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