C ommon C ompression S pring T erminology Symbol Units Description Formula D mm, in Mean diameter D = De - d De mm, in External diameter De = D + d Di mm, in Internal diameter Di = D - d d mm, in Wire diameter d = De - D E, Mpa, PSI Modulus of elasticity E = δ/ε L1, L2 N, Lbf Target loads (related to target heights/lengths) Fc Th N, Lbf Theoretical load/force at set solid Fn N, Lbf Load/force related to Ln (smalled length) fe Hz Natural frequency k - Stress correction factor L0, FL mm, in Free length/height D1, D2 mm, in Target Length/distance (related to target loads) Lc mm, in Solid length Lc = d(n+ni+nm) Ld mm, in Length of wire Ld = p D [ 2 + nm + n / Cos(z)] LK mm, in Buckling length Ln mm, in Smallest allowed operating length (geometric) Ln = d (n + ni +nm) + Sa Lr mm, in Smalled allowed operating length (stress) M g, lb Mass M =Ld p π d 2 10 -3 / 4 m mm, in Spring pitch m = [ L0 - d (ni + nm) ]/n N Number of cycles n Number of active coils n = G d 4 / (8 R D 3 ) ni Coils related to the ends nm Number of dead coils nt Total number of coils nt = n + nm + 2 R, KSR N/mm, Lbf/in Spring Rate R = G d 4 / (8 n D 3 ) or (L1-L2) / (D1-D2) Rm Mpa, PSI Ultimate tensile strength Sh mm, in Spring travel Sh = D1 - D2 W Nmm, Joule Stored energy W = 0.5(L1+L2)(D1-D2) w - Spring index w = D / d
Closed and Ground Closed and ground ended com- pression springs are also com- mon but they are more expensive. Closed and ground ends will help your compression spring stand vertically straight on a flat surface when the slenderness ratio is too large.
Closed and Squared Closed and squared end com- pression springs are the most common. This end type allows the spring to stand vertically when placed on a flat surface. The last coil on either end is closed. This end type is suited for compres- sion springs with a low slender- ness ratio.
Open Ended Open ended compression springs are uncommon since the spring will not be able to stand unless supported by a shaft or mandrel. There is a pitch between each coil on an open ended compression spring.
Double Closed Double closed ends are very simi- lar to closed and squared ended compression springs. Instead of the spring having one closed coil at the ends, it has two. They are used to provide stability when your spring has a high slender- ness ratio. This end type helps prevent buckling.
C ommon S pring D esign M aterials
Corrosion Resistance
Material
Stress
Application
Brass
Low
Yes
Water resistant Electrical connectivity Corrosive environments Large diameters
Phosphor Bronze Low
Yes
Stainless Steel (302/304) Oil Tempered
Low
Yes
Medium
No No No
Hard Drawn MB Medium
Low cost
Music Wire
High
High stress
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