When I started writing my gear book back in 1986, the mission was to provide enough information for a person new to the gear trade to tell if someone was sending them on a “snipe hunt.” For those of you who never had the pleasure of attending summer camp, a “snipe hunt” as a popular way to keep the youngsters busy looking for a non-existent bird so the counselors could goof off. My duties had recently expanded to include sales department management and my team complained about the time needed to educate new buyers and engineers about our products. Management tasked me with developing some advertising pieces to be left with customers and that grew into a book.
Some of these “newbies” were so green they barely knew a gear when they saw one. And for some of them this was understandable as our products included almost every type of gear imaginable. We made everything from a simple spur gear to a complex differential counting mechanism so the first step required making sketches of the basic types of gears so they could at least point to what they thought they needed.
We sorted them out by shaft orientation: parallel shafts, intersecting shafts, skewed shafts, and no shafts at all. Each sub-group has a number of types that have unique characteristics that influence the design of the gear set and its application for particular uses. As St. Thomas Aquinas famously said “things are right or wrong depending upon their circumstances.” A parallel shaft gearbox could theoretically use spur, helical, herringbone, double helical, internal spur, internal helical, spur epicyclic, helical epicyclic, or even double helical epicyclic and each design could “work” but their volumes, weight, cost, and efficiency would be much different.
The marketplace decides which type of gear is “best.” Over time competitive products converge on a “solution” which tends to make them very much alike. This convergence annoys many engineers so we occasionally attempt to “re-invent wheel” if there is a technological breakthrough available. Sometimes, such as the shift from rear wheel drive automobiles to front wheel drive the “innovation” even works.
Your first step in becoming a gear expert is to be able to recognize the many types of gears by sight.