Figure 6-2.4-Spur gears coupling two parallel shafts.one to the other by spur gears. This setup is shown infigure 6-2.Spur gears are wheels with mating teeth cut in theirsurfaces so that one can turn the other without slippage.When the mating teeth are cut so that they are parallelto the axis of rotation, as shown in figure 6-2, the gearsare called straight spur gears.When two gears of unequal size are meshedtogether, the smaller of the two is usually called a pinion.By unequal size, we mean an unequal number of teethcausing one gear to be a larger diameter than the other.The teeth, themselves, must be of the same size to meshproperly.The most commonly used gears are the straight spurgears. Often you’ll run across another type of spur gearcalled the helical spur gear.In helical gears the teeth are cut slantwise across theworking face of the gear. One end of the tooth, therefore,lies ahead of the other. Thus, each tooth has a leadingend and a trailing end. Figure 6-3, view A, shows youthe construction of these gears.In the straight spur gears, the whole width of theteeth comes in contact at the same time. However, withhelical (spiral) gears, contact between two teeth startsfirst at the leading ends and moves progressively acrossthe gear faces until the trailing ends are in contact. Thiskind of meshing action keeps the gears in constantcontact with one another. Therefore, less lost motion andsmoother, quieter action is possible. One disadvantageof this helical spur gear is the tendency of each gear tothrust or push axially on its shaft. It is necessary to puta special thrust bearing at the end of the shaft tocounteract this thrust.You do not need thrust bearings if you useherringbone gears like those shown in figure 6-4. Sincethe teeth on each half of the gear are cut in oppositedirections, each half of the gear develops a thrust thatcounterbalances the other half. You’ll find herringbonegears used mostly on heavy machinery.Figure 6-3.-Gear types.6-2
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