A bolt is not something most of us spend a lot of time thinking about, but if we didn’t have them, every appliance and gadget we own would literally fall apart.
The first bolt is believed to have been used in 400 BC. However, it took nearly 2,000 years for the bolt and its accompanying nut to be made on a machine.
Source: Nord Lock The History of the Bolt
What does a bolt look like?
A bolt is very similar to a screw, in that it has the same shape and is used to fasten materials together. The bolt itself is threaded and meant to be paired with a nut. It is important to make sure that both the nut and bolt are the same size so that they will fit together well without any extra space.
There are different bolts to choose from. They vary in use depending on their shape, head type, and size. Read on to see what type of bolt is right for you and your project.
Related: Types of Lug Nuts | Types of Construction Nails | Parts of a Screw | Types of Fence Gate Latches | Rock Bolts | Shoulder Bolts | Lag Bolts
Different Types of Bolts with Pictures
Basic bolts consist of a head and cylindrical body that has part of its length (or shaft) threaded (if it’s completely threaded, it’s a screw). The bolt screws into a nut which has internal threads to match the bolt’s. A flat washer is often used to prevent the nut loosening.
Allow objects to be attached to concrete, with the bolt head normally placed in the concrete before it has cured so the threading end is exposed and ready for use. There are different kinds of anchor bolts, with the simplest and strongest being cast-in-place.
With cast-in-place bolts, the embedded end has a regular hexagonal bolt and washer, flange, or 90-bend, with the load-transfer mechanism being in the mechanical interlock. There are also post-installed anchors, that can be placed in concrete after drilling.
Learn more about the anchor bolts here.
Have the washer permanently attached, and the threading on these is reversed. These are designed for use in certain tools, such as the miter saw, to make sure the tool stays secure and the blade doesn’t fall out.
Find out more about arbor bolts here.
Have a smooth rounded head and a square section that will keep the bolt from turning; these also have a threaded section for a nut. Carriage bolts are also called round head square neck bolts and are used to fasten metal to wood.
Normally there are strengthening plates on either side of wooden beams, and then those beams are fastened with the carriage bolts. These are also used for needs like locks and hinges.
Read about carriage bolts here.
Have a large flat head and are usually used in conveyor system setups.
Read more on elevator bolts here.
Don’t have any heads at all, instead consisting just of a machine threaded body and a wood threaded screw tip. Technically, hanger bolts are studs, and they can be used to suspend objects from wood or attach objects to wood.
There are threads at each hand of a hanger bolt, lag threads on one end to screw into the wood and machine screw threads on the other end to accept the nut or thread into an already threaded hole. Screw-in-stud mountings can usually hold about 300 pounds.
Hex bolts have a hexagonal head and a threaded body. The section right under the head may not be threaded.
Read all about hex bolts here.
Shaped like the letter J, and are used for tie-downs, helping to hold down materials or equipment to keep it safe.
Also called lag screws and aren’t really bolts, with a hex bolt head and thread screw tip so it can be used in wood materials.
Help to stabilize walls in tunnel construction.
Sex bolts or Chicago bolts
Have both a male and female part with interior threads and bolt heads on both ends. These are commonly used for paper binding.
Shoulder Bolts or Stripper Bolts
Help create a pivot or attachment point. They have a broad smooth shoulder but a small threaded end.
Shaped like the letter U, with the two straight sections threaded.
Bolt Types by Shapes and Usage
Bolts can also be characterized by their shapes and usage.
These are one of the most common kinds of fastener and will be used in many projects as a standard bolt. Hex bolts have three standard lengths or grades and different diameters for the shank, ranging from 1/4″ to 4″.
Compared to the square bolt, the hex head offers more strength, it is easier to assemble, torque application is easier, and there is more area for the manufacturer’s identification.
Unlike most traditional bolts, bent bolts are not straight. The end is bent or shaped to meet special requirements, such as in a right-angle bend or in the shape of an eye.
Bent bolts are used in a variety of industries, including waterworks, steel fabrication, metal building, and other special kinds of work.
