14 THE DISTRIBUTOR’S LINK Laurence Claus Laurence Claus is the President of NNi Training and Consulting, Inc. He has 25 years of experience with a medium sized automotive fastener manufacturer, holding positions including Vice President of Engineering, General Manager, Director of Quality, Director of New Business Development and Applications Engineer. In 2012 he formed NNi offering technical and business training courses as well as technical consulting, expert witness and consultation work. He can be reached at 847-867-7363 or by email: Lclaus@NNiTraining.com. You can learn more about NNi at www.NNiTraining.com. FATIGUE IN FASTENERS In 1999 General Motors refreshed its full size truck and SUV offering. The previous model, the GMT400, was reinvented and emerged for the 1999 model year as the GMT 800. At the time, this was one of the most significant launches in General Motors history. With this new truck platform came many changes and upgrades from the previous models. One of these upgrades would be an entirely new and more powerful disc braking system. With the new brakes came all new components. To abridge the story a little, the brake supplier that had been awarded the business started development of the new brakes with its chosen component suppliers including a cold header to supply the Brake Caliper Pins. During the course of development the Caliper Pin design transitioned head design from a hex flange head to an internal Torx® round washer head. This head change combined with a material upgrade spooked the existing cold header to the point that they requested to be relieved of the responsibility of supplying the part. That set the brake supplier into a frenzied search for a new supplier, which culminated in them awarding the part to the company I worked for at the time. As deadlines were looming, we turned up the heat and produced PPAP parts in record time. The brake supplier took these parts, assembled them into brakes and began the arduous task of validation. Brake Caliper Pins generally receive greater scrutiny than most cold headed components just because of the criticality of the task they are to perform. Therefore, it was no surprise that these new brakes were thoroughly tested including some TECHNICAL ARTICLE new and unique test protocols. One of these tests was a durability test that simulated a fully loaded full-size pickup truck descending steep mountain grades with frequent application of the brakes. Unfortunately, during this test, the Brake Caliper Pins were occasionally breaking prior to meeting the minimum number of test application cycles. Analysis would determine that the Caliper Pins were breaking by reverse bending fatigue. Before one gets the idea that the Caliper Pins were in some way flawed, they were not. In fact, analysis determined that the severe load placed on the caliper assembly by this test protocol caused the caliper bracket, which wasn’t substantial enough, to flex, bending the pins up and down and ultimately resulting in reverse bending fatigue. The expert consensus was that if the bracket were stiffer the problem would be solved. Unfortunately, redesigning the bracket was out of the question because of cost and timing. Therefore, the brake supplier approached us and requested that we do whatever it took to “improve” the Caliper Pins to be able to withstand the rigors of this test protocol. So with that as an introduction, we can begin asking some questions like: ¤ What is fatigue? ¤ What does fatigue look like? ¤ How might you reduce the risk of fatigue in a fastener? We will explore these and other questions in this short article. CONTINUED ON PAGE 104
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