Are you confused about the difference between right hand and left hand helical gears? Using the wrong gear type can cause excessive axial thrust forces, reduced efficiency, and premature failure.
In this article, we’ll clarify the difference between right hand and left hand helical gears, and explain how the direction of the axial thrust force is impacted by the hand of the gear.
What Is Right Hand Helical Gear
A right hand helical gear — sometimes shortened on shop drawings to a right gear (right hand helical gear) — is a type of cylindrical gear with teeth that are angled and twisted around the gear’s axis in a right-handed or clockwise direction (a right hand helix at the tooth flank). The angle and twist of the teeth allows for smoother, quieter operation compared to spur gears. Right hand helical gears are commonly used in applications that require high speed, high power transmission, or where noise reduction is important.
The teeth of a right hand helical gear are cut at an angle to the axis of the gear. This angle, known as the helix angle, typically ranges from 15 to 45 degrees. The greater the helix angle, the greater the overlap between mating teeth, which results in smoother gear operation and increased load carrying capacity.
Right hand helical gears are often used in parallel shaft configurations. When two right hand helical gears mesh together, they produce an axial thrust load in opposite directions.
What Is Left Hand Helical Gear
A left hand helical gear is similar to a right hand helical gear, except that the teeth are angled and twisted around the gear’s axis in a left-handed or counterclockwise direction (a left hand helix at the tooth flank). Like right hand helical gears, left hand helical gears offer smoother, quieter operation compared to spur gears.
The helix angle of a left hand helical gear is also typically between 15 and 45 degrees, with larger angles providing greater overlap between mating teeth and smoother operation. However, the direction of the tooth angle is opposite that of a right hand helical gear.
Left hand helical gears are often used in conjunction with right hand helical gears in applications where the axial thrust loads generated by the gears need to be balanced. By using a right hand helical gear on one shaft and a left hand helical gear on the other, the axial thrust loads cancel each other out, eliminating the need for thrust bearings.
The Difference Between Right Hand and Left Hand Helical Gear
The primary difference between right hand and left hand helical gears lies in the direction of the axial thrust force generated when the gears mesh together. The magnitude of that thrust is set by the helix angle through Fa = Ft · tan(β), where Ft is the tangential force at the pitch circle and β is the helix angle. At β = 15° the axial thrust is about 27% of Ft; at 30° it climbs to 58%; at 45° it equals Ft. So a 30° helical gear pair driving a conveyor needs the thrust bearing rated for roughly 58% of the tangential drive force — a number that has to be designed in, not discovered during commissioning.
In a right hand helical gear, the axial thrust force is directed away from the gear and along the shaft axis. This means that in a pair of mating right hand helical gears, the axial forces are directed away from each other, pushing the gears apart.
Conversely, in a left hand helical gear, the axial thrust force is directed toward the gear and along the shaft axis. When a left hand helical gear meshes with a right hand helical gear, the axial forces are directed toward each other, cancelling out the net axial thrust on the system.
