A beginner's guide to spring selection

Springs are fundamental mechanical components which form the basis of many mechanical systems. A spring can be defined as an elastic member which exerts a resisting force when its shape is changed. Most springs are assumed linear and obey Hooke's Law.



A spring is capable of absorbing energy at a relatively large deflection and then storing some or all of it as strain energy. If the spring is unloaded some or all of the stored energy is released. A spring can therefore be described according to its energy-storing and energy-absorbing characteristics.

Mechanical springs may be used to apply force, store or absorb energy or provide elastic movement. They are frequently key elements in the operation of safety devices and their failure can often have damaging results.

Selection criteria should include: space available; magnitude and direction of loads and deflections; accuracy and reliability; environmental conditions; tolerances; and costs.

Spring dimensions may be specified simply by: wire diameter (or cross-sectional area); coil diameter; free length and number of coils. For a full specification, however, it is important to follow guidelines and complete spring data sheets as laid out in the relevant standard.

Loading conditions are usually the first design priority because at one end of the scale, performance under load may be unimportant, whereas at the other, detailed spring rate data may be needed. The two most common types are helical compression and extension springs, the former having flat end coils and the latter either hooks or eyes for load transfer (which are an extra cost factor).

Compression springs should close solid without damage and extension springs need either large safety factors or mechanical stops to prevent overstressing. Springs must normally be constrained at one or both ends so that the risk of lift or slide is minimised, although some traverse or eccentric loading may be desirable to suit a particular application.

End fixing for compression springs may require a pad to evenly distribute loading and prevent damage to the base structure especially if it is thin sheet or plastic. Where springs are fitted over a rod or inside a hole or tube, care must be taken with tolerances (especially if temperature rises are expected), to prevent jamming or scuffing.

Where very close fitting is needed, springs can be supplied with the outside and ends pre-ground (barrelled and faced). One of the most important long term considerations is corrosion and a s a general rule, springs at risk should be located within a protective surround and, if they are made of ferrous metal, be lightly coated with oil or grease. Electroplating may be specified as an alternative to lubrication but its effect may be to cause an increase in spring rate.

If blind holes are used for end fixings in wet environments, these may become moisture traps and should therefore be avoided. The same applies in situations where electrolytic corrosion may occur between the spring and the material to which it is fixed. High temperatures can cause tempering of the spring material and undesirable expansion may also effect physical performance.

When selecting a spring for an application, a design engineer may be asked to specify the spring in a number of different ways. Here are just a few:

* End type (open, squared, ground)
* Number of nested springs
* Minimum and maximum outside diameter
* Diametrical clearance between nested springs
* Minimum and maximum wire diameter
* Minimum number of active coils
* Maximum working strength
* Spring force at working length and permissible variation
* Spring constant and permissible variation
* Material modulus
* Spring displacement during work
* Solid length factor, residual stress, type of use

Author
Tom Shelley

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