Everything You Need to Know About Socket Screws

Socket screws are a type of screw designed and manufactured with a hexagonal (hex) internal drive built into the head. As such, they are intended to be tightened and loosened with an Allen wrench or hex key, as opposed to a traditional screwdriver. This is the key feature that sets a socket screw apart from other screw head drive styles such as the Phillips head screw, slotted and flathead screws, and Pozidriv screws, to name just a few.

Another common distinction is that socket screws are often squared off at the tip. These are generally referred to as flat point socket screws and they tend to be the most widely used type found in many common applications. However, it's worth noting that various other designs are also available.

In most other regards, there isn’t typically much physical difference between socket screws and regular screws for the majority of everyday uses. So what exactly is the point of a socket screw, and when should you use one?

In short, socket screws combine some of the increased performance features of other common hex fasteners - nut and bolt systems being the obvious point of reference - with the convenience and ease of use of a standard format screw set. This middle ground can be a useful compromise in many situations, particularly when working in confined spaces or areas without easy access to secure externally wrenched hex fasteners.

The potentially higher torque and clamping force that can be achieved with a socket screw over a regular screw makes them a preferable option in some applications. When compared to regular screws and bolts in a like for like test, socket screws can often deliver some or all of the following advantages in relevant scenarios:

Since socket screws typically offer greater clamping force than standard screws and many other small-format fastener types, fewer socket screws can be used in a given application to achieve the same overall clamping force on the joint or workpiece. Using fewer screws means:

• Reduced hardware costs
• Fewer holes to drill and tap into mounting surfaces, saving time and reducing potential damage to components
• Lower overall weight of the finished piece

The trend towards lightweight constructions when using socket screws is further enhanced due to their smaller individual size than many other screw types and fastening components - especially compared to nut and bolt fixing arrangements. In particular, the cylindrical heads and internal drives of socket screws are far smaller in diameter than fasteners with external hex heads, and therefore need less space (with no additional room needed to manoeuvre a wrench) for installation.

The name 'cap screw' was traditionally used as an umbrella term, meaning any one of the various screw types chosen for fastening applications where the use of a nut and bolt installation was deemed unsuitable. However, today, it tends to refer specifically to a particular style of screw head.

Any number of different screw types and shapes can incorporate elements of the basic cap screw head design - you might find anything from shoulder screws and countersink screws, through to button and fillister head screws described broadly as cap screws. A socket head cap screw is also commonly referred to as a socket cap screw, Allen bolt, or simply a socket screw. In many ways, it is very much the standard version of a socket screw.

Socket head cap screws are widely used in industrial production and assembly lines, including in the automotive sector, furniture manufacture, machine tooling applications and steel fabrication. This is chiefly due to the fact that they are easy to install and tighten or loosen, as well as providing impressive clamping strengths while retaining a discreet and tidy finished look.

As with the vast majority of all socket screws, the socket head cap screw features a cylindrical head with an internal hex drive hole and is often threaded all the way along the full length of the shank. Similarly, they are primarily designed to be used in assemblies where there isn't sufficient clearance to install a conventional nut and bolt or to manoeuvre a standard socket wrench.

The average socket cap screw head is around 1.5 times the diameter of the screw shank, and about as high as the shank is wide. However, alternative head designs for socket head cap screws include low head, button head and flat head varieties, intended for driving into countersunk holes and other application-specific mounting positions.

When looking to buy socket screws, note that the stated length of the product is typically measured from immediately under the head to the tip. This is the case for most screw types, but some specific varieties - thumb screws, for example - will often give stated length measurements that include the height of the head in the overall specification.

Most socket screw types are available in either metric or imperial measurements for length and diameter, depending on the particular manufacturer or supplier in question. Many leading brands and socket screw manufacturers will often provide a socket screw sizing chart or similar tool that can be referred to at the point of purchase.

While relatively small socket screw sizes (of diameters ranging from M2 to M6, and lengths of around 3mm to 20mm) tend to be more in demand across a wider range of tasks and applications, the full range of available sizes is considerable. Larger, heavier gauge and extra-long socket screws are not at all uncommon, particularly in widths of around M8 to M20, and lengths of anywhere up to 130mm.

Extra-long socket screws are often found in electrical plug sockets. Common electrical socket screw sizes include 40mm, 50mm, 60mm, 75mm and 100mm.

There are many different materials used in the manufacture of socket screws. The most common materials are steel and stainless steel but alternatives include brass, aluminium, black nickel, zinc-plated and chrome socket screws. The ideal material to choose will depend on the specific application where the socket screw will be used.

Some of the most common socket screw materials are detailed below.

Aluminium Socket Screws

Aluminium socket cap screws are often preferred for applications in which weight is a key consideration. This might include applications ranging from the automotive industry through to uses such as installing hardware and trim on moveable furniture, cabinets, doors and windows.

Despite only being around one third the weight of steel, aluminium is durable and tends to have lower shear strength. While aluminium offers decent performance in terms of corrosion resistance in more challenging environments, it is less hardwearing than stainless steel.