SPRING 2017

26 THE DISTRIBUTOR’S

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: ExpertBoltGuy@gmail.com or visit www.BoltFailure.com. WHAT FASTENER DISTRIBUTORS NEED TO KNOW ABOUT KEEPING FASTENERS TIGHT Many times a customer has a particular problem with keeping parts together. The fastener and joint loosens and they may seek your advice on how to prevent this from happening. There are a host of reasons why parts loosen; from not being able to initially tighten the nut and bolt properly to severe vibration problems. First, there is no single product that will serve the needs for vibrational resistance for all applications without having some limitations. This is why there are such a variety of products on the market. In some cases, the hardness and softness of the joint materials must be considered to avoid embedment. In other cases, there may be times where we intentionally want to have the fastener mechanically dig into the material to provide a solid grip. The type of external loading will also influence our choices of what method we choose to help keep the parts together. Axial loading, for instance, is a force perpendicular to the joint. It may be steady for awhile, and then provide intermittent impacting loads. When an impact load hits the joint, it will cause a momentary loss of clamp load due to joint compression as the joint absorbs the load. After the impact load dissipates, the joint will relax again to its normal position. However, during the brief moment the joint was compressed, the nut has the desire and ability to rotate down the helix angle of the fastener. This may be only a few degrees of rotation, but over time and frequency of impact or vibrational loads, the nut will continue to rotate off, causing a considerable loss of clamp load until the joint is totally loose. Transverse loading, or shear, will cause the parts to CONTRIBUTOR ARTICLE slide. If the loads shift to the left, the nut will also move to the left which will cause loosening. The intent here is to stop the parts from sliding or any movement. To do this, the mating surfaces must be rough to increase surface friction. The transverse loads must overcome the joint friction to cause movement. In this case, we need a strong fastener and locking nut. One basic tenet is: if you don’t want the nut to come off, lock the nut. First, one must be careful when using the term ‘lock’ to describe a feature, such as a lock nut. This implies a permanent condition relating to the performance of the connection. Law suits have been filed for use of terminology like this, so the proper description is ‘prevailing torque’. This term will describe an entire genre of nuts which will provide a type of interference friction between the threads of the fastener and nut that will cause an increase in both the assembly and disassembly torque, which is sustainable for several applications and removals. Most prevailing torque (PT) nuts are metal and produce a drag against the mating threads, which is felt as an increase in torque effort while the nut is being run up the threads of the fastener. This drag is dissipated when the nut contacts the joint surface and the fastener is tightened in tension. The ‘locking’ element may be caused from distorting the body of the nut at the sides, having a slotted or castle-like top, to a crowned top which has crimps on the top that deflect the first few threads. Some designs and crimping locations differ between manufacturers. CONTINUED ON PAGE 112