The ultimate goal in designing a spring is to achieve the desired spring performance in order to support application specific requirements. Ken Massett reports.

There are many factors that go into achieving spring performance. The first thing that the engineer should consider is preventing spring failure. Spring design should be considered early in the product design process to ensure that room is allocated for the appropriately sized spring cavity. Designing early also has a number of other advantages; it will often prevent prototype delays, which can slow up projects, allows time for testing a number of different springs so the most economical spring for the application is selected.

There are a number of things that can cause problems when designing a spring. The whole assembly should be carefully thought out and each assembly component should be considered. In certain situations one neglected key factor would send the engineer back to square one in the design process. Before designing a spring into the application, the engineer should have realistic goals and have an understanding of the design parameters that will impact the springs life.

A spring has three main functions: to absorb energy and mitigate shock; apply a definite force or torque; and to support moving masses or isolate vibration (Wahl 1963).

There are many different types of springs: compression springs, extension springs, torsion springs, leaf springs and washers.

Stress factors

Environmental or stress factors should be considered when designing a spring for certain applications. Selecting the correct material is a key factor in eliminating spring stress. Selecting the proper material for a specific application requires general knowledge of what materials there are and the limitations of each one. There are many other factors that need to be considered in determining the spring stress, listed below are some of them (Wahl 1963):

• Type of loading: static or fatigue; number of cycles
• Corrosion effects
• Creep or load loss at load-elevated temperature
• Change in torsional modulus with temperatures
• Vibration and impact effects
• Effects due to load eccentricity, tolerances, etc.
• Knowledge of applied loads
•  Stress calculation method

Material selection is a crucial decision in designing a spring that will work the right way in the application. There are some variables that need to go into choosing the perfect material. Listed below are some of the considerations:

• Chemical and physical characteristics
• Elastic modulus
• Magnetic characteristics
• Heat treatment
• Environmental considerations
• Stress Relaxation
• Corrosion
• Spring Wire 
• Cost & Availability

Fatigue life

A springs fatigue life is one of the last things to consider when designing a spring. Material flaws pits or steams are just some of the considerations that can be associated with fatigue life. Selecting spring tempered wire or shot peening can increase the fatigue life of a spring Another variable that is commonly associated with increasing fatigue life is compressing a spring beyond its yield point, or preseting the spring.

Wave springs offer the advantage of space savings when used to replace coil springs. By reducing spring height, wave springs can reduce the spring cavity by up to 50percent. With a smaller assembly size and less material used in the manufacturing process, a cost saving is realised.

Wave springs operate as load bearing devices. They take up play and compensate for dimensional variations within assemblies.

A virtually unlimited range of forces can be produced whereby loads build either gradually or abruptly to reach a predetermined working height. This establishes a precise spring rate in which load is proportional to deflection.

Loads and spring rates are more accurate, more predictable and may be toleranced better than 50percent tighter than stampings. The force associated with Smalley wave springs will increase at a uniform rate throughout most of the available deflection.

When a spring is produced from full-hard, pre-tempered raw material, there is no risk of distorting the spring during a hardening heat treatment. By contrast, subsequent manufacturing procedures for stamped wavy washers can lead to problems such as fatigue, cracking and inaccurate or inconsistent loading between springs. All told, the metallurgy, the mechanical properties and the uniform dimensional stability of Smalley wave springs provide a component for precise applications.

There are a number of different elements that go into designing a spring and achieving the desired spring performance. Smalley has engineers available to assist with this process.

Engineers have been trained to design and solve thousands of tough applications facing OEMs in nearly every industry. Whether it is high-temperature or a cryogenic environment Smalley has resolved complex design issues. 

Ken Massett is with Smalley Steel Ring Company, Lake Zurich, IL, USA. www.smalley.com