NSK Washers

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 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 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.

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 Outdated components such as spring rings are not permitted.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.

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 17976:2025-03 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.

Slip resistance refers to the maximum shear force that a screw connection can transmit before components begin to slip relative to one another. Especially with short clamping lengths, the use of spring-loaded elements such as tension washers (DIN 6796) or NSK washers significantly increases resistance to slippage. In addition to functional advantages, these elements also support lightweight design goals and CO₂ reduction. Evaluation is performed, for example, in accordance with VDI 2230 Part 1. See also Eisenbahningenieur Technical Report.

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 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.

Screw connections with special hole geometries significantly alter the mechanical behavior of the joint due to their segmented contact area, negatively affecting surface pressure, pressure distribution, and elastic resilience while simultaneously promoting settlement, creeping, and loosening effects. Notches on the hole flanks and the absence of lateral screw guidance are particularly critical, which is why it is necessary to evaluate additional factors such as contact area, deflection, and resistance to loosening. Since no standard specifications exist, tested securing elements such as NSK L, NSK B, and NSK EL are recommended for mechanical and electrical applications.