Extension Springs

Extension springs are closed coiled helical springs and can come in a wide variety of sizes. They absorb and store energy as well as offering resistance to a pulling force. They are wound to oppose extension. Extension springs are often tightly wound in a no-load position and are attached at both ends to other components with hooks, eyes or other attachments. The initial tension, which is the effort required to pull the coils apart, determines how tightly together an extension spring is coiled. This calculation formula is the same one used with compression springs. It is measured by checking two load or deflection points. Initial tension is necessary in extension springs to help maintain control of free length. The best range to keep initial tension is between 40% and 80% of the material strength divided by the spring index. The designer can help select the proper load value to correspond to the stress in this range.

The loops or hooks on either end of the extension spring can come in a variety of shapes as well. The most used types are called German (twist) and English (cross) but there are many variations of these two. These designs can be made completely on a spring machine, however many manufacturers still use secondary equipment.

Unlike compression springs, extension springs do not have a solid stop to prevent overloading. Because of this, stress levels are lower for extension springs than those of compression springs. An extension spring that fails will drop whatever load has caused it to fail, because of this, extension springs are generally given three-fourths of the maximum working stress of other, similar compression springs.

Extension springs are used in various products, a few of their common uses include- automobile interiors and exteriors, garage door assemblies, vise-grip pliers, carburetors, trampolines, washing devices, farm machinery and some toys.

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