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.