SEMS Screws

SEMS screws are machine screws pre-assembled with a captive, free-spinning washer that cannot fall off. The washer is installed before the thread is rolled, permanently trapping it on the screw. This eliminates the handling of loose washers, speeds assembly, and guarantees the washer is never forgotten. We stock SEMS screws with several washer types and head styles.

Why SEMS Screws

By combining screw and washer into one captive part, SEMS screws cut assembly time, reduce parts inventory, and prevent the common error of a missing washer. They are favored in electronics, appliances, and high-volume assembly where speed and consistency matter.

Washer Types

Lock washer SEMS: a split spring washer for vibration resistance — the most common. Flat washer SEMS: distributes load and protects the surface. External tooth SEMS: serrated teeth bite into the surface for electrical bonding and strong anti-loosening grip. Internal tooth SEMS: teeth hidden under the head for a cleaner look. Some combine flat and lock washers.

Head Styles and Materials

Available in pan, hex washer, and truss heads with Phillips, slotted, or hex drives. Steel with zinc plating is the standard; 18-8 stainless provides corrosion resistance, though external-tooth grounding performance differs.

Dimensional Standards

SEMS screws follow machine screw dimensional standards (ASME B18.6.3 inch) for the screw, with the washer captive per the assembly specification.

Selecting the Right SEMS Screw

Choose the washer type for your need — lock for vibration, external tooth for electrical bonding, flat for load distribution. Match thread size to the tapped hole or nut, select head style for your tooling and clearance, and pick material and finish for the environment. For grounding applications, external-tooth steel SEMS provide the best electrical contact.

Use the filters to narrow by size, washer type, head style, and material.