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What are the different types of nuts and bolts?

What does a bolt look like? Let’s start with a nuts-and-bolts definition. Bolts are threaded fasteners with external male threads. They mate with nuts, which have female – meaning, internal – threads.

What are nuts used for? Both the bolt and the nut grip the materials being fastened, creating a bolt joint, with the nut also preventing axial movement.

The effect of the bolt joint comes down to the axial clamping force provided by the nut and the shank of the bolt, which acts as a rod that presses the joint against sideways shear forces. This is why so many bolt shanks are threadless – it makes for a stronger rod.

When to use bolts vs screws

It’s often assumed that the difference between hex bolts and screws lies in the tools used to install each – a screwdriver for a screw and a wrench for a bolt. This isn’t always the case, however. Bolts can have heads that we associate with screws and require a screwdriver for installation. Even some screws use nuts, so we’re dealing with gray areas.

The decision of which you choose really comes down to the application and the materials you’re fastening. For lightweight materials, such as plastics, plywood and drywall, screws are best. Most of the time, that is. Bolts also come in plastics, but these are mostly used for electronics, as they’re lightweight, corrosion resistant and provide excellent insulation.

For heavy-duty applications and heavier materials, such as concrete and metals, go with bolts.

Types of bolt heads

Bolt head styles are designed for the bolt’s intended function while enabling the installation tool to grip the head. Below are examples of different types of bolt heads. As seen here, bolts can have slots or drives, just as screws do.

The vast majority of nuts, no matter what type, are hex shaped. This is because the six sides make it easy to turn. It only takes a one-sixth turn for the nut to reach the next flat parallel. A nut with fewer sides takes more time to install. Other shapes are available, which are for specific needs.

For the same reason you use them with screws. Technically you don’t need washers for nuts and heavy hex bolts, but we recommend that you still use them.

a silver washer

How do washers work with bolts?

Washers evenly distribute the nut’s load and protect the surface that you’re fastening from damage. It also gives your nut a smooth surface to push against, which helps the fasteners remain tight instead of loosening. In a few cases, you’ll need to put the washer on the bolt side, but only if it’s the bolt that requires turning.

How to choose the right nut for your bolt

Nuts and bolts fasten together with their threads. The weakest shear plane in the thread profile is where failure can start. That is, the weakest material determines the strength of the connection. The point is, nuts and bolts should be made of the same alloys. Not only that, but your nut should meet or exceed the maximum tensile strength of the bolt, which is the amount of pull the bolt can withstand.

If safety is a key consideration in your application, then you want your nut to be stronger than your bolt. In these instances, the Industrial Fastener Institute (IFI) recommends that your nut should exceed your bolt’s tensile strength by 20%.

How to tell the grade of a bolt

Bolt grades indicate the strength of your fastener. Understanding bolt grades is critical to choosing the right fastener. Typically, identification markings on bolt heads include the grade and the manufacturer’s mark. The grades are indicated by raised dashes or numbers.

Bolts are one of the most common elements used in construction and machine design. They hold every­thing together – from screws in electric toothbrushes and door hinges to massive bolts that secure concrete pillars in buildings. Yet, have you ever stopped to wonder where they actually came from?

When was the first bolt made?

While the history of threads can be traced back to 400 BC, the most significant developments in the modern-day bolt and screw processes were made during the last 150 years. Experts differ as to the origins of the humble nut and bolt. In his article “Nuts and Bolts”, Frederick E. Graves argues that a threaded bolt and a matching nut serving as a fastener only dates back to the 15th century. He bases this conclusion on the first printed record of screws appearing in a book in the early 15th century.

However, Graves also acknowledges that even though the threaded bolt dates back to the 15th century, the unthreaded bolt goes back to Roman times when it was used for “barring doors, as pivots for opening and closing doors and as wedge bolts: a bar or a rod with a slot in which a wedge was inserted so that the bolt could not be moved.” He also implies that the Romans developed the first screw, which was made out of bronze, or even silver. The threads were filed by hand or consisted of a wire wound around a rod and soldered on.

According to bolt expert Bill Eccles’ research, the history of the screw thread goes back much further. Archimedes (287 BC–212 BC) developed the screw principle and used it to construct devices to raise water. However, there are signs that the water screw may have originated in Egypt before the time of Archimedes. It was constructed from wood and was used to irrigate land and remove bilge water from ships. “But many consider that the screw thread was invented around 400 BC by [Greek philosopher] Archytas of Tarentum, who has often been called the founder of mechanics and considered a contemporary of Plato,” Eccles writes on his website.

The history can be broken down into two parts: the threads themselves that date back to around 400 BC when they were used for items such as a spiral for lifting water, presses for grapes to make wine, and the fasteners themselves, which have been in use for around 400 years.

Moving forward to the 15th century, Johann Gutenberg used screws in the fastenings on his printing presses. The tendency to use screws gained momentum with their use being extended to items such as clocks and armor. According to Graves, Leonardo da Vinci’s notebooks from the late 15th and early 16th centuries include several designs for screw-cutting machines.

What the majority of researchers on this topic do agree on, though, is that it was the Industrial Revolution that sped up the development of the nut and bolt and put them firmly on the map as an important component in the engineering and construction world.

The first industrial production of bolts

The “History of the Nut and Bolt Industry in America” by W.R. Wilbur in 1905 acknowledges that the first machine for making carriage bolts and screws was made by Besson in France in 1568, who later introduced a screw-cutting gauge or plate to be used on lathes. In 1641, the English firm, Hindley of York, improved this device and it became widely used.

Across the Atlantic in the USA, some of the documented histories of the bolt may be found in the Carriage Museum of America. Nuts on vehicles built in the early 1800s were flatter and squarer than later vehicles, which had chamfered corners on the nuts and the flush was trimmed off the bolts. Making bolts at this time was a cumbersome and painstaking process.

In Britain in 1760, J and W Wyatt introduced a factory process for the mass production of screw threads. However, this milestone led to another challenge: each company manufactured its own threads, nuts, and bolts so there was a huge range of different sized screw threads on the market, causing problems for machinery manufacturers.

When were screw threads first standardized?

It wasn’t until 1841 that Joseph Whitworth managed to find a solution. After years of research collecting sample screws from many British workshops, he suggested standardizing the size of the screw threads in Britain so that, for example, someone could make a bolt in England and someone in Glasgow could make the nut and they would both fit together. His proposal was that the angle of the thread flanks was standardized at 55 degrees, and the number of threads per inch, should be defined for various diameters. While this issue was being addressed in Britain, the Americans were trying to do likewise and initially started using the Whitworth thread.

In 1864, William Sellers proposed a 60-degree thread form and various thread pitches for different diameters. This developed into the American Standard Coarse Series and the Fine Series. One advantage the Americans had over the British was that their thread form had flat roots and crests. This made it easier to manufacture than the Whitworth standard, which had rounded roots and crests. It was found, however, that the Whitworth thread performed better in dynamic applications and the rounded root of the Whitworth thread improved fatigue performance.

Introducing the ISO metric

During World War I, the lack of consistency between screw threads in different countries became a huge obstacle to the war effort; during World War II it became an even bigger problem for the Allied forces. In 1948, Britain, the USA and Canada agreed on the Unified thread as the standard for all countries that used imperial measurements. It uses a similar profile as the DIN metric thread previously developed in Germany in 1919. This was a combination of the best of the Whitworth thread form (the rounded root to improve fatigue performance) and the Sellers thread (60-degree flank angle and flat crests). However, the larger root radius of the Unified thread proved to be advantageous over the DIN metric profile. This led to the ISO metric thread which is used in all industrialized countries today.

Those working in the industry have witnessed much fine-tuning of bolts during recent decades. “When I started in the industry 35 years ago the strength of the bolts was not as fully defined as it is today,” recalls Eccles. “With the introduction of the modern metric property classes and the recent updates to the relevant ISO standards, the description of a bolt’s strength and the test methods used to establish their properties is now far better defined.”

As the raw materials industry has become more sophisticated, the DNA of bolts has changed from steel to other more exotic materials to meet changing industry needs.

What metals are used to produce bolts?

Over the last 20 years, there have been developments in nickel-based alloys that can work in high-temperature environments such as turbochargers and engines in which steel doesn’t perform as well. Recent research focuses on light metal bolts such as aluminium, magnesium, and titanium.

Today’s bolt technology has come a long way since the days when bolts and screws were made by hand and customers could only choose between basic steel nuts and bolts. These days, solutions like Nord-Lock have invented significant improvements in bolting technology, including wedge-locking systems. Customers can select pre-assembled zinc flake coated or stainless-steel washers, wheel nuts designed for flat-faced steel rims, or combi bolts, which are customized for different applications. Superbolt multi-jackbolt tensioners and Boltight hydraulic tensioners have also been added for use in heavy industries, such as offshore, energy, and mining, taking a huge step in becoming a world leader in bolt securing.
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