Using Custom High-Strength Bolts

Nuts and bolts have been used to hold things together for hundreds of years. You can walk into any hardware store and find hundreds of bolts of various lengths, diameters, materials, and shapes.

Bolts are a unique type of part because they allow you to assemble and disassemble components quickly and easily, compared to screws and rivets which provide a more permanent solution.

Understanding how bolts work in different applications and ordering what you need can mean the difference between your project being successful or falling short. This article discusses the use of bolts, different materials, and why high-strength bolts are used.

What are bolts used for?

When compared with other types of fasteners, nuts and bolts have the advantage that they can be disassembled whenever needed. If you have a system that needs to be taken apart for maintenance or repairs, bolts make your life much easier. Other fasteners such as rivets are designed for permanent installation.

Bolts also offer a much stronger connection compared to nails or screws.

There are many different types of bolts, and each has different characteristics that make them usable across a wide range of industries.

Here are a few types of bolts that are commonly used:

Carriage Bolts: This is a type of self-locking bolt that affords a level of security by using a rounded head, which means you can only remove or loosen these bolts from the nut side.
Double End Bolts: These are also called stud bolts and feature a threaded portion on both ends of the bolt without a traditional head.
Flange Bolts: These contain a washer-like surface or flange beneath the head of these fasteners that increases the surface area of the clamping load. This reduces potential damage to the surface where these fasteners will mate. Flange bolts are often used in plumbing and automotive assemblies.
Shoulder Bolts: These are also called stripper bolts, they have a threaded bolt section of smaller diameter than the shoulder of the bolt (the section of the bolt between the head and the beginning of the threaded portion). Shoulder bolts are useful for functioning as a shaft or axle that can contain a rotating part, such as a bearing or a bushing.
Socket Head Bolts: These bolts feature a recessed head that is tightened using an Allen wrench or hex socket tool. The head style of these bolts can vary from a cylindrical profile to a flat-head countersunk style to a domed button head design, depending on the desired use. A deeper recessed Allen socket head allows for increased torque to be applied to the bolt while minimizing the risk of stripping or damaging the head.

What's the best material to use?

ASTM/SAE grade bolts can be fabricated from a number of material choices. These materials are the most common for high-strength bolts:

Low carbon steel
Medium carbon steel
Low carbon martensite steel
Weathering steel
Alloy steel
Medium carbon alloy steel

Bolts come in many varieties, such as T-Head or T-Slot bolts, and can be produced from a wide range of materials, but steel is the most common material used for high-strength applications.

In some cases, bolts will be plated with zinc or galvanized to increase the corrosion resistance. Zinc is the most common coating used today because it offers a decent level of corrosion resistance at an affordable price. Galvanized coatings offer a greater degree of resistance and are typically used where parts are exposed to moisture or salt, such as coastal areas.

How strong do structural bolts need to be?

High-strength bolts are made to withstand high amounts of pressure. A typical range for proof strength for steel is 50 to 100 kpsi (pound force per square inch). For example, a bolt with a 1 square inch cross-sectional area of steel can hold up to 50,000 to 100,000 pounds.

How is the strength tested?

The American Society for Testing Materials (ASTM) sets most of the standards for bolts in the U.S., including dimensions, materials, and finishing. A bolt’s overall mechanical strength is measured by the amount of pressure at which it breaks, while the yield strength is the level at which it bends or deforms.

Although the overall, or tensile, strength is always higher than the yield strength, for a machine to run properly the bolts should be kept under the yield level. Bolts that have a wide gap between their tensile and yield strength are considered flexible and will stretch further before breaking.

Steel bolts are graded according to newtons per millimeter squared for bolt tensile strength as compared to yield strength. For example, a steel bolt with a rating of 5.5 will provide roughly 500 n/mm2 (72.5 kpsi) of bolt tensile strength and approximately 400 n/mm2 (58 kpsi) of yield strength.

The higher the tensile strength, the tighter the bolt joint is, and the more torque the bolt can handle. The SAE (Society of Automotive Engineers) assigns bolts seven different grades, between 2 and 8.

A grade 8 bolt is the highest grade SAE bolt. Grade 8 bolts have a minimum tensile strength of 150 kpsi. Other bolt materials, such as stainless steel, alloys, and composite materials are rated in a similar fashion.

Grade 9 structural bolts are one the strongest structural bolts used today. A grade 9 bolt has a tensile strength of 180 kpsi. These bolts are ideal for heavy-duty applications and are often used in heavy machinery such as bulldozers. These bolts are considerably stronger than their grade 8 counterparts and are also more resistant to corrosive elements since they’re made with yellow zinc chromate.

How does threading impact the strength?

When ordering custom bolts, you’ll also need to consider the threading. Fine threaded fasteners are a better choice when specific measurements are required because the nut can be tightened closer.

Fine threads also provide greater tensile strength, since there is less material removed to make the threads. The shallow pitch of fine threads makes them much less susceptible to loosening from vibrations. Maintains joint tightness and clamping force better due to the smaller helix angle of the thread. Aerospace applications generally use fine thread fasteners due to their increased strength.

Larger threads are less susceptible to being nicked or damaged, so they do not have to be “handled with care” as much as fine threads. Coarse threads are much less likely to experience thread galling than fine threads. Larger thread allowances accommodate thicker plating, coatings and are therefore less likely to seize in corrosion-prone applications.

Order custom bolts for your next project

Do you need high-strength bolts for your project?

Spex manufactures made-to-order bolts that are machined to your exact specifications. We machine a wide range of materials, including steel, stainless steel, carbon steel, and superalloys like Monel, Inconel, and Hastelloy. We also supply plated bolts that have higher corrosion resistance for harsh environments.

Contact our team to get started on your next project.