Elastic Resilience

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.

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.

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.

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.

"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.

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.

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.