SUMMER 2024

14 THE DISTRIBUTOR’S

14 THE DISTRIBUTOR’S LINK Guy Avellon Guy Avellon has been in MRO and Fastener Distribution for over 30 years, in such positions Sales Engineer, Chief Engineer, Manager of Product Marketing, Product Engineering & Quality and Director of Quality & Engineering. He founded GT Technical Consultants where he performs failure analysis, lectures on fastener safety, works for law firms and designs/audits Quality systems. He is a member of SAE, is Vice Chairman of the ASTM F16 Fastener Committee, Chairman of the F16.01 Test Methods Committee and received the ASTM Award of Merit in 2005. Guy can be contacted at 847- 477-5057, Email: ExpertBoltGuy@gmail.com or visit www.BoltFailure.com. WHAT DISTRIBUTORS NEED TO KNOW ABOUT THE TECHNIQUE WHEN TIGHTENING A BOLTED CONNECTION For the past several articles we have defined torque and the different ways it is used in maintenance, assembly lines and structural applications. Our objective has been to create a joint connection that will perform in the face of environmental changes while enduring external forms of shock loading or other forms of abuse. A somewhat reliable connection may be made by tightening a single bolt as users will still seek out the proper torque value to use with their fasteners. However, torque is fickle and as we have seen, there are many variables to consider. One of which is when the joint has multiple bolts in the connection and the other is how the fasteners are tightened. Customers may still experience some bolt failures even when using the ‘proper’ torque values. However, if their technique of applying the torque is not compatible with the bolted connection and joint conditions, some failures will occur. When a bolt is tightened it experiences a torsional twist. Therefore, as soon as the wrench stops turning the torsional force dissipates but the clamping preload will decrease a slight amount as the bolt regains its original shape from being twisted. If a bolt is tightened onto a hardened joint surface, the amount of relaxation from compression of the joint, washer and threads will amount to approximately 10-15% CONTRIBUTOR ARTICLE within the first 90 seconds. This loss is very significant if there are multiple bolts in the joint and/or a gasket. Other factors contributing to preload loss can be from fluctuating temperatures, condition of the joint surfaces, corrosion and joint rigidity in relation to the bolt grade and amount of torque applied. When a softer material is introduced in the connection, such as a non-hardened washer, lower strength nut than the bolt or a gasket, the drop in preload is much faster and greater than 15%. Materials are elastic but they are also compressive. The compressed materials like to return to their previous dimensions but sometimes do not. This action of relaxation of the bolt and joint is the reason why after a fastener has been tightened, the fastener can be rotated a little bit more a few moments after all the other fasteners have been tightened. With the exception of a gasket or other soft material, once the fastener is retightened it will regain most or all of the preload it lost. The fastener and joint will now be stabilized from further loss of preload. Multiple Fasteners In A Joint An example of bolt and joint relaxation can be illustrated by using the multiple bolt pattern in Figure 1. This represents either a solid flange without a gasket, or a wheel. CONTINUED ON PAGE 104