for example, all make use of this high initial tension, low- rate concept, whereas a spring balance requires zero initial tension.

TORSION SPRINGS 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 they can be stressed higher than compression springs, although they can easily be overstressed. It is therefore important that sufficient residual range is always designed into the spring, which normally means allowing a torque of 15% greater than that actually required.

SPRING SOLUTIONS 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 > 14



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