Difference between herringbone and double helical gears

Herringbone and double helical gears are constructed by combining two identical helical gears of the same pitch circle diameter, module, and a number of teeth, but both gears have an opposite hand of a helix. This offers an advantage of balancing the axial thrust in both the direction by developing opposite thrust reaction providing higher power transmission capacities. 

What is herringbone gear?

No gap is provided between two halves so teeth with left-hand helix touch the teeth with right-hand helix. Its manufacturing is difficult and requires a dedicated machine. 

What is double helical gear?

Small relief gap is provided between two halves so teeth with left-hand helix do not touch the tooth with right-hand helix. Its manufacturing is comparatively easy as it can be cut by hobbing, shaping and even by milling. 

Let us have a deep insight into the difference between herringbone and double helical gears. 

Difference :

• In the case of double helical gears, there is a groove between two helical gears, while herringbone gears are called a gear without a groove.
• A double helical gear is cut on a single gear blank, by a hob with a tool run-out groove between the hands of helices whereas herringbone gear is cut by two cutters, which reciprocate 180⁰ out of phase to avoid clashing. 
• Both the gears allow higher helix angle because there is no thrust force and angle of helix is about to 20⁰ to 45⁰.
• Herringbone gear has less axial strength thus can be used where space is limited whereas double-helical gear requires more axial space due to the presence of relief gap.