Axial thrust in helical gears happens because the teeth are cut at an angle to the gear’s shaft, creating a force that pushes the gear sideways along its axis. When two helical gears mesh together, the angled teeth act like a ramp or screw thread, redirecting part of the rotational force into a sideways push.
This is completely different from spur gears with straight teeth. In spur gears, all the forces stay in the gear’s plane – there’s no sideways push at all.
The mechanics are straightforward. When helical gear teeth mesh, the contact force between them acts perpendicular to the tooth surface. Since the tooth surface is tilted at the helix angle, this force splits into three components: tangential (which drives rotation), radial (which pushes the gears apart), and axial (which creates the thrust along the shaft).
You can calculate the exact axial thrust using a simple formula: Fa = Ft × tan(β), where Ft is the tangential force and β is the helix angle. The bigger the helix angle, the stronger the axial thrust becomes.
Design Factors Influencing Axial Thrust
- Helix Angle: The helix angle is the single most important factor determining axial thrust. Larger angles create more thrust since Fa = Ft × tan(β). Most designers limit helix angles to 45 degrees or less because beyond that, the thrust forces become impractically large. Common angles range from 15 to 30 degrees, balancing smooth operation with manageable thrust loads.
- Single vs. Double Helical Gears: Single helical gears create unbalanced thrust that must be supported by bearings. Double helical (herringbone) gears eliminate this problem entirely by using two sets of opposing teeth that generate equal and opposite thrust forces, canceling each other out. This clever design transmits torque without any net axial thrust, eliminating the need for expensive thrust bearings.
- Gear Hand and Pairