12 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 FASTENER DISTRIBUTORS NEED TO KNOW ABOUT SPRING WASHERS The Belleville washer is known by several names, such as; a compression washer, conical spring, conical compression washer, spring disc, or the most common, the Belleville spring washer. The washer is named after Julien Francois Belleville, who was granted a patent in 1861. Even so, little is known about how it is used and its existence is often forgotten. The Belleville spring is conical in shape. Being a spring, it will compress at a given rate producing very high loads in a very small space that are constant during periods of joint relaxation which will help keep the joint together. The Belleville washer is very advantageous when multiple factors are involved, such as with thermal expansion, softer materials, dissimilar metals, gasketed joints and with multiple bolts. Depending on materials and strength, they may be used in critical and non-critical applications. Some of the available materials range from high carbon 1075 steel, 6150 alloy steel, 301 and 17-7PH stainless steels and H-13 tool steel. Inconel X-750 and X-718 are also used. Spring material choice is based on application, environment and operating temperatures. The Belleville spring washer is designed to prevent loosening from a variety of factors, such as vibration, embedment relaxation into softer materials and / or thread flanks, bolt creep, stress relaxation, thermal expansion and relaxation as well as elastic interaction. From prior discussions, we know that a fastener will lose between 10-15% of its initial preload just from normal relaxation soon after it is tightened. In a multiple bolting connection, adjacent fasteners can lose over 50% on solid joints and over 75% with a gasketed connection CONTRIBUTOR ARTICLE due to an elastic interaction between the fasteners, even if criss-cross torqueing is used. This is because when the second bolt is tightened, it further compresses the joint and allows the first bolt to relax and so on with the other bolts in the connection. However, cross torquing helps but loads will still change in flanges under pressure and temperature variables. Bolt creep and stress relaxation happens when the joint experiences elevated temperatures. During this steady high temperature exposure, the bolt will relax over a period of time. Creep is the amount of tension loss the bolt experiences, which is a factor of the bolt material, temperature and the time exposed to that temperature. Differential thermal expansion happens when the joint heats up from the thermal resistance experienced in an electrical connector, friction from compressed air or fluids flowing through pipes and from heated fluids conducting heat in flanges and connectors. Thermal conductivity will cause expansion of the parts closest to the heat source. These parts will naturally expand faster than parts further away. When parts cool, they all cool at different rates, which depends upon the material and the temperatures. For instance, in an electrical connection there is either aluminum or copper. Each will expand faster than the steel bolt, unless it is bronze. When the heat source stops, the aluminum and copper will cool and contract faster than the steel bolt, causing a loose joint. During this time when the joint is loose, the joint can leak until parts expand again and the nut is susceptible to loosening. Electric bus bars are very susceptible to thermal loosening. CONTINUED ON PAGE 104
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