Multifunctional Screw-Lock Systems

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.

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.

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.