Locking-edge washers
Locking-edge washers are disc-spring-like, one-piece securing elements made of spring steel or stainless steel with radial locking edges that prevent loosening through a purely mechanical form fit. They do not increase elastic resilience and are suitable only for dynamically loaded mechanical connections. Locking-edge washers are not suitable for electrically conductive connections (electrical potential equalisation and grounding screw connections) or electrical contact glands. NSK washers represent the multifunctional, standards-compliant further development.
- Design and Construction
Lock-edge washers are disc-spring-like, one-piece screw-locking elements made of hardened spring steel or stainless steel, featuring radially aligned, acute-angled deformation ridges (lock edges). These edges are designed so that, when the screws are tightened, they plastically indent into the contact surfaces (screw head/nut and backing surface). The element functions purely mechanically without any additional spring effect and—if teckentrup is the manufacturer—complies with the former BN 208 010 standard of the German Railway.
- Principles of Operation
The locking-edge washer primarily provides a form-fit locking mechanism and secondarily a force-fit mechanism due to its disc-spring-like design. The locking edges reliably prevent the screw or nut from rotating by engaging positively, while the limited spring effect does not allow for a significant increase in elastic resilience.
- Applications
Locking-edge washers are used in mechanical screw connections, particularly in applications with high vibrations or dynamic transverse loads, as screw locking elements to prevent self-acting loosening by rotation. They are available for property classes 5.6 through 12.9 as well as in stainless steel variants (A2/A4).
Lock washers are unsuitable for compensating for loosening effects—similar to a tension washer (DIN 6796)—for electrically conductive connections (potential & ground equalization) or electrical contact glands.
- Advantages
- Effective protection against self-acting loosening by rotation
- Constant coefficients of friction
- Minimal installation effort
- Cost-effective
- Protection against short-term overloading
- Disadvantages
- No compensation for slackening effects
- No increase in elastic resilience
- Possible damage to contact surfaces
- Limited scope of application exclusively to mechanical anti-loosening devices
- Standard-based evaluation
Locking-edge washers were classified according to DIN 25201-4; however, the validity of this standard has been superseded by DIN EN 17976. The required residual spring force according to DIN 267 Part 26 is not achieved by locking-edge washers, which limits their use in safety-critical applications. Test requirements according to DIN 25201-4 Annex B may be met, depending on the manufacturer.
- Design Considerations and Further Development
For applications requiring increased resilience as well as combined anti-loosening and anti-backlash protection, locking washers are insufficient. The NSK washer is a technically superior, multifunctional advancement that, in addition to preventing rotation, can also compensate for loosening effects and complies with current standards such as DIN EN 17976. Due to their identical dimensions, locking-edge washers can be easily and seamlessly replaced with NSK washers.
Back to the glossary overview
Electrically Conductive Connections
Electrically conductive screw connections ensure low-resistance transfer of current while providing mechanical strength, even through insulating layers. The NSK-K contact washer combines electrical conductivity, spring action, and anti-loosening protection—ideal for earthing and equipotential bonding. See also NSK-E Technical Report.
Electrical Contact Glands
Electrical contact glands transmit current via the clamped contact surfaces of the conductor materials, not via the screw. They require stable contact pressure and a minimal number of interface surfaces. NSK-E/EL washers offer elastic resilience, anti-loosening protection, and safety in accordance with standards—ideal for modern applications. See also NSK-E Technical Report.
Elastic Resilience
Elastic resilience has a significant influence on the preload force and sliding resistance of bolted joints. It is determined by the screw, the backing surfaces, and suitable spring elements. Lock washers conforming to DIN 6796 or NSK washers enhance this effect and reduce loss of preload, especially with short clamping lengths. Unsuitable elements are excluded; variants that comply with standards can be verified by measuring the residual spring force. See also Railway Engineer Technical Report.
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.
DIN 6796
DIN 6796 defines lock washers as friction-lock spring elements for screws in property classes 8.8 through 10.9. They compensate for settlement through elastic restoring force; however, they do not prevent the screw from loosening under transverse loads and are suitable only for axially loaded connections.
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.
DIN EN 17976
DIN EN 17976:2025-03 defines new Europe-wide standards for screw connections in rail vehicles, replacing the previous DIN 25201. The focus is on a dual-security approach to prevent slackening and unscrewing. Outdated components such as spring rings are not permitted. For electrical glands, only single securing elements are allowed. Based on the defined properties of spring elements, elastic resilience and slip resistance can be increased. See also Eisenbahningenieur Technical Report.
DIN 25201
DIN EN 17976:2025-03 fully replaces the national standard DIN 25201 and standardizes design, safety, and functional requirements for bolted joints in rail vehicle construction across Europe. For the first time, the new standard defines specific requirements for residual spring effect during slackening—in addition to the already detailed requirements for protection against self-acting loosening by rotation—which is central to lightweight construction and operational safety. See also Eisenbahningenieur Technical Report.
