Engineering Spring Design Considerations Operating Environment There are a several key factors to consider when it comes to choosing the right material for your application. One of the factors is defining the operating environment. Extreme temperatures, corrosive substances, dynamic loading (fatigue), stress, and other operating conditions must be considered for optimal spring performance. Stress 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 Smalley utilizes the minimum tensile strength value (found in our Materials Table on page 128) to approximate yield strength due to minimal elongation of hardened flat wire used. Static Operating Stress In static applications, the calculated operating stress should be no greater than 100% of minimum tensile strength. This can vary based on the acceptability of permanent set, relaxation, loss of load, and/or loss of free height. Dynamic Operating Stress In dynamic applications, the calculated operating stress should not exceed 80% of minimum tensile strength. Refer to the “Cycle Life and Fatigue” section to the right and Table 2 for further fatigue guidelines. Residual Stress/Pre-Setting Increasing the load capacity and/or fatigue life can be achieved by compressing a spring beyond its yield point, known as “presetting”. Preset springs are manufactured to a higher than needed free height and load, and then compressed beyond the stress limit of the material. Both the free height and load are reduced, and the material surfaces now exhibit residual stresses, which enhances spring performance. Table 2 Where: σ = Material tensile strength S1 = Calculated operating stress at lower work height (must be less than σ ) S2 = Calculated operating stress at upper work height Fatigue Guidelines X < 0,40 0,40 - 0,49 0,50 - 0,55 0,56 - 0,60 0,61 - 0,67 0,68 - 0,70 > 0,70 Estimated Cycle Life Under 30000 30000 – 50000 50000 – 75000 75000 – 100000 100000 – 200000 200000 – 1000000 Over 1000000 Cycle Life and Fatigue Cycle life or fatigue cycling is the number of strokes a spring can withstand before permanent deformation or breakage. Fatigue cycling is an important consideration in wave spring design. An analysis should include whether the spring deflects full stroke each cycle, only some fraction of a full stroke, or possibly a combination of both due to part wear and or/temperature changes, for example. The fatigue guidelines in Table 2 provide a conservative approach to calculate cycle life between two work heights. Although these methods of fatigue analysis have proven to be a good approximation, testing in the application is recommended whenever cycle life is critical. Formula: Fatigue Stress Ratio = X = (σ-S1) (σ-S2) smalley.com • +1 847.719.5900 Free Samples Easy Customization 136
