A socket cap screw, also known as a socket head cap screw, socket screw, or Allen socket bolt, is a type of cap screw with a cylindrical head and hexagonal drive hole. They are typically driven with a socket wrench or Allen hex key and usually have knurled heads. The term socket cap screw typically refers to a threaded fastener whose head diameter is nominally 1.5 times or more than the screw shank (major) diameter.Socket head cap screws are used for applications with limited space. Their cylindrical heads and internal wrenching features (mostly hexagon socket) allow them to be used in locations where externally wrenched fasteners aren’t desirable. They are used for critical vehicle applications, machine tools, tools and dies, earth moving and mining machinery, and a wide range of engineering applications. The most important reasons for the increasing use of socket head cap screws in industry are safety, reliability and economy.The following is a precise description of a socket head cap screw along with technical info including chemical composition, hardness values, tensile strength, and yield strength. An externally threaded fastener with unified threads, a cylindrical head with a flat chamfered top surface, knurled cylindrical sides and hexagonal recess, made from alloy steel. Applications/ Advantages Ideal for precision assembly work with close tolerances and applications needing a well tooled appearance. Supplies greater tensile strength than equivalent sizes of Grade-5 or Grade-8 hex head cap screws while requiring less surface area or counterbore since the fastener is internally wrenched. Material Cap screws shall be made from an alloy steel which conforms to the following chemical composition requirements (per product analysis)– Carbon: 0.31% minimum; Phosphorus: 0.040% maximum; Sulfur: 0.045% maximum. Also, one or more of the following elements shall be present in sufficient quantity to meet the strength requirements listed below: chromium, nickel, molybdenum or vanadium. Heat Treatment Cap screws shall be heat treated by oil quenching from above the transformation temperature and then tempered at a temperature not lower than 650°F. Hardness 0 through 1/2″ diameters: Rockwell C39 minimum; 5/8″ diameter & larger: Rockwell C37 minimum Cap screws of a length 3D or greater, where D equals the nominal diameter of the screw, are subject to tensile strength, yield strength, elongation and reduction of area testing. Tensile Strength 0 through 1/2″ diameters: 180,000 psi. minimum; 5/8″ diameter & larger: 170,000 psi. minimum Yield Strength 0 through 1/2″ diameters: 162,000 psi. minimum; 5/8″ diameter & larger: 153,000 psi. minimum Elongation 10% minimum.
There are several head styles of socket cap screws that can be used depending on the application. Below are the most common:
Socket Cap Screw:
A socket cap screw, also known as a socket head cap screw, socket screw, or Allen socket bolt, is a type of cap screw with a cylindrical head and hexagonal drive hole. They are typically driven with a socket wrench or Allen hex key and usually have knurled heads. The term socket cap screw typically refers to a threaded fastener whose head diameter is nominally 1.5 times or more than the screw shank (major) diameter.
Button Head Socket Cap Screw:
A button socket cap screw, also known as a button socket head cap screw, socket screw, or Allen button socket bolt, is a type of cap screw with a button head and a hexagonal drive hole. Socket wrenches and hex Allen keys drive these screws. Most button socket cap screws feature clean heads with no stamping or marking for a nice clean, finished look.Larger head diameter makes it more appropriate for holding thin materials like sheet metal guards.Because of its internal hex drive style it’s ideal for tamper-proofing applications.Good substitute for other drive styles that are prone to stripping like Phillips and slotted. Button head socket cap screws have a low profile dome head that provides an aesthetically pleasing appearance and extra clearance when countersinking isn’t possible. Their design inherently limits tensile strength so use with covers, machine guards, ornamental fixtures, etc., and in other light-duty, non-critical assemblies that do not require the full strength of a socket head cap screw.
Flat Head Socket Cap Screw:
A flat socket cap screw, also known as a flat socket head cap screw, socket screw, or Allen flat socket bolt, is a type of cap screw with a hexagonal drive hole. They are driven with a socket wrench or Allen hex key and have an undercut of 82 degrees. They are designed to countersink and be flush with the material they are being driven into.Due to their reduced tensile strength, use with hinges and in other moderate fastening applications where socket head cap screw strength is not required.
Low Head Sockets
Low Head Socket head cap screws are high strength, precision fasteners that are designed to be used in applications where head height is a problem. Because of their reduced head height and smaller socket size, they cannot be preloaded as high as a standard socket head cap screw. Therefore they should not be subjected to dynamic loads. Similar to a standard alloy socket cap screw except the head height is 50% of the standard’s and the socket size is smaller. The low-head style is normally supplied only in plain steel.
Although standard socket head cap screws are normally produced of high strength alloy steel, they can also be made in a variety of materials such as stainless steel, 316 stainless steel, silicone bronze, brass, and others.
Socket head cap screws can be used in a variety of materials depending on the specific need and application. In some cases strength is most critical while in other applications corrosion resistance is most important. In other circumstances strength and corrosion resistance are both required. Standard alloy steel socket head cap screws in all head styles are standardly supplied in black oxide. Black oxide is not a corrosion resistant coating so if corrosion resistance is required then an additional plating process has to be applied. The most common plating for a socket head cap screw is going to be zinc plating.