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Extension Springs

Extension springs have a wide range of applications and uses. They are closed-coiled spring that are used where a force is required to pull between two points - the opposite of a compression spring, where the force is repulsive. As for compression springs, they are relatively easy to design and manufacture although the type of end hook or loop can be important. For more detailed information on extension springs contact our design department or consult BS1726 : Part 2 : 1988.

Extension springs can be made from round or square wire, although round is considerably cheaper and easier to manufacture. We keep a vast selection of material sizes and types in stock, from standard spring steels to nickel alloys, all of which can be used to manufacture extension springs.

Extension Spring

When ordering an extension spring, consider the following factors:

Wire size - Specify either metric or Imperial measurements or Standard Wire Gauges. We stock all the standard wire sizes and also a large range of non-standard wire sizes for specific requirements.

Body Diameter - Specify the outside diameter of the main part of the spring, or the size of the hole it is to go into (if any). Usually, the diameter of the end loops or hooks is the same as that of the body of the spring but if not, specify their diameter too.

Number of coils - The number of active coils is inversely proportional to the rate (strength) of an extension spring. A large number of coils will result in a fairly weak spring. A small number of coils will give a relatively strong spring.

Extension Spring

Length - Specify the free length of the spring - either the body length (BL), inside loop length (FL IL) or the free length overall.

Initial Tension - This is the force required to just open the coils of an extension spring. This property of the spring can be varied to a certain extent if required - contact our design department for more details.

Rate - The rate (strength) of the spring depends on the above factors and can easily be calculated. However, sometimes the rate is the starting point from which the design can be based. This is particularly useful if the forces required at particular working lengths are critical. Rate is defined as the force that has to be applied to the spring in order to produce a unit deflection and is measured in either Newtons per mm (N/mm) or pounds per inch (lb/in).

Hook / Loop types - The following are examples of end arrangements for extension springs:

Machine loop. Crossover loop. Extended round loop.
Machine loop. Crossover loop. Extended round loop.
example
Coned end with swivel loop. Plain-end spring with threaded insert.
Coned end with swivel loop.
example
Plain-end spring with threaded insert.

The most common loops/hooks are the machine and crossover loop. More "exotic" loops or hooks are generally not used unless there is a specific reason for doing so.

Index - This is the ratio of the mean diameter to the wire diameter. A low index indicates a tightly wound spring: a relatively large wire size wound around a relatively small diameter mandrel giving a high rate. A high index indicates a more loosely wound spring: a relatively small wire size wound around a relatively large diameter giving a low rate.

Stresses - Extension springs can take a permanent set if stretched beyond the safe deflection. Our computer programs can warn our customers what this maximum safe deflection is and prevent damage to the spring during operation. For more information contact our design department.

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