When left-handed and right-handed helical gears with the same twist angle arecombined, it is called a double helical gear. Double helical gears transmit rotational motion between two parallel shafts and have all of the advantages of helical gears. Double Helical gears are strong and quiet with the successful ability to cancel the thrust forces with both right and left-hand twists by combining helical gears.
Following are the advantages of Double Helical Gear:

  1. A double helical gear is like a worm gear but with the teeth on two opposite sides of the gear, providing a reduction ratio of 1:2.
  2. With very high load capacity double helical gears are suitable for heavy-duty applications.
  3. The Large number of teeth in double helical gears helps in the reduction of heat build-up which makes a double helical gear last longer.
  4. An increased reduction ratio in comparison to the standard worm gear, makes double helical gear strong and quiet.
  5. Its usabilitywith both parallel and non-parallel shafts makes helical gear more versatile.
  6. The tooth profile of a double helical gear is more efficient than that of a worm gear.
  7. Double helical gears have fewer parts in comparison to worm gears and they produce low noise.
  8. When cutting curves, double helical gears are better than spur gears.
  9. Very smooth at gear operations, simple and easy to use makes a double helical gear preferred choice for most applications.
  10. Double helical gears are more efficient because of the way they transfer power between the input and output shafts, providing a significant amount of power to the output shaft with less mechanical energy required at the input.
  11. They have a high-efficiency factor that is determined by their ratio, which is often significantly higher than other types of gear teeth (such as spur or bevel).

So to sum up:
Sometimes, a double helical gear counteracts or nullifies the forces to overcome axial thrust. With the same helix angle with opposite hands(1,2), the total face is made into two halves. The axial thrusts are opposite one another and force is contained in the gear, not transmitting to the bearing.
With high loading capability, steady transmission, and other advantages double helical gears are widely used for power transmission in gas turbines, movers, pumps, fans, generators, and compressors in marine ships and construction machines.