Principles of spring design: navigating stress, strain, and fatigue for enhanced durability

Springs are fundamental mechanical components used to store and release energy through deformation. Their design requires a deep understanding of various mechanical principles, including stress, strain, and fatigue. In another blog we’ll explain more about the basics of mechanical springs. Let’s dive into these topics and explore how to design springs that last.

Stress and strain in springs

Stress

Stress in a spring is the internal force per unit area induced by external forces or deformations. When a spring is loaded, it experiences a combination of tensile-, compressive and shear stress.

For most helical springs, shear stress is the dominant type. The maximum shear stress τ can be calculated using the formula:

where:

• ( F ) is the applied load.
• ( Dm ) is the mean coil diameter.
• ( d ) is the wire diameter.
• ( K ) is the Wahl correction factor, accounting for direct shear and curvature effects.

Strain

Strain is the deformation per unit length caused by applied stress. For springs, it reflects how much the spring will deform when subjected to a specific load. Strain ((\epsilon)) can be expressed as:

where:

• (ΔL) is the change in length.
• (L0) is the original length.

Hooke’s Law

Under elastic conditions (i.e., the deformation is reversible), Hooke’s Law describes the relationship between stress and strain:

σ= E * ε

where:

• ( σ ) is the normal/tensile stress in MPa
• ( E ) is the Young’s modulus (a measure of the material’s stiffness).
• ( ε ) is the strain

Understanding fatigue life

Fatigue

Fatigue is the progressive and localized structural damage that occurs when a material is subjected to cyclic loading. Over time, such cyclic stresses can cause a spring to crack and ultimately fail, even if the stresses are well below the material’s ultimate tensile strength.

Fatigue life

The fatigue life of a spring describes the number of cycles it can withstand before failure. Several factors influence fatigue life, including:

1. Material properties: materials with higher fatigue limits are more resistant to cyclic stresses.
2. Surface finish: smoother surfaces reduce stress concentrations and improve fatigue life.
3. Load magnitude and variability: higher loads or variable loads reduce fatigue life.
4. Environmental factors: corrosion or extreme temperatures can accelerate fatigue failure.

Design considerations to mitigate wear and failure

To enhance the durability and performance of springs, it’s crucial to consider:

Material selection

Choosing a material with high fatigue strength and good corrosion resistance is essential. Common materials include:

1. Music wire: high tensile strength and excellent fatigue life.
2. Stainless steel: good corrosion resistance for harsh environments.
3. Alloy steels: enhanced fatigue and wear resistance properties.

Design for fatigue life

1. Stress concentration: avoid sharp corners and notches. Use smooth transitions and fillets to reduce stress concentrations.
2. Surface treatments: shot peening, coating, or polishing the surface can significantly improve fatigue resistance.
3. Load optimization: design the spring to operate within safe stress limits, avoiding excessive loads or sudden changes in load.
4. Preventive maintenance: regular inspection and maintenance can detect early signs of wear and prevent catastrophic failure.

Environmental considerations

Design springs to withstand specific environmental conditions. For example:

1. Corrosion protection: use coatings or corrosion-resistant materials in corrosive environments.
2. Temperature effects: consider material properties at operating temperatures since high or low temperatures can affect material performance and fatigue life.

Redundancy and Safety Factors

In critical applications, consider using redundancy and safety factors to ensure reliability. For instance, designing springs to handle loads well above the maximum expected load can prevent unexpected failures.

Understanding these principles and applying them thoughtfully to spring design can lead to highly reliable and long-lasting springs, tailored to their specific applications.