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Six Fascinating Facts about Fasteners

By Engaged Expert Ryan Castells Advanced Materials

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It’s always a surprise when you learn something new or interesting about a subject you thought you knew everything about. Well fasteners are pretty cut and dry, right? Read on for a few facts about fasteners that may amaze you! 

Fastener Fact 1

If you’ve ever designed a part with a tapped hole, you may have wondered, “How many threads do I need to make a strong connection?” The answer is that it varies, but six at most.  Because bolts stretch slightly when load is applied, the loading on each thread is different. When you apply a tensile load on a threaded fastener, the first thread at the point of connection sees the highest percentage of the load. The load on each thread decreases from there, as seen in the table below. Additional threads beyond the sixth will not further distribute the load and will not make the connection any stronger. 

Fastener Load Table

Fastener Fact 2

There is a common misconception that black-oxide alloy steel socket head cap screws (SHCS) are ‘grade 8’. This is believed because grade 8 fasteners are so widely available that the label has become associated with all high-strength fasteners. Technically speaking, to be considered ‘grade 8’, a fastener has to meet industry standards for various characteristics. Three of the most important physical properties of SHCS are inconsistent with the ‘grade 8’ classification: tensile strength, hardness, and markings on the bolt head. SHCS are actually stronger than ‘grade 8’, and have more in common with grade 9 fasteners.

Fastener Fact 3

When a bolted connection will be subjected to a fatigue loading, you want to tighten the bolt up to its yielding point for maximum strength. A bolt will experience zero change in load if the applied tensile force is less than the compressive force of the connection. So, a tightly fastened connection is better suited to withstand fatigue loading than a loose connection because the bolt itself will not sense the fatigue load, only the constant force applied due to the clamping of the joint. To ensure that the connection is properly fastened, you can look up the recommended torque for a given fastener type in a table like the one found here. If the applied torque is critical for your application, make sure that you apply the recommended torque to the head of the bolt, rather than the nut. Torquing the nut can result in different nut factors and change the torque required to achieve proper pre-load. 

Fastener Fact 4

Have you ever seen a fastener labeled with a 2A or 3B rating and wondered what that meant? That number-letter combo is used to indicate the thread class of the fastener. Thread classes include 1, 2, 3 (loose to tight), A (external), and B (internal). These ratings are clearance fits which indicates that they assemble without interference. Classes 1A and 1B are rarely used, but are a good choice when quick assembly and disassembly are a priority. Classes 2A and 2B are the most common thread classes because they offer a good balance between price and quality. 3A and 3B are best used in applications requiring close tolerances and a strong connection. Socket cap and socket set screws are usually class 3A. 

Fastener Fact 5

All fasteners are available with either coarse or fine threads, and each option has its own distinct advantages. Finely threaded bolts have larger stress areas than coarse bolts of the same diameter, so if you are limited on the bolt size due to dimensional constraints, choose a fine thread for greater strength. Fine threads are also a better choice when threading a thin walled member. When you don’t have much depth to work with, you want to utilize their greater number of threads per inch. Fine threads also permit greater adjustment accuracy by requiring more rotations to move linearly. 

On the other hand, coarsely threaded bolts are less likely to be cross threaded during assembly. They also allow for quicker assembly and disassembly, so choose these when you will be reassembling a part often. If the threads will be exposed to harsh conditions or chemicals, a coarsely threaded fastener should be considered for its thicker plating or coating. 

Fastener Fact 6

When designing a clearance hole for a bolt, it helps to refer to a chart to pick the correct hole size. A useful reference can be found here. Similarly, when pre-drilling a hole that is to be tapped, it helps to have a chart to refer to the appropriate size pilot hole. Such a chart can be found here.

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