Flat strip parts can be very complicated in their form. Inside many 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 obstacle to strip design is the imagination of the designer, and the practical limitations of manufacture.
The simplest strip spring is probably a leaf spring operating as a cantilever, with simple-to-calculate loads and deflections. 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 rates 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.
Due to the wide variety of strip parts it is difficult to discuss them in any great detail, also the complexities of many of the equations fall outside the bounds of this Guide. When designing a strip component it is good practice to ask the advice of a spring designer.
The simplest part to produce is the most economical to produce in small quantities. But even complex parts, when produced on production tooling, can also be produced at low cost.
Good Design Practice
Material hardness is very important when designing a flat strip component. The hardness of the material affects the minimum bend radius. Below is a table of ‘minimum bend radius’ for a number of materials.
t = thickness.
Note: The direction of rolling is along the strip.
The bend radius refers to the inside bend radius.
Going below the above figures would prove to be difficult, and may lead to cracking of the material on the outside bend radius of the material.
As can be seen, 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 the minimum bend radius, it is good design practice to orientate the component by 45º relative to the rolling direction.
Avoid punched holes or slots too close to the edge of the component or another hole.
This can cause the hole to deform the edge or the other hole.
Avoid punched holes or slots on a bend or too close to a bend. This may cause the hole to stretch and affect the smoothness of the bend.
When forming a bend in a spring material it is 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. This must be taken into account when designing the tooling and consequently when designing the part.
It is best to consult the spring designer/manufacturer when applying tolerances to a spring component. Standard drawing tolerances can increase the cost of the component.
Article written by David Banks-Fear and published on MDF by kind permission of Southern Springs & Pressings Limited.
David Banks-Fear is a Mechanical Design Forum Group member. He is a technical author and consultant design engineer with nearly 40 years of experience. He and his design team are available to assist with any technical design issues with springs, pressings and precision engineered parts. Email: [email protected]