Figure 6-4.—Herringbone gear.Figure 6-3, views B, C, and D, also shows youthree other gear arrangements in common use.The internal gear in figure 6-3, view B, has teethon the inside of a ring, pointing inward toward theaxis of rotation. An internal gear is meshed with anexternal gear, or pinion, whose center is offset fromthe center of the internal gear. Either the internal orpinion gear can be the driver gear, and the gear ratiois calculated the same as for other gears—by countingteeth.You only need a portion of a gear where themotion of the pinion is limited. You use the sectorgear shown in figure 6-3, view C, to save space andmaterial. The rack and pinion in figure 6-3, view D,are both spur gears. The rack is a piece cut from agear with an extremely large radius. The rack-and-pinion arrangement is useful in changing rotarymotion into linear motion.Figure 6-5.-Bevel gears.THE BEVEL GEARSo far most of the gears you’ve learned abouttransmit motion between parallel shafts. However,when shafts are not parallel (at an angle), we useanother type of gear called the bevel gear. This typeof gear can connect shafts lying at any given anglebecause you can bevel them to suit the angle.Figure 6-5, view A, shows a special case of thebevel gear-the miter gear. You use the miter gears toconnect shafts having a 90-degree angle; that meansthe gear faces are beveled at a 45-degree angle.You can see in figure 6-5, view B, how bevelgears are designed to join shafts at any angle. Gearscut at any angle other than 45 degrees are bevelgears.The gears shown in figure 6-5 are straight bevelgears, because the whole width of each tooth comes incontact with the mating tooth at the same time.However, you’ll run across spiral bevel gears withteeth cut to have advanced and trailing ends. Figure6-6 shows you what spiral bevel gears look like. Theyhave the same advantage as other spiral (helical)gears—less lost motion and smoother, quieteroperation.Figure 6-6.-Spiral bevel gears.6-3
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