Industrial bolts are also marked by a top portion which is commonly known as the head.
Unfortunately, there is a great deal of semantic confusion between the terms bolt, screw, and stud. Even more unfortunately, the confusion exists because there are no sharp delineations or divisions between these fastener categories. This blurring of technological definitions can be traced back to the history of industrial fasteners (to be discussed further below). Originally, screws were simply threaded fasteners while bolts were simply unthreaded fasteners. Increasing technological sophistication eventually destroyed this simple demarcation. Today, bolts and screws are commonly viewed as interchangeable with each other. For the sake of clarity, the most frequent differences between fasteners commonly called “bolts” and those common called “screws” are:
• The characteristic that most distinguishes a bolt from a screw is technically not part of the bolt at all. The vast majority of bolts are used in conjunction with metal nuts (which fasten opposite the head), while many screws are not.
• Often, bolts and screws are simply differentiated by their application than by any physical difference. Many times, bolts are used with nuts to fasten pieces in components that have been assembled already and possess great amounts of strength. On the contrary, many screws are placed into tapped holes of components that have not yet been assembled.
• While bolts have previously been defined as a threaded fastener, it is important to note that some specific bolts contain shanks with unthreaded portions. Screws are usually fully threaded.
• Bolts generally have smaller pitches (distances between thread crests) as opposed to screws.
While this list is by no means exhaustive or inviolable, it provides a basic starting point for differentiating between bolts and nuts. Studs constitute another specific type of fastener made of threaded rods known; they are either completely covered or covered at the ends in threads. Although studs are sometimes identified as “stud bolts”, it is better to place them in their own separate category. For sake of clarity, this article will focus on those fasteners which can be identified as “bolts” according to the preceding specifications.
Fasteners are among the oldest inventions of man. Evidence exists of the use of a very primitive “water screw” used in ancient Egypt. This screw would have been made of wood and applied for land irrigation purposes. Interestingly, the subsequent development of fasteners can (very roughly) be broken into discrete stages. The appearance of screw threads can be traced back to Archimedes (287 BC-212 BC), who applied them to lifting applications involving water. Later, the Romans developed an early, unthreaded bolt in order to bar doors or provide pivot points for opening them. The Romans were also (most likely) responsible for developing some of the first threaded fasteners, which consisted of bronze and were threaded via hand filing or wire wrapping and soldering.
The earliest depictions of a threaded fastener and nut combination occur in an early 15th-century “Renaissance” era book. Evidence of the use of fasteners increases from this point on. Johann Gutenberg, for example, depended on various bolts and or screws for his printing presses later on in the same century. Around the advent of the 16th century, Leonardo da Vinci sketched several designs for screw-cutting machines. However, although the first screw-making machine can be traced back to the French inventor Bessop (1568), the true development of bolts would not occur until the Industrial Revolution of the mid-19th century.
Up until the Industrial Revolution, fastener usage was largely hampered by inefficient, manual production processes. In 1760, however, some Englishmen (the Wyatts) developed a factory-driven process for producing screw threads en masse. Although this development greatly spurred the popularity of industrial fasteners, manufacturing continued to suffer from a lack of standardization among threads. Standardization was eventually proposed and developed by Joseph Whitworth of Britain (1841) as well as William Sellers of America (1864). The complete standardization of threaded fasteners was spurred by continuing problems of incompatibility during the two world wars. In 1948, a “unified thread” was developed as an industrial standard by the major Anglo-American countries (i.e. Britain, the USA, and Canada).
The bolt fabrication process begins when a manufacturer selects the preferred raw material. This raw material always comes in the form a wire rod. Bolts are typically made of a few different metals. Steel accounts for the vast majority of bolt production (including carbon steel, alloy steel, martensitic stainless steel, and austenitic stainless steel). When steel is not applied to bolt production, a specific reason usually exists. For example, nickel-based alloys are ideal for producing bolts which function well under high temperature conditions. Brass, aluminum, magnesium, titanium, and even nylon are some of the other materials used for bolt production.
