Spring washers in accordance with DIN 137 / DIN 6904
Spring washers conforming to DIN 137/DIN 6904 provide only limited spring forces that are not effective over the long term for preventing loosening and experience failure when used to prevent unscrewing. Due to their lack of effectiveness, the withdrawal of the standards (since 2004), and potential liability risks, they are considered technically obsolete. Their use is no longer state-of-the-art and is not recommended. Spring washers are now being replaced by more effective securing elements, such as NSK washers.
- Design and Function
Spring washers are one-piece screw locking elements made of spring steel with a corrugated (Type A) or domed (Type B) geometry. Their shape creates a small frictional preload, which is theoretically intended to compensate for settlement and relaxation effects.
However, an effective residual spring force is not maintained over the long term, which is why the relevant standards were removed as early as 2004.
Spring washers do not prevent self-acting loosening by rotation and, due to their localized contact area, cause plastic deformation of the contact areas.
- Application Limits
- Suitable exclusively for screws ≤ property class 5.8
- Effect is further reduced with A4 stainless steel
- Not suitable for safety-critical and electrical screw connections
Use is no longer technically justifiable
- Advantages
- Simple, cost-effective solution
- One-piece component
- Disadvantages
- No standard-compliant locking effect; placebo effect
- Risk of damage to contact surfaces due to uneven pressure distribution
- No effect during short-term overloading
- Use with flat washers exacerbates settling
- Standards Classification
The DIN 137 and DIN 6904 standards were removed in 2004 because they do not provide sufficient locking effectiveness.
Spring washers are no longer considered state-of-the-art; their use carries liability risks.
- Design Considerations
Spring washers do not meet the requirements of modern screw-locking systems.
Their use—even in combination with other washers—is not recommended.
We recommend switching to NSK washers, which comply with standards and prevent both slackening and unscrewing (types E/EL for electrical applications, S/M/L for mechanical applications).
Back to the glossary overview
NSK Washers
NSK washers are one-piece, multifunctional screw-locking elements that provide both a force-fit and a form-fit to prevent slackening and unscrewing. They increase elastic resilience and anti-seize properties; up to property class 12.9, they are also suitable for electrically conductive connections (earthing & electrical potential equalisation) as well as electrical contact glands, and comply with various standards, such as DIN EN 17976, DIN 25201, DIN 50343, and DIN 267 Part 26; they are considered state-of-the-art and offer assembly-friendly, standards-compliant, and cost- and weight-efficient solutions.
Loosening
Self-acting loosening by rotation refers to the rotational movement of a screw or nut without an external torque load, resulting from the loss of preload and anti-slip strength, preceded by slackening and triggered by dynamic transverse loads. The self-loosening mechanism proceeds in several phases—beginning with relative rotational movements and bending stress (Phase 1), through thread slippage and head tilting (Phase 2), to rotation caused by stored torsion (Phase 3), and finally to complete loosening due to loss of friction (Phase 4). To ensure safety, securing elements such as anti-loosening elements or multifunctional locking elements such as NSK® washers are required; in addition to preventing loosening, these elements also increase slip resistance and resilience, thereby improving overall resilience—especially with short clamping lengths.
Relaxation
During the relaxation phase, relaxation—defined as a time-dependent reduction in stress at constant elongation—leads to a gradual loss of preload force. This is caused by microplasticity or viscoelastic flow at contact surfaces. This impairs sliding resistance and function, which is why, according to VDI 2230, suitable materials and designs are required.
Electrical Screw Connections
Electrical screw connections ensure the transfer of current and mechanical clamping. A distinction is made between indirect current transmission via electrical contact glands and directly current-conducting connections, known as electrically conductive screw connections. Settlement behavior, corrosion, and thermal influences require a targeted design using NSK washers. See also NSK-E Technical Report.<
Slackening
First, the slackening phase occurs in the slackening mechanism of bolted joints. During slackening—or in the slackening phase of bolted joints—the preload force decreases due to embedding, relaxation, and creeping. The sliding resistance is maintained, so that operational forces continue to be transmitted through clamping. Embedding occurs through the plastic leveling of rough surfaces immediately after assembly. Relaxation and creeping lead to further force reduction over the long term. Screw connections can be secured against slackening using special screw locking elements or multifunctional screw locking elements.