There are different kinds of bent bolts:
- Anchor bolts — usually used to attach structures to concrete and for all other types of projects.
- U-bolts — around bent bolts which can be used to attach pipe or steel round bar to a round wood or steel post; these are often used with tie rods or all-thread rods to tighten a connection; for hanging wrought iron pipe in mechanical installations; these can also be embedded in concrete and used as anchor bolts.
- Eyebolts — most often used to attach the eye to a structure so that ropes or cables can be tied to it.
- Offset eye bolts (also called tie bolts) — in conjunction with tie rods or all-thread rods, these are used to tighten a connection.
Bent bolts can also be specially manufactured in different shapes to meet specific needs.
These are made to look good, with a rounded head that has a more aesthetically pleasing appearance.
The body of the bolt is threaded in the normal way, but when you get to the shank, there are ribs or flats that will keep the bolt from turning once it has been tightened. Some carriage bolts are simply pressed into place, while others have a pre-punched square hold.
Track bolts are actually used for railroad tracks. They are also called fang bolts or rail anchor bolts, and they are used to fasten rail joints to link rails or to fix steel rails and rail chairs to railroad ties.
These are very strong, high-quality bolts that are specifically resistant to vibrations. Track bolts come in various sizes and shapes, including
- Button head oval neck track bolts
- Diamond neck track bolts
- Rail bolt NF F50-008
- Clip bolt with BSW thread
- Fish bolt used for rail joints (with rail nuts and spring washers)
- Clip bolt HS26 & HS32
- Clamp bolt and inserted bolt for Russia
- Anchor bolt
- Special bolt
- Tunnel bolt
Because of their tight safety requirements, track bolts have special requirements. They must be the right shape and size, and they must always be tightly in place.
They need to be installed with AREMA spring washers, and the areas must be cleaned and lubricated properly. If bolts are missing or ineffective, there are rules in place to make sure the situation is remedied.
Square bolts are commonly used for many kinds of projects and have a square head and threaded body.
These bolts are often used for aesthetic purposes, such as to provide a rustic look to your project or to match existing bolts (or other kinds of fasteners) in an older structure. They come in multiple strength grades and range from 1/2″ to 1-1/2″ in diameter.
Round bolts look good, giving a smooth, finished look to projects at the end. They are similar to carriage bolts, but they differentiate themselves by not having the square neck under the head.
They are characterized by their smooth round head, and they are tightened by torquing the nut that goes with them. Round bolts are often used in wood connections.
Aircraft bolts actually include several kinds of fasteners that have a 12-sided head and offer greater strength than standard bolts. The head allows for a much better grip during assembly, with so much extra area, and also allows higher levels of torque when tightening it.
While there is a wide variety of different kinds of aircraft bolts, an aircraft bolt can also be any bolt that meets DoD standards.
Plow bolts are characterized by their flat, countersunk head, square neck, and unified thread pitch. The plow bolt is also similar to the carriage bolt, but the plow bolt differentiates itself with its concave or flathead.
The odd design allows for flush mounting, and sometimes there is also a key that will keep the plow bolt from rotating. Because of this, plow bolts are often used in plows, road graters, bulldozers, and a variety of other pieces of heavy-duty industrial equipment.
Lag bolts have a square head with threaded conical points and are commonly used for projects involving wood or masonry. They are applied and secured with an expansion anchor and are very sturdy.
They can connect very heavy lumber and other heavy materials, and are much larger than regular wood or sheet metal screws. Lag screws are only available with hex heads, while the other screws come in a wider variety of shapes.
Flanged bolts, like many of the other bolts here, have a large head which makes it easier to secure the bolt and identify the object. The circular flange under the head acts as a washer, distributing the load, actually providing about four times the bearing area of a standard hex screw.
Flange bolts are often used in vehicle frames, especially trucks. Flange bolts that aren’t serrated are usually called frame bolts.
Elevator bolts come with their own special washers and are characterized by their square necks, flat heads, and partial threads. The square head can be countersunk and will resist turning, making it easier to turn the bolt.