Although industrial bolts vary widely, the fundamentals of their production are strikingly similar. Roughly speaking, the bolt production process can be broken down into three major parts: shaping (including cold forging, head making, and threading), heat treating, and surface treatment.
Once straight wire rod is cut to length, it is ready to be shaped – specifically via cold forging. Cold forging involves forming a raw metal into a new shape using mechanical pressure and plastic deformation at room temperatures with pre-formed, rod-shaped dies. The metal is squeezed through and between two dies until it takes their shape (as opposed to chemical or heat treatment). Turning (spinning and cutting a bolt at high speed) and drilling (cutting holes in the bolt) are combined with standard cold forging processes to achieve more complex shapes. Once shaped, the newly formed rods are cut into smaller pieces and sent through another die that will form their heads.
Heat treatment forms the second basic step in the bolt production process. The main purpose of heat treating bolts is to harden them. A rod can be placed in a furnace for up to thirty hours; afterward, it is often immersed in a bath of sulfuric acid, which removes all potential rust particles. (Long bolts, defined as bolts that are ten times as long as they are wide, usually suffer from deformation after heat treatment; thus, a straightening process is often applied to them.)
While not a completely separate step, the process of threading bolts should be discussed here since threading can take place before or after heat treatment. For the majority of bolt production, threads are formed (by rolling or cutting) on “soft” steel prior to heat treatment. (Rolling depends on yet another type of die to form threads on the bolt.) While pre-heat threading is easier and cheaper, threading a bolt after heat treatment may be preferable due to better durability and improved stress or fatigue performance.
Once a bolt has been shaped and heat treated, it is surface treated for various resistance and performance properties. Since bolts are typically used in highly corrosive environments, corrosion resistance is the main priority in surface treatment. Zinc (in the form of an electrolytic coating or flakes) is the most popular choice for surface treatment, followed by phosphate. To disallow the resurgence of rust, a manufacturer will coat wire rod in phosphate which doubles as a lubricant.
One optional step following the major steps of bolt production is assembly (when advanced bolts need to be combined with other components, such as brackets). Patching is another procedure sometimes applied to bolts after surface treatment, in order to impart locking patches (nylon layers which improve grip) or liquid patches (which improve torque).
As extremely common industrial fasteners, industrial bolts are used to hold together materials in industries from at-home and commercial building construction to aircraft building, automotive manufacturing, and military vehicles and equipment.
Bolts are partly valued for their versatility; they are compatible with tools ranging from high pressure drills to handheld screwdrivers and Allen wrenches. Examples of specific applications for industrial bolts include attaching tires to cars and facilitating various forms of underwater construction, such as shipbuilding. (The latter is facilitated by specialty bronze bolts, an example of product-specific material selection.) More specific examples of bolt applications can be found in the following section.
Like most industrial products, bolts vary widely in a number of ways. Bolts can vary by head type, pitch measurement, and the major diameter of their external thread.
Bolt heads are classified by their shape and size. Some common head types include binding heads, flat heads, oval heads, pan heads, taper heads, truss heads, and socket heads.
The pitch is a measure of the root, which is distance between separate thread crests. (Crests are another name for ridges.) This is measured along the thread axis using a tool called a thread pitch gauge.
Diameters are understood based on the helix angle, which is the measured angle of the threads, (which will either go up to the right or up to the left) and the thread angle (which is a measured, v-shaped angle between the crests.) Bolts are also often named or classified according to their use, shape, or size. Some common representatives of bolts which follow these type of classification systems follow below.
Bolts Classified by Shape
J bolts and U bolts are both named after the letter of the alphabet they most closely resemble. Like J bolts and U bolts, eye bolts are also named after their shape. Threaded at one end, they are specifically named after the circular eye that sits at the other end in place of a head. They are most often applied to rigging and lifting applications which involve wire, rope, etc.
