Herringbone Gears Information
Herringbone gears, also called double helical gears, are gear sets designed to transmit power through parallel or, less commonly, perpendicular axes. The unique tooth structure of a herringbone gear consists of two adjoining, opposite helixes that appear in the shape of the letter 'V'.
Herringbone gears mate via the use of smooth, precisely manufactured V-shaped teeth. Like helical gears multiple teeth are engaged during rotation, distributing the work load and offering quiet operation. However, due to their tooth structure, herringbone gears nullify the axial thrust typical of helical gears.
The gear set's teeth may be manufactured so that tooth-tip aligns with the opposite tooth-tip, or so tooth-tip aligns with the opposite gear's tooth trough.
Herringbone Gear Can Crusher Video Credit: Steven Garrison via Youtube
Herringbone gears are produced in mirrored pairs. Their complex tooth profile makes them more expensive than most gear options. It is common to place two opposite-hand helical gears adjacently and mill a centered, flat groove; this does not affect their performance. Perpendicular herringbone gears are rare, but gained prominence in Citroën vehicles.
Gears mate via teeth with very specific geometry. Pitch is a measure of tooth spacing and is expressed in several ways.
Pressure angle is the angle of tooth drive action, or the angle between the line of force between meshing teeth and the tangent to the pitch circle at the point of mesh. Typical pressure angles are 14.5° or 20°.
Helix angle is the angle at which the gear teeth are aligned compared to the axis. Both angles present on the herringbone tooth must be accounted.
Selection tip: Gears must have the same pitch and pressure angle in order to mesh.
Consider the gear center, bore diameter and shaft diameter. The gear center can be a bored hole or an integral shaft. The bore diameter is the diameter of the center hole. The shaft diameter is the diameter of the shaft for gears with an integral shaft. Herringbone gears can be mounted on a hub or shaft. A hub is a cylindrical projection on one or both sides of a herringbone gear, often for the provision of a screw or other shaft attachment mechanism. Hubless gears are typically attached via press fit, adhesive or internal keyway.
Shaft mounting choices include the following:
Keyway: One or more cutouts exist in the gear bore for exact mounting on the shaft.
Set Screw: The gear is attached to the shaft by screws through the hub.
Split: The hub is split into several pieces that are tightened down by a separate clamp to grip the shaft.
Simple Bore: A straight bore designed for adhesive attachment.
Workload and Environmental Specifications
Application requirements should be considered with the workload and environment of the gear set in mind.
Power, velocity and torque consistency and output peaks of the gear drive so the gear meets mechanical requirements.
Inertia of the gear through acceleration and deceleration. Heavier gears can be harder to stop or reverse.
Precision requirement of gear, including gear pitch, shaft diameter, pressure angle and tooth layout. Herringbone gears' precise teeth can make them expensive.
Gear lubrication requirements — some gears require lubrication for smooth, temperate operation.
Mounting requirements — application may limit the gear's shaft positioning.
Noise limitation — commercial applications may value a smooth, quietly meshing gear. Herringbone gears offer quiet operation.
Corrosion resistance — gears exposed to harsh environments or chemicals should be especially hardened or protected.
Temperature exposure — some gears may warp or become brittle in the face of extreme temperatures.
Vibration and shock resistance — heavy machine loads or backlash, the deliberate surplus space in the circular pitch, may jostle gearing.
Operation disruption resistance — It may be necessary for some gear sets to function despite missing teeth or misalignment.
Gear composition is determined by application, including the gear's service, rotation speed, accuracy and more.
Cast iron provides durability and ease of manufacture.
Alloy steel provides superior durability and corrosion resistance. Minerals may be added to the alloy to further harden the gear.
Cast steel provides easier fabrication, strong working loads and vibration resistance.
Carbon steels are inexpensive and strong, but are susceptible to corrosion.
Aluminum is used when low gear inertia with some resiliency is required.
Brass is inexpensive, easy to mold and corrosion resistant.
Copper is easily shaped, conductive and corrosion resistant. The gear's strength would increase if bronzed.
Plastic is inexpensive, corrosion resistant, quiet operationally and can overcome missing teeth or misalignment. Plastic is less robust than metal and is vulnerable to temperature changes and chemical corrosion. Acetal, delrin, nylon, and polycarbonate plastics are common.
Other material types like wood may be suitable for individual applications.