24 Nov

Helical or coil springs designed for tension


     COMPRESSION SPRINGS are open-coil helical springs wound or constructed to oppose compression along the axis of wind. Helical Compression Springs are the most common metal spring configuration. Generally, these coil springs are either placed over a rod or fitted inside a hole. When you put a load on a compression coil spring, making it shorter, it pushes back against the load and tries to get back to its original length. Compression springs offer resistance to linear compressing forces (push), and are in fact one of the most efficient energy storage devices available.

Springs are unlike other machine/structure components in that they undergo significant deformation when loaded – their compliance enables them to store readily recoverable mechanical energy. In a vehicle suspension, when the wheel meets an obstacle, the springing allows movement of the wheel over the obstacle and thereafter returns the wheel to its normal position. Another common duty is in cam follower return – rather than complicate the cam to provide positive drive in both directions, positive drive is provided in one sense only, and the spring is used to return the follower to its original position. Springs are common also in force- displacement transducers, eg. in weighing scales, where an easily discerned displacement is a measure of a change in force. 

The simplest spring is the tension bar. This is an efficient energy store since all its elements are stressed identically, but its deformation is small if it is made of metal. Bicycle wheel spokes are the only common applications which come to mind. 
Beams form the essence of many springs.  The deflection   δ of the load   F on the end of a cantilever can be appreciable – it depends upon the cantilever’s geometry and elastic modulus, as predicted by elementary beam theory. Unlike the constant cross- section beam, the   leaf springshown on the right is stressed almost constantly along its length because the linear increase of bending moment from either simple support is matched by the beam’s widening – not by its deepening, as longitudinal shear cannot be transmitted between the leaves.

 The shortcoming of most metal springs is that they rely on either bending or torsion to obtain significant deformations; the stress therefore varies throughout the material so that the material does not all contribute uniformly to energy storage. The wire of a   helical compression spring – such as shown on the left – is loaded mainly in torsion and is therefore usually of circular cross- section. This type of spring is the most common and we shall focus on it. 
The (ex)tension spring is similar to the compression spring however it requires special ends to permit application of the load – these ends assume many forms but they are all potential sources of weakness not present in compression springs. Rigorous duties thus usually call for compression rather than tension springs.
A tension spring can be wound with initial pre-load so that it deforms only after the load reaches a certain minimum value. Springs which are loaded both in tension and in compression are rare and restricted to light duty.

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Posted by on November 24, 2011 in Final Year Projects


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