Shoulder bolts are named after their shafts, which are nicknamed shoulders. This name derives from the fact that their pivot mounting shafts are not threaded all the way to the top, stop before the head, and are smooth-sided and slightly wider than the threaded area. Shoulder bolts, which may have a round or square head, are typically made from stainless steel. Shoulder eye bolts are used for rigging scenarios where tension occurs angular to the bolt itself.
Square bolts are types of threaded fasteners which derive their name from square shaped heads and fit easily with wrenches. Similarly, hex bolts are named after their six-sided head. Although square bolts are still used, hex bolts have become a much more popular alternative.
Toggle bolts are unique in the sense that they are part of a larger assembly. The label “toggle” refers to the hinged mechanism that is attached to this type of bolt. This type of bolt is used for mounting applications (e.g. for drywall), where the entire bolt is compressed to fit through a hole before the bolt secures the toggle against the wall.
Bolts Classified According to Usage
Anchor bolts are used to anchor supports to a foundation. Anchor bolts are able to do this by unthreaded, “L”-shaped structures which sit opposite threaded ends. Examples of items they are used with include street signs, columns, and traffic sign posts.
Expansion bolts are used to enable expandable features of certain seating and flooring, such as those found in theaters, auditoriums, and lecture halls. Composed of a nut, metal cone, lead sleeve, and taper head, they are specifically named after their attachments, which expand as the bolt is forced into a surface.
Lag bolts are named after their function: they “lag” together wooden beams or posts and serve other carpentry duties. The use of the word “lag” in this context comes from the early use of these bolts, when they fastened items called lags. (This term primarily referred to the vertical wooden posts of buildings, barrels, casks, and the like.) Lag bolts, which either have a hexagonal or a square head, are used on applications involving wood (e.g. deck construction) and are driven with either a wrench or a socket.
Stove bolts are machined, fully threaded fasteners often used to secure metal. Their heads can be flat, round, or oval with slots. Unlike some other bolts, they do not always require nuts and washers.
Carriage bolts are a very common industrial bolt marked by rounded heads with square-shaped shafts immediately underneath. Carriage bolts are fully threaded and also applied to wood construction. The squared shaft underneath the head keeps the bolt from turning while the bolt is in use.
Tap bolts are a type of hex bolt that is fully threaded. They derive their name from being applied to drilled or tapped holes in several different fields (including construction, agriculture, the automotive field, etc.)
Hanger bolts have the unique role of joining metal to wood. They are often used in overhead products. Hanger bolts usually come either fully threaded or with a plain center.
Elevator bolts possess thin round heads. They derive their name from their original usage in grain elevators and conveyer systems. Similar to carriage bolts, some elevator bolts have a square neck under their round head. In place of a square shaft, some elevator bolts possess teeth instead.
Standard bolts are made from aforementioned materials like stainless steel, aluminum, and titanium. However, specialty fasteners or bolts for advanced applications also exist. Examples include bolts designed to work in the military and aircraft industries. In these industries, it is imperative for bolts to meet specific standards that match the gravity and sensitivity of the applications they serve. Usually, these bolts must also be made with special anti-corrosive materials, receive strengthening and protective coatings, and undergo rigorous quality testing.
As with most other industrial products, it is very important to choose the right type of bolt for a specific application. The strength of a particular bolt is of primary importance in this regard. Choosing a steel over aluminum bolt, for instance, often translates into increased durability over a long period of time. Fortunately, many industry standards have improved in recent history to make assessing and properly applying bolt strength far easier than it was in the past.
To learn what bolts will best serve their application, or how to fabricate the most durable bolts, interested parties should contact an experienced industrial bolt manufacturer or supplier. Standard considerations for finding a quality industrial supplier apply, such as investigating a supplier’s ability to meet customer specifications, their track record of customer service (e.g. turnaround/on-time delivery record), etc. Customers should always discuss industry standards with suppliers and practices a supplier uses to adhere to those standards. For example, ISO 898-1 covers grades of steel that are used across the fastener industry and is an example of a standard that would be prudent to investigate along with your supplier.