High Speed Helical Gear, Precision Helical Gear

The teeth on helical gears are cut at an angle to the face of the gear. When two teeth on a helical gear system engage, the contact starts at one end of the tooth and gradually spreads as the gears rotate. Two mating helical gears must have equal helix angle but opposite hand. They run smoother and more quietly. They have higher load capacity, are more expensive to manufacture. Helical gears can be used to mesh two shafts that are not parallel and can also be used in a crossed gear mesh connecting two perpendicular shafts. They have longer and strong teeth. They can carry heavy load because of the greater surface contact with the teeth. The efficiency is also reduced because of longer surface contact. The gearing is quieter with less vibration.
One interesting thing about helical gears is that if the angles of the gear teeth are correct, they can be mounted on perpendicular shafts, adjusting the rotation angle by 90 degrees.

A helical gear is similar to a spur gear except that the teeth of a helical gear are cut at an angle (known as the helix angle) to the axis (or hole). Helical gears are made in both right and left hand configurations. There are two configurations of helical gear connections: Those connecting parallel shafts and those connecting nonparallel shafts.
Helical Gears Connecting Parallel Shafts Helical gears connecting parallel shafts will run more smoothly and quietly than spur gears, particularly when the helix angle is great enough to ensure that there is continuous contact from one tooth to the next. A pair of helical gears used to connect parallel shafts must have the same pitch, pressure angle and helix angle, but they will be opposite hand gears (that is, one will be a left-hand gear; The other a right-hand gear).
Helical Gears Connecting Non-Parallel Shafts Helical gears used to connect non-parallel shafts are commonly called spiral gears or crossed axis helical gears. If the shaft angle is 90 degrees, the gears will be of the same hand and the sum of the helix angles will be equal to the shaft angle (90 degrees).
Helical gears used on non-parallel shafts must have the same normal pitch and normal pressure angles. They may, however, be of the same or opposite hand depending on the shaft angle.