The axial loads produced by helical gears can be countered by using double helical or herringbone gears. These arrangements have the appearance of two helical gears with opposite hands mounted back-to-back, although in reality they are machined from the same gear. (The difference between the two designs is that double helical gears have a groove in the middle, between the teeth, whereas herringbone gears do not.) This arrangement cancels out the axial forces on each set of teeth, so larger helix angles can be used. It also eliminates the need for thrust bearings.
Besides smoother motion, higher speed capability, and less noise, another advantage that helical gears provide over spur gears is the ability to be used with either parallel or non-parallel (crossed) shafts. Helical gears with parallel shafts require the same helix angle, but opposite hands (i.e. right-handed teeth vs. left-handed teeth).
When crossed helical gears are used, they can be of either the same or opposite hands. If the gears have the same hands, the sum of the helix angles should equal the angle between the shafts. The most common example of this are crossed helical gears with perpendicular (i.e. 90 degree) shafts. Both gears have the same hand, and the sum of their helix angles equals 90 degrees. For configurations with opposite hands, the difference between helix angles should equal the angle between the shafts. Crossed helical gears provide flexibility in design, but the contact between teeth is closer to point contact than line contact, so they have lower force capabilities than parallel shaft designs.
Helical gears are often the default choice in applications that are suitable for spur gears but have non-parallel shafts. They are also used in applications that require high speeds or high loading. And regardless of the load or speed, they generally provide smoother, quieter operation than spur gears.
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