Compression springs push things apart. Extension springs pull things together. But extension springs have a peculiar property that compression springs don't: they resist being pulled apart even before you start pulling. That's initial tension, and it's the defining feature that separates extension springs from every other spring type.

An extension spring is a helical coil wound with its coils touching each other. During winding, the wire is twisted such that the coils press against each other with a residual force. To open the spring at all, you must overcome this preload — typically 10-30% of the spring's maximum working load. Below that force, the spring doesn't extend; it just sits there like a rigid rod.

The force equation reflects this: F = F₀ + kx, where F₀ is the initial tension and k is the spring rate. Compare that to a compression spring's simple F = kx. That non-zero y-intercept trips up engineers who model extension springs as ideal linear springs and can't figure out why their mechanism binds at rest.

The end loops are the weak point. Unlike compression springs (which just bear against flat surfaces), extension springs need hooks, loops, or threaded inserts to transmit load. A standard machined half-loop or full loop over center creates a stress concentration at the transition bend where the coil straightens into the loop. Fatigue failures almost always start here — not in the body of the spring. For high-cycle applications, use swivel hooks or threaded plugs to reduce bending stress at the ends.

Real-world example: the screen door closer. When the door is fully closed, the spring is at its installed length — still under initial tension, pulling the door snug against the frame with maybe 5 lbf even at rest. Open the door 90°, and you stretch the spring another few inches, adding another 15 lbf via the spring rate. Let go, and both forces pull the door shut. Without initial tension, the door would flop loosely against the frame when closed.

Rule of thumb: for reliable operation, install extension springs so that the working range stays between 20% and 80% of maximum extension. Below 20%, you're operating near the initial tension threshold where behavior is inconsistent. Above 80%, you're stressing the end loops toward failure. Also: never fully compress an extension spring by pushing on it — the coils aren't designed to bear compressive load and will buckle sideways.

Extension springs also can't be shot-peened effectively (the touching coils shield each other), so their fatigue life is inherently lower than equivalent compression springs. If your application demands millions of cycles, consider a compression spring with a reversed mechanism instead.