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Socket Head Cap Screws

Socket head cap screws are high tensile strength precision fasteners that have a small cylindrical head with a hexagon socket and external machine screw threads. They are used with bearing caps, machine assemblies, punch press tooling, shaft collars and other applications. Except for four fractional sizes—1/4, 5/16, 7/16 and 9/16"—their maximum head diameter is the same as the maximum width across flats of like-sized hex head cap screws. Because they are internally wrenched, heads can be recessed in counterbored holes or protrude above the mating surface and be closely spaced. Their high strength allows for fewer screws or smaller size screws to be used, which can reduce assembly size and weight while delivering holding power at the lowest cost per pound of fasteners. There is another potential cost savings: fewer screws means fewer drilled and tapped holes are needed. Officially known as 

hexagon socket head cap screws

, they are usually mated with internally threaded (tapped) holes rather than nuts. Other head styles are listed below.

Unlike other screws, there are two different series of socket head cap screws available. Identified as 1960 and 1936 series, their names are based on historic dates and refer to differences in dimensions and not screw material (four-digit designations are also used to identify materials by the American Iron and Steel Institute [AISI], hence the reason for possible confusion). 1960 series is considered the standard in the U.S. and should be used for new designs while 1936 series screws can be found in limited sizes but are intended for replacement purposes. To overcome limited performance of some 1936 series sizes because the relationship between socket size and head and shank diameters was inconsistent, optimized designs based on geometry, material strength and application were developed by one socket screw manufacturer and accepted as industry standards in 1960 thus replacing ASA B18.3, which was designated as an American standard in 1936. The resulting difference between the two series is that the head diameter and hexagon socket size is larger for some 1960 series screws, especially larger screw sizes. Table 1 below summarizes the dimensional differences.

Threads are Class 3A UNRC (coarse) and UNRF (fine) for sizes #0 through 1", Class 2A UNRC and UNRF for sizes over 1" through 1 1/2" and Class 2A UNR for sizes larger than 1 1/2". The number of threads per inch identifies the screw as being coarse or fine and the "R" indicates that the thread root has a continuous rounded contour (in contrast, the root may be flat for UN threads). Class 3A "threads provide for applications where closeness of fit and/or accuracy of thread elements are important" and do not allow for plating thickness. Some screw sizes are available with British Standard Whitworth (BSW), coarse, and British Standard Fine (BSF) threads. Threads are standard right-hand. Flat countersunk and socket head cap screws may be fully threaded or partially threaded and longer lengths may be offered in both versions. Button heads, because of their short length, are fully threaded.

Sizes—listed as a whole number for small sizes and as fractional inches for large sizes—typically range from #0 to #12 and 1/4" to 2 1/2" and lengths span from 1/16" to 20". Measure the length of button, low and socket head cap screws from under the head to the tip. Flat countersunk heads are measured from the top of the head to the tip.

The top of the cylindrical head on a socket head cap screw is flat and chamfered, the side may be plain or knurled and the bearing surface is flat. Use a hex key—also called an Allen wrench—or hex bit to drive a standard socket head cap screw. Some are tamper resistant (hex with center pin for example) and require a special driver bit.

Hexagon socket button head 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.

Hexagon socket flat countersunk head cap screws have a flat head and a standard 82° underhead angle. Countersinking allows the head to be flush with the mating surface. 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 hexagon socket cap screws have a reduced height cylindrical head and are used with thin material and where head clearance may be an issue. Still considered a high strength fastener, they are recommended for light-duty applications free from high dynamic loads because their reduced head height and smaller socket size may prevent achieving the proper preload.

In inch series, there is only one strength grade of alloy steel socket head cap screw and it exceeds the strength of SAE J429 Grade 8 hex head cap screws. In addition, at least one manufacturer claims to offer a higher tensile strength than that specified by ASTM A574. Despite their high tensile and yield strengths, they are not intended to resist shear loads. It is therefore recommended that assemblies be dowel pinned in applications where high shear loading can occur. Alloy steel button, flat countersunk and low heads and stainless steel button, flat countersunk, low and socket heads all have a lower tensile strength rating than alloy steel socket head cap screws. CAUTION: Be extremely careful when working with metric socket head cap screws because they are available in three different strength grades: Property Class 8.8, 10.9 and 12.9. Always check the head marking and replace metric socket head cap screws with the same Property Class.

The most common materials are alloy steel, 18-8 and 316 stainless steel. Other materials include nickel-copper (NI-CU) alloy and silicon bronze. Either thermal or chemical black oxide is the typical finish for steel, which offers minimal rust protection, but zinc plated and zinc yellow plated finishes are available and provide moderate corrosion resistance.

When required to resist vibration or lock the screw in position without seating, prevailing torque is accomplished by a nylon patch or nylon pellet within the threaded area. Lock washers suitable for use with socket head cap screws are called high collar (also "hi collar" and "hi-collar") and are split (helical spring) type with a small outside diameter (OD) and increased thickness to preserve their locking ability.

For specifications relating to socket head cap screws refer to: American Society of Mechanical Engineers Standard ASME B18.3, Socket Cap, Shoulder, Set Screws, and Hex Keys (Inch Series); American Society for Testing and Materials Standard ASTM A574, Alloy Steel Socket-Head Cap Screws; and American Society for Testing and Materials Standard ASTM F837 Stainless Steel Socket Head Cap Screws.

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