Elevator bolts come in different shapes and sizes and even have different ways of attaching to the surface, so it is important to know you are choosing the correct one for the job you are currently doing.
Elevator bolts are useful because they can be used on soft surfaces, even softwood, while still remaining sturdy and safe. Elevator bolts are essential in the creation of some conveyer belt systems and are also used for lifts, which have an increased load capacity.
These kinds of bolts are also used in vehicle floors, such as RVs and campers because regular bolts wouldn’t be able to handle the constant bending and flexing.
Countersunk bolts are also called stove bolts when they work together with machine screws. Countersunk bolts are used to flush-mount assemblies, and are available in different shapes and sizes depending on the current need. These kinds of bolts are often used in bridge decking, walkways, and railing.
Types of Bolt Material
The bolt material will depend on what the bolt is used for, especially what kinds of materials are being fastened. There are varying levels even among the different materials that will make the bolt stronger. For instance, a Grade 2 bolt is made out of standard hardware grade steel.
Grade 5 alloy steel is medium carbon zinc-plated alloy steel that was treated with heat in order to increase hardness. Grade 8 steel has been hardened even more so it can be used in particularly demanding applications like automotive suspensions.
This is the most common material used to make bolts and will work for most purposes. About 90% of all bolts are made using regular steel.
With its inherent strength and low price, steel can be used in most jobs unless there are problems with the environment or with the materials you’re using for the project.
Stainless steel is a steel alloy and offers some extra strength and protection. For instance, as an alloy, stainless steel has corrosion resistance, which is useful for certain kinds of construction where the bolts will be exposed to different weather.
Because it is actually an alloy, stainless steel won’t lose its special properties even if it is scratched or damaged. Because of that, stainless steel bolts are a popular choice in areas close to water or in areas with high levels of pollution.
Another alloy, bronze also contains copper and sometimes other metals and non-metals. Bronze may be used when the product needs to be waterproof. Specifically, bronze bolts are usually used in marine environments, especially with wooden boat construction and repair.
Nylon is obviously much lighter than the other materials here, and is used for that characteristic and when the final product needs to be waterproof. Because they aren’t as strong, nylon bolts won’t be able to hold as much weight as most other kinds of bolts.
They are, however, useful for holding materials such as plastic, wood, rubber, particleboard, and metal. They are frequently used in home appliances, like washing machines, and with automotive parts.
Titanium is lightweight, resistant to corrosion and strong. Bolts made for Titanium are used where there’s a requirement for a high strength to weight ratio and can be up to 4 times stronger than stainless steel. These bolts are mainly used in airplanes, ships and submarines.
Choosing the Right Bolt Types
There are many different kinds of bolts, in different shapes and sizes, and it becomes even more confusing when you see that there is overlap between different types of bolts and what they are called.
Some bolts will offer more strength for use in certain materials, while a different bolt that might normally be considered stronger wouldn’t work well at all in that situation.
There is also the matter of finding the right size for your project, which can be confusing if you haven’t done it by yourself before.
Choosing the correct bolt size
You will need to know both the length and the diameter of any fasteners you intend to use. If you have a fastener, you measure it from the outer thread on one side to the outer thread on the other side.
That is called the major diameter and will be the bolt size you need. The length is the entire length, from the flat part of the head to the tip or blunt end of the screw or bolt end.
Choosing the correct bolt size is very important.
Choosing the incorrect bolt size can result in a variety of increased costs, such as in the form of wasted time — and we all know that time is money, waste in the cost and misuse of materials, and the possible loss in sales if you plan on selling your work.
One of the most common mistakes is to oversize the bolt, so be aware of the actual requirements that your bolt needs to fulfill, and then choose your bolt size accordingly.
Choosing the right material
When choosing a bolt based on material, remember that stronger isn’t necessarily better if the materials don’t work well together. Also, remember the advantages and disadvantages of each, and notice the big difference in pricing with some of the materials.
It doesn’t necessarily make sense to spend more on the bolts than the other materials. For example, stainless steel is corrosion-resistant, but it is a much more expensive material. Make sure to choose the bolt within the right price range and right material for you and your project.