Appreciating the Spur Gear

When the average civilian hears the word “gear,” if they think of mechanical things at all, they envision a big spur gear, perhaps in a clock or with a movie hero trying to escape being crushed between meshing teeth. My first non-toy exposure to gears was an extra credit assignment to draw a cast spur gear in the 7^th grade drafting class at Alexander Graham Bell Junior High School in Milwaukee. My hometown was serious about gears in 1964. It may surprise some of you that a cast tooth was still relevant over a century after gear hobbing became possible. The “cutting edge” (pun very much intended) of technology always gets the headlines, but the trailing end sticks around for a long time. Walk through most gear shops and you will be forced to agree; we do not abandon things that work well simply because a shiny object is waved in front of us at a trade show. Don’t be surprised if — rebranded as “near net shape” — cast teeth make a comeback via “additive manufacturing” (3D printing). But I digress. Spur gears are far from obsolete and a fledgling gear expert would be wise to fully understand the positive and negative characteristics of this venerable gear type. While it was once used because of the limits of machining technology, it remains popular in many applications because of its simplicity, ease of mounting, and well understood capabilities. That simplicity derives from the lack of a helix angle. No need for a trig function on your calculator unless you are calculating the separating forces. No thrust forces or overturning moments to factor into your bearing selection. No concern over compatible helix hands or balancing those thrust forces. No transverse pressure angle or transverse diametral pitch or module. So simple, a 12 year old can understand them.