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NOTE: Bold face diametral pitches and modules designate preferred values.

2.7 Gear Types And Axial Arrangements     in accordance with the orientation of axes, there are three catagories of gears:     1. Parallel Axes Gears     2. Intersecting Axes Gears     3. Nonparallel and Nonintersecting         Axes Gears

Spur and helical gears are the parallel axes gears. Bevel gears are the intersecting axes gears. Screw or crossed helical, worm and hypoid gears handle the third category. Table 2-3 lists the gear types per axes orientation.     Also, included in Table 2-3 is the theoretical efficiency range of the various gear types. These figures do not include bearing and lubricant losses. Also, they assume ideal mounting in regard to axis orientation and center distance. Inclusion of these realistic considerations will downgrade the efficiency numbers.

	Table 2-3 Types of Gears and Their Categories
	Categories of Gears Types of Gears Efficiency (%) Parallel Axes Gears Spur Gear Spur Rack Internal Gear Helical Gear Helical Rack Double Helical Gear 98 ... 99.5 Intersecting Axes Gears Straight Bevel Gear Spiral Bevel Gear Zerol Gear 98 ... 99 Nonparallel and Nonintersecting Axes Gears Hypoid Worm Gear Screw Gear Gear  30 ... 90 70 ... 95 96 ... 98

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