FORMULAS FOR COMPRESSION SPRING DESIGN


Compression spring formulas including spring index formula, Wahl factor formula, spring rate formula, shear stress at spring body formula, spring outer diameter at solid height formula, spring stability condition and Hooke's law.

Parameter Formula
Spring outer diameter [OD] OD = D + d
Spring inner diameter [ID] ID = D - d
Spring index [C] C = D/d
Wahl factor [Kw] $${ K }_{ W }=\frac { 4C-1 }{ 4C-4 } +\frac { 0.615 }{ C } $$
Shear stress at spring body (corrected with Wahl factor)- used for unprestressed springs [τs] $$\tau ={ K }_{ W }\frac { 8FD }{ \pi { d }^{ 3 } } $$
Shear stress at spring body  (uncorrected )- used for prestressed springs [τs] $$\tau =\frac { 2C+1 }{ 2C } \times \frac { 8FD }{ \pi { d }^{ 3 } } $$
Spring rate [k] $$k=\frac { { d }^{ 4 }G }{ 8{ D }^{ 3 }{ N }_{ a } } $$
OD at solid height [ODat solid] $${ OD }_{ at\quad solid }=\sqrt { { D }^{ 2 }+\frac { { p }^{ 2 }-{ d }^{ 2 } }{ { \pi }^{ 2 } } } +d$$
Spring stability condition $${ L }_{ f }<\frac { \pi D }{ \alpha } { \left[ \frac { 2(E-G) }{ 2G+E } \right] }^{ 1/2 }$$
Hooke's Law $$\Delta F=k\Delta x$$

Type of Spring Ends
Parameter Open or plain (Not ground) Open or plain (Ground) Squared or closed (Not ground) Squared or closed (Ground)
Total coils [Nt] Na Na+1 Na+2 Na+2
Free height [Lf] pNa+d p(Na+1) pNa+3d pNa+2d
Solid height [Ls] d(Nt+1) dNt d(Nt+1) dNt
Pitch [p] (Lf - d) / Na Lf / (Na+1) (Lf -3d) / Na (Lf -2d) / Na
Guidelines for Dimensional Characteristics of Compression Springs
Source : From Design Handbook [Ref 1] page 32

List of Parameters
Symbol Definition
OD Spring outer diameter
ID Spring inner diameter
D Spring mean diameter
d Wire diameter
p Pitch
Lf Spring free length
Ls Spring solid height
F Axial force
Fs Force at solid length
ΔF Force difference (Ex: F2-F1
Δx Deflection (Ex: L2-L1)  

Reference: