Written by Dr. Jody Muelaner
A socket cap screws are machine screws with a cylindrical barrel shaped head containing a hexagonal socket. The head has a smaller diameter than most other machine screw heads, typically just 1.5 times the nominal thread diameter. Socket cap screws may also be referred to as a socket head cap screw, a cap head socket screw or hex socket head screw.
Fastening for these components requires no external clearance, which is typical of screws such as a hex head for it to accept a spanner or socket. These properties mean that socket head cap screws are well suited for use within confined spaces.
Socket cap screw are often used with the head recessed within a counterbored hole, leaving the end of the head flush with the surface of the part being fastened. This produces a clean appearance and leaves no protrusions that could catch or be damaged, making them popular in sporting equipment such as bicycles.
Socket cap screws are often installed with a flush head within a counterbore, which means the dimensioning convention differs from bolts and screws that would usually be installed with the head proud of the mounting surface. The length of most machine screws and bolts is given as the total length of the shaft, including both the threaded portion and the unthreaded shank, but excluding the head.
With socket cap head screws, however, the stated length is the overall length including both shaft and head. This is also how countersunk head screws are dimensioned.
Additionally, socket cap screws are often used in more demanding mechanical applications, so they are often only available in higher grades. Machine screws and bolts are graded by strength, using two numbers separated by a point. This point is a separator and should not be confused with a decimal point. The first number is the ultimate tensile strength (UTS) in MPa divided by 100 and the second number is the ratio of yield strength to the UTS.
Typical bolts are supplied as grade 5.8, 8.8, or 10.9. Socket cap screws are often available in grade 12.9, meaning it has a UTS of 1200 MPa and will yield at 90% of this (1080 MPa).