Fastener Design Course Part -5 : Fasteners Tension | Design Criteria | Grip Length | Shear Loading
Channel: Fasteners
Date: 2021-03-28

The correct tension of a fastener is a critical factor in the assembly of most machinery and equipment, as well as in the construction of many different buildings. In general, the fastener tightening process is the act of tightening the fastener/bolt/nut to a specific tension to yield a desired clamping force.
Not only is tension of a fastener of critical importance, but it is also one of the most misunderstood and least understood parameters used in engineering. A misunderstanding of tension of a fastener is a major contributor to failure in engineering design.

How fastener tension affects fastener diameter and length?
 When you first install a new fastener, it's important to make sure that the fastener tension is correct. Fastener tension can be determined by the amount of deflection of the fastener when a determined load is applied. A fastener that is correctly tensioned will deflect slightly and then return to its original position when the load is removed.

Fasteners grip length is the actual length of the fastener when installed. The fastener is stretched by the torque applied to it. The higher the torque, the more the fastener is stretched and the longer it becomes. Thus, the fastener grip length is longer than the thread length.
The length of a fastener (the distance from the bolt head to the end) is measured in imperial units from the underside of the head to the end of the bolt. The fastener grip length is measured from the underside of the head to the end of the bolt when a compressive load is applied. Length measurements are typically in thousandths of an inch.

One of the most common fatigue failures is a fastener being pulled from a hole due to the thread pulling out of the material. For some materials, the thread material may pull out of the hole, but the bolt may remain in place. There are also a number of fastener types that have the material surrounding the fastener itself. These include self-locking nuts, collarless bolts, and threaded inserts. For these, the fatigue failures often result in the fastener being pulled away from the surface. (In addition, there are often other forms of fatigue failures that are not as obvious). For fatigue resistant bolts, the holes need to be designed to have sufficient mass to resist fatigue failure, and the fastener must have the proper amount of friction

Fastener design is the process of using engineering principles to design fasteners for use in a product, system, or structure. One of the best ways to learn fasteners, in particular, is to sit down with an experienced practitioner and listen to him or her explain the ups and downs of their career. Richard T. Barrett is an aerospace engineer (aerospace engineer) at NASA Lewis Research Center in Cleveland. He has been involved in aerospace and rocketry for over forty years, including work on the Space Program, rocket engines, and launch pads. He is one of the world's experts on the subject of fasteners and fastener design.

Center tapped threads, acme threads, stub acme threads, buttress threads, cold rolled threads are no big deal since they are used in nearly every type of engineering project that requires fasteners. Some of these fasteners are used in more complicated projects than others. For example, you can find tapped threads, acme threads, stub acme threads, buttress threads, cold rolled threads used in the Lincoln Tunnel and the Brooklyn Bridge.

Source: http://www.sti.nasa.gov​
https://www.youtube.com/channel/UCI9BX-UcPOhsfr0fTklLytQ

This course is a comprehensive study of fastener technology. While this course is designed for aerospace engineers, mechanical engineers will find it beneficial. This course is about materials, plants and coatings. There is a summary of locking methods, threads, joint stiffness, rivets and inserts. There is also a discussion of lubricants and design considerations.

This course is presented as a comprehensive reference on aeronautical fasteners technology, and covers fasteners materials, fasteners plantings and coatings, fasteners locking methods, fastener threads, fastener joint stiffness, fasteners rivets, fasteners inserts, fastener nut plates, fastener thread lubricants, and design criteria. The course is presented with lecture, charts, illustrations, and real-world experiences, designed to help you understand aerospace fasteners and how to select the right technology for your application.

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