All springs are constructed to have an initial tension, that force that keeps the coils together in a set position. We know, according to Hooke’s Law, when a spring is stretched or compressed the necessary force to do so will vary in a linear way, proportional to its displacement.

The tension force is the force that is transmitted through a string, rope, cable or wire when it is pulled tight by forces acting from opposite ends. The spring force is the force exerted by a compressed or stretched spring upon any object that is attached to it.

The spring is in equilibrium (non accelerating), hence T=R. Thus Tension is same as the Restoring force.

Extension Spring (also known as Tension Spring) – A spring whose length, in the direction of the applied force, increases under the application of that force.

The slope of a straight line from the origin to the value of the load at the point S is the model spring constant k1. A spring with a large initial tension would exert a larger restoring force for a given elongation than one with the same spring constant and small initial tension.

The main function of a tension spring is to allow for an outside force to create tension, and then use this energy to pull the components back together by reverting to its original shape. Tension springs are typically wound very tightly and are coiled closely together when at rest.

Formula for Initial Tension

1. d = Wire size.
2. D = Mean Diameter.
3. N = Number of active coils.
4. Rf = Rate of extension spring (in lbs./inch)
5. S = Stress (lbs. / Sq. Inch)
6. P = Load (lbs)
7. M = Moment (inch-lbs.)
8. D / d = Index correction (developed from Wahl Factor)

The load applied to extension springs stretches its tightly wound coils apart and spreads them open. Tension springs resist the pulling force that is placed upon them and this creates a sling back effect when the force is removed, causing it to return to its original shape.

How are tension springs made? Even though tension springs differ from compression springs, they are usually made from similar materials including copper, titanium and steel. Steel is the most durable of these, and there are different types of steel used, such as; stainless, cold rolled and alloy steel.

The most common spring types include:

• Compression Springs.
• Extension Springs.
• Torsion Springs.
• Constant Force Springs.
• Belleville Springs.
• Drawbar Springs.
• Volute Springs.
• Garter Springs.

Here are some typical places you will find springs:

• Ball point pen.
• Car shocks.
• Wire bound notebooks.
• Mattresses.
• Wind up toys.
• Pool diving board.
• Watches.
• Door locks.

Following are the five different types of Suspension springs used in automobile vehicles:

• Leaf springs.
• Coil spring.
• Torsion spring.
• Air bags.
• Rubber spring.