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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.  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. Carbon Steels 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 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. Copper alloys 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  In our latest blog Tim Page analyses the ‘torsion spring’ and the difference in their mode of operation in comparison to extension and compression springs.
Torsion springs are stressed in bending, whereas compression and extension springs are stressed in torsion, in effect they are a wound-up cantilever. As a torsion spring can supply or withstand torque they require some form of spring leg. The type of spring leg depends on its application and can vary from a simple tangential straight leg or it can become a lot more complex. In order to reduce manufacturing tolerances and difficulties – the simpler the better! The torsion spring is unable to withstand as great a deflection if it is not operated in a wind-up condition. The spring designer will specify the spring requirements as a load applied to the legs and it’s necessary to convert this into a torque. When torque is applied to a torsion spring the body length will increase by one wire size for every 360 degree of deflection, this can result in the spring binding if not enough space has been allowed, with the application resulting in spring failure. As most torsion springs work over a shaft, if the mean diameter of the spring decreases and not enough clearance is allowed between the shaft and the spring, the spring will bind onto the shaft. Consequently the legs will take all of the torque resulting in a permanent set. When applying tolerances to a spring component it’s essential that this is factored in with the spring design and manufacture. We hope you found this post on ‘Torsion springs’ informative, please do get in touch if you would like to know more, email us or call 01425 611517. In our next blog Jon Davies will review ‘flat strip materials’. Tim Page, Managing Director  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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