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
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
In our latest blog, Steve Blunt examines the popular compression spring.
Widely used throughout the industry, compression springs are very popular due to the relatively simple method of production and their excellent static and dynamic properties. As the most commonly used helically wound spring, this blog will look at compression springs.
Just like extension springs, compression springs are stressed in torsion. Just like a torsion bar, when wound helically they can reduce the space taken up. This does limit the materials which can be used due to their ultimate tensile strength.
The stress limit in torsion is 49% of the ultimate tensile strength when an unprestressed compression spring is manufactured from a BS5216 spring steel. The minimum working position (at most) of a compression spring is usually 84% of the total deflection available. To go further than this would result in the coils contacting each other, reducing the effective number of active coils leading to an increasing spring rate. Also, fretting (wear) can happen between the faces of the coils as they make contact.
If there are two or more working positions the free length tolerances are generally not specified as they will be determined by the tolerances on the working loads (unless they are required for assembly purposes).
Compression springs have a number of options for end coil formation, a closed and ground spring will require more manufacturing time than a simple closed or open spring. However, a ground spring will provide more stability as the wire diameter to mean diameter ratio is high, the end formation can be considered as relatively stable.
The end formation of a spring will affect its tendency to buckle which renders the spring useless in most applications. If the end formation has closed coils, this can reduce the number of active coils (those that deflect and contribute to the spring’s rate).
When measuring the number of active coils with a spring, there can be a certain level of uncertainty, thus the British Standard tolerances do not apply to a spring with less than 3.5 total coils.
There are a number of factors to take into account when deciding on the best type of spring to use, these can include anticipated working requirements; its fittings; the wire diameter; the spring diameter and the cost of the end product:
• Throughout the life of the spring it’s important to maintain a force within close limits, if the temperature is to increase the amount of relaxation that will take place should be quantified.
• Any environmental factors that may affect the performance of the spring or the end product must also be factored in. These can include corrosive environments, elevated temperatures, the ability to conduct electricity and magnetic fields which will all affect the choice of material
• When applying tolerances to a spring component it’s best to consult the spring designer/manufacturer as standard drawing tolerances can increase the cost of the component.
Now that you know a little about the production process of a compression spring, please do get in touch if you would like to know more, email us or call 01425 611517.
Next month our Production Manager, Mike Hales will look at ‘the use of conical compression springs’.
Steve Blunt, Director of Quality
Spring materials are some of the strongest materials used, they are chosen for their strength as they are designed to work to greater working stresses than most other components. This can be seen in a helically wound compression spring which can be stressed to 70% or more of the ultimate tensile strength of the material.
Many spring materials are required to work in extreme environments such as high or low temperatures and they will be pressurised under large loading.
Often used for their magnetic and electrical capabilities, there are many materials available in the production of a reliable and long-lasting spring. We will look at the most commonly used materials in wire springs (strip materials will be looked at in a later blog).
The most popular material of choice is high carbon spring steel, they are relatively low-cost and are widely available. They are also the strongest materials to be used in spring production.
If the wire size is 1.60mm the range of material grades taken from BS5216 could be used as follows:
The material grades also refer to the surface finish and the dynamic qualities are:
EN10270-1 is the most up to date specification for high carbon spring steel however BS5216 is still widely recognised and used in our industry.
During the drawing process the mechanical strength is obtained as the size of the wire decreases, as this happens the ultimate tensile strength of the material increases.
Some of the above grades can include a zinc or aluminium/zinc coating when pre-drawn. This will allow for sufficient corrosion protection for non-arduous applications, otherwise the components will need electroplating to produce a corrosive protection.
Pre-hardened and tempered steels
Low alloy or carbon pre-hardened and tempered steels are another option for spring materials, they can be drawn annealed and then hardened later on during the wire manufacturing process allowing them to be stronger than cold drawn materials above the size of 2.00mm.
The hardening process results in the mechanical strength of these materials allowing the ultimate tensile strength to not be dependent on wire size. Larger section materials can actually obtain a higher ultimate tensile strength.
With excellent static and dynamic properties they are prone to corrode easily without any surface protection.
If the materials are pre-hardened and tempered there are many standards relating to whether they are carbon steel or one of the many low alloy steels. The most popular alloys used are silicon chrome (BS2083 685A55) or chrome vanadium (BS2083 730A65).
Where the corrosive or relaxation resistance requirements are too great or the working temperatures are too high stainless steels are used.
With many grades of stainless steel available they vary in their mechanical properties and corrosion protection. They tend to be approx. 10% weaker than spring steels of the same size, however there are precipitation hardened grades that are nearly of equivalent strength.
The stainless grades used are usually 301S26, 302S26 both similar having 17%/18% chromium and 7%/8% nickel respectively. However, for greater corrosion resistance, especially in salt water, grades 316S33 and 316S42 are used as they have molybdenum added for improved resistance to chlorides. Stainless steel grades are covered by EN10270-3 however BS2056 is still widely used in our industry.
Stainless steels go through a cold drawing process, just as carbon steels, to increase their tensile strength. During this process the materials become slightly magnetic, if low magnetic permeability is required there are two stainless grades that can be used, these are 305S11 and 904S14,
Where greater strength is needed, precipitation-hardened stainless steels can be used. Once the springs are manufactured they are heat treated at 480°C. This causes small precipitates to grow through the material, increasing the ultimate tensile strength. For example, in the as drawn condition, 1.00mm wire has a minimum ultimate tensile strength of 1710 N/mm2, while after heat treatment this is increased to 2030N/mm*. A cost of this is slightly inferior corrosion performance to 302S26.
Now that you know about the correct materials for a spring, please do get in touch if you would like to know more, email us or call 01425 611517.
Next month our Director of Quality, Steve Blunt will look at ‘the production of compression springs’.
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.