6 Fascinating Facts about Fasteners
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.
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, although you still need to make sure the bolt is properly sized. . 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.
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.
6. When designing a clearance hole for a bolt, it helps to refer to a chart to pick the correct hole size. 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.