In this article, we’ll cover what a bolt is, what types of bolts there are, and what the properties of each type are. The table of contents is below:
About Bolts
A bolt is a cylindrical threaded fastener with a head (hexagonal, etc.) that joins parts together in a removable way using a nut, with tightening torque usually applied to the nut.
What’s the Difference Between a Bolt and a Screw?
Bolts, when tightened with a wrench, provide much more clamping force than screws. They’re also larger in size than screws.
Bolts generally clamp together with a nut. Screws, on the other hand, are usually threaded directly into a body.
NOTE: According to Turkish standards, both are technically called “civata” (bolt). The distinction given above generally reflects terms used in the market.
Bolts and screws can be made by hand. But meeting industrial demand this way isn’t feasible. Also, the extra labor and material waste make manual production very costly. That’s why bolts and screws are made automatically, without removing material, on special machines. Special machines have been built for this purpose that can produce thousands of bolts and screws per hour.
Not all bolts are used under the same conditions. Some are used in very cold environments, others in hot ones. The connection needs to stay secure under these conditions too. That’s why bolts need to be manufactured with technical and environmental properties suited to the working environment.
NOTE: Under no circumstances should the threaded portion of a bolt be extended afterward with a die. Since bolts are thread-rolled on automatic machines, the root of the thread isn’t a full thread profile. Because of this, a die will cut and damage the threads instead of forming them properly. A secure connection can’t be made with a bolt whose threads have been damaged this way.
Bolt Types (by Head Shape)
Hex Head Bolts
Hex head bolts are frequently used in machining for joining machine parts, and in construction for assembling steel structures. Depending on the requirements of the job, bolts are made with either normal or fine threads.
The bolt’s threaded length is determined by its intended use. Depending on the application, it may be made with a partial thread or fully threaded along its length.
Allen (Socket Head) Bolts
Also known as an Allen bolt. When the space around the bolt head is too tight for a wrench to fit, or when the bolt head shouldn’t stick out, cylindrical socket-head (Allen) bolts, or their smaller counterparts, Allen screws, are used.
Countersunk Head Bolts
Countersunk socket-head bolts have a thinner head than cylindrical head bolts. When joining thin materials where the bolt head shouldn’t protrude, countersunk head bolts are used. The countersunk shape of the bolt head also keeps it centered in the hole as it’s tightened.
Slotted Head Bolts
Cylindrical head bolts with a screwdriver slot cut into the head are used for connections small enough to be tightened and loosened with a screwdriver.
Sheet-metal-thickness parts are joined with cylindrical head screws; thicker parts are joined with countersunk or oval-countersunk head screws.
Phillips (Cross) Head Bolts
Phillips-head screws provide stronger tightening than slotted screws of the same size. The main advantage of Phillips-head screws is that the head doesn’t get damaged while tightening or loosening.
Since the tightening force is distributed across four arms via the cross-shaped screwdriver tip, this achieves strong tightening while keeping the head from wearing out.
Bolt Types by Shank Shape
Stud Bolts (Headless Bolts)
A stud bolt is a headless bolt threaded on both ends. Studs are used in connections that are removed and reinstalled frequently. One end is tightened very firmly into the machine body so it stays fixed. After the part to be attached is placed over it, it’s secured with a nut threaded onto the stud.
When the joined parts need to be removed, only the nuts are taken off. The stud stays in place. This method prevents the threads in the main body from wearing out.
Stretch (Tension) Bolts
Used for large shaft lengths that experience dynamic loading, such as connecting rods and high-pressure flange connections. During tightening, its long, thin shank stretches elastically. Because of this, stretch bolts don’t rely on friction alone for security. Its shank diameter is around 90% of the thread’s minor diameter. For these bolts to do their job properly, they need to be installed with a high preload tension.
Waisted (Reduced-Shank) Bolts
Waisted bolts are designed to prevent permanent physical changes in a connection that could otherwise result from forces stressing the joint axially.
A waisted bolt’s shank diameter can be reduced to as little as 10% of the thread root diameter. Under the forces acting on the bolt, this thinned section stretches slightly. Once the force is removed, the bolt returns to its original shape. This happens because of the elasticity of the bolt material. Waisted bolts don’t need an additional locking system to prevent loosening — being very firmly tightened and elastic solves this problem on its own.
Fitted (Reamer) Bolts
Some machine elements that transmit large torques are joined with special bolts that have precision-ground shanks with tight tolerances. The holes the bolts pass through are also reamed. (See: Reamers and Reaming)
If elements that transmit large forces aren’t joined this precisely — meaning the bolt hole is oversized — the bolt suddenly encounters a large shear force the moment it starts operating. Bolts repeatedly subjected to these sudden shear forces have a short lifespan. They can fail unexpectedly, taking the system down with them.
Special-Purpose Bolts
Used in the automotive sector, heavy equipment, vehicles of all kinds, industrial machinery, and anywhere a fastening is needed. Depending on the grade, they’re made from materials like C35, 41Cr4, or 42CrMo4. We call them “special” because they’re made in dimensions different from standard bolts.
For example, depending on nominal size, the d2 diameter can be larger or smaller. There may be a cotter pin hole at the tip. The same bolt may have different thread pitches cut at different sections.
An eye bolt is an example of a special-purpose bolt. Here, we see a sample application together with a wing nut.
Thread Standard Compatibility
Bolts are made to different thread standards, like Metric or Whitworth, and mixing them up can damage the threads.
Strength Class Marking
Codes like 8.8 or 10.9 stamped on the bolt head indicate the material’s tensile and yield strength.
Variety of Head Shapes
Bolts are made with different head shapes — hex, cylindrical, countersunk — tailored to the application.
Related Questions
Bolts are designed to work together with a nut, and a wrench can apply a large amount of torque to that nut. Screws, on the other hand, are usually threaded directly into a body and aren’t intended to generate this level of clamping force.
Manual production is both time-consuming and wastes a lot of labor and material, which drives the cost up significantly. Special machines make the process economical by producing thousands of bolts per hour, both quickly and at consistent quality.
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