Loosening
Self-acting loosening by rotation refers to a rotational movement of the screw or nut without an external torque load resulting from the loss of preload and sliding resistance, as described in DIN 25201-4. A prerequisite for self-acting loosening by rotation is a prior loosening process that ends below the minimum clamping force and leads to relative rotational movement at contact points due to dynamic transverse loads.
- In Phase 1, slip begins at the joint surfaces when the friction limit is exceeded, leading to bending stresses in the screw and potential damage due to cyclic bending stress.
- Phase 2 is characterized by thread slippage and tilting of the screw head, which reduces transverse stiffness and promotes permanent failure.
- In Phase 3, additional stresses and moments arise due to stored torsion, causing initial rotation and a significant reduction in the preload.
- Phase 4 describes complete loosening, during which the stored reverse torques are no longer held by friction, leading to complete rotation of the joint.
To prevent self-acting loosening by rotation, suitable securing elements such as wedge washers, NSK®-Washers, or adhesive threaded connections are required, and their effectiveness must be tested in accordance with DIN 25201-4, Annex B. The resilience of a connection is primarily ensured by maintaining sliding resistance and elastic resilience, while anti-backlash devices only minimize risk and do not provide complete protection.
Multifunctional securing elements, such as NSK-Washers, combine protection against slackening and unscrewing with increased elasticity and sliding resistance, thereby enabling a secure design even with short clamping lengths. From a design perspective, screw connections should generally be designed to be slip-resistant in accordance with VDI 2230 or supplemented with securing elements based on computational or experimental verification to reliably rule out function-critical failure modes.