Avoiding fatigue performance in the manufacture of flat strip products


There are many different methods which can be applied to avoid fatigue performance in the function of flat strip products.
The fatigue performance can be greatly affected by the edge and surface condition of the material, it’s possible to purchase some strip materials with a dressed or rounded edge which can greatly improve performance. However, if the components are punched out of the material the edge finish will depend upon the performance of the tooling.
Flat strip parts can be very complicated in their form, when used inside products such as mobile phones, computers and medical equipment there are a wide variety of shapes all formed from a simple coil or sheet of flat material. 
Many flat strip parts are designed to perform more than one mechanical function thereby reducing the number of components.
The number of different variations of strip parts is virtually infinite, the only obstacles to strip design is the practical limitations of the manufacturing process. A leaf spring operating as a cantilever, with simple to-calculate loads and deflections is probably the simplest strip spring which can be produced.
Many strip parts are, in effect, made up of a number of sections operating as cantilevers. Strip springs are not limited to just simple cantilevers. There are spring washers such as disc springs which are able to provide a high spring rate over a small movement, and constant force springs, used in seat belt retention, devices that are able to provide an almost constant force over a large deflection.
As there is such a wide variety of strip parts, it is difficult to examine them individually. In the strip designing process, it’s always good practice to seek advice of a spring designer. The more simple design will be more economical to manufacture in small quantities. However, more complex parts produced with tooling can also be manufactured at a relatively low cost.
Material hardness is very important when designing a flat strip component as the
hardness of the material affects the minimum bend radius. 
The orientation of the bend on the strip affects the minimum bend radius, if a component requires bends perpendicular to each other with radii close to the minimum bend radius, it’s good design practice to orientate the component by 45º relative to the rolling direction.
If punched holes or slots are too close to the edge of the component or another hole this can cause the hole to deform the edge or the other hole. It’s also best to avoid punched holes or slots on a bend or too close to a bend as this may cause the hole to stretch and affect the smoothness of the bend.
When forming a bend in a spring material it’s important to remember ‘Spring Back’, depending on the hardness of the material, all spring materials will exhibit some form of ‘Spring Back’. 
For instance, when forming a bend of 90º, the material will return to an angle greater than 90º. The spring back will also affect the radius of the bend and this must be taken into account when designing the tooling and consequently when designing the part.
Due to the complexity of strip parts, the calculations of force and stress are much more complex than those for helical compression, extension and torsion springs.
Due to the nature of flat strip design, it’s best to validate the spring design by
manufacturing several samples. These can then be tested to verify the performance.
We can design and manufacture bespoke flat strip products in our specialist tool production area using the latest machinery and techniques.