Extension Springs, or tension springs, are springs which absorb and store energy by offering resistance to a pulling force. Typically, extension springs are made from round wire and are close wound with initial tension. Extension applications include tape cassette players, balance scales, garage doors, washing machines and applications which requiring various types of tensioning devices. Various types of ends are used to attach the extension spring to the source of the force.
Initial Tension of extension springs
Most extension springs are wound with initial tension. This is an internal force that holds the coils tightly together. The measure of the initial force and just start coil separation. Unlike a compression spring, which has zero load at zero deflection, an extension spring can have a prelude at zero deflection.
This build-in load, called initial tension, can be varied within limits, decreasing as the tension spring index increases. There is a range of stress (and, therefore, force) for any spring index that can be held without problems. If the designer needs an extension spring with no initial tension he should design the spring with space between the coils.
Unlike compression springs, extension springs don't have a solid stop to prevent overloading. Because of this design stress levels are lower for extension springs than for compression. A special type of extension spring is called a drawbar spring, it has a solid stop and is a type of compression spring with special hooks.
Measuring Rate of extension springs
Extend spring to a length (L1) such that definite coil separation occurs and measure the load (P1).
Extend spring further to a second length (L2) and again measure the load (P2).
Calculate rate by dividing the load difference by the length difference in: R = (P2 - P1)/(L2- L1)
Measuring Initial Tension - Simplified Way for extension springs
Establish exact initial length (Li) of spring by applying enough load to get slack out but not enough to separate coils.
Extend spring to length (L1) sufficient to open coils and measure load (P1).
Extend spring to length (L2) such that second deflection equals first deflection and measure load (P2).
Since the two deflections are equal, proof can be shown that initial tension is as follows:
Pi = 2P1 - P2
Graphic layout of Specificiations for Extension Springs