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.
In our latest blog Steve Blunt, Director of Quality will review the different types of tooling available when manufacturing flat strip components.
If just a small batch of components are needed, for example as a ‘prototype sample’ it’s possible to produce these without needing any tooling.
Wire-eroding can produce these with standard tooling utilised to form the parts to the required dimensions. It’s a time-consuming process but it allows us to produce parts for testing without our customers having to invest in production tooling.
When a larger volume of components are required they can be blanked out on tooling and formed in subsequent operations on separate equipment. The cost of tooling is relatively small and will increase the production time in comparison to the previous process.
In cases of medium to high volume production, the flat strip component is manufactured complete on a single piece of equipment. This can be achieved through the use of progression tooling.
The developed components are not completely blanked out when producing them on progression and multi-slide tooling. A small section of material is left to carry the part into the subsequent forming stages.
When using progression tools, the material indexed forward to each forming stage. As it progresses through the tool, the component experiences a sequence of forming operations until it’s fully formed. The last stage cuts out the section of material that has carried the component forward.
Although these tools are complicated to design they will produce finished parts at very high speeds resulting in very low unit prices. CAD technology can allow us to design tooling for strip components precisely, and in a cost-effective manner.
Next month our Production Manager, Mike Hales will look at the benefits of prestressing.
Just like wire, there are a wide range of materials available in the production and design of flat strip products.
Strip materials can be obtained in different grades of hardness, and some spring materials are able to be heat treated to increase their strength and hardness. We will look at carbon steels, stainless steels and copper alloys only in this blog as there are such a vast range of materials out there.
The different grades of carbon steel strip are classified according to the carbon content, the manufacturing method, and if heat treatment is used in the process.
When formability is need, annealed carbon steel strip is used. After forming, if a heat treatment is used this will increase the materials strength and hardness. If formability is not required there are heat treated grades of spring steel.
Clock springs and seat belt retaining springs utilise these materials in their hard condition. They provide a good surface finish, uniformity of mechanical properties and precision thickness tolerances. Carbon steel springs will corrode readily so they do require some form of protection if working in a harsh environment.
Stainless steels are widely used for their corrosion resistance, their ability to withstand elevated temperatures and their resistance to relaxation.
They are generally obtained in the hard rolled condition, there is a need to take into account the effect of spring hardness when strip components are designed and manufactured from stainless steels.
Stainless steels are around 20% weaker than heat treated springs steels of the same size. During the cold rolling process the hardness of stainless steel is produced causing the stainless steel to be slightly magnetic.
When it is necessary to have high electrical and thermal conductivity and/or non-magnetic properties it is essential to use copper based alloys.
Another benefit of copper alloys is their good atmospheric corrosion resistance, as the majority of copper-alloy strip components are used as electrical contacts, many copper parts are electro-plated. With its high tin content, phosphor bronze has the high tensile strength in comparison to other copper alloys resulting in it being the most commonly used.
Beryllium copper is a precipitation hardening material and can be purchased in a variety of harnesses, depending on the amount of heat treatment carried out. As one of the most expensive copper alloys, it can be precipitation hardened and used to greater working stresses than the other alternatives.
Next month our Director of Quality, Steve Blunt will look at ‘tooling materials’.
Jon Davies, Sales Manager
Southern Springs & Pressings manufacture a wide range of springs, wire forms, flat strip components and tailor made metal products to meet your needs. We also provide specialist services such as tooling, assembly and design solutions to help deliver your products to market.