Everywhere you look you can find an application for bolts, whether it is inside your car, fridge, or even a bridge you cross bolts are used everywhere in most applications. Bolts come in different grades, sizes and are intended for various applications. It is important that the correct bolt is used in specific applications to prevent the failure of equipment or structures. We will discuss how bolts are graded, and how to identify the grade of bolts.
Factors that determine a bolts grade
Size
A bolts size will have a role in its yield point, and proof of load. We can compare the proof of load of two US bolts coarse threaded of 1/4″ and 5/16″. The 1/4″ bolt has a proof load of 7.79kN while the 5/16″ bolt has a proof load of 12.9kN. That’s a difference of 5.11kN.
Material
Not all bolts are made of the same material. Depending on your application a bolt could be made of different grades of steel. You can also commonly find bolts made of stainless steel and brass to help prevent rust and corrosion. There are other types of materials used to create bolts, but the most commonly used material is steel.
How to identify the grade or class of a bolt?
SAE
In this article, we will only cover the 3 most common bolts under the SAE standard. These bolts are grade 2, grade 5, and grade 8. SAE bolts are identified by the head markings on the top of the bolt. For example grade 2 bolts will have no markings or radial lines. Grade 5 bolts will have 3 radial lines. Grade 8 bolts will have 6 radial lines. Below is a picture showing these markings with respect to their bolts.
Metric
Metric graded bolts use a class system. The 3 most common classes of bolts are class 8.8, class 10.9, class 12.9. Metric graded bolts are identified by the head markings on the top of the bolts, and the number will be marked. A class 8.8 would have an 8.8 mark, a class 10.9 would have a 10.9 mark, and a class 12.9 would have a 12.9 mark. This is easier to identify than an SAE-graded bolt for most people.
Aside from the head marking to identify a bolt, most manufacturers will have a head marking to identify themselves as the manufacturer of a bolt. You can find a proof of load chart for US bolts here. You can also find proof of load chart for metric bolts here.