SEMS Screw

SEMS screws, also known as screw and washer assemblies, are fasteners with one or more washers pre-assembled onto the screw prior to threading. The washer is permanently captivated because the screw is threaded after the washer is placed, making the washer’s inner diameter smaller than the screw’s major diameter. This integrated design eliminates the need to handle separate washers during assembly, reducing assembly time, preventing washer loss, and minimizing foreign object damage (FOD) in critical applications.

Key Features & Benefits

Reduced assembly time: No separate washer handling or installation required
Prevents washer loss: Washers are permanently captivated and cannot fall off
Lower inventory count: Single component replaces separate screw and washer
FOD reduction: Eliminates loose washers that could become foreign object debris, especially critical in aerospace and electronics

Available Head Styles

Head Type	Description
Pan Head SEMS Screw	Low-profile rounded head with flat bearing surface; most common general-purpose style
Hex Head SEMS Screw	External hex drive for high torque applications; suitable for wrench installation
Hex Washer Head SEMS Screw	Hex head with integrated flange; provides larger bearing surface without additional washer
Custom Heads	Manufactured per customer drawings

Washer Configurations

Washer Type	Description
Plain Washer (Flat Washer)	Single flat washer; provides load distribution and protects mounting surface
Spring Washer (Split Lock)	Helical split washer; provides vibration resistance and prevents loosening
Internal Tooth Washer	Serrated teeth facing inward; hides teeth for appearance and prevents snagging
External Tooth Washer	Serrated teeth facing outward; provides greater torsional resistance due to larger radius
Conical Washer	Bellville-style conical spring; provides high retained clamp load and compensates for thermal cycling
Combination (Split + Flat)	Spring washer paired with flat washer; common for applications requiring both locking and load distribution

Applicable Standards

Standard	Description
ASME B18.13	Screw and Washer Assemblies – SEMS (Inch Series). Covers sizes No. 0 through 1/2″ diameters
ASME B18.13.1M	Screw and Washer Assemblies – SEMS (Metric Series). Covers M5 to M12 machine screws and tapping screws
JIS B1187 / JIS B1188	Washer-integrated hexagon head screws and cross-recessed head screws – Japanese standards for SEMS