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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 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.

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.

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.

Creeping is a time-dependent, plastic, and irreversible deformation that occurs during the relaxation phase of a bolted joint under constant stress as a result of dislocation movements in the material. It occurs under long-term loading, particularly when the time-dependent strength or fatigue strength is exceeded. In bolted joints, creeping leads to a loss of preload over time.