July 26, 2023
In my last blog we went over “pitch” descriptions for spur gears. Engineers being stubborn individuals, it has not been possible to reach a consensus on a single set of tooth proportions or pitches for use in the United States. Some people prefer the simplicity offered by the metric “module” system, but in future posts you will see that such “simplicity” can get complicated too. At least consensus was reached on the use of an involute curve for the actual tooth profile. Otherwise I would have to bore you with a discussion of the “composite” profile or one of the other profile curvatures that were once considered. By the early 1920s, the involute curve became “standard” but designers were unable to agree on pressure angles or tooth depths. Some were convinced that there was magic in the 14.5 degree value that was beloved by pattern makers because the sine of 14.5 was very close to ¼, and that made lofting the tooth profiles easier. Others advocated for the more “logical” 20 degree system. Theorists also debated 22.5 and 25 degree systems. There is no “gear cop” to enforce the adherence to an official system. Companies and industries made their own decisions and tooled up to implement those decisions. You are free to do so also, but it is important that you understand the implications on performance and tool availability. One prominent firm had its own tooth form — a legacy of gear cutting machines they built in-house. Over time they developed an almost religious belief in this system and many people trained within the organization did not know they were “different” from the rest of the industry. Marketing foolishly thought that being “unique” insulated them from pirated parts. Engineering just “plugged and chugged” designs using the proprietary software written in-house. Imagine their surprise that others had little trouble reverse engineering gear sets to fit in their products. Or their embarrassment when some of their engineers did not know how to translate their drawings into the format required for third-party-supplied software. Learn from their mistakes! Be as different as you like, but understand how you are different and have valid reasons for being that way. “That is just the way we have always done it” will not merit any respect from your peers.