Introductory Video

Mechanical Fasteners

Mechanical fasteners are devices that use force and deflection to implement locating functions, which means they align and secure the parts of a mechanical system. Mechanical fasteners can be classified into two categories: integral fasteners and discrete fasteners. Integral fasteners are formed as part of the components they join, such as tabs, slots, and snap-fits. Discrete fasteners are separate pieces that are added to the components, such as threaded fasteners, rivets, and pins.

Threaded fasteners are discrete fasteners that achieve locating by joining two parts together with a helical ridge called a thread. Threaded fasteners enable assembly and disassembly of the parts without damaging them. A common type of threaded fastener is a bolt, which is a cylindrical rod with a head at one end and a thread at the other end. Bolts are used with nuts, which are also threaded devices that fit onto the bolts and tighten them. Bolts and nuts have different grades, which indicate their strength and resistance to various loads and environments. The grades are defined by standards organizations such as the Society of Automotive Engineers (SAE), the Society for Testing and Materials (ASTM), and the International Organization for Standardization (ISO). A great comparison of Grades and their specification can be found here. Some examples of bolt types are round head bolts, which have a dome-shaped head, and studs, which have threads on both ends. Some examples of nut types are square nuts, which have four flat sides, and hex nuts, which have six flat sides. Nuts can also be single-threaded or double-threaded, depending on whether they have one or two threads per inch.

Another type of threaded fastener is a screw, which is similar to a bolt but has a pointed tip and is usually driven into a pre-drilled hole in the component. Screws can also be used with nuts or washers to increase their clamping force or prevent loosening. Screws have various types, depending on their function and design. Some examples are machine screws are: cap screws, which have a cylindrical head with a hexagonal or socket shape; set screws, which have no head and are used to secure a shaft or collar; Sems screws, which have a washer attached to the head; and tapping screws, which create their own threads in the component as they are driven in. Screws also have various heads and tip end conditions, depending on how they are driven and what kind of hole they fit into. Some examples of screw heads are slotted, Phillips, hexagonal, and Torx. Some examples of tip end conditions are blunt, pointed, self-drilling, and self-tapping.

Terminology

The pitch of a thread is the distance between two adjacent threads measured along the axis of the fastener. The lead of a thread is the distance that the fastener advances along its axis in one turn. The diameter of a thread is the distance between the crests or roots of two opposite threads measured perpendicular to the axis of the fastener. For example, a 1/4-20 UNC bolt has a nominal diameter of 1/4 inch, a pitch of 1/20 inch, and a lead of 1/20 inch. UNC stands for Unified National Coarse, which is one of the standards for imperial units of measurement. Another standard is UNF, which stands for Unified National Fine, which has smaller pitches than UNC. For metric units of measurement, there are different standards such as ISO metric screw threads, which use millimeters as the unit of length. For example, an M6x1 bolt has a nominal diameter of 6 mm, a pitch of 1 mm, and a lead of 1 mm.

Rivets

A rivet is another type of discrete fastener that consists of a cylindrical shaft with a head at one end and a tail at the other end. Rivets are used to join two or more components together by inserting them through holes in the components and deforming the tail to form another head on the opposite side. Rivets can be made of various materials such as steel, aluminum, copper, or plastic. Rivets have some advantages over threaded fasteners such as being cheaper, lighter, more resistant to vibration and corrosion, and more aesthetically pleasing. However, rivets also have some limitations such as being permanent (cannot be disassembled without damaging them), requiring access to both sides of the components (cannot be used in blind holes), requiring special tools for installation and removal (such as hammers or rivet guns), and having lower strength than some threaded fasteners.