What is a socket head cap screw?
A socket head cap screw features a cylindrical head and a hexagonal recess. This specialised drive requires a hex wrench or Allen key to tighten or loosen the screw. Hex socket cap screws have threads along their length and can either be fully or partially threaded, depending on the application. In this guide, we’ll take a closer look at these screws, covering:
Advantages of socket head cap screws
Socket screws are the preferred screw when bolts and nuts can't be used. The hex socket (Allen) screw drive gives the screw six stress-bearing sides for use with an Allen wrench (hex key). This enables the screw to resist cam-out better than most other drives.
Compared to other screws, hex socket screw types of the same size enable you to achieve higher torque and clamping force. This, in turn, allows you to use fewer socket head screws than you'd have to install otherwise. Fewer pieces of hardware reduce the weight of your finished application. Fewer screws also reduce your costs and the time needed to drill holes and tap into surfaces.
Socket head cap screws are also known by different names such as Allen head screws, Allen drive screws, hex socket screws, hex cap screw, hex socket head cap screw, hex socket head screw and hex socket head cap screws.
Many people in the trades simply call it a socket screw, but be aware that this is also the name for a different screw type used for light sockets. Whichever name you use, socket head screw types are preferred when bolts and nuts can't be used. The cylindrical head style of the socket head screw has a low profile compared to other fasteners, making it ideal for use in tight spaces.
Types of socket head cap screws
There are several socket head types to choose from. Hex socket screw materials are available in plastic and metal.
There are several types of socket screws to choose from. While standard socket screws are easy to find, you could also consider non-standard socket screws. These are screws modified in some way from established standards. Our team of experienced engineers can design and prototype screws from you from concept. Learn more about our custom solutions.
The head is 1.5 times the nominal thread diameter, making it a smaller diameter than most other machine screw heads. When in use, the socket head cap screw is recessed within a counterbored hole so that the head is flush with the surface of the component being fastened. This result is a clean appearance. There are no protrusions on the socket cap screw that could catch skin or clothes.
A hex socket button head cap screw is sometimes referred to as a dome or round screw. Button head socket screws characterised by their wide, load-bearing surface, which helps them resist loosening. The look is popular for aesthetic purposes, while the lack of sharp edges offers a measure of safety. The bulbous socket button head enables more torque than normal for greater clamping force. For best results, use a retaining washer to lock in place.
Countersunk socket screws
Also called a countersunk hex head screw, or flat head socket screw. The shoulder slopes at an 82-degree angle if imperial screws and around 90 degrees if metric screws. The head is designed to sit flush with its surface or just below.
Applications vary, depending on the dimensions of the socket countersunk head cap screw. For instance, the shorter variety are ideal for light-duty applications that need a strong joint. Otherwise, you’ll find these screws used in the automotive industry, industrial equipment, fixings and electronic applications.
Used to generate a compressive load, which is opposite to a fastener tensile load created by bolts and most other screws. Socket head set screws hold parts beneath the cup – the concave, hollowed end – resisting rotational or lateral movement and standing up to vibration. When choosing your socket head set screw size, keep in mind that the screw diameter should be roughly one half of the shaft. The Allen key set socket head screw, as it’s sometimes called, is intended for light loads. For extreme conditions, socket head cap screws are your best bet.
These types of socket head screws have a partially smooth shaft that enables some rotation and movement around the screw axis while the threaded end will usually use a nut to secure the screw's position. Socket shoulder screws are typically used when pre-tapped holes need alignment with some tolerance. They’re used in equipment and machines with components that require the ability and room to swing, slide or pivot. Hex socket drives resist cam-out better than other drives.
You can learn more about the different screw types and types of socket heads in What are screw heads, drives and threads?
Socket head cap screw dimensions
The dimensions are crucial to the functionality and compatibility with the corresponding hex head cap screw components. Hex head cap screw dimensions are typically defined by these elements:
- Thread size: This is the diameter of the screw's shank. Hex socket screw sizes reflect this dimension. For example, you can get an M3, M4, M5, and M6 socket head cap screw, etc. in metric. Imperial size examples include #6-32, #8-32, and #10-24.
- Length: This is the distance from underneath the screw head to the end of the threaded portion. Note: This is the case whether we’re talking about machine screws, hex socket screws, long socket screws, and even extra-long socket screws, which are used as wall socket screws.
- Head diameter: The diameter of the screw head.
- Head height: This is the distance from the top of the head to the bottom of the socket.
- Socket depth: This is the depth of the socket in the head of the screw.
How to measure socket head screws
Simply take the measurements of the components listed above. We explain how to measure screws in What are screw heads, drives and threads?
You can learn more about sizing, along with standards, by checking out What is the difference between metric and standard fasteners?
Also, check out our guide, What are screw thread sizes?
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