Understanding the anatomy of a threaded fastener is essential for engineers, procurement specialists, and anyone working with mechanical assemblies. This guide defines the 10 key structural terms used to describe a standard threaded fastener—from head to tip.
10. Head
The head is the topmost portion of the screw, designed to provide a contact surface for applying torque during installation or removal. It comes in various shapes—such as hex, pan, countersunk, or round—to accommodate different drive tools and application requirements.
9. Across Flat
Across flat denotes the distance between two opposite parallel surfaces on a hexagonal head. This dimension is the standard reference for specifying wrench or socket sizes, ensuring proper tool engagement during installation and removal.
8. Bearing Surface
The bearing surface is the flat underside of the screw head that makes direct contact with the workpiece or mating surface. This area is where the clamping force is applied, holding the assembled materials firmly together and distributing the load evenly across the contact zone.
7. Fillet Radius
The fillet radius is the curved transitional area between the screw head and the shank. This rounded contour is engineered to minimize stress concentration at the junction, thereby enhancing the overall fatigue resistance and mechanical strength of the fastener.
6. Body Shank
The body shank is the main cylindrical portion located between the head and the threaded section of the screw. It is generally smooth and free of threads, serving as the primary structural backbone that provides strength and stability to the fastener under load.
5. Grip Length
Grip length refers to the unthreaded section of the screw, typically measured from the underside of the head to the starting point of the thread. This length is crucial because it determines how much of the screw actually clamps and secures the joint or material being fastened together.
4. Run Out
Run out is the transition zone where the threaded portion of the bolt meets the smooth shank diameter. It marks the exact point where the thread form ends and the unthreaded body begins, playing a critical role in determining the bolt's grip and load distribution characteristics.
3. Thread
The thread is the helical ridge that wraps around the screw shank. When the bolt is rotated, this spiral structure converts rotational motion into linear motion, effectively driving the screw forward into the material or mating component to create a secure fastening.
2. Thread Length
Thread length is the portion of the screw that carries the helical threads. In the case of a fully threaded screw, this measurement extends from directly beneath the head all the way to the end of the thread. For partially threaded screws, it is the distance from the thread start point to the tip.
1. Chamfered End
The chamfered end refers to a tapered or beveled finish at the tip of the screw. This design feature helps the screw engage smoothly with the mating component—such as a nut or tapped hole—allowing for easy starting and insertion without cross-threading or damage to the workpiece.