Coil Springs - Design & Specifications

The same design formulas and procedures that apply to cold coil springs also apply to hot coil springs with these two differences:

1.  The effective modulus of rigidity for hot coil springs is reduced by 5 to 10 percent.  Hot-rolled bars use a modulus of 10.5x10⁶ psi.  If turned or centerless ground bars are used, the apparent modulus can be increased to 11x10⁶ psi.

2.  The calculated height of hot coil compression springs is lower than the comparable cold coil springs because the taper forging operation produces a thinner end section.  The solid height, which is one half a wire diameter smaller than for cold coil springs, is calculated
as follows: H = d(N - 1/2)

When coil spring rate is specified, it should apply only in the deflection range from 20 to 60 percent of total deflection.  The tolerance on spring rate should not be less than +- 10 percent.

The magnitude of allowable stress for hot coil springs is limited by the grade and condition of material used, the mechanical properties developed by heat treatment, and the kind of service the spring must provide.  For static loading or variable but infrequent loading at relatively low stress ranges, the important considerations are the solid stress and the maximum working stress. 

Specifications for Coil Springs

In Ordering Coil Springs Give The Following Information As Completely As Possible:

• Free Length, Maximum, Minimum.
• Controlling Diameter, Outside Diameter Maximum. Inside Diameter Minimum. Pitch Diameter. Works Inside (Dia. Hole). Works Over (Dia. Shaft).
• Number of Coils.
• Wire Size. Decimal size if possible. Material, Kind and Grade.
• Loads at deflected positions.
• Style of Ends, (see illustrations). Right or Left Hand Wound.
• Finish. Plain unless otherwise specified.
• Maximum Solid Length.
• Frequency of Coil.

How to Determine Rate For Coil Springs

Rate which is the change in load per unit deflection, may be determined by the following procedure:

1. Deflect spring to approximately 20 percent of available deflection and measure load (P₁) and spring length (L₁).
2. Deflect spring to approximately 80 percent of available deflection and measure load (P₂) and spring length (L₂). Be certain that no coils (other than closed ends) are touching L₂.
3. Calculate rate (R) lb./in. (N/mm)
   R = (P₂ - P₁) / (L₁ - L₂)

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