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###### The Nuts and Bolts | Seek n' Geek III

Top left image from Unlock Vibration Proof Fasteners

Top right from Charles Roberts

It’s all about the “nuts and bolts.” According to the Google ngram viewer, the phrase nuts and bolts was first used in 1828 and really took off in the 1940s. Everybody knows a bolt is some type of connector with threads, but it’s crucial to understand that bolts work by providing a compressive force between two surfaces. The system can be modeled as springs – the bolts are springs in tension and slightly elongated and the plates are in compression and slightly compressed. The initial compression inputs this stored energy into the system. The resulting connection relies most on the friction force between the surfaces, not primarily on the shear strength of the bolt itself. These principles are illustrated in this cool Youtube animation.

Image from Fastenal Company

In the recent movie Deep Water Horizon one of the scenes shows the bolts on a piece of equipment shearing as the tower crumbles. Check it out in the trailer at 1:13. After talking about this failure and numerous other examples in my structural materials course, I decided to investigate bolt physics and lay out some personal guidelines for selecting and inspecting bolts in future work. Engineers take responsibility for the machines they design, test and work with. Understanding the physics and potential failures of equipment and structures is essential to safety (which also often means not having to clean up huge messes which costs huge amounts of money.)

I decided to pick the brain of Professor Eagar who has analyzed a lot of different failures. Professor Eagar recommended John Bickford’s An Introduction to the Design and Behaviors of Bolted Joints as the best reference on bolts. Below is a shot of my high level notes while reading the first chapter which gives an overview of the book. The two primary types of bolted joints are tension joints and shear joints. One more specific type of shear joint is called a friction-typeslip-critical joint, which relies on friction between the plates instead of the force of the pin.

Author Neville Sachs (he wrote a text titled Practical Plant Failure Analysis: A guide to Understanding Machinery Deterioration and Improving Equipment Reliability) wrote a helpful article titled “Failure Analysis of Mechanical Components” for a website called Maintenance World.

Many systemic factors lead to failure. As Mr. Sachs articulates, “time, temperature, amperage, voltage, load, humidity, pressure, lubricants, materials, operating procedures, shifts, corrosives, vibration” all impact the actual operating conditions, compared to the design conditions. Types of failures include “overload…fatigue…corrosion-influenced fatigue…corrosion…[and] wear.” [Sachs, 2004] Additional types of failures include vibration loosening, hydrogen embrittlement, and failure due to combined loading in bending cases.

This article by Fastenal Company gives an overview of mechanical properties of steel fasteners, joint design and tensioning methods and is a helpful reference for selection.

A short dictionary of some key vocabulary:

• ROOT CAUSE FAILURE ANALYSIS (FCFA) – investigating how parts fail by diagnosing mechanical causes

• PRIMARY FAILURE – a part or component that fails first and leads to secondary failures

• STRESS CONCENTRATIONS – physical geometry that causes significantly higher local stresses

• PROOF LOAD – maximum tensile force that can be applied to a bolt that will not result in plastic deformation (typically 85-95% yield strength)

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