Operating Stress

Compressing a wave spring creates bending stresses similar to a simple beam in bending. These compressive and tensile stresses limit the amount a spring can be compressed before it yields or “takes a set”. Although spring set is sometimes not acceptable, load and deflection requirements will often drive the design to accept some set or “relaxation” over time.

Maximum Design Stress

Static Applications: Smalley utilizes the Minimum Tensile Strength found within our tables of standard springs to approximate yield strength due to the minimal elongation of the hardened flat wire used in Smalley products. When designing springs for static applications we recommend the calculated operating stress be no greater than 100% of the minimum tensile strength. However, depending on certain applications, operating stress can exceed the minimum tensile strength with allowances for yield strength. Typical factors to consider are permanent set, relaxation, loss of load and/or loss of free height.

Dynamic Applications: When designing wave springs for dynamic applications, Smalley recommends that the calculation of operating stress not exceed 80% of the minimum tensile strength.

Residual Stress and Pre-Setting

Increasing the load capacity and/or fatigue life can be achieved by compressing a spring beyond its yield point or “presetting”. Preset springs are manufactured to a higher than needed free height and load and then compressed solid. Both the free height and load are reduced and the material surfaces now exhibit residual stresses, which enhance spring performance.

Fatigue

Fatigue cycling is an important consideration in wave spring design and determining precisely how much the spring will deflect can greatly impact the price of the spring. An analysis should include whether the spring deflects full stroke or only a few thousandths each cycle or possibly a combination of both as parts wear or temperature changes.

Fatigue Stress Ratio Estimated Cycle Life

.00 < X < .4 Under 30,000 cycles
.40 < X < .49 30,000 to 50,000 cycles
.50 < X < .55 50,000 to 75,000 cycles
.56 < X < .60 75,000 to 100,000 cycles
.61 < X < .67 100,000 to 200,000 cycles
.68 < X < .70 200,000 to 1,000,000 cycles
.70 < X Over 1,000,000 cycles
Formula