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Helical Spring Design

INTRODUCTION

A helical spring is a spiral wound wire with a constant coil diameter and uniform pitch.   The most common form of helical spring is the compression spring but tension springs are also widely used. .   Helical springs are generally made from round wire... it is comparatively rare for springs to be made from square or rectangular sections.  The strength of the steel used is one of the most important criteria to consider in designing springs.  Most helical springs are mass produced by specialists organisations.  It is not recommended that springs are made specifically for applications if off-the-shelf springs can be obtained to the job.

Note: Excelcals has produced a set of excel based calculations which contain much of the content found on this page. Excelcalcs - Helical springs

 Compression Springs Tension Springs

Nomenclature

 C = Spring Index D/d d = wire diameter (m) D = Spring diameter = (Di+Do)/2 (m) Di = Spring inside diameter (m) Do = Spring outside diameter (m) Dil = Spring inside diameter (loaded ) (m) E = Young's Modulus (N/m2) F = Axial Force (N) Fi = Initial Axial Force (N)      (close coiled tension spring) G = Modulus of Rigidity (N/m2) K d = Traverse Shear Factor = (C + 0,5)/C K W = Wahl Factor = (4C-1)/(4C-4)+ (0,615/C) L = length (m) L 0 = Free Length (m) L s = Solid Length (m) n t = Total number of coils n = Number of active coils p = pitch (m) y = distance from neutral axis to outer fibre of wire (m) τ = shear stress (N/m2) τ i = initial spring stress (N/m2) τ max = Max shear stress (N/m2) θ = Deflection (radians) δ = linear deflection (mm)

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 Index

The spring index (C) for helical springs in a measure of coil curvature ..

For most helical springs C is between 3 and 12

Spring Rate

Generally springs are designed to have a deflection proportional to the applied load (or torque -for torsion springs).   The "Spring Rate" is the Load per unit deflection.... Rate (N/mm) = F(N) / δ e(deflection=mm)

Spring Stress Values

For General purpose springs a maximum stress value of 40% of the steel tensile stress may be used. However the stress levels are related to the duty and material condition (ref to relevant Code/standard). Reference Webpage Spring Materials

Compression Springs- Formulae

a)   Stress

A typical compression spring is shown below

Consider a compression spring under an axial force F.   If a section through a single wire is taken it can be seen that, to maintain equilibrium of forces, the wire is transmits a pure shear load F and also to a torque of Fr.

The stress in the wire due to the applied load =

This equation is simplified by using a traverse shear distribution factor K d = (C+0,5)/C.... The above equation now becomes.

The curvature of the helical spring actually results in higher shear stresses on the inner surfaces of the spring than indicated by the formula above.  A curvature correction factor has been determined ( attributed to A.M.Wahl). This (Wahl) factor K w is shown as follows.

This factor includes the traverse shear distribution factor K d.. The formula for maximum shear stress now becomes.

A table relating KW to C is provided below

C34567 89101112131415 16
Kw1,581,41,311,251,21 1,181,161,141,131,121,111,1 1,11,09

b)   Deflection

The spring axial deflection is obtained as follows.

The force deflection relationship is most conventiently obtained using Castigliano's theorem. Which is stated as ... When forces act on elastic systems subject to small displacements, the displacement corresponding to any force collinear with the force is equal to the partial derivative to the total strain energy with respect to that force.

For the helical spring the strain energy includes that due to shear and that due to torsion.
Referring to notes on strain energy Strain Energy

Replacing T= FD/2, l = πDn, A = πd2 /4 The formula becomes.

Using Castiglianos theorem to find the total strain energy....

Substituting the spring index C for D/d The formula becomes....

In practice the term (1 + 0,5/C2) which approximates to 1 can be ignored

c)  Spring Rate

The spring rate = Axial Force /Axial deflection

In practice the term (C2 /(C2 + 0,5)) which approximates to 1 can be ignored

Compression Spring End Designs

The figure below shows various end designs with different handing.   Each end design can be associated with any end design.  The plain ends are not desirable for springs which are highly loaded or for precise duties.

The table below shows some equations affected by the end designs...

Note: The results from these equations is not necessarily integers and the equations are not accurate.   The springmaking process involves a degree of variation...

 Term Plain Plain and Ground Closed Closed and Ground End Coils (n e ) 0 1 2 2 Total Coils (n t ) n n+1 n+2 n+2 Free Length (L 0 ) pn+d p(n+1) pn +3d pn +2d Solid Length (L s ) d(n t +1) dn t d(n t +1 dn t Pitch(p ) (L 0-d)/n L 0/(n +1) (L 0-3d)/n (L 0-2d)/n

Helical Extension Springs

The formulae provided for the compression springs generally also apply to extension springs.

An important design consideration for helical extensions springs is the shape of the ends which transfers the load to the the spring body.  These must be designed to transfer the load with minimum local stress concentration values caused by sharp bends.   The figures below show some end designs.. The third design C) design has relatively low stress concentration factors.

Extension Spring Initial Tension

An Extension spring is sometimes tightly wound such that it is prestressed with an initial stress τ i . This results in the spring having a property of an initial tension which must be exceeded before any deflection can take place.   When the load exceeds the initial tension the spring behaves according the the formulae above.  This relationship is illustrated in the figure below

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The initial tension load can be calculated from the formula.... T i = π τ i d 3/ ( 8 D)

Best range of of Initial Stress (τ i) for a spring related to the Spring Index C = (D/d)

 C = D/d Best Initial Tension Stress range = τ i (N/mm 2 ) 3 140 205 4 120 185 5 110 165 6 95 150 7 90 140 8 80 125 9 70 110 10 60 100 11 55 90 12 45 85 13 40 75 14 35 65 15 30 60 16 25 55

If the coils in a tension spring are not tightly wound, there is no initial tension and the relevant equations are identical to those for the spring under compression as identified above.

The equations for tension springs with initial tension are provided below

Helical Compression Springs (Rectangular Wire)

Spring Rate and Stress

 Rate (N/mm) = K 2 G b t 3/ (n D 3) Stress (N/mm 2) = K W .K 1 F D /( b t 2 )
 D = Mean Diameter of spring(mm) b = Largest section dimension(mm) t = Smallest Section dimension(mm) n = Number of Active turns F = Axial Force on Spring K 1 = Shape Factor (see table) K 2 = Shape Factor (see table) K W = Wahl Factor (see table) C = Spring Index = D/(radial dimension = b or t)
 b/t 1 1.5 1.75 2 2.5 3 4 6 8 10 K 1 2.41 2.16 2.09 2.04 1.94 1.87 1.77 1.67 1.63 1.6 K 2 0.18 0.25 0.272 0.292 0.317 0.335 0.385 0.381 0.391 0.399

Conical Helical Compression Springs

These are helical springs with coils progressively change in diameter to give increasing stiffness with increasing load.  This type of spring has the advantage that its compressed height can be relatively small.  A major user of conical springs is the upholstery industry for beds and settees.

 D1 = Smaller Diameter D2 = Larger Diameter

Allowable Force on Spring...
Fa = allowable force (N)..τ = allowable shear stress (N/m2)

Stiffness of Spring...

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