Advice for spring design and specification

Although the development of springs could be described as evolution rather than revolution, in recent years manufacturing processes and changes in end use criteria have impacted on the specification of these ubiquitous products. For example, medical, aerospace, food and toy applications demand special load characteristics and treatments, such as ultrasonic cleaning, while RoHS and WEEE have influenced the types of finishes offered.

So what are the key factors that need to be considered when specify springs today? The first step is to match the application to the spring type and there are many types from which to choose.

The most popular types are compression, extension, torsion and disc springs. Added to these are conical, swivel hook, battery and drawbar springs. More recent additions include continuous-length extension springs and light-pressure springs as featured in Lee Spring's latest stock spring catalogue.

Spring types

Compression springs offer resistance to a compressive force applied axially. Coiled as constant-diameter cylinders, common forms include conical, tapered, concave and convex, as well as combinations of these. Most compression springs are manufactured in round wire but square, rectangular, or wire with special sections can be specified.

Generally these springs are specified to work in a bore or over a rod. They can be supplied with end coils closed and ground square for optimum alignment and reduced solid height. Springs can also be pre-stressed during manufacture to maintain their length at elevated temperatures.

Accurate design of compression springs requires knowledge of both the potential and the limitations of available materials, together with 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 (see right).

Extension springs absorb and store energy by offering resistance to a pulling force. Various types of ends are used to attach this type of spring to the load. The variety of extension spring ends is limited only by the imagination of the designer. Ends can 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. If possible, machine loops and cross-over loop types should be specified, as these are the most cost-effective to produce.

Most failures of extension springs occur in the area of the end, so in order to maximise the life of a spring, the path 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.

Most extension springs are wound with initial tension - an internal force that holds the coils together tightly. In practice, this means that, before the spring will extend, a force greater than the initial tension must be applied. 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.

Counterbalances, electrical switchgear and tensioning devices all make use of high initial tension, low-rate springs, whereas a spring balance requires zero initial tension.

Torsion springs have ends that are rotated in angular deflection to offer resistance to externally applied torque. The wire itself is subjected to bending stresses rather than torsional stresses. Springs of this type usually are close-wound. They reduce in coil diameter and increase in body length as they are deflected, so it is essential to allow for these factors, particularly if they are to be used over a mandrel.

The types of ends for a torsion spring also must be considered carefully. Designers should also check nominal free-angle tolerances relating to application requirements in manufacturers' data.

Torsion springs are stressed in bending and not torsion and, 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 allowing a torque 15 per cent greater than that required.

Disc springs, also known as Bellville spring washers, are used where a compression spring application requires a high load in a small space. The conical configuration 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 are designed to provide efficient and reliable contacts in most situations where portable power is required - for example, in self-contained battery compartments. Generally they are offered in several mounting configurations and 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 new Lite Pressure range. Designed to deliver 7-35kPa pressure @ 80 per cent deflection, these springs are manufactured in 316 stainless steel and are suited to applications such as valves, pistons, syringes, motor brushes, dispensers, contacts and toys.

If a suitable spring cannot be found from stock, remember that most springs can be custom-designed and manufactured.

Material choice

Most stock springs are manufactured in music wire, stainless steel, oil-tempered MB and chrome silicon steel.

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

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

Battery springs are produced in music wire and nickel coated, as 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 Lee Spring's 316 stainless steel springs are passivated and ultrasonically cleaned to offer medical- and food-grade levels of cleanliness. Other special finishes may be specified.

Spring performance is affected by temperature, which should not exceed 120degC for music wire, 260degC for stainless steel and 245degC for chrome silicon steel.

Avoiding failures

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. Care should also be given to the operating conditions, particularly service temperature and presence of water or solvents.