Spur gears have been around “forever.” That ancient astronomical device recovered from the Mediterranean Sea a few years ago shows the antiquity and utility of the simple hand-filed tooth. Scientists are still speculating over the use of the device, but had no trouble building replicas. This situation illustrates a fundament split in the gear trade between instrument and power transmission applications. Instruments include clocks, watches, and measuring devices of amazing accuracy with little more than a dividing head and some shaped files. Metal was scarce; and besides, you could not get a blank big enough to make a pinion for a waterwheel. The power transmission specialists of the time had to whittle teeth out of wood and mount them in a wooden hub. The inclines in Pittsburgh still use that configuration today. A nice story can be found in the Gear Technology online archives:
Digital has rendered analog obsolete in the instrument world. Spur gears remain popular in engine timing gears because they do not generate a thrust force on camshafts, crankshafts, or pumps. As rotational speeds increase, spurs become more attractive for the same reason; and likewise, for epicyclic designs where high power density is achieved via having multiple meshes working together.
But even with the deeper-than-standard whole depths of high contact ratio design, spur gears cannot match the ability of helical gears to spread the load over many teeth. Total contact ratio is a major factor in the sound produced by a pair of gears; a spur set gets no help from its face contact ratio. The introduction of that pesky helix angle changes the transverse pressure and allows for lower numbers of pinion teeth before undercutting occurs.
Some theorists model a helical gear as a number of spur gear wafers mounted with radial variation on a common shaft, but that in my opinion, is an over-simplification. The sound produced by a conventional spur gear at speed reflects its transition from single tooth contact to double tooth contact, and back again. For a true helical design there are never less than two teeth in contact — and that changes almost everything about their performance.
Understanding spur gear dynamics is required for gear expert status, no matter what your application is. All the fancy modifications you read about are an attempt to adjust those dynamics. Until you master spur gears, those technical discussions will remain gibberish.