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SEMS - Screw and Washer Assemblies
SEMS are screw and washer assemblies. The washers are free turning and captive (held in place under the head by the threads, they aren't intended to be removed). SEMS offer three distinct advantages: 1) they save time—it's faster to install a screw/washer assembly than individual screws and washers; 2) they ensure that a washer is always installed; and 3) it's easier to find an errant screw/washer assembly than a really small washer that got away. SEMS is a generic term—not a brand name—that enjoys usage in the U.S.
They are available in various drive style, head, thread and washer type combinations. For example, the screws can be machine screws, hex cap screws, tapping screws or socket head cap screws. The washers can be external tooth or internal tooth, split lock (helical spring), conical spring or plain (flat) washers. Only the most popular varieties will be found in stock.
SEMS with internal tooth and especially external tooth lock washers are very common. The teeth create a ratchet action by biting into the screw head and bearing surface. External tooth provides greater resistance to loosening because the teeth are on the largest radius of the washer, which better withstands twisting. Internal tooth is used when the teeth and any associated surface marks need to be concealed but avoid oversized holes because the teeth won't properly engage with the mating surface.
Steel and stainless steel are common materials and zinc plating is the typical finish for steel.
Although machine screw threaded SEMS can be mated with nuts, they are often used with internally threaded (tapped) holes.
Refer to American Society of Mechanical Engineers Standard ASME B18.13, Screw and Washer Assemblies - SEMS, for specifications relating to SEMS.
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