General Spring Design

Step 1:

Spring Material:
The first step in the spring design process will be to select a material.  See our Spring Material page for the advantages and disadvantages of each spring material.

Step 2:

Spring envelope: 
Once a material is selected, it will be necessary to determine what loadsand deflections are required for the design.

At this point, it is important to remember that for a given set of loads and deflections, the size of the spring envelope available will dictate the stresses on the spring and therefore the life of the spring. 

It is all too common to allow too small of an area for the spring.  It is possible to make some improvements with special materials, shot peen and press, but for the most part, the available envelope for the spring will dictate stresses.

If there is not room for a spring large enough for the loads and deflection, then complete redesign of the mating parts may be the only option available. Contact Rockford Spring for help with your spring design before locking in the envelope size.

Step 3:

Tolerancing: 
For a typical compression spring or extension spring design, the designer should specify a load or a load and a rate, but avoid tolerancing the free length. 

For most applications it is the load and or rate that is important, not the free length.  Leaving the free length and number of coils reference allows the spring manufacturer something to adjust in order to hold the loads.

For the same reasons, the designer should avoid specifying a tolerance on a torsion spring free position. Avoid specifying a solid height unless it is specifically              needed in the spring's application. 

Designing a spring with the Max solid right at the calculated solid (or worse, below) can increase manufacturing cost.  The designer should, when ever possible, allow a half of a wire diameter for max solid above calculated solid to allow for variation in the amount of grind and the amount of coils.  See the SMI "Spring Design Handbook" for commercial spring tolerances.

Step 4:

Linear Range: 
Any loads specified for a compression spring should be within 15% and 85% of the full deflection range. 

The first part of the deflection and the last part as the spring approaches solid are not linear.  Close load tolerances cannot be held outside of the linear range of the spring. The linear part of the spring is only between 15% of deflection and 85% of deflection.

Step 5:

Buckling: 
Compression springs with lengths greater than four times the spring diameter can buckle. 

There are several factors used to determine if a spring will buckle.  Slenderness ratio, grind, how ends are held and how far the spring is compressed are a few of the factors.  Rockford Spring can help you determine if your spring is susceptible to buckling. 

Step 6:

Static Loading: 
For springs experiencing low numbers of cycles and slow speeds, the springs can be considered in a static situation.  

The following charts give the maximum percent of minimum tensile strength allowable for compression springs and torsion springs. 

For extension springs, the values for compression springs can be used for the body.  The hooks have a combination of bending stresses and torsional stresses.  The bending stresses in the hooks should be kept 10% under the bending stresses allowable for a torsion spring and the torsional stresses in the hooks should be kept 10% under the allowable torsional stresses for a compression spring.  (before set removed values)

Step 7:

Cyclic Applications:
Stresses must be lower for cyclic applications to avoid metal fatigue.  See our page onFatigueand our page on Dynamic loading.