Fasteners are generally manufactured in accordance with two distinct sizing systems. The first fastener system is measured in inches and is known as the imperial system, due to its institution by Imperial England in 1824. This system categorizes steel material according to “grades”. Although originally instituted in England, and spread during English colonization, most countries no longer use this system, with the exception of the United States, Liberia, and Myanmar.
The second fastener system is the metric system, originally created in 1618, the metric system was developed specifically for standardization, i.e. its more intuitive nature (whereas the imperial system was rooted in tradition and based on using the human body for measurements, i.e. it’s less intuitive conversions). The metric system was designated as the standard measurement for lengths in 1960 according to the System of International Units, which specifies the standard units for a range of variables, such as temperature, pressure, etc, and quickly became the dominant measurement system in the world. Metric fastener material is categorized by “classes”. The fastener industry uses the term property classes because they describe the properties of the fasteners; most notably, strength.
The Fastener Backstory
The threaded bolts and nuts of today are a remnant of the industrial revolution — when the first mass-produced fasteners appeared. More specifically, a British company began the serialized production of screw bolts in 1760. While others soon followed suit, a problem emerged — different companies were making different-sized fasteners, which made it far more difficult to assemble machinery with them.
Fast forward to 1841, after the institution of the imperial system, and a solution was found — all sizes of fasteners were standardized across Britain. The pitch of the thread was set at 55 degrees, and certain bolt diameters had a set number of threads.
Initially, the USA only copied the British imperial system, but after a couple of years, they made a slight change to their standard by altering the thread by five degrees. This seemingly innocent change would represent issues a century later when the Allies would wage two World Wars together. After WWII, Canada, Britain, and the United States adopted a common fastener standardization which was applied using imperial measurements.
The advent of the ISO system for all kinds of international measurements has ushered in an age of the metric system; though that still hasn’t happened in North America. And while threads have been completely standardized by metric designs, other variables have not — which is why we have metric property classes today, along with imperial grades.
The United States is home to two grading systems that still use imperial grades:
- Society of Automotive Engineers (SAE)
- American Society for Testing and Materials (ASTM)
Both of these associations use number scales for their bolts — higher tensile strengths are depicted with higher numbers. The SAE scale is used for diameters of fasteners up to an inch and a half, with Grade 8 being stronger than Grade 5.
Every single SAE fastener contains letters showing you who the manufacturer is, along with markings that denote tensile strength. If a bolt or a nut has no visible markings, that means you’ve got a Grade 2 — the very lowest tensile strength level according to the SAE system.
On the other hand, a fastener containing three radial lines shows tensile strength at Grade 5. Finally, a Grade 8 fastener has six radial lines.
When you’re dealing with ASTM fasteners, the situation is much easier — they use simple alphanumeric codes. So, for example, if you were dealing with an ASTM A325 bolt that’s standardly used with structural steel joints — you’d see the A325 letters on the bolt. As you might imagine, this makes them far easier to identify.
Next up, we’ve got the metric fasteners — most often used for machinery and factory plants. Their scale is quite simple to understand, and it shows the bolt diameter, ranging from M5 to M30. It’s easy enough — the M30 bolt comes with a thread diameter of 30mm.
When it comes to the tensile strength of these fasteners, each of their heads should have a numerical marking depicting the strength and the ID code of the manufacturer.
These numerical markings show the maximum load-bearing capacity of the fastener; in other words, the biggest load per square millimeter that the fastener is capable of carrying before breaking. If you’ve got a class 8.8 bolt, that means that it’s capable of bearing 80kg per square millimeter. And the .8 number is there to tell you that the fastener will begin bending or stretching at 80% of the specified load. So, if you’ve got a 10.9 bolt, then it is capable of bearing 100kg per square millimeter and will begin bending or stretching at 90% of the specified load and so forth with class 12.9.
It should be noted that many smaller fasteners (like screws) are not graded, because there’s no need for their strength to be specified. Of course, fasteners used for specialized purposes and those of larger sizes absolutely must comply with specific strength requirements. That’s why we have the property classes for metric fasteners across the globe, and grades for imperial inch fasteners in the U.S.