![]() The following table is adapted from Barrett, from MIL-HDBK-60, and from the Machinery's Handbook: The accuracy of the preload that is applied during installation is highly dependent on the tightening method employed. Barrett provides a comprehensive treatment of thread locking mechanisms.Īccording to the Machinery's Handbook, preload relaxation occurs within hours after installation, and a preload loss allowance of approximately 10% is sufficient as a general rule. Preload relaxation can be mitigated through the use of thread-locking mechanisms including locking adhesives, lock nuts, lock washers, lock wire, and locking pellets/patches. Other factors which contribute to preload relaxation include embedment and creep. Over time this torsion will dissipate and causing relaxation of the preload force. If the preload is achieved by applying torque to the bolt or nut, then this will result in torsion on the fastener which will act to increase the bolt tension and thus the preload force. Temperature fluctuations will result in expansion and contraction of the joint members and can cause either an increase or a decrease in the preload force depending on the relative lengthening and shortening between the fastener and the clamped parts. There are many factors which can result in a "relaxation" or a loss of the preload applied during installation. ![]()
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