8th March 2017
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.