In Spring Designs: Choice Of Operating Stress

In settling on spring designs the designer must choose the operating stress that fits his spring configuration and material choice. The solution must meet the chosen conditions of load, travel, and available space.

The under designed spring not only uses more pounds of material, it costs more to manufacture and requires more operating space. By designing a spring with high, but appropriate, operating stresses you can create a considerable savings.

Note that if too little space is allowed for spring operation it may be difficult or impossible to design a workable spring within the stress limitations. This may make it necessary to go to a more costly spring material to attain higher stress carrying ability. It may also force the designer to redesign the entire mechanism.

This is why it is important that spring designs be considered early in the total design analysis.

Almost all springs are stressed in torsion or bending, or a combination of both. This table lists the principal direction of stress application for most common spring types.

Bending or twisting determines the nature of stress and the modulus E or G to be used in calculating loads. The table below indicates the recommended stress levels.

The recommended stress data presented in the table above are based on the strength of the materials.

The stress data will result in spring designs which set less than 2% in service.

The residual stress, maximum design values are selected whenever space limitations force a high design stress.

Buckling and shape considerations impose a configuration that is relatively low stressed for many flat springs. Most of these types of springs are made from steel that is quenched and tempered to a hardness range of Rockwell C43-48. This hardness must be measured on a scale appropriate to the stock thickness.

springs design stresses

Note the stress values listed in the tables for design stresses for cyclic services shown here. These are for springs undergoing non-reversed, constant deflection loading with a stress range varying from zero to maximum. Life or maximum stress can be increased by reducing the stress range. A Goodman diagram combined with an S-N curve for the spring material being used can predict the life of a spring at any stress range.

For each material size and stress type (torsion or bending) there is an S-N curve. The abscissa is used as a double scale- a log scale for the number of cycles, N, and a linear scale labeled minor stress of the same rate as the ordinate or major stress scale.

A 45 degree line is drawn from the origin of the plot. The point ”A” marked on this line corresponds to the ultimate strength of the material. This is the tensile strength for structures in bending and the torsional strength for structures in torsion. You can estimate torsional strength as two-thirds of tensile strength.

These two lines make up the combined S-N and Goodman diagrams for a given material and stress type.

To determine a minimum service life draw a vertical line from the appropriate value on the ”N” scale to its intersection with the S-N curve ”B”. At this intersection draw a horizontal line to the intersection with the ordinate or ”major stress” scale ”C”. From that point draw a straight line to the tensile strength point ”A”. Along this line AC are the combinations of major and minor stress that will meet the desired cycle life of the spring.

Surface quality of the spring wire is extremely important to cycle life. Surface defects such as cracks, pits, and notches may cause premature failure.

The appropriate material for the fatigue application of the part needs to be chosen in the design phase. (See the chart on this page)

Note: The fatigue data given in the tables on this page for design stresses refers to springs working at room temperature and in non-corrosive environments. They also represent a range from zero stress to the maximum. In this case the mean stress is one-half the maximum stress.

If the mean stress is zero (a fully reversed stress cycle), the maximum stress would be 65% of that listed. Few spring designs are used in reverse cycling.

If springs are designed to the highest stress levels shown in the tables you need to anticipate that there will be some setting in use.

Shot peening is one of the best methods for increasing fatigue life of springs. Note the increased design levels for shot peened springs shown on this page. Shot peening is also relatively inexpensive. The use of shot peening is only restricted by spring designs that distort or tangle. This operation needs to be discussed with the spring manufacturer before placing on the spring design.

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