 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. Working stress 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. Wear 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 Tim Page, our Managing Director, will look at situations that require the manufacture of conical compression springs.
When space is limited, or a spring application requires a non-linear spring rate, a conical spring is recommended for use. When the spring is coiled it creates the non-linear spring rate, once the spring is deflected the coils begin to contact. As the larger coils move further, they have the lowest spring rate, they will contact sooner. This results in the number of active coils reducing and then increasing the spring rate. A compressed conical spring can also be coiled so the coils lie inside of each other, this will result in the spring having a solid length of one wire diameter. This can be very useful when space is restricted. Designing a conical compression spring is much more complex than that of parallel sided springs. An approximation of the spring’s behaviour can be sought from the calculations, as small changes in the pitch of the spring can result in large changes in the load/deflection characteristics. What are the benefits of nested springs? A nesting spring is one that has one or more springs sitting inside a larger spring. The spring manufacturer can design a spring to achieve more loadbearing material into a fixed space. This will result in the nested spring being able to support a greater load than one spring on its own could withstand. The working life of the spring can be increased as the working stresses within each nested spring are reduced, the length of the spring can also be reduced by the spring manufacturer resulting in less chance of buckling. Tangling in operation can occur if the springs adjacent to each other are not coiled in different directions. In situations where high loads and long fatigue life are needed, particularly in a small space, then nested springs are widely used. Next month our Sales Manager, Jon Davies will look at ‘wire forms’. 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 Mike Hales, our Production Manager reviews the ‘extension spring’ and examines the differences to other springs, such as the compression spring.
The direction of load application and the method of application differentiates the helical compression and helical extension spring. To apply force, special end forms generally have to be used, either utilising the formed end coils or special screwed-in inserts. A more complex end formation will result in greater manufacturing tolerances, when calculating the formulae for extension springs they are very similar to those of compression springs but they include an extra property called initial tension. This is the force which holds together coils of an extension spring. At the beginning part of the curve of an extension spring the tension is not constant throughout the spring, as a force is applied it must exceed the initial tension before the spring deflects. The relationship between the mean diameter and the wire diameter (spring index), the material strength and the manufacturing process will all affect the amount of initial tension that can be coiled. There is always a preferred value of initial tension, however outside of this range can lead to larger manufacturing tolerances. The spring designer can produce springs with large initial tension but with a low spring rate. This will result in a nearly constant load/deflection characteristic, this can be seen in electrical switchgear, tensioning devices and counter balances. In situations where the initial tension is not required, the spring needs to be coiled with a slight pitch. This will then result in a linear spring rate such as the governor spring in a diesel engine. In the design process of an extension spring the maximum working position is at most 85% of the total possible deflection, it’s important to maintain a force with close limits throughout the life of the spring and to also quantify the amount of relaxation that will take place (especially if the temperature is elevated). Heat treating the spring can reduce the amount of initial tension, if an extension spring is heat treated than the maximum allowable stresses must be reduced. Any environmental factors which may affect the spring’s performance (corrosive, elevated temperatures, ability to conduct electricity and magnetic fields) must be factored in when choosing the correct materials. Extension springs should not be stressed as highly as compression springs, if the spring is operating dynamically more care needs to be taken with the spring’s design. We hope you found this post on ‘extension springs’ informative, please do get in touch if you would like to know more, email us or call 01425 611517. In our next blog Tim Page will take a look at Torsion Springs. Mike Hales, Production Manager |
AuthorSouthern 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. Archives
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