Safety washers work by employing a combination of force and positive locking to prevent the unintentional loosening of bolted joints, even under dynamic loads like vibration or thermal expansion. This dual approach ensures maximum safety and reliability for critical connections.
Understanding the Core Mechanisms
The effectiveness of safety washers, such as SCHNORR Safety Washers mentioned in the reference, stems from their ability to apply both continuous force and a physical lock against rotation.
1. Applying Force (Friction-Based Resistance)
Safety washers generate and maintain a high level of clamping force within the bolted joint. This force contributes to the overall friction between the fastener (bolt head or nut) and the mating surface, resisting rotation.
- Spring Action: Many safety washers, particularly conical spring washers or disc springs (like SCHNORR washers), are designed to act as springs. They are compressed during tightening, storing potential energy. If the bolt assembly experiences slight relaxation due to settling, embedment, or minor thermal changes, the washer expands, continuously exerting an axial force. This maintains the preload and the critical friction required to keep the joint tight.
- Increased Friction Surface: Some washers increase the effective friction area or introduce materials with high coefficients of friction.
2. Positive Locking (Mechanical Interlock)
Beyond just friction, safety washers also provide a positive lock, which is a physical barrier designed to actively resist loosening rotation.
- Serrated Surfaces: As seen in SCHNORR washers, serrations (teeth) on the washer's surfaces bite into both the underside of the bolt head or nut and the surface of the bolted component. This mechanical interlock prevents relative rotation between these surfaces, even if the clamping force slightly decreases.
- Wedge-Locking Action: Certain safety washer systems use a pair of washers with opposing cams and serrations. When vibration or dynamic loads attempt to loosen the joint, the cams ride up against each other, effectively increasing the tension in the bolt rather than allowing it to unwind. The serrations on the outer surfaces ensure the washers remain fixed to the mating parts.
How Force and Positive Locking Safeguard Your Bolted Joint
The combined action of force and positive locking creates a highly secure fastening solution:
- Vibration Resistance: The continuous spring force ensures that the joint remains under preload, while the positive locking mechanism actively resists the rotational forces induced by vibration, which are a common cause of fastener loosening.
- Compensation for Relaxation: As components settle or deform slightly after initial tightening (embedment relaxation), the spring action of the washer compensates by maintaining the clamping force, preventing a loss of preload.
- Thermal Stability: In environments with fluctuating temperatures, materials expand and contract. Safety washers can accommodate minor changes in joint length, maintaining tension and preventing loosening.
- Reliability for Critical Applications: This dual mechanism is crucial in applications where fastener failure could lead to significant safety risks, operational downtime, or structural damage.
In essence, safety washers don't just add friction; they actively combat the forces that cause loosening by maintaining a strong clamp while simultaneously creating a physical barrier against rotational unwind.
Common Types of Safety Washers and Their Principles
| Safety Washer Type | Primary Mechanism(s) | How it Works |
|---|---|---|
| SCHNORR (Conical Spring with Serrations) | Force (Spring) + Positive Locking (Serrations) | Combines the elastic recovery of a conical spring to maintain preload with serrations that bite into surfaces, resisting rotation. This is the mechanism highlighted in the reference. |
| Wedge-Locking Washers | Positive Locking | Two washers with cammed surfaces that lock together when rotational forces try to loosen the bolt, creating a tensioning effect rather than unwinding. Serrations grip the mating surfaces. |
| Serrated Locking Washers | Positive Locking | Teeth on one or both sides bite into the fastener head/nut and the joint material, creating a mechanical lock. |
| Split Lock Washers | Friction / Spring | The split ends dig into the nut/bolt head and the joint surface, providing some frictional resistance and a slight spring action. Generally less effective against severe vibration. |
| Spring Washers (Curved/Wave) | Force (Spring) | Exert a continuous spring force to maintain preload and compensate for minor joint relaxation. Primarily friction-based. |
Why Use Safety Washers?
- Enhanced Security: Prevents catastrophic failures due to loosening.
- Reduced Maintenance: Decreases the need for frequent retightening.
- Increased Lifespan: Protects bolted joints from wear and tear caused by movement.
- Cost-Effectiveness: A small investment compared to the cost of equipment damage or downtime.
Safety washers are an indispensable component in many industries, from automotive and aerospace to construction and machinery, ensuring the integrity and longevity of bolted connections.
