SUMMER 2012

44 THE DISTRIBUTOR’S

44 THE DISTRIBUTOR’S LINK Industrial Fasteners Institute 636 Oak Tree Blvd. • Independence Ohio 44131 Phone: 216.241.1482 • Fax: 216.241.5901 www.indfast.org LENGTHEN THE BOLT TO STOP THE LOOSENING by Joe Greenslade Director of Engineering Technology, IFI Lengthen the bolt and stop the loosening. What a strange but true concept understood by few. During my forty-plus years in the fastener industry I have had many questions about how to effectively stop problem bolt loosening. When an application has a bothersome loosening issue the first thought is to add more fastener components such as lock washers, nuts with locking features, or adhesives, but sometimes just lengthening the screw or bolt will work more effectively. Few users or suppliers of fasteners really understand why fasteners work to hold things together. Most assume that torque is why fasteners work, but torque is only a “means to an end” which is bolt stretch. Friction between components do cause some degree of resistance to loosening, but the major reason fasteners work is because they act just like heavy coil springs. When the installer rotates the part by applying torque, stretching forces are exerted on the screw or bolt. When the rotating stops the stretch remains. So long as that stretched condition is maintained the joint stays tight. When the stretch goes away so does the tightness and assemblies become loose. The effective performance of most types of fasteners is based on something called “Hooke’s Law”. Dr. Robert Hooke was an English scientist who stated in 1660 that materials, including steel, stretch a predictable amount in direct proportion to the amount of pulling force that is applied to that material. This is referred to technically as linear-elasticity. Dr. Hooke’s original work was on springs, but it was later learned that his law explains many mechanical material reactions including how bolts and screws work. It is now universally accepted that steel stretches 0.001 inches per inch for every 30,000 pounds per square inch (psi) force that is linearly exerted on the metal component. The distance that this relates to in fasteners is the distance from the bearing surface of the screw or bolt to where the screw or bolt’s thread engages the nut or tapped assembly component. I will refer to this as the screw or bolt’s “free-length”. This simply means that the longer the screw or bolt is the more stretch-length is achieved per unit of applied force. please turn to page 153

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