NSK Washer / SK Washer

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.

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.

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.

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.

During the settling phase, the preload of a bolted joint initially decreases due to embedding effects. This “embedding” occurs through the plastic leveling of surface roughness in the contact areas, the loaded flanks of the mated threads, and other interface surfaces. The amount of set depends on the number of interface surfaces and the surface roughness. The settling process begins as early as the assembly stage and is completed within a few hours.

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.

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.

Corrosion protection for screw locking elements is more than just a material or a coating. What matters most is the tested condition of the finished fastener—including its surface, coefficient of friction, spring effect, and locking function.

teckentrup SLI offers three versions optimized for corrosion protection:
zinc-lamellar-coated spring steel / C60 for cost-effective applications requiring high mechanical load-bearing capacity, A4 stainless steel / NIRO / 1.4401 for particularly demanding corrosive environments, as well as zinc-nickel coatings for connections with electrical conductivity.

"Mechanical fasteners—Technical delivery conditions—Part 26: Spring steel locking washers for bolted joints" specifies requirements for conical spring steel locking washers designed to compensate for a loss of preload due to embedding or creeping. It specifies materials, hardness (420–490 HV), surface finishes, test methods (e.g., embedding, endurance, and spring force tests), as well as packaging and marking. The standard ensures the quality and functional reliability of the washers.