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An extension spring, or which is also known as a tension spring, is a helically wound to oppose resistant forces having consistent mechanical energy to return to its no load, compressed position. The ends are attached to components intended to move apart providing a reliable return force. This is a type of coil spring that is closely wound when at rest. They are wound in either a clockwise or counter-clockwise direction. The load is applied laterally; therefore the wind direction is usually unimportant. The internal force pulling the coils together is known as initial tension. As the extension spring is being pulled outward, the resistance to the initial tension increases. It is the initial tension that determines the load capacity of the extension spring. The initial tension can be engineered to meet your load requirements. Extension springs can be overextended and will then not return to the original at rest position so proper sizing is important. Types of Ends Extension springs offer a variety of ending inserts, loops, hooks, and eyes to aid in their attachment to the component. The most common ends are: • Full loops • Threaded inserts • Rectangular ends • Teardrop-ends • Expanded and reduced eyes • Long round end hooks over center • Half loops • Machine cut plain loops • Double full-loop over center • Small offset hook • Machine half hook • Coned end with swivel bolt • Coned end with swivel hook • Drawbar • Y-hook over center Each of these helically coiled spring end is designed to cater to a specific situation. How They Work, Applications Extension springs resist a force pulling the spring apart. To accomplish this, it has to have a hook or other special “end” on each side of the spring to connect it to the load. A predefined load can be built into the spring when it is wound. This is a load that must be overcome before the spring starts to deform – that is stretch. These defined loads are part of the design specification of an extension spring. Accordingly these springs are used in applications that need a force to return the unit from an extended position. A trampoline is a good example of extension springs in use. The springs all around the surface extend as the load is applied, store the energy applied and return the surface and the jumper to the upper rest position – with a bounce! Other common applications are screen and storm doors, garage doors, toys, farm machinery and thousands of other uses. The plumbing snake is a common albeit mechanically unconventional use for these springs. A long, plain end extension, tension spring is fed through pipes to dislodge pipe congestion.
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