This article takes an in-depth look at types of bolts.
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Bolts are tools used to fasten and secure the faces of assemblies. They are threaded and come with a matching nut. Their purpose is to connect aligned unthreaded holes by being placed through the aligned holes and secured with the bolt’s nut.
The shaft is a cylindrical member that bears the shear forces acting on the bolt and prevents the radial movement of the parts. The bolt shaft has two portions:
The fastened parts may produce or be exposed to vibration, which increases the chance of the bolt loosening. If a bolt has no shank (i.e., the bolt shaft entirely consists of threading), the vibration may push back out of the fastened parts.
The bolt head is responsible for bearing the axial forces that hold the fastened materials. A wrench or a driver grips and applies sufficient torque on the bolt head to tighten or loosen the bolt. Bolt heads come in a variety of profiles and types; some of them are:
Bolts are typically used with the following mechanical devices:
The nut is the female member of a bolted joint that contains a threaded hole. It provides clamping force and prevents axial movement in order to hold multiple parts together. A combination of two forces secures the parts. The nut exerts the axial force, which compresses the fastened parts. This axial force results in a frictional force between the bolt and nut threads, preventing the bolt and nut from being unscrewed. Mechanical locking devices (e.g., lock washers, jam nuts, and split pins) and thread-locking fluids are used if the parts are exposed to prolonged and continuous vibration and impact.
A washer is a flat, thin plate with a concentric hole at its center. It is typically placed under the bolt head and the nut. Its primary function is to distribute the load of the threaded fastener over a larger area. Thus, the deformation of fastened materials is prevented. Washers made with softer materials such as plastic, rubber, and urethane can effectively dampen vibration, thus preventing the threads from loosening. Special types of washers can prevent liquids from penetrating the bolt joint, creating a waterproof seal.
The following are the fastener terminologies used when talking about bolts:
The shank diameter is the diameter of the smooth or unthreaded portion of the bolt shaft. It is often referred to as the nominal diameter or body diameter.
The major diameter is the largest diameter of the bolt thread. It is the distance between two opposite crests or the topmost surface of the thread. The major diameter is often equal to the nominal diameter.
The minor diameter (or root diameter) is the smallest distance of the bolt thread. It is the distance between two opposite roots or the bottommost surface of the thread.
The pitch diameter is the diameter halfway between the major and minor diameters.
Basic Dimensions of a Bolt Thread
The nominal length is the total axial length of the bolt shaft, which is equal to the sum of the grip and thread lengths. The grip length is the length of the unthreaded portion, while the thread length is the length of the threaded portion measured from the first complete thread next to the shank to the end of the bolt shaft.
The thread pitch is the axial distance between two adjacent crests. This specification is used in metric fasteners and is normally expressed in millimeters.
Metric fasteners are manufactured in accordance with the ISO and DIN standards. How do you read metric bolt sizing?
If you encounter a bolt with M10-1.0 x 2.0 sizing:
You may also encounter a bolt specification like M12-50. The thread pitch is not specified in this specification, which means the bolt has a coarse thread. The nominal diameter in this example is 12 mm, while the bolt length is 50 mm.
The TPI is the number of threads per bolt thread inch. This specification is only used in imperial or US fasteners. How do you read imperial bolt sizing?
If you encounter a bolt with ¼”-20 x 2” sizing:
Right-handed threads are tightened when rotated in a clockwise direction. It is the most common thread configuration. Meanwhile, left-handed threads advance axially when turned in a counterclockwise direction. These threads are used when the application of pressure causes a right-handed thread to become loose. Left-handed threads are used in bicycle pedals, vehicle wheels, circular saw blades, and gas supply valves.
A chamfer is a slightly beveled edge near the bolt tip. It is created by cutting a portion of the first thread near the bolt tip. Some bolts have a chamfered end in order to insert a nut and fit into holes easily. The chamfer angle is measured from the normal to the bolt axis.
Below are different types of bolts.
Anchor bolts are designed to join components into concrete surfaces. They are commonly used in the construction industry. They are usually found in the bases of steel poles. They are also used in attaching façade systems to reinforced concrete walls. There are two classifications of anchor bolts:
There are two common types of anchor bolts:
Arbor bolts have a washer permanently attached to their heads. They commonly have a dark or black finish. Their essential feature is their reversed or left-handed threads, making them suitable for power tools such as miter saws. These bolts keep the blade of a miter saw in place.
Blind bolts are suitable when access to one of the sides of the fastened materials is restricted. The blind bolt is installed by inserting it into a predrilled hole and rotating it by 1800 to expand the locking anchor in the blindside of the bolt. The nut at the visible side is then rotated by a spanner while preventing the rotation of the installed bolt. This mechanism produces a durable joint that can be made in a few minutes.
Carriage bolts have a shallow dome head and a flat tip. The bolt shaft is partially threaded. A square neck is present under the head to keep the bolt from turning during the tightening of the nut. They are commonly used in joining metal and wooden parts together. They are used in furniture, patios, frames, and decks.
Elevator bolts are characterized by flat, thin, and round heads, which sit perfectly with the connection surface and distribute the load evenly over a larger area. The bolt shaft is partially threaded, and the threads are coarse. A square neck is present under their head to keep the bolt from turning while the nut is tightened or removed. Elevator bolts are ideal for elevators, pulleys, and conveying systems and do not interfere with their operations.
Eye bolts feature a fully or partially looped shank and a thread on their straight end. A round or square neck may be present between the loop and the thread to impart a higher bending resistance. The looped shank can lift weights to a certain extent. Some types of eye bolts can serve as a connection point for lifting, hoisting, pulling, and rigging applications. They can also be used in organizing and routing items such as cables, wires, hoses, and ropes to eliminate occupational hazards.
Flange bolts have a circular washer-like flange under their head, which distributes the clamping force over a greater area. In vehicles, they protect the engine and transmission systems by holding the outer casing tightly. In piping systems, they firmly hold the ends of two flanged pipes together.
Serrated flange bolts are almost similar to standard flange bolts in construction, except that the circular flange under their heads has teeth that grip onto surfaces firmly. These teeth bite onto surfaces to resist vibration effectively and prevent threads from loosening. Serrated flange bolts are commonly used in mechanical and plumbing applications and electronic product casings.
Hanger bolts are headless bolts that consist of a lag screw thread at one end and a machine bolt thread at the other. They are ideal for fastening wooden and metallic materials together. The lag screw thread has a pointed tip which enables them to tap holes into the wooden part. The machine bolt thread is inserted into the predrilled holes of a machine. A nut is inserted and tightened on the machine bolt thread to complete the setup.
Hex bolts are characterized by their forged hexagonal head, which can be easily gripped and turned by a wrench. They are commonly used to fasten machine parts and structural components. In addition, they are suitable for joining parts made of metal and wood. Hex bolts are available in a wide variety of standard sizes.
J-bolts are characterized by their round or J-bend. They do not have a bolt head, but they can accept a nut at their threaded end. They are commonly used in the construction industry. Their bent end can be cast in a concrete slab as an anchor bolt. They are used in securing walls to concrete and roofing applications.
Lag bolts or lag screws are technically screw fasteners because they do not accept a nut. Unlike other bolts with a blunt tip, they have a pointed end which enables them to create a threaded hole as they are rotated on their heads. Lag bolts are usually provided with a hexagonal or square head. They are commonly used in fastening wooden materials.
Machine bolts typically have a hex or square head, which a wrench or a socket can tighten. They have a rounded or non-chamfered end and are not designed to accommodate a washer. Compared to machine screws, they are available in larger sizes or diameters. They are typically used in joining metal to metal and metal to wood. They can also be used in fastening wooden materials. They offer higher rigidity and load capacity than lag bolts but require access to both sides of the fastened materials.
Pentagon® tamper proof bolts, also known as penta bolts, are a type of industrial fastener made of stainless steel. They have a five-sided head in the shape of a pentagon and come in many sizes. The five-sided design of the penta bolt prevents the accumulation of debris or dirt, which can be easily swept away. When used to secure ground surfaces, penta bolts are recessed to avoid tripping hazards.
The penta bolt is widely used for city, municipal, and government infrastructure applications such as manhole covers, waterworks, valve boxes, meter boxes, ground vaults, electrical junction boxes, and concrete enclosures. They are categorized as security bolts since it requires a specially designed tool to loosen or remove them.
Penta head bolts are manufactured as machine bolts, washer head bolts, washer head captive style, and lag bolts in various sizes for all types.
Plow bolts are similar to carriage bolts except for their countersunk heads, which are ideal for making mechanical connections with even surfaces. They also feature a square neck to ensure that the bolt will not rotate as the nut is tightened. Plow bolts are used in heavy equipment such as snowplows, tractors, excavators, and scoop shovels.
A rock bolt is a special type of anchor bolt used to stabilize a rock structure to prevent it from crumbling and falling on workers and tourists. It works by knitting the rock mass together. Rock bolts are typically made from steel and fiberglass and are typically stabilized by adhesives once installed.
Sex bolts are fasteners with a barrel-shaped female component with internal threads covering the threaded male component. The female component serves as the nut and is almost the same length as the male component. Both components have a flange that serves as the bearing surface and makes the connection surface more even. These bolts are ideal for fastening materials sensitive to abrasive parts. They are used in paper binding, helmets, railing systems, furniture, playground equipment, signages, bathroom partitions, and other products.
Shoulder bolts are machine bolts with an unthreaded shoulder under their heads, which are larger in diameter than their threads. The shoulder diameter is considered the nominal diameter of the bolt. When installed, the shoulder extends beyond the surface in which the thread is embedded. The shoulder acts as the shaft for bearings and bushings, the axle for rolling parts, the guide for sliding elements, and pivot points. It is also used to align molds and dies accurately.
Socket head bolts feature a recessed slot in their bolt heads which accommodates an Allen wrench or a socket tool. The torque applied by these tools on the recessed slot minimizes the risk of damaging the bolt head. Several socket head styles are available, such as cylindrical profile, flat-head countersunk profile, and domed profile. Socket head bolts are also known as socket head cap screws.
Square bolts are machine bolts with a square bolt head that offers a large bearing area for positive gripping. They were once widely used before hexagon heads became the industry standard because they could be turned easily by a variety of tools. Square bolts are still available nowadays for decorative purposes; they provide a rustic look and match the bolts in older structures.
Structural bolts are used with hex nuts to connect structural steel members and are made to meet ASTM standards A563 and A194. They are graded by their size range, proof load, and their minimum yield and tensile strength. Structural bolts are categorized as high-strength bolts because of their exceptional tensile strength and ability to form secure and tight joint connections. In addition, they can withstand higher torque, which is essential for steel constructions.
SAE Grade 9 structural bolts, also called grade 9 hex cap screws, are the strongest form of structural bolt, with a tensile strength of 180,000 PSI. They are used in highly stressful applications where heavy-duty bolts are required, such as manufacturing machinery and bulldozers. In addition, grade 9 structural bolts are made of yellow zinc chromate, which makes them corrosion-resistant.
Stud bolts are headless bolts with threads on both ends of their shaft. The thread may continuously run the shaft (fully threaded) or have an unthreaded portion in the middle of its shaft (partially threaded). The fastening is made by installing a hexagon nut on each end. They are also used in connecting equipment, parts, and permanent fixtures.
There are two types of partially threaded stud bolts:
T-head bolts have a large T-shaped or square-shaped head. The head can be easily inserted into T-shaped slots that sit flush or below the surface of the surrounding material. T-head bolts can be used in fastening lift guide rails.
Tap bolts are fully threaded hex bolts. They are designed to fasten parts through their unthreaded holes, which are tightened by a nut. A washer can be installed underneath the hexagonal head. Tap bolts are used to adjust the tension of a pulley and attach motors to machines.
Toggle bolts are used to hang items such as plants, bookshelves, and ceiling fans on hollow walls and ceilings. They feature a set of spring-loaded wings that expand inside the hollow wall. The wings brace against the wall, spreading the load onto a greater area. They also increase the loading capacity compared to a regular bolt.
Tork bolts, also known as shear bolts or breakaway bolts, are a unique type of security bolt that come with both flat or countersunk heads and button heads. The structure of a tork bolt is unlike any other form of bolt since they have one head placed on top of another with a breakaway connector. The upper head is hexagonal for easy installation using traditional wrenches.
With a tork bolt, the hexagonal head is designed to shear off or break away when the head's maximum torque is achieved. This configuration permanently places the bolt, making it difficult to remove using conventional tools. When a wrench separates the head from the bolt, the bolt left behind can no longer be gripped, making the bolt exceptionally secure.
Since installing tork bolts does not require a special tool, they are an easy solution for attaching products, equipment, or components that could be vulnerable to tampering using any other forms of bolts. Their purpose is to protect secured items against unauthorized relocation, tampering, or interference.
Track bolts are designed to connect and secure railway tracks onto their surface. A washer is frequently added to reduce the intense vibration effects brought by a passing train. Some track bolts may be cast into concrete. Track bolts are commonly made from steel to impart high strength, which is treated for enhanced corrosion resistance.
U-bolts are metal rods bent in a U-shape with threaded ends that accommodate a nut. The U-bend perfectly fits around pipes and tubes; hence, U-bolts are commonly used in hanging pipes and tubes in ceilings, walls, and other overhead structures.
The types of bolts are:
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