A cap screw is a screw that is threaded along all or part of its length with various types of drive heads, called the cap. The term cap screw is sometimes used interchangeably with "bolt". In the past, the term "cap screw" was restricted to threaded fasteners with a shaft that is threaded all the way to the head, but that has been revised to include many graded cap screws which may only have a partial thread depending on length and diameter.
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What drive types are used on Cap Screws?
There are many drive types used on Cap Screws. The two most common are Hex Cap Screws and Socket Cap Screws. A Hex Cap Screw will have a hexagonal head (five flat sides) and is driven by a spanner wrench of the specific size for the head. A Socket Cap Screw can have a variety of drive types that will be built into the center of the head. The most common is Allen or Hex drive, this is a five sided indentation in the center of the head. Other common drive types on a Socket Head Cap screw are Torx Drive (a five sided star) and Robertson Drive (also know as square drive). Other less common types are Poly Drive, Spline Drive and Double Hex Drive.
What types of head styles are used on Cap Screws?
The Hex Cap Screw describes the drive type and head style. It is a hexagonal head with a flat top and bottom and five flat sides. In Socket Head Cap Screws there are several head styles just as there are different drive types. The most common Socket Head Cap Screw head style is the standard cylindrical head that is flat on top and bottom. The two other common styles are the Button Head Socket Cap Screw and the Flat Head Socket Cap Screw. The Button Head has a round head on top and a flat bottom. The Flat Head is flat across the top but is campfered on the bottom, or commonly know as "countersunk". This design allows the screw to fit flush with the material the screw is mated with when screwed into a mating countersunk hole.
What are Graded versus Non-graded Cap Screws?
A non-graded Cap Screw is any cap screw that has not been tested and certified to industry standard specifications. These are many times referred to as "Commerical Grade". Graded Cap Screws fall into three main catagories under U.S. specifications, these are grades 2, 5 and 8. Grade 2 are generally used on wood and general hardware applications. Grade 5 are used in small engine repair and automotive applications. Grade 8 Cap Screws are used in more critical automotive applications and other applications where their high tensile strength is required. There are also equivalent but different grades in Metric Cap Screws.
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