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Distributor's Link Magazine Summer 2020 / Vol 43 No3


48 THE DISTRIBUTOR’S LINK PENN ENGINEERING 5190 Old Easton Road, Danboro, PA 18916 TEL 1-800-237-4736 FAX 215-766-0143 EMAIL WEB WHEN TO USE SCREWS OR PEM ® TACK SOLUTIONS by Brian Bentrim Overview There are many cases where two or more items need to be held together, and many product designers have historically defaulted to using a screw; however, a screw is frequently not the optimal solution. This paper explores and contrasts two joining techniques: screws (defined as a machine thread with a head and driver that is mated to a nut or threaded hole) and PEM® Tack solutions (TackPin®or TackSert® fasteners). Many factors should be considered when choosing a fastener to hold two or more items together, including: ¤ Mechanical performance characteristics – Clamp requirements – Joint strength and failure modes ¤ Manufacturing and assembly – Required preparation – Assembly requirements – Assembly failures – Rework after failure ¤ Use features – Need for removability/reuse – Aesthetics and form factor In some cases, screws are the right solution. In fact, they may be the only solution. But in many cases, bypassing the screw in favor of a Tack solution from PennEngineering® can deliver dramatic improvements. Mechanical Performance Characteristics The performance of a joint can be measured in several ways. This paper will consider the joint as a construct intended to hold two or more components and will examine the fastening elements themselves. Clamp Load A threaded joint will typically provide clamp load: A lasting force joining the mated components and resisting working loads and vibrations. This is accomplished by engaging a screw into a nut or threaded hole and applying torque to stretch the screw and compress the joined members. The materials are deformed to a point below the yield strength within the elastic zone of the materials, which causes the clamp force. This is called generating a preload. Clamp load is typically only required if the joint is expected to see cyclical loading or loads that will attempt to separate the components in a case where any separation could result in failure. In most light-duty applications, clamp is not necessary. However, to generate clamp, there must be enough length between the head of the screw and the start of the mating threads. Without enough length, the stretched and compressed elements are unlikely to develop a clamp, and if they do develop a clamp it is unlikely that it can be maintained. TECHNICAL ARTICLE CONTINUED ON PAGE 120


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