Extension and Torsion Springs: Initial Tension, Hooks and Handedness

**It is force wound in, not force applied.** When an extension spring is coiled, the wire can be fed on with the coils pressed hard against each other. The residual twist in the wire keeps them pressed together, and that press has to be overcome before the spring will open at all. **It is a property of the coiling, not of the design.** You cannot order a particular initial tension the way you order a rate; you specify a range and the spring maker hits it by feel and by machine setting. Typical values are between about 10% and 25% of the maximum working load, and the tolerance on it is wide. **It cannot be adjusted afterwards.** Stretching a spring to 'loosen' it removes initial tension by taking a set, and takes free length with it. The spring is then a different spring, permanently. **A compression spring has no equivalent**, and a compression spring that seems to have one is a compression spring with coils touching — which is a fault. **Why it is useful.** A spring that does nothing until a threshold force is a very handy thing: a screen door that stays shut until you push, a latch that holds until it is asked. And on a machine it means the spring is doing nothing at all in the rest position, which is one fewer thing creeping. **Why it is a nuisance.** It makes a single force-and-length measurement useless, it has a loose tolerance, and it drifts in service as the spring relaxes.

**Machine loop or machine hook.** The last coil bent up into the axis and left as a loop, or bent further into a hook. Cheap, standard, made on the same machine as the spring. The bend radius is small, so these are the weakest ends there are — fine for light loads and wrong for heavy ones. **Crossover loop.** The end coil crosses the axis and loops on the far side. Slightly better aligned with the load and no stronger where it matters. **Extended hook.** The end wire is carried out beyond the body before being formed. Lets the bend radius be generous, which is the whole point, and adds length. **Side loop.** The loop is formed to one side rather than on the axis. Easy to hook onto something, and it loads the spring off-centre, which adds a bending moment to the body. Avoid where the load is high. **Threaded insert or swivel end.** A plug screwed into the end coils, with a tapped hole or an eye. No bent wire at all, so the ends are no longer the weak point and the load is taken into the body directly. More expensive, and the correct answer whenever the hook stress is the thing limiting the design. **The choice, briefly:** light duty and cost matters, use a machine loop. High load or many cycles, get the bend radius up or get rid of the bend. And whatever you choose, make the two ends the same orientation unless you have a reason — a spring with its loops at 90 degrees to each other twists as it is pulled.

**Get the hand right first.** A torsion spring must be loaded so that the coils wind TIGHTER. Fit it backwards and the coil opens out under load, the ends unwind, the diameter grows into whatever is around it, and the rate is wrong because the coils are no longer helping each other. Left hand and right hand springs are not interchangeable and look almost identical in the box. **Size the shaft for the wound-up diameter**, as the notebook computes — then take about another 10% of the wire diameter as clearance on top. A shaft that fits the relaxed spring will be gripped at full deflection. **Support both legs properly.** A torsion spring leg bearing on a sharp corner concentrates the load at one point and will eventually notch itself there. Give each leg a flat face or a rounded post to bear against. **Do not let the legs slide.** If a leg can creep along its bearing surface, the effective leg length changes and so does the torque, and the spring will appear to have an inconsistent rate. Capture the ends, or bend them so they cannot travel. **Allow for the spring getting LONGER.** As the coils wind tighter the spring also grows in length by roughly one wire diameter per turn wound on. If it is trapped between two faces it will bind. **Never let it go past its designed turns.** A torsion spring wound too far yields in bending on the inside fibre and takes a set, which shows up as the free angle between the legs having opened out. That is the measurement to check: not the torque, the free angle.