Bolts are essential fasteners used in countless applications across various industries. With many bolt types and grades available, determining the right one for your specific needs can take time.
In this ultimate guide to bolts, we’ll dive into markings, grades, and strengths to help you make an informed decision. We’ll explore how to choose the correct bolt for your application, the different types of bolts available, what their markings indicate, and some frequently asked questions.
How to Choose the Right Bolt
When selecting a bolt, think about the specific requirements and demands it will face. Common questions to consider include:
What is the bolt size?
What material is the bolt made from?
Where will the bolt be used?
What strength of bolt is required?
Are there any special requirements such as security, aesthetics or anti-tamper capabilities?
The type of bolt you should choose will also depend on the industry and the application. Examples of different industries and common use cases include:
Construction: Bolts used in construction must withstand significant loads and resist corrosion from exposure to the elements. High-strength structural bolts are commonly used.
Automotive: Bolts in the automotive industry must handle vibration, high temperatures and various fluids. Grades 5 and 8 are often used for their strength and durability.
Machinery: Industrial machinery requires bolts that can cope with high stresses and dynamic loads. Grade 8 bolts or Metric Class 10.9 are frequently used.
If you’re unsure about the specifications needed for your application, consult us at C.M. Mockbee for the insights.
Types of Bolts
There are numerous types of bolts, each designed for specific purposes and applications. Some common ones include:
Hex Bolts: The most common type features a hexagonal head and is available in various sizes and grades.
Hex Cap Screws: Similar to Hex Bolts, Hex Cap Screws have more precise tolerances and are often fully threaded. They typically have a washer face under the head and are commonly used in machinery and equipment assembly. Some common fastener specifications unless otherwise specified: ASTM A449 and SAE J429 Grade 2, 5, and 8.
Structural Bolts: Also can be described as Heavy Hex Bolts. These high-strength fasteners are designed for steel-to-steel connections in building and bridge construction. They feature a heavy hex head and shorter thread length, meeting strict standards for load-bearing applications.
Carriage Bolts: These have a smooth, rounded head with a square neck underneath to prevent rotation during tightening. They are ideal for wood and metal applications where a flush surface is desired.
Hex Lag Screws: Lag Bolts is often used to describe this product. Lags have a pointed tip and coarse threads designed to be driven into wood without a nut. They provide strong holding power in wood-to-wood and metal-to-wood connections.
Anchor Bolts:Anchor Bolts attach structural elements to concrete foundations. They are embedded into the concrete and provide a means of securing other components.
Tension Control Bolts: Also known as TC Bolts, these feature a dome-shaped head and are driven using a specialized electric shear wrench. They are commonly used for high-strength, steel-to-steel fastening and are sometimes even preferred over large hex bolts.
Shoulder Bolts: These bolts feature a smooth, unthreaded shoulder between the head and threaded portion. They are used for pivoting or sliding applications.
Elevator Bolts: An elevator bolt is a threaded fastener with a thin, flat head and a square neck that’s used to secure parts in tight spaces with moving parts, like in elevators, conveyor belts and securing flooring.
Step Bolts: They are commonly used on steel communication and electrical transmission towers. They are similar to Carriage Bolts in design, but have a lower profile head
Markings and What They Mean
Bolt markings provide essential information about the bolt’s grade, strength and material. The most common grading systems for bolts are:
Society of Automotive Engineers (SAE): Numbers represent SAE grades, with higher numbers indicating greater strength. Grade 2 bolts are made from low-carbon steel, while Grade 5 and 8 bolts are made from medium-carbon steel and alloy steel, respectively.
Metric: Numbers designate classes like 8.8 or 10.9 in the metric grading system. The first number is the nominal tensile strength in 100 megapascals, and the second is the yield strength as a percentage of tensile strength. For example, a Class 10.9 bolt has a nominal tensile strength of 1,000 megapascals and a yield strength of 90% of its tensile strength.
American Society for Testing and Materials (ASTM): Bolts are marked with the ASTM designation, like A325 or A490. If applicable, this designation is followed by a grade based on the steel’s mechanical properties and chemical composition. A325 and A490 are high-strength structural bolts. Different variations are made from low-carbon, medium-carbon or atmospheric corrosion-resistant steel. A490 is used in more demanding structural applications where greater load-bearing capacity is necessary, as it boasts a tensile strength of 150,000 pounds per square inch (psi). Grade B is medium-strength, medium-carbon steel, while Grade C is high-strength, high-carbon steel.
Grade markings on bolts are typically found on the bolt head and may include additional information, such as the manufacturer, material or country of origin. Some standard markings include:
Grade 5 Head Marking
Grade 8 Head Marking
Metric Class 8.8
Some manufacturer’s add their logo or initials as a brand marker that can be helpful as it allows you to trace the fastener back to the manufacturer if there is a defect.
While different grading systems exist, many of them have correlations that can help compare bolt strengths across standards. For example, ASTM A325 bolts are similar in strength to SAE Grade 5 bolts. Both are medium-strength fasteners commonly used in structural assemblies.
ASTM 490 bolts correspond roughly to SAE Grade 8 bolts and are both high-strength fasteners used in demanding applications. Metric Class 8.8 bolts are similar in strength to SAE Grade 5 bolts, while Class 10.9 bolts are comparable to SAE Grade 8.
Learning these correlations can be extremely helpful when substituting bolts, working on international projects or comparing specifications.
FAQs
To help you better understand how to pick bolts for your application, here are some answers to commonly asked questions:
What is bolt tensile strength? A bolt’s tensile strength is the maximum load it can withstand before breaking when pulled apart. The bolt’s grade, size and material determine its tensile strength.
What are the four parts of a bolt? The four main parts of a bolt are the head, shank (unthreaded portion), threads and point (tip).
What are the grades of bolts? Common bolt grades include SAE Grade 2, 5 and 8 and Metric Class 8.8, 9.8, 10.9 and 12.9. Higher grades indicate greater strength.
How are bolts classified? Bolts are classified by their grade (strength), type (hex and carriage), size (diameter and length), thread pitch and material.
What is the strongest grade for bolts? The strongest commonly available bolt grades are SAE Grade 8 and in Metric Class 12.9.
Trust the Bolt Experts at C.M. Mockbee Co.
It’s important to work with experts when determining a strong bolt for your application. At C.M. Mockbee Co., we have over 100 years of experience providing high-quality fasteners and exceptional customer service. Our expert staff is ready to assist you in selecting the perfect bolts for your needs through custom threaded solutions or our VMI program.
With extensive inventory and local delivery services, we’re committed to meeting your fastener requirements. Call us today at 513-621–3864 or contact us online to schedule an appointment. Experience the difference our expertise can make in your projects.
