TechArchive

This article was originally written in the period 1995-2000

Bryan Harris, Managing Director of Harris Springs, looks at the case for improved spring specifications and design.

Any engineering component can easily become the weak spot in an assembly if it is incorrectly specified. Bearings, for instance, have to be specified against a knowledge of working load, speed range and operating conditions. Get these wrong and problems are sure to follow, so few designers are averse to seeking specialised assistance from bearing manufacturers.

Springs are a similarly specialised area of mechanical design. However, it is less likely that technical assistance from spring makers will be sought. As a result, the field performance of a product may fall short of expectations.

Often the fault will lie, not with the spring but with the design specifications to which it is produced. This may well be traceable to a lack of knowledge of constraints that apply to spring design. Indeed, general awareness of the relevant British Standard (BS1726) is lamentably low.

No-one expects designers to be expert in every field but there is a mistaken inclination to treat springs as commodity items rather than engineering components. When a remedy is sought, it results in higher development costs than would be necessary if the help of specialist spring suppliers had been sought during product development.

The basic point, regularly overlooked, is that standard linear drawing tolerances are irrelevant to spring specification. In practice, no spring tolerances should be applied without first considering the operational effect that they will have in the particular application.

As an example we will take a conventional constant rate compression spring, 50mm long, manufactured to give a rate of 1N per mm compression. If the designer specifies a manufacturing tolerance of +/-1mm on length then an acceptable batch may contain springs between 49mm and 51mm length.

This is fine, so long as the implications have been thought through. If deflection to 20mm is required, then the nominal load will be 30N and variation will be +/-3.33 per cent, which may be acceptable. If, however, the application requires spring deflection to 45mm then the load will vary about the nominal (5N), by up to 1N, or +/-20%, which is unlikely to be acceptable in a load critical application.

When we move to extension type springs, an added complication arises in that each requires a certain load to be applied before it will begin to stretch. This load is approximately proportional to the spring index – the ratio of coil diameter to wire diameter – but is nevertheless an additional variable that requires consideration.

As we can see, spring specification is demonstrably not as straightforward as it might at first appear. Reliable, consistent operation is dependant on having available the maximum amount of application data to help design the spring properly. Simply treating it as a commodity will almost certainly lead to problems.

Fortunately, there is considerable assistance available from British Standards to the form of BS1726. As standards go, it costs little but deals with spring specification clearly and comprehensively, and could save a lot of development time. In addition, specialist spring manufacturers like Harris Springs have computer aided spring design packages available which, given the necessary information, can develop a spring design and specify sensible tolerances based on customers data.

With design complete, it is then possible to control the manufacturing process very tightly. On a compression spring in particular, length is crucial and much work has gone into the development of gauging systems. Harris Springs uses proximity type length probes operating in cycle in conjunction with microprocessor based gauging and statistical process control systems. These link in with the machine control systems for automatic adjustment whilst automatic sorting facilities make it possible to guarantee that every spring in a batch will comply with the designed tolerances.

Thus it is possible to develop a spring that will match the customers requirements exactly and then produce it with consistency.

• Harris Springs
• Tel: 0118 934 0024
• Fax: 0118 934 1365