EVOLUTION OF A DESIGN

Most likely you have, right at this moment, at least one machine design project in progress. Maybe you were the originator of the design, but I suspect you inherited this design from others. I further suspect that you have already identified elements of the design you feel could be improved. You might be under pressure from customer service or marketing to respond to some need for change. In responding successfully, either to your own observations for change or to those of others, the design will evolve. Recognizing that the evolutionary design process is decidedly complex, with a seemingly random sequence of steps, the primary purpose of Standard Handbook of Machine Design is to make the information you need as readily accessible and usable as possible. As an example of how a design can evolve, and to provide perspective on how the information in this Handbook has traditionally been used, let me review for you a project I was given in my first job as a mechanical engineer. It involved the positioning of a microwave feed horn for a 30-ft-diameter antenna dish.The original gn (not mine, by the way) called for a technician to climb up onto a platform, some 20 ft off the ground, near the backside of the feed horn.The technician had to loosen a half dozen bolts, rotate the feed horn manually, and then retighten the bolts. This design worked quite well until several systems were sold to a customer providing telecommunications along the Alaskan oil pipeline. Workers were not really safe going out in below 0°F weather,with snow and ice on everything. As a result of their concerns for safety, this customer asked that we provide remote positioning of the feed horn from the nearby control room. The critical design requirement was that the positioning of the feed horn needed to be relatively precise. This meant that our design had to have as little backlash in the drive mechanism as possible. Being a young engineer, I was unaware of the wide variety of different drive systems, in particular their respective properties and capa- bilities. I asked one of the older engineers for some direction. He suggested I use a worm drive since it cannot be back driven, and loaned me his copy of Joseph Shigley’s book, Mechanical Engineering Design. He said that Shigley’s book (a precursor to this Handbook) had been his primary source of information about worm drives, and a wealth of other machine design information. As it turned out, the resulting design worked as required. It not only pleased our Alaskan customer but became a standard on all antenna systems. I did not get a promotion as a result of the success of this new design, nor did I receive a raise. However, I was proud, and, as you can surmise, still am. I credit this successful design evolution to the material on worm drives in Shigley’s book. And there is more to this story. The worm drive gearbox we ultimately purchased contained a plastic drive element. This allowed the backlash to be greater than what could be tolerated in positioning accuracy and did not provide the necessary strength to break the feed horn loose from a covering of ice.The original manufacturer of the gearbox refused to change this drive element to metal for the units we would be buying. If we made the change ourselves, they said, the warranty would be voided. However, after absorbing the wealth of information on worm drives in Shigley’s book, I felt confident that we could make this substitution without endangering the reliability of the unit. Also, because of Joseph Shigley’s reputation in the mechanical engineering community and the extensive list of references he cited, I never felt the need to consult other sources. Another aspect of this story is also important to note. In addition to the information on worm drives, I also used Shigley’s book to find comprehensive design information on the many other machine elements in the new design: gear train geometry, chain drives, couplings, roller bearings, bolted joints, welds, lubrication, corrosion, and the necessary stress and deformation calculations I needed to make. All this information, and much more, was contained in the First Edition of the Standard Handbook of Machine Design, which Joseph Shigley coauthored with Charles Mischke. Now in its Third Edition, this Handbook includes the information machine design engineers have come to trust.We hope you will find this information invaluable as you constantly strive to improve your designs, whether by your own initiatives, or for other reasons.
Thomas H. Brown, Jr., Ph.D., P.E.
Faculty Associate
Institute for Transportation Research and Education
North Carolina State University
Raleigh, North Carolina
Source: STANDARD HANDBOOK OF MACHINE DESIGN
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