Double helical gears have one a right-hand helix and the other a left-hand helix. The teeth of two rows are separated by a groove used for tool run out. Axial thrust which occurs in case of single helical gears, two rows of teeth cancel each other. This can be run at high speeds with less noise and vibrations. Figure 15.20 shows the basic structure of double helical gear and herringbone or double helical gear.
Figure 15.20 (a) Double Helical Gearing with Two Pairs of Opposed Gears, (b) Herringbone Gears Having Opposed Teeth Joined in the Middle
Herringbone gears have opposed teeth to eliminate side thrust loads the same as double helical, but they are joined in the middle of the gear circumference. This arrangement makes herringbone gears more compact than double helicals. However, the gear centres must be precisely aligned to avoid interference between the mating helixes.
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