• The formulas below give good results for moderately stressed springs.
  
  
• A constant factor of 26.4 is used to compensate for cross curvature occurring in the extension form.
  
  
• The spring obeys Hooke's Law as an increment is deflected from curved to straight.
  
  
• The spring is usually not attached to storage drum and at least 1.5 turns should remain on storage drum at full deflection.
  
  
• Caution must be used while using a highly stressed spring like constant force spring as it has an inherently limited fatigue life and its failure can result in harm to equipment or personnel.

Constant Force Spring Nomenclature:

P = Load, lbs

E = Modulus of elasticity, psi

S = Stress, psi

b = Material width, in.

t = Material thickness, in.

R_n= Natural radius, in (R_n= Natural Inside Diameter ÷ 2)

R₂ = Storage drum radius, in.

S_f= Stress factor

Formulas: