Technical Information

Basic Relations

Free Height = Travel + Solid Height

Load = Load rate x Deflection

Solid Load = Load rate x Travel

Basic Equations

For Round Bar:
Load Rate (R) = (Gd4) / (8ND3)
Stress (S) = (PD / .393d3)K or (GdF / (Pi)ND2)K
Curvature Correction (K) = (4C - 1) / (4C - 4) + (0.615 / c)
Spring Index (c) = D / d

G = Modulus of Elasticity (11,200,000 for steel)
R = Load Rate
d = Bar Diameter
D = Mean Diameter (O.D. minus Bar Size)
N = Active Turns (Total Turns less 1.5)
P = Applied Load
F = Axial Deflection
c = Spring Index (D / d)
K = Curvature Correction Factor
Pi = Pi (3.14)

Suggested Limits on Proportions

In designing springs, certain practical limits on proportions should be followed whenever possible. These are as follows:

  • Outside diameter = 4 to 8 times bar diameter
  • Free height = 1 to 4 times outside diameter
  • Minimum solid height = 5 times bar diameter

Readily available bar sizes should be used unless large quantities are involved. Double and triple concentric nests may be used for more capacity in limited space.


COMPRESSION SPRINGS: A compression spring is an open-coil helical spring that offers resistance to a compressive force applied axially. Compression springs are usually coiled as a constant-diameter cylinder.

EXTENSION SPRINGS: Extension springs are springs which absorb and store energy by offering resistance to a pulling force. Various types of ends are used to attach the extension spring to the source of the force.

TORSION SPRINGS: Torsion springs, whose ends are rotated in angular deflection, offer resistance to externally applied torque. The wire itself is subjected to bending stresses rather than torsional stresses, as might be expected from the name.

SOLID HEIGHT: Height of spring when loaded to bring all coils in contact.

STATIC LOAD: The dead weight supported by the spring, no motion being involved.

LOADED HEIGHT: Height of the spring when carrying the static load.

SOLID LOAD: The load required to bring the coils in contact with each other.

TRAVEL: The movement between free height and solid height.

DIAMETERS: Outside diameter must be given if spring is fit into a tube. Inside diameter must be given if spring is to fit over a rod or another spring.

LOAD RATE: The load per unit of deflection-nomally expressed in lbs/inch. This is a basic property of the spring.

SOLID STRESS: The stress range between spring free and spring solid.