Joint decisions

When comparing its importance to effort expended, few aspects of product development are more out of balance than joint design. An ultrasonic sensor measures tension of a 5/16-18 Grade 8 bolt installed in a part-handling system. The screen shot is of a reference and test echo along with a bolt-tension reading of 1,563 lb. The bolt was initially torqued to 25 lb-ft, which in theory gives a 4,700-lb clamp load. A screenshot of the fastening/joining screening utility from Archetype Joint. Design decisions made early in product development are generally the most influential. But those involving fasteners rarely address whether a particular fastening method is best for the application, or if a joint is needed at all. Threaded fasteners, for instance, tend to be the attachment method of choice because they are easy to specify and viewed as inexpensive. In reality, fastener unit cost is about onetenth to one-third of total installed cost. Perhaps a better approach is to select fastening and joining methods by a process of elimination, not from the refining of "established" methods. A software screening utility from does just that. It looks at joint metrics including permanence, relative movement, and component materials, and flags incompatible options, leaving potential designs that warrant a second look. This is important because most products contain nearly as many fasteners as they do other components. In automotive final assembly, for example, fasteners comprise nearly three-fourths of all installed components and consume about 66% of labor hours. No reliable correlation exists — across product classes — between fastener-to-component ratio and assembly time associated with fastening. One reason: The true cost of fastening and joining may be largely hidden. Costreduction efforts typically start with a so-called Pareto analysis of the bill of material that emphasizes "high-impact" components. Such analyses may ignore fasteners because they generally