A bolt is a form of threaded fastener with an external male thread. Bolts are thus closely related to, and often confused with, screws. Bolts are for the assembly of two unthreaded components, with the aid of a nut. Though, the distinction between a bolt and a screw is commonly misunderstood. Many threaded fasteners can be described as either screws or bolts, depending on how they are used.
Bolts are often used to make a bolted joint. This is a combination of the nut applying an axial clamping force and also the shank of the bolt acting as a dowel, pinning the joint against sideways shear forces. For this reason, many bolts have a plain unthreaded shank (called the grip length) as this makes for a better, stronger dowel.
Bolts use a wide variety of head designs, as do screws. These are designed to engage with the tool used to tighten them. Some bolt heads instead lock the bolt in place, so that it does not move and a tool is only needed for the nut end.
Common bolt heads include hex, slotted hex washer, and socket cap.
Carriage bolt - Bolt with a smooth rounded head and a square section to prevent turning followed with a threaded section for a nut.
Elevator bolt - Bolt with a large flat head used in conveyor system setups.
Hex bolt - Bolt with a hexagonal head and threaded body. Section immediately under head may or may not be threaded.
J bolt - Bolt shaped like the letter J. Used for tie downs. Only the non curved section is threaded for a nut to be attached.
Lag bolt - Not a true bolt. Hex bolt head with thread screw tip for use in wood.
Rock bolt - Used in tunnel construction to stabilize walls.
Chicago Bolt - Bolt that has a male and female part with interior threads and bolt heads on either end. Commonly used in paper binding.
Shoulder bolt or Stripper bolt - Bolt with a broad smooth shoulder and small threaded end used to create a pivot or attachment point.
U Bolt - Bolt shaped like the letter U where the two straight sections are threaded. A straight metal plate with two bolt holes is used with nuts to hold pipes or other round objects to the U-bolt.
Fasteners are manufactured in a wide range of materials from common steel to titanium, plastic and other exotic materials. Many materials are further separated into different grades to describe specific alloy mixtures, hardening processes, etc. In addition, some materials are available with a variety of coatings or platings to enhance the corrosion resistance or alter the appearance of the fastener.
Stainless steel is an alloy of low carbon steel and chromium for enhanced corrosion characteristics. Stainless steel bolts are highly corrosion resistant. Because the anti-corrosive properties are inherent to the metal, it will not lose this resistance if scratched during installation or use.
It is a common misconception that stainless steel bolts are stronger than regular steel bolts. When a stainless steel bolt is compared to a regular steel bolt, the stainless alloys used in bolts are slightly stronger than an un-hardened (grade 2) steel but significantly weaker than hardened steel bolts. Unless great care is taken, stainless steel bolts are susceptible to seizing up during installation, a phenomenon known as galling. Most stainless steel bolts are much less magnetic than regular steel fasteners though some grades will be slightly magnetic.
Steel is the most common bolt material. Steel bolts are available plain as well as with various surface treatments such as zinc plating, galvanization, and chrome plating.
Steel bolts are commonly available in 4 grades: Grade 2, Grade 5, Grade 8, and Alloy Steel. Many other grades exist but are used far less often. Grade 2, 5, and 8 are usually plated with a slightly blue-ish or yellow zinc coating, or are galvanized, to resist corrosion.
Silicon bronze, often referred to simply as bronze, is an alloy made mostly of copper and tin with a small amount of silicon. Bronze bolts are used primarily in marine environments. It is preferred over stainless in wooden boat construction and re-fastening due to its superior corrosion resistance and its higher strength. Bronze bolts are similar to copper in colour and is also sometimes seen in fine woodworks where it is used for its appearance.
Brass is an alloy of primarily copper and zinc. Brass is highly corrosion resistant and electrically conductive. However, its use as a fastener is somewhat limited due to its relative softness. It is used primarily for its appearance.