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FALL 2017

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Distributor's Link Magazine Fall Issue 2017 / Vol 40 No4

126 THE DISTRIBUTOR’S

126 THE DISTRIBUTOR’S LINK LAURENCE CLAUS WHY FASTENERS FAIL PART 2 - OVERLOAD, FATIGUE AND CREEP from page 14 This accident serves as a poignant illustration of what happens when a fastener is unwittingly overloaded because users naïvely expect them to perform beyond their capabilities. Unfortunately this underappreciation of the critical function that fasteners are being asked to perform is an all too often event, although, fortunately, most do not end in as tragic a fashion as the Senior Road Tower collapse. In Part 2 of this series on Why Fasteners Fail, we will take a look at overload, fatigue and creep failures. These can all be characterized as mechanical failures and are generally preventable when care is taken by the designer to build-in a sufficient safety factor and risk mitigation, and by the user to apply proper tightening techniques. Overload When an engineer talks about stresses or loads, he is referring to the forces that are acting on a system or part. On fasteners, these loads experienced in service, Service Loads, may come from any number of inputs. Take for example, a structural bolt holding together part of the bridge deck support structure. This bolt will carry some of the constant load provided by the weight of the bridge deck above it, but also the fluctuating loads created when a truck bounces over the bridge or a heavy wind buffets the bridge causing it to slightly bend and deflect. When an engineer talks about strength, they are referring to the limits a part or material will have when exposed to a stress or load. In other words, a material’s strength is the point at which an applied stress will begin to have an adverse impact. There are many types of strength that can be associated with a fastener although yield strength, tensile strength, and proof strength are the three most common. The yield strength is the point at which the fastener begins to permanently stretch, the tensile strength at which the fastener breaks, and the proof strength a value slightly below yield that provides confidence that the part will meet the proper yield strength value without actually having to destroy the part. Overload is an apt word to describe this failure mode. In essence it means that the stresses or loads exerted on the part exceed the part or material’s strength limits and it fails. Fasteners can be overloaded in tension, shear, or torsion. In fact, many of us can probably share experiences from tinkering in the garage where we gave the wrench one last tug to get the screw or bolt really tight only to come away with two pieces. In another example, there is a special variety of fastener known as a shear bolt that is specifically designed to overload and break in shear before another, more expensive component is damaged. Snow blowers commonly use this technique to break the auger free from the drive shaft and prevent damage to the drive mechanism should the user suck up a stick, stone, newspaper, or other obstruction that is not snow. Returning to our discussion of fundamentals in Part 1, one may recall that materials may break in either a ductile or a brittle fashion. Recognizing which of these modes of failure occurs is often important when investigating a failure and attempting to get to the root cause. Overload failures may occur in either a ductile or brittle fashion which is one of the primary clues that a failure analysis will attempt to discover. Overload may occur for a variety of reasons. One might think that perhaps the most common reason would be the utilization of a lower strength fastener than is required. Naturally, this does happen. As was illustrated by the Senior Road Tower accident; users, engineers, or designers simply fail to appreciate or understand the loads that will be experienced by a part or system. Perhaps more often, though, fastener overload failures are caused by load shifting. Take, for example, a pipe flange that is joined over its periphery with eight bolted joints. Assuming that they are equally spaced around the periphery, each joint will be sharing one-eighth of the load. CONTINUED ON PAGE 188

THE DISTRIBUTOR’S LINK 127 Tel: 1-800-926-1495 Fax: 1-888-526-4566 Email: info@chrislynninserts.com www.chrisslynninserts.com FOOD FOR THOUGHT... SALES WHERE TO SPEND YOUR SELLING TIME DURING SLOW PERIODS? There are a few fastener products that sell as well during a bad economy as they do when manufacturing is doing well. Threaded inserts is one of those products. Precision Helical Wire and Rock Solid K-type threaded inserts are the most common used inserts that meet Engineering, Military, Manufacturing and general maintenance and repair specifications. During a good economy approximately 50% of inserts sold are being used in new machinery, automotive, power generation, etc as part of the design. The reason is that manufacturers design new products with thinner and softer materials to save on cost & reduce the weight of their product. When this is done inserts are used to maintain or increase the strength of the threads being used. During a bad economy companies repair instead of replace their machines and equipment. When the threads are damaged on this equipment inserts are used to repair them. In short, inserts do not have the ups and downs of the economy. They are sold every day/week/ month of the year regardless of the state of the economy. Add the Chrislynn Threaded Insert Line to your high volume sales products. Become a distributor today.

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