Some of you are probably calling forth the gear deities to smite me for promoting a “free thinking” approach to helix angles; I understand completely. As with the mispronunciation of “helical,” tolerance is not just a technical term to argue about at a non-conforming material review.
There once were “valid” reasons for every one of the “family recipe” limits on helix angle. Unless you are obnoxious about promoting your particular “rule,” you have my permission to continue following it. Just don’t try to get it slipped into a standard without actual science to support it.
The history of how these rules came about is quite interesting. At my initial stop in this adventure, we had a 3-inch-thick book of possible transverse DPs that could be used in new designs. Being young and impressionable, I accepted this as carved-in-stone “science.” After all, the “book” was the result of 76 years making the best gears in the entire world.
A few years later, I had design problems that fell between the lines in that big old book and had to look in other books for guidance. My mentor was shocked at my solution and grilled me on my heretical calculations. Our boss got involved. An expert from the gear technology department was consulted. The room awaited my execution with great anticipation.
Instead, the expert said “The kid’s numbers work out. We’d have to buy a standard 3-module metric hob and figure out the right change gears, but the computer didn’t crash so we will authorize this. Who let him have access to the library?”
That big book, it turned out, was pre-calculated for our “standard” proprietary tooth form and the change gears that fit our hobbing machines. Nothing prevented us from using other cutting tools — except laziness. If you ever had to calculate change gears — my punishment for coloring outside the lines — you understand the restrictions. It was worse back in those pre-computer days. You kids will never know how we suffered.
The integral helix angle folks have my respect though; you simplify that number and complicate other aspects of the calculations. Mandatory rack offset coefficients, anyone? Gee, another potential blog topic!