A hex head screw is a common mechanical fastener distinguished by its six-sided, or hexagonal, head shape. This geometry provides a large surface area for the driving tool to grip the fastener securely. The design allows for the application of significant rotational force, known as torque, without stripping or deforming the head. This makes the hex head highly effective for creating strong, secure joints in various construction and mechanical assemblies.
Anatomy and Identification
The defining feature is the six-sided external drive, which is turned from the outside perimeter rather than an internal recess like a Phillips or Torx drive. This configuration provides superior resistance to cam-out, which occurs when the driving tool slips out under high torque. The flat sides of the hexagon distribute force evenly across the tool contact points, ensuring maximum power transfer from the wrench or socket.
Below the head is the shank, the main body of the fastener that extends into the material. Shanks are either fully threaded, meaning the threads run from the tip up to the head, or partially threaded. Partially threaded fasteners feature a smooth, unthreaded section near the head. This allows the joint to be pulled tightly together by the threads while the smooth section resists shear forces and improves the alignment and stability of the connection.
Hex head fasteners are manufactured from various materials to suit different environments and strength requirements. Common materials include low-carbon steel for general use and stainless steel for corrosion resistance in wet or outdoor settings. Finishes like zinc plating or hot-dip galvanization are applied to steel fasteners to create a sacrificial layer that protects the underlying metal from rust. The choice of material directly impacts the fastener’s tensile strength and its ability to withstand environmental stress.
Common Types and Applications
A technical distinction exists between a hex head screw and a hex head bolt. Generally, a screw is designed to thread directly into a material or a pre-tapped hole. A bolt is designed to pass through an unthreaded hole and be secured by a separate nut. This difference primarily relates to the intended function within the assembly.
One specific type is the hex cap screw, a high-strength fastener manufactured to precise dimensional tolerances. These fasteners often feature a washer face under the head to provide a smooth bearing surface, ensuring even load distribution. Hex cap screws are employed in machinery, automotive engines, and other applications requiring a high degree of clamping force and reliability.
The lag screw is a specialized hex head fastener designed for heavy-duty wood construction. It features a coarse, deep thread and a sharp gimlet point, allowing it to be driven directly into wood without the need for a nut or through-bolting. Lag screws are commonly used for securing deck ledger boards, timber framing, and other structural connections where high pull-out resistance is necessary.
Tools and Sizing
The external hex head requires tools that grip the outside of the fastener, such as an open-end wrench, a box-end wrench, or a socket wrench. The socket wrench is preferred because it completely surrounds the head, providing the most secure grip and allowing for the highest torque application. Using the correct size tool is necessary to prevent rounding the corners of the head, which compromises the fastener’s integrity and makes future removal difficult.
Hex head fasteners are measured using three primary dimensions: the nominal diameter of the shank, the overall length, and the thread pitch or thread count. Sizing follows either the imperial system (measured in inches and threads per inch) or the metric system (measured in millimeters and thread pitch). Matching the tool size to the head size is important, as the head dimension is standardized based on the shank diameter to ensure compatibility with standard wrench sets.