Helical gears and spur gears are two of the most common types of gears and can be used in many of the same applications. Spur gears are simple and inexpensive to produce, but helical gears offer some important advantages over spur gears.
The teeth of a helical gear are placed at an angle (relative to the gear axis) and take the shape of a helix. This allows the teeth to engage gradually, starting as point contact and progressing to linear contact as engagement progresses. One of the most obvious advantages of helical gears compared to cylindrical gears is lower noise, especially at medium-high speeds. Additionally, with helical gears, there are always more teeth in the mesh, which means less load on each individual tooth. This results in a smoother transition of forces from one tooth to another, thus reducing vibration, shock loads and wear.
But the steep angle of the teeth also causes sliding contact between the teeth, which produces axial forces and heat, decreasing efficiency. These axial forces play a significant role in the selection of helical gear bearings. Because bearings must resist radial and axial forces, helical gears require thrust or roller bearings, which are typically larger (and more expensive) than the plain bearings used with spur gears. Axial forces vary in proportion to the width of the tangent of the helix angle. Although larger helix angles provide greater speed and smoother motion, the helix angle is typically limited to 45 degrees due to the production of axial forces.
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 opposing pointers mounted back to back, although they are actually worked from the same gear. (The difference between the two designs is that double helical gears have a groove in the middle, between the teeth, while herringbone gears do not.) This arrangement cancels out the axial forces on each set of teeth, so you can use angles larger propellers. . It also eliminates the need for thrust bearings.
In addition to smoother movement, greater speed capability and less noise, another advantage that helical gears offer over spur gears is the ability to be used with parallel or non-parallel (crossed) shafts. Helical gears with parallel axes require the same helix angle but opposite sides (e.g., right teeth versus left teeth).
Gears with parallel axes have opposite pointers, while gears with perpendicular axes have the same pointer.
Image credit: JC Leonard
When crossed helical gears are used, they can be in the same direction or in opposite directions. If the gears have the same pointers, the sum of the helix angles must equal the angle between the axes. The most common example of this is crossed helical gears with perpendicular axes (i.e. 90 degrees). Both gears have the same pointer and the sum of the helix angles is equal to 90 degrees. For opposite configurations, the difference between the helix angles must be equal to the angle between the axes. Crossed helical gears offer flexibility in design, but the contact between the teeth is closer to point contact than linear contact, so they have lower force capabilities than parallel-shaft designs.
Helical gears are often the default choice in applications suitable for spur gears but with non-parallel shafts. They are also used in applications that require high speeds or loads. And regardless of load or speed, they generally provide smoother, quieter operation than spur gears.
