Helical Gears have a cylindrical shape, but their teeth are set at an angle to the axis. They are capable of smoother and quieter action than spur gears. When their axes are parallel, they are called parallel helical gears, and when they are at right angles they are called helical gears.
Double helical gear :
In a variation on the basic helical gear known as the double helical gear or herringbone gear, the gear has two sets of teeth side by side, slanting at different angles. This reduces the amount of thrust, eliminating the need to use specialized bearings. Herringbone gears may have a v-shaped pattern, or two sets of angled teeth with a groove in the middle, depending on the age of the gears and where they were fabricated. Older fabrication techniques usually required the presence of a trough in the middle of the double helical gear, while newer gears tend to lack this feature.