Understanding Axial Thrust in Gears

When gears transmit power, forces are generated on the gear teeth. These forces can have different components depending on the gear type. One important component is axial thrust, which is a force directed along the axis of the shaft the gear is mounted on. This axial thrust needs to be managed as it can put extra load on bearings and the shaft itself.

How Different Gears Handle Axial Thrust

• Spur Gears: These are the simplest type of gear, with teeth parallel to the axis of rotation. When spur gears mesh, they generate force primarily in the tangential and radial directions relative to the pitch circle. There is no axial component of force, so spur gears do not produce axial thrust on the shaft.
• Helical Gears: Unlike spur gears, helical gears have teeth that are cut at an angle (helix angle) to the axis of rotation. This angled tooth engagement provides smoother and quieter operation than spur gears, but it introduces a force component in the axial direction. Therefore, helical gears generate axial thrust on the shaft. The direction of the thrust depends on the helix angle (left hand or right hand) and the direction of rotation.
• Bevel Gears: Bevel gears are used to transmit power between shafts that intersect, usually at a 90-degree angle. Due to the conical shape and angle of the teeth, bevel gears generate both radial and axial thrust forces on the shafts they are mounted on.
• Herringbone Gears: Herringbone gears are essentially like two helical gears placed side-by-side, but with opposing helix angles. One half of the gear has a left-hand helix, and the other half has a right-hand helix. When this gear meshes with another herringbone gear, the axial thrust generated by one half is opposed and effectively cancelled out by the axial thrust generated by the other half. This clever design means that herringbone gears do not produce net axial thrust on the shaft, even though they have angled teeth like helical gears.

To nullify or cancel out the axial thrust that would otherwise be produced by angled gear teeth, a gear design that combines opposing helix angles is used. This is precisely the principle behind the herringbone gear.

Therefore, to nullify the thrust on the shaft resulting from the gear's operation, a herringbone gear is used because its dual-helix design internally balances the axial forces.