In Chap. 8 , as the last one of the three chapters dedicated to gears, more gear types and gear train types are introduced. They include helical, bevel, worm and worm gears. The geometry of gear teeth in these gears is more complicated than that of the spur gears and as a result more parameters need to be introduced in order to capture it. Compared to spur gears, helical gears are less noisy. That is because in helical gears load transfer among gear teeth happens more gradually than in spur gears. Also helical gears have a higher load carrying capacity. In helical gear trains, the gear axes do not intersect; so they may be either parallel or skewed (i.e., they are not on the same plane). Bevel gears, worm and worm gears provide more options in terms of speed ratios and angles of the power transmission shafts with respect to each other. In bevel gear trains the shafts are intersecting, whereas in worm and worm gears they are skewed. An important application of bevel gears is their use in the differential. A differential allows wheels on the same axle in a car to spin at different speeds during a turn: the inner wheel turns slower and the outer wheel turns faster. Consequently, minimum tire slipping happens during a turn.