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HomeMachine_Parts_Index Spring Index

Helical Spring Design

Introduction.... Nomenclature.... Spring Index .... Spring Rate.... Spring Stress Values.... Compression Spring Formula.... Compression Spring End Designs .... Extension Springs.... Rectangular Section Springs.... Conical Helical Springs....

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 thecompression spring but tension springs are also widely used. . Helical springs are generally made from round wire... it is comparativelyrare for springs to be made from square or rectangular sections. The strength of the steel used is one of the most important criteria toconsider in designing springs. Most helical springs are mass produced by specialists organisations. It is not recommended that springsare 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

Nomenc lature

C = Spring Index D/d

d = w ire 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 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 f ibre of

w ire (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)

L = length (m)

Note: metres (m) have been shown as the units of length in all of the variables above for consistency. In most practical calculationsmilli-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 arerelated 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 maintainequilibrium 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 theformula 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

C 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Kw 1,58 1,4 1,31 1,25 1,21 1,18 1,16 1,14 1,13 1,12 1,11 1,1 1,1 1,09

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 ... W hen forc es ac ton elastic systems subjec t to small displac ements, the displac ement c orresponding to any forc e c ollinear withthe forc e is equal to the partial deriv ativ e to the total strain energy with respec t to that forc e.

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. Theplain 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 processinvolves 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

Helic al 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 springbody. These must be designed to transfer the load with minimum local stress concentration values caused by sharp bends. Thefigures below show some end designs.. The third design C) design has relatively low stress concentration factors.

Extension Spring Init ial 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 initialtension the spring behaves according the the formulae above. This relationship is illustrated in the figure below

>

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/dBest 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 thespring under compression as identified above.

The equations for tension springs with initial tension are provided below

Helic al Compression Springs (Rec tangular W ire)

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 turnsF = Axial Force on SpringK 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 1.0 1.5 1.75 2.0 2.5 3.0 4.0 6.0 8.0 10.0

K 1 2.41 2.16 2.09 2.04 1.94 1.87 1.77 1.67 1.63 1.60

K 2 0.18 0.25 0.272 0.292 0.317 0.335 0.385 0.381 0.391 0.399

Conic al Helic al Compression Springs

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

Links to Spring Design

1. Excelcalcs - Helical springs ...A excel based Spreadsheet Calculation tool2. tpcdayton ...Anti-Vibration Mountings3. Acxess Springs ...Information on Design, Materials etc (Imperial)4. Harris Springs ...Information on Design, Materials etc (Imperial) and a catalog5. Earthlink- How to Make Springs ...Good simple comprehensive information6. Lee Springs ...Spring Supplier + useful technical Information7. Springmasters ...Spring Supplier + comprehensive range of springs with sizes and ratings

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Last Updated 23/ 02/ 2011