A Socket Head Bolt, commonly known as a Hex Screw or Allen screw, is one of the most widely used types of fasteners in industrial production and assembly lines. They are some of the strongest screws available (link to tensile strength and torque specification chart), inexpensive and easy to install using a hex wrench or hex key.
These fasteners have a cylindrical head, with a flat chamfered top surface and knurled cylindrical sides, with a hexagonal recess that accepts a hex wrench drive. They can be installed and tightened from both ends, but are commonly tightened from the head, as this allows for a more secure fit with less strain.
Socket cap screws are available in a wide range of sizes and styles, from standard socket countersunk heads to button-shaped domes. They are commonly paired with a washer or barrel nut, and can be used to fasten shaft collars, gears and knobs on shafts. Socket caps are available with either fully threaded or partially threaded shafts. Fully threaded socket caps can be installed all the way into a tapped hole, while partially threaded versions have unthreaded sections that require the use of a nut to install.
Most socket screw designs are made from corrosion-resistant materials, such as steel and various grades of stainless steel. However, some are also available in high-strength alloys and titanium alloys. The tensile strength of a socket screw is determined by the design of its head, while the shoulder, which supports shear loads, is precision ground and heat treated.
Socket screw head shapes vary by application and function, with the most common being a standard socket cap screw with a small rounded head and tall vertical sides. Other types of socket screw heads include low head and button head. The low head variety has a smaller head with shorter vertical sides, and is designed for applications where there’s not enough clearance to accommodate a standard socket cap screw’s head. The button head is a similar design, but with a lower profile that’s ideal for preventing the screw from snagging or catching on moving machinery.
Another type of socket screw is the set screw. This style has a sharp point that creates a compression lock with the mating application. This provides the most axial holding power, and is often found in shaft collars, crash bars on exit doors and knobs of many types. They are rated at 10,000 psi lower than standard socket cap screws due to their head configuration and are often used in lighter duty applications.