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Distributor's Link Magazine Spring 2021 / Vol 44 No 2


26 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: or visit WHAT FASTENER DISTRIBUTORS NEED TO KNOW ABOUT FASTENER FAILURES No one likes to have a customer tell them that their products failed and are no good. Especially if the customer begins to tell other users or contractors that they were unsatisfied with the products purchased from your company. Situations like this need to be handled very quickly. There are many ways a fastener can fail but the majority of failures I have investigated were due to installation or application errors. Less than 1% was the cause of a material defect. Therefore, it is imperative that a dialogue begins with the customer to determine the conditions of how the fasteners were used. The following outlines the causes and effects of various installation errors. The most common failures come from fasteners being overloaded or underloaded. Overloaded By being ‘overloaded’ it means that the bolt or cap screw has been stretched past its yield strength. This type of failure can be identified visually by a ‘dog bone’ appearance of the threaded portion, much like taffy when pulled. An overloaded fastener will also cause a loose connection. If the service loads exceed the proof load CONTRIBUTOR ARTICLE or yield strength, the fastener does become slightly longer, but remains in the connection. When the external service loads are removed, the joint returns to a relaxed condition but the bolt remains slightly longer and appears to be loose. The bolt may not have a ‘dog bone’ appearance but will have a change in thread pitch. It is always best to check the thread pitch of the fasteners as the thread deformation may not always be visually apparent. Checking the thread pitch of a questionable fastener is done with a thread pitch gauge. However, when one is not readily available, the thread pitch may always be compared using a new bolt of the same diameter and thread pitch. Simply place the entire length of the new bolt on top of the entire length of the questionable bolt. It is very important to check the entire length of the threads because there will be no thread pitch change of the threads where the nut held the bolt or the first five or so threads of a cap screw in a tapped hole. This is because all of the stretching in tension and thread deformation occurs between the nut and bolt head. Overloaded Causes [1] Improper Fastener Grades ¤ Here, the bolt is not strong enough for the applied loads and will yield. ¤ The bolt may fail or be loose when the external loads are removed. ¤ If the nut does not match the grade of the bolt, the nut will fail. CONTINUED ON PAGE 108


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