By Dr. Jody Muelaner

A bolt is a mechanical fastener with a threaded shaft. Bolts are closely related to screws, which are also mechanical fasteners with threaded shafts. These types of fasteners are typically inserted through two parts, with aligned holes.

According to some definitions, whether something is a bolt or a screw depends on how it is used. A bolt is inserted through parts which all have unthreaded holes and a nut is then screwed onto the bolt to provide a clamping force and prevent axial movement. A screw may first pass through a first part with a clearance hole but its threads mate with threads in one of the parts being fastened. A screw may cut its own threads or mate with a threaded part.

In practice this definition is rarely used. The term bolt is usually used for a fastener which has only part of its shaft threaded. Fasteners with their entire shaft threaded are normally called screws. The unthreaded part of a bolts shaft is called the shank. The shaft of the bolt prevents radial movement of the parts, while the head of the bolt and the nut, if fitted, prevents axial movement. The unthreaded shank provides an interface with the parts that is more precise and less abrasive. The shank also does not contain stress concentrations that could lead to failure, it is therefore important that the shank extends well beyond the interface between parts if a significant shear force will be placed on the bolt.

Bolts often rely on axial force causing sufficient friction at the threads to remain in place. A torque is applied to the head to generate this axial force. The force acts between the bolt head and whatever the bolt is screwed into, whether that is a nut or one of the parts being fastened. This causes elongation of the bolt and compression of the parts containing clearance holes. Alternatively some form of locking nut or thread-locking adhesive may be used to prevent the bolt from loosening.

The most common type of bolt is the hex bolt. This has a hexagonally shaped head, providing flat surfaces for tools to apply torque when fastening. An easier to manufacture square head was used for older bolts and this is still used today for applications where very high torque must be applied using a spanner. Many other types of bolts are available for specific applications, for example:

• Carriage bolt: The head is rounded with a square section of shank immediately below which locks into the part allowing a nut to be tightened without holding the bolt.
• Shoulder bolt: The shank has a significantly larger diameter immediately below the head and then steps down to the threaded diameter.
• J bolt: The head is replaced by a hook formed from the shank.
• Sex bolt: This is really a type of elongated nut with an elongated body designed to fit inside the hole of a part, acting as the shank of a bolt.

Bolts are graded according to their strength, using two numbers separated by a point. This grade is often stamped on the head. The point is not a decimal but acts as a separator. 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. Common classes are 5.8, 8.8, and 10.9. For example, a grade 8.8 bolt has a UTS, the load at which it would fail, of 800 MPa and will yield at 80% of this value (640 MPa).