Guidelines for Bolt Connections: Correct Selection and Application of Flat Washers and Spring Washers

In bolt connections, flat washers and spring washers are two of the most common and critical accessories. While they may appear simple, the choice of whether to use them and their correct application directly impacts the reliability of the connection, its ability to resist loosening, and the safety of the connected components. This article will clearly explain the differences between them, selection criteria, and application scenarios.

I. Flat Washer: Increasing Area, Protecting Surfaces

Core Function:

The primary function of a flat washer is to distribute pressure. By increasing the contact area between the bolt head/nut and the surface of the connected part, it achieves the following:

• Protects Soft Materials: Prevents the bolt from crushing or damaging softer connected surfaces like aluminum, plastic, or composite materials during tightening.
• Compensates for Uneven Surfaces: Provides a better bearing surface on slightly angled or uneven surfaces.
• Acts as a Spacer: Can sometimes be used as an adjustment shim.

How to Choose a Flat Washer?

• Material Compatibility:

  • Typically should match the material of the connected parts (e.g., steel, stainless steel, aluminum alloy) to avoid electrochemical corrosion.
  • For conductivity requirements, copper washers can be chosen.

• Size Compatibility:

  • Inner Diameter: Should be slightly larger than the thread or shank diameter to ensure easy insertion.
  • Outer Diameter: Should be chosen with a larger outer diameter to provide optimal support when connecting soft materials or when used in conjunction with a spring washer.

• Special Types:

  • When using bolts with large fillets, flat washers with chamfered inner holes should be used to prevent interference.
  • For critical large-diameter bolts or connections requiring extremely high resistance to crushing, steel flat washers are essential.

• Advantages and Limitations:

  • Advantages: Effectively protects contact surfaces, distributes pressure, low cost.
  • Limitations: Does not provide anti-loosening functionality and cannot dampen vibrations.

II. Spring Washer: Prevents Loosening, Absorbs Shock, Dynamic Locking

Core Function:

The primary function of a spring washer is to prevent loosening and absorb shock. Its split-ring spring structure creates continuous elastic force after tightening, providing a preload force opposite to the nut's tightening direction, thereby:

• Increasing Friction: Effectively resists nut loosening caused by vibration, impact, or load variations.
• Providing Cushioning: Absorbs some vibrational energy, protecting the connection.

Key Application Points:

• Spring washers are typically not used alone and should be combined with a flat washer.
• The correct installation sequence is: Connected Part → Flat Washer → Spring Washer → Nut. The flat washer contacts the workpiece surface to protect it, while the spring washer provides the anti-loosening force under the nut.

Advantages and Limitations:

• Advantages: Good anti-loosening and shock absorption performance, easy installation, cost-effective.
• Limitations: Its performance highly depends on material and heat treatment processes. Poor-quality spring washers are prone to fracturing and failure. Therefore, sourcing from reputable suppliers is crucial.

III. How to Choose? A Practical Guide Based on Conditions

In practical applications, the selection of washers primarily depends on the specific operating conditions:

• Static, Light Load, No Vibration: Using only a flat washer is sufficient. Its role is to protect the surface and distribute pressure.
• Presence of Vibration, Impact, or Load Variations: The combination of "Flat Washer + Spring Washer" is essential. The flat washer is responsible for protecting the contact surface, while the spring washer primarily provides anti-loosening and shock absorption capabilities.
• Soft or Easily Damaged Base Material: A flat washer must always be used to prevent crushing damage, regardless of whether it is combined with a spring washer.
• Special Performance Requirements (e.g., Conductivity, Sealing): Specialized washers with specific functions, such as copper washers or sealing washers, should be selected directly.

IV. Conclusion & Best Practices

For the vast majority of industrial applications subject to vibration or alternating loads, the combination of a "Flat Washer + Spring Washer" is the optimal and most reliable choice. This combination simultaneously addresses:

• Protection (Flat Washer)
• Anti-Loosening (Spring Washer)
• Vibration Resistance (Spring Washer)

Final Recommendations:

To ensure long-term reliability of the connection, always:

1. Clarify requirements based on operating conditions (static/dynamic load, vibration level) and materials (soft/hard).
2. Select washers with matching dimensions and appropriate materials.
3. Prioritize purchasing washers, especially spring washers, from quality-reliable suppliers. Their heat treatment quality directly affects anti-loosening service life.

Correctly selecting and using washers, though a detail, is a crucial step in ensuring the safety and reliability of the entire mechanical connection.