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 preload at zero deflection.
This build-in load, called initial tension, can be varied within limits, decreasing as the 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.
Extension Spring End Specifications
Most extension spring failures occur in the area of the end. To maximize the life of the extension spring, the path of the wire should be smooth and gradual as it flows into the end. Tool marks and other stress concentrations should be held to a minimum. A minimum bend radius of 1 1/2 times the wire diameter is recommended.
EXTENSION SPRINGS – Custom made – bespoke design.
Our extension springs are made to customer supplied specifications/drawings/ samples for applications such as agricultural springs, horseboxes, ramp springs, trampoline springs and many applications requiring a return or extension spring with loops / hooks at the ends. ‘
Materials are generally Stainless steel type 302 or 316, or high carbon alloys