SUMMER 2019

26 THE DISTRIBUTOR’S

26 THE DISTRIBUTOR’S LINK Bruno Marbacher Bruno Marbacher earned his mechanical engineering degree in Switzerland, he also holds a business degree. He started out as a tool and die maker (poly-mechanic) and over the years he has held various management positions in quality and engineering. During his time in America he has developed and given numerous seminars on topics related to the proper use of mechanical fasteners and machine elements, and assists engineers in solving fastening/assembly issues. His has groomed and directed many young engineers in fastening/assembly technology. He now offers his 40 years of experience through writing and lecturing. TECHNICAL BRIEFING: SCREWS & NUTS - CRITICAL ELEMENTS FOR ANY ASSEMBLY Dear Reader, I have worked in the fastening technology field for over 40 years, I have learned a thing or two. I will share that experience and knowledge in tech briefs. In this article we talk about another essential fastening element, The screw - the bolt. It will provide readers with general insight into the important aspects of screws and bolts. Definition - Bolt vs Screw Based on Machinery’s Handbook, bolts are typically used to assemble two or more unthreaded components. Screws are used with internally threaded components. This doesn’t mean that the components used with screws must be threaded, the screw may create the mating thread. Well, there are always exceptions to this rule, a socket head cap screw may be used with a nut. It’s still called a screw. BOLT For screws and or bolts to function they need the following features: TECHNICAL ARTICLE SCREW ¤ The screw’s head holds the clamped parts on one side. ¤ The drive (slot, hex, etc.) is used to drive the screw home and tightens it. ¤ The shank length provides the screw with the necessary length to fasten’ things. ¤ The screw thread always engages in a mating internal thread, either a nut thread or a tapped hole. Thus, the clamped parts are held together from the opposite side. ¤ Screws may be threaded up to the head of the screw. this does not weaken the screw. The weakest section is still always in the thread. For stainless steel screws the weakest area is in the unthreaded shank. ¤ Screws, bolts need a drive system so they can be driven in and properly tightened. What drive system is being used, is often a question of the applied assembly method. (Manual or automated assembly, accessibility to the joint, type of design, etc.). In this article we focus on external hex drives. Radius Screws must have a radius in the shank to head junction, not too small, not too big. A small radius may create a stress riser, and lead to fatigue fractures. Too large. the screw may not be seated properly and may rest on the radius instead of the bearing area of the screw. CONTINUED ON PAGE 134