A Double Helical Gear, also known as a herringbone gear, is a type of gear that is used to transmit power and motion between parallel shafts. It is similar to a helical gear, but with a central groove that runs down the middle of the gear, dividing it into two halves.

The unique shape of the Double Helical Gear allows it to transmit power more smoothly and quietly than other types of gears. The helical teeth of the gear are cut at an angle to the axis of the gear, which creates a diagonal contact between the teeth of the two gears in mesh. This results in a gradual engagement of the gear teeth, which reduces noise and vibration.

One of the disadvantages of a Double Helical Gear is that it is more difficult to manufacture than other types of gears. The gear teeth must be cut in opposite directions on each half of the gear, which requires specialized equipment and expertise. Additionally, the central groove between the halves of the gear can make it more difficult to lubricate the gear.

Double Helical Gears are used in a variety of industrial applications, including power generation, mining, and heavy machinery. They are often used in applications where quiet operation and smooth transmission of power are important.