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 some instances a spring is required to operate in a corrosive environment which means some form of surface protection is needed. The options of protection available will depend on the application of the spring.
If cost is an issue, or where the material needs to be of a required strength, than it may not be possible to design a spring with materials that will not corrode.
An obvious choice is the use of nickel alloys, they are excellent for corrosion resistance, however the cost can be prohibitive. A simple method is to simply oil or grease the springs, this should give a sufficient level of corrosion protection for springs in transit or when in storage (as long as the storage conditions are not too testing).
Another effective method of protecting springs from corrosion is through either plastic coating or painting. The only issue with using either of these methods is that the protection is only effective until it is damaged, it will then be liable to corrosion underneath the finish.
The easiest option is to manufacture the spring from carbon steel, wire drawn with a galvanised coating which should be sufficient enough in most circumstances, if not, a better protection is required. A popular method is to use a metallic finish which can be obtained by electroplating the spring. In order to ensure the maximum corrosion resistance it’s vital to use the correct electroplated metal.
Zinc plate and cadmium (rarely used due to its toxicity) corrode in preference to steel and will protect the surface even when the coating is damaged.
Nickel, copper and chromium plate will lead to the steel corroding when damaged so is not recommended. Nickel plate is only recommend when the component needs to undergo soldering and so it’s widely used in the electronics industry. There is a risk of hydrogen embrittlement when electroplating which can lead to component failure when it’s loaded.
A de-embrittlement process must be carried out in order to minimise the risk, this process involves the component being held at an elevated temperature of 190-200°C for up to 24 hours to allow the hydrogen to dissipate.
If low alloy spring steels such as BS2083 685A55 are to be used then they should not be electroplated under any circumstances due to the high risk of hydrogen embrittlement.
In contrast, a mechanical zinc or zinc alloy plate will provide zero risk of hydrogen embrittlement and an equally effective corrosion resistance, all be it costly.
The other options available include coating the spring with a resin impregnated with zinc flakes, they can have either a black or silver finish. They provide a superior protection to mechanical or electroplating and avoid the risks associated with hydrogen embrittlement.
One of the key factors that a spring user requires is reliability, there has been much research into the behaviour of springs under fluctuating loads to ensure a spring is long-lasting.
If a spring will only be operated for less than 10,000 cycles during its lifespan then fatigue does not need to be taken into account. Any spring that will be performing for more than 10,000 cycles can be affected by fatigue and this needs to be factored in during the design process.
The factors that can affect the fatigue performance in springs include working stress, material surface quality and wear.
When in operation, springs will generally work between two fixed positions. In order to predict the working life of the spring the working stress at these positions can simply be calculated.
A common cause of failure in an extension spring is when the loop of the spring breaks off in the area where the hook meets the body of the spring, this is the area of highest stress. Loops are subjected to bending and torsion stress, when they are formed in extension springs small tooling marks are unavoidable. These marks can increase the likelihood of a failure at this point, sometimes loops are formed using bends that are too small and a small radius can increase stress. Different fatigue performance can be caused by different types of end loop, this can be solved by using a loop with a transition radius between the spring body and the end loop of approximately the body radius.
When an extension spring is required to work dynamically, it must be noted that an extension springs can have around 20% lower performance with regard to fatigue than compression springs.
Material surface quality
The material surface quality is very important when trying to avoid the risk of spring failure, fatigue cracks generally originate from the surface of the material, so the greater the surface quality the more chance of improved fatigue performance.
There are a number of methods that can improve the surface quality, the most popular of these is shot-peening. This involves firing small rounded beads of material at the surface of the spring which will lead to a small residual compressive stress this then lowers the chance of a fatigue crack appearing.
The process of shot-peening is generally carried out only on compression springs and large leaf springs, otherwise the shot could become trapped in the coils of close wound torsion and extension springs. Also, the inside face of the coils would not be peened and this would eliminate the benefits of the process.
The wear of springs can be caused in a manner or ways, when operating it’s important that the maximum deflection of the spring should not exceed 85% of the available deflection. If a spring is working close to its solid (coil bound) length, the number of active coils will reduce due to coils coming into contact with each other and there is a chance that this could cause wear to the contacting faces.
Another cause of wear is when a spring works over a shaft or in a bore, if a spring is able to contact either the shaft or the wall of the bore then the wear can lead to early fatigue. If the inside diameter of the spring is worn this can increase the failure as this is where the working stresses are at their greatest.
Torsion springs have a lower fatigue performance in comparison to compression and extension springs, mainly due the friction and wear between the spring and that shaft. It’s hard to eliminate this with torsion springs however it can be reduced with an effective design.
There are other factors that can affect the fatigue performance such as corrosion, material cleanliness and speed of operation.
In our latest blog Mike Hales, our Production Manager, will look at the benefits of prestressing when manufacturing springs.
Prestressing can increase the load-carrying capability and the spring’s ability to withstand stress ultimately improving the fatigue life of a spring.
Dimensional changes will take place when a spring is prestressed during its manufacture. The process of prestressing a compression spring is relatively simple. The manufacturing process involves the spring being coiled, stress relieved and ground. After this process has taken place, the spring is placed on a press and compressed to a fixed or solid position which is greater than its maximum working position.
Repeating this process at least three times will result in the spring being shorter than the coiled spring, with the correct initial set-up it will be possible to achieve the required final length.
Tension and torsion springs can also utilise prestressing during the spring manufacturing process. When it comes to the manufacture of tension springs, the amount of initial tension is reduced and is therefore not often carried out.
In order to successfully prestress torsion springs, special jigs are required. The leg relationship will change (the number of coils slightly increases).
As prestressing is an additional operation in the manufacture of a spring, this will increase its unit cost. However, the benefits of prestressing during the manufacturing process will generally outweigh the additional cost.
Next month our Managing Director, Tim Page will look at the use of conical compression springs.
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.
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.