Flat-strip spiral springs: the limits that define a good design

The flat-strip spiral spring, also called a clock spring or motor spring, looks simple: a steel strip wound in a spiral between an arbor and a housing. But anyone who has produced one knows the design lives on crossed limits: the strip has to fit the space, deliver the requested torque, accept the winding turns and not take a permanent set along the way.

Those limits have names, and working with them explicit changes the quality of the conversation between design and factory.

Set torque: the stress ceiling

Load the strip past a certain torque and it never returns to its free position: it has taken a permanent set. That ceiling comes from the strip section (width times thickness squared) and the material's allowable bending stress.

The pair that matters: the application's working torque must sit comfortably below the set torque. A 10% cushion already absorbs material batch variation.

Turns to solid: the space ceiling

As the spring is wound, every turn of charge pulls one more coil of strip into the pack. The radial space between arbor and outer diameter holds a finite number of coils: strip thickness times coil count cannot exceed the available annulus.

Two design numbers fall out: the space fill at rest (how much of the annulus the strip already occupies) and the turns to solid (how many winding turns fit before the strip stacks coil against coil). Fills around 50% usually balance travel and material.

The dangerous crossing: pack first, or set first?

The two ceilings cross in a treacherous way. If the torque needed to pack the strip solid is higher than the set torque, the spring can be overstressed before it reaches its physical end of travel: nothing stops it from crossing the limit. In those cases the mechanism needs a travel stop, or the design needs to change.

Our studio computes both torques and warns automatically when the packing torque exceeds the set torque, along with the travel-stop recommendation. It is the kind of check that prevents the classic field return: the spring that came back bent.

Strip proportions and caging

• Width over thickness between 3 and 15: below that the section turns almost square and loses efficiency; above it, the strip tips over during winding.
• Length over thickness between 200 and 3000: strips too short barely deflect; too long wastes material and space.
• Usable travel up to 2 or 3 turns: beyond that the coils can buckle out of plane and tangle, the so-called caging. More free coils or more outer diameter solves it.

Everything checked while you design

In the studio these limits appear as engineering results (set torque and turns, packing torque and turns, space fill) and as manufacturability warnings whenever a proportion leaves its range. The 3D view shows the spiral with the real strip, and the quote comes from the same screen. Designing inside the limits stops depending on the memory of whoever got burned last.