Spiral Spring Handing and End Fixing - Assembly Traps
Spiral Spring Assembly Traps
,Wire Forming End Fixing
,Spring Handing Assembly Traps
Spiral Spring Handing and End Fixing: Where Assembly Goes Wrong
A spiral spring is a simple part with two critical interfaces: how it is wound, and how both ends are held. When a spiral spring fails in the field or a batch is rejected at assembly, the coil itself is rarely the cause. The cause is normally that the spring was wound the wrong way, or that the end fixing was designed without the clearance the spring needs to work.
This article sets out the two parameters that should be on every spiral spring drawing, the three common end forms, and the assembly errors we see most often.
Handing: Left or Right Wind
Handing describes the direction the strip or wire travels as it coils inward from the free outer end. Viewed from the front face, a spring that winds clockwise going inward is right hand; anticlockwise is left hand. The two are not interchangeable: mounting a right hand spring in a housing designed for left hand reverses the direction of the reaction torque, so the load transfers to the wrong end fixing and can double the stress at that point.
Handing is easy to miss because a mirror image spring often still fits the housing. It simply works badly, and the failure appears later as an end break rather than as an obvious assembly fault.
Our recommendation: state the handing on the drawing as well as on the purchase order, and mark the outer end with a paint dot during packing for high volume assembly lines.
Three End Forms and Their Clearances
Straight end in a slot
The simplest and cheapest form. The straight leg drops into a milled slot; the slot width should be 0.1-0.2 mm wider than the wire or strip thickness, no more. A slot that is too tight grips the leg and prevents the spring from unwinding freely, which shows up as a torque spike at the start of travel. A slot that is too loose lets the leg hammer and wear the slot corner, which is a common cause of end breakage after a few thousand cycles.
Looped end over a pin
A closed loop anchored on a pin gives a defined pivot and tolerates small alignment error. Loop inside diameter should clear the pin by 0.1-0.3 mm. If the loop is tight on the pin, the spring cannot rotate as it winds, and the loop root becomes a fatigue site. If the clearance is excessive, the spring shifts radially and the torque curve becomes inconsistent from unit to unit.
T-end or hooked end in a keyway
Used where the spring must transmit higher torque. The bend radius at the root of the T should not be smaller than half the wire or strip thickness; sharper bends crack during forming or shortly after. The keyway should be deburred, because a burr at the keyway edge acts as a chisel on the end at every cycle.
Arbor and Inner End Fit
The inner end locates on the arbor or shaft. Clearance of 0.1-0.3 mm is the normal working range for wire diameters up to 3 mm. Below 0.1 mm the spring grips the arbor, torque rises and the innermost coil yields. Above 0.3 mm the inner end slips, and the spring loses pre-tension, so the first part of the travel delivers nothing.
Specification Window
| Parameter | Range / Note |
|---|---|
| Wire or strip thickness | 0.15 - 12 mm |
| Grades | 201, 302, 304, 316, 430 |
| Handing | Left or right, stated on drawing |
| End forms | Straight, looped, T-end, to drawing |
| Slot clearance | 0.1 - 0.2 mm over wire thickness |
| Arbor clearance | 0.1 - 0.3 mm |
| Root bend radius | Not less than 0.5 x wire thickness |
| Surface | Bright, passivated, or as specified |
Applications
- Cable retractors and constant force take-up reels
- Appliance cord and hose management
- Seal and gasket energising springs
- Instrument and timer mechanisms
- Medical device and diagnostic closures
Four Assembly Errors We See Repeatedly
- Handing reversed during a supplier change, with no drawing note to catch it
- Slot width copied from a previous part and 0.4 mm too wide, causing leg hammer
- Keyway left with a burr from machining, cutting the end within days
- Arbor machined to nominal spring inside diameter with no clearance, so the spring grips and torque is 20-30% above target
What to Put on the Drawing
Handing, end form and its clearance, arbor clearance, root bend radius, working torque at a stated number of turns, and the direction of rotation in service. With these six items the spring can be made right the first time instead of being diagnosed after a rejected assembly lot.
TOPONE has produced stainless steel wire since 1993 and supplies wire and formed spiral springs from 0.15 mm to 12 mm, so wire tolerance and end form are controlled in one production chain.
