Development of the acetal spring

Acetal Contact Sprocket Dimension spring surface teeth a-a 

However, steel springs had a number of drawbacks, and the acetal spring was designed to overcome them. The drawbacks were  

The approach was therefore to make a prototype to the new shape and subject it to performance testing against the current performance specification for the switch unit. 

The prototype was made manually from acetal homopolymer sheet, with advice from the polymer supplier (Du Pont) on the most suitable grade for such a purpose. Its thickness was determined by the thickness of the sprocket teeth 3 mm sheet was used (approx 1/8 inch). The shape, as already stated, was drawn to fit the available space. In use, it will be strained by the rotation of the teeth (Fig. 6.2(b)) which push against the central section, the actual elastic strain occurring in the curved, spring-shaped ends. The critical design parameters were (a) the profile in the centre where the sprocket teeth contact the spring (b) the width of the spring section (dimension a-a. Fig. 6.2(b)). 

These are critical because the switch must operate with a positive action between positions, within the specified torque limits and without slackness. If there is too much material in the spring section (a-a), too much torque will be required to turn the switch, i.e. the spring will be too strong. If the central section contact profile is wrong the teeth will not slide evenly, and the action will jam or be slack after the click of the action. 

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