What You Should Know About Bolt & Fastener Grades

They say the devil is in the details. This is especially true when overlooking them might cause catastrophic failure.

The topic of bolt & fastener grades isn't exactly glamorous, but without the vital knowledge of their appropriate application and capability, there's a good chance your project could come apart at the seams.

In an industrial setting, the last thing you want to do is use a bolt or fastener that isn't up to the task. A chain is only as strong as its weakest link. When the chain is a building full of people, realizing it's held together by the wrong grade of bolt might keep you up at night.

That's easily avoided, however! Keep reading and we'll explore the different types of bolts and fasteners available and where they should and shouldn't be.

What Are Bolts?

Put simply, a bolt is a type of threaded fastener that allows the joining of separate entities, whatever they may be. Force is applied through leverage directly to a matching female thread in the form of a nut. This force becomes locked in place through friction.

The generic term ‘bolt’ is sometimes used interchangeably with setscrews. The difference between these two products is that a bolt has an unthreaded shank under the head, whereas a setscrew has a thread that runs the entire length of the shank.

For example, to join two girders together all you need to do is drill the appropriate holes, slip the bolt through and tighten the nut. The force applied to the nut translates directly to the force the girders pull together.

The force applied upon the threads stays constant as long as the material the bolt joins to keeps its form. The same applies to the material of the bolt itself.

The type of bolt used for each job is of paramount importance. They're designed for a myriad of stresses, environments and applications. Using the wrong bolt for the job spells an easily avoidable disaster.

The Basics

This is pretty basic but without a reliable foundation we'd have nothing to build upon. Oftentimes, when you neglect the basics you end up being burned.

You can hire the best architects and engineers in the world, the most proficient builders, site managers, logistics teams… but supply them with the wrong bolts and it'll all be for naught. A costly, embarrassing mistake, to say the least.

Fortunately, because their use is so important and ubiquitous they're tested thoroughly for each application. All you need to do is select the appropriate grade of bolt.

What are the different bolt grades?

Grades of bolt are exactly what they sound like. Depending on what you've got planned, there's an appropriate bolt for the job. Stick to the grading system and you can't go wrong!

Bolts are commonly graded by material, size and mechanical properties i.e yield strength, proof load and tensile strength. Consult your engineer to ensure the correct grade of bolt is used throughout your project.

Metals have different tolerances for bearing loads, and there are different types of loads to bear. The five types of loads are known as tension, compression, shear, bending and torsion.

The correct bolt for the job is the bolt that can comfortably withstand each type of load where necessary and maintain a safety factor. So long as these tolerances aren't exceeded, the bolt will remain securely fastened in place.

It is important to not confuse the terms ‘Grade’ and ‘Class’ when referring to bolts. Grade refers SAE fasteners while Class refers to Metric fasteners.

SAE Grading System

One of the systems used to determine bolt grades was agreed upon and put into place by the SAE, the Society of Automotive Engineers. They've got it all worked out.

The system differentiates bolts with a simple naming scheme. Each bolt has a number, and the number denotes the strength of the bolt. For example, the three most commonly used bolts are known as 2, 5 and 8. Their grade can be easily distinguished by a glance at their head.

No markings mean it's a grade 2 - this bolt have a minimum tensile of 60,000 pounds per square inch, this is the average hardware bolt found at department stores commonly used for DIY purposes.

Grade 5 bolts are differentiated by three raised lines on their heads. These have a minimum tensile strength of 105,000 pounds per square inch and are what is most often used to bolt cars and other similar structures together that are likely to experience constant daily stresses.

Grade 8 bolts are designed to deal with heavy loads and high stress, are ideal for structural loads and have a minimum tensile strength of to 150,000 pounds per square inch of pressure. These can be spotted by their six raised marks.

Metric Grading System

These high tensile bolts are slightly different and are marked by numbers on their heads separated by a decimal point. Graded by the ISO, the International Standards Organization, these numbers denote two separate pieces of information available at a glance; their load strength and the spread between breaking and bending tolerances.

Class 8.8 bolts are made of medium carbon steel, both quenched and tempered for hardness, they're used in all industries other than those that require resistance to extremes in temperature.

Class 10.9 bolts are of a tempered steel alloy, similarly tempered and quenched but are more suited to bolting larger parts together than Class 8.8 and exhibit greater wear resistance and load capacity, essentially making them heavier duty.

Class 12.9 bolts are heavier duty still, with a minimum proof load of 970 MPa. One MPa, or Megapascal is an abbreviation for one million pascals. A single pascal is one newton of force per square meter. The more pascals a bolt is rated to, the stronger it is.

All of these bolts are available in different lengths, though their thread and diameter are often determined by their grade.

Mild steel grade bolts are also available for their ductility where the need for slight malleability is necessary, allowing deformity without fracture.