| The Institute of Spring Technology (IST) Ltd, UK, has recently been presented with a SMART award by the DTI for a feasibility study into predicting the non-axial forces characteristics of a spring, based upon end coil movement. Traditionally, it has been believed that the line of force within a compression spring operates along the centre line of the spring. With the development of a sophisticated testing machine by IST, it has been shown that the line of force can take many forms, depending upon the component to which it is fitted. These forces being a shear force, a tilting moment and a twisting moment. This work is aimed at providing a low cost solution to the understanding and minimising of these non-axial forces in springs.
As industry reduces the over engineering of products, they are increasingly encountering the problems due to these forces which may be exhibited in the hystersis of a system, additional wear and variability in performance. National standards for springs do not cover non-axial forces but do allow for the tolerancing of the squareness of springs in the free position and some designers will use this in the incorrect belief that the more square a spring is the lower the non-axial forces. However, recent work has shown that spring squareness in the free position does not relate to the level of non-axial forces exhibited by the spring under load. It is the squareness under load that is the most influential factor.
This project will investigate the feasibility of producing an attachment for standard load testers such that the spring ends can move during the test. By allowing the force to move the spring ends and measure the movement using sensors, it will then be possible to utilise the axial spring rate, the compressed length and the lateral rate to calculate a predicted non axial performance instead of using a specialist machine with multiple load cells.
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