 # Spiral Spring Design

### Spiral Spring Design

INTRODUCTION

A spiral spring consists of a strip or wire wound in a flat spiral .     This is subject to a torque to produce an angular deflection.     A typical spiral spring is a clock spring

Nomenclature

 D = Outside diameter of spring (m) b = Width of spring strip (m) d = Inside diameter of spring (m) t = thickness of spring strip (m) n = Number of turns of spring k = Spring rate = M /θ Nm/rad. E = Young's Modulus (N/m2) M = Moment/torque on spring = F.D / 2(Nm) L = Length of strip (m) G = Modulus of Rigidity (N/m2) I = Second moment of intertia of spring strip (m4) F = Force to deflect spring N y = distance from neutral axis to outer fibre of wire/strip = y/2 (m) θ = Deflection (radians) α = Tensile/compressive stress resulting from spring deflection (N/m2

Note: metres (m) have been shown as the units of length in all of the variables above for consistency.   In most practical calculations milli-metres will be more convenient. Spring Rate

The spring rate k is defined on this webpage as the torque (Nm)per unit angular deflection (θ ). Spiral Spring Formulae

length of Strip Spring Rate Spring sureface stress 