Here There Be Dragons

On the AGMA Helical Gear Rating Committee we frequently deal with how to warn users away from potentially dangerous designs. Some years ago our wonderful chairman colorfully declared we should just adopt the warning from ancient maps of the then “flat world” and label the areas of concern as “here there be dragons.” Eventually some explorer ventured “off the map,” and if they came back the map got revised to reflect their account. The same thing happens in design, of course. Usually it is the result of an error or operator abuse. We once visited a customer about a competitor’s gearbox that was failing every three or four months. It did not take an expert to figure out why; the maintenance foreman complained about having to special order a coupling to connect the four-inch-diameter output shaft of the problem gearbox to the NINE inch diameter input shaft of the flattener drive. Somewhere along the line the requested 2.0 service factor got inverted and a .5 service factor was applied to the reducer. This was a solid “data point” that our allowable stresses were not too conservative. That was a pretty obvious problem, although it was pointed out that the reducer did operate for a decent interval under such a huge overload. We built a replacement drive with a 3.0 service factor and nine-inch-diameter output shaft that continues to run flawlessly twenty years later. It was not the situation where you wanted to explore the exact border of the dragon lair. We celebrate the successful explorers in history classes and name things after them. Very few of the people who ventured “off the map” and never returned are remembered at all. As engineers and designers, the lesson in that is to have a back-up plan that allows you to return to a safe place if things do not quite work out. Someone has to take a chance if the map is ever to expand, so we cannot all remain safely on the beach.