Sems are machine screws and freely-rotating, captive washers manufactured as one assembly. The term Sems is recognized in the US as a generic term applicable to screw and captive washer assemblies, according to the American Society of Mechanical Engineers (ASME). Sems are usually used as locking screws, with the washer providing the locking action. Machine pre-assembly of these two parts may provide cost savings to the end user, especially in high-volume applications for machine screws.
InStock Fasteners offers sems with both external tooth and internal tooth lock washers. Philips drive pan head is the most common style, though round head slotted drive is also available.