COMPRESSION SPRINGS BASIC INFORMATION AND TUTORIALS

What Are Compression Springs? How Compression Spring Works?

The four types of ends generally used for compression springs are illustrated below. A spring with plain ends has a non interrupted helicoid; the ends are the same as if a long spring had been cut into sections.

A spring with plain ends that are squared or closed is obtained by deforming the ends to a zero-degree helix angle. Springs should always be both squared and ground for important applications, because a better transfer of the load is obtained.

Table 10–1 shows how the type of end used affects the number of coils and the spring length. Note that the digits 0, 1, 2, and 3 appearing in Table 10–1 are often used without question. Some of these need closer scrutiny as they may not be integers.

This depends on how a springmaker forms the ends. Forys3 pointed out that squared and ground ends give a solid length Ls of Ls = (Nt − a)d where a varies, with an average of 0.75, so the entry dNt in Table 10–1 may be overstated.

The way to check these variations is to take springs from a particular spring maker, close them solid, and measure the solid height. Another way is to look at the spring and count the wire diameters in the solid stack.

Set removal or presetting is a process used in the manufacture of compression springs to induce useful residual stresses. It is done by making the spring longer than needed and then compressing it to its solid height. This operation sets the spring to the required final free length and, since the torsional yield strength has been exceeded, induces residual stresses opposite in direction to those induced in service.

Springs to be preset should be designed so that 10 to 30 percent of the initial free length is removed during the operation. If the stress at the solid height is greater than 1.3 times the torsional yield strength, distortion may occur.

If this stress is much less than 1.1 times, it is difficult to control the resulting free length. Set removal increases the strength of the spring and so is especially useful when the spring is used for energy storage purposes. However, set removal should not be used when springs are subject to fatigue.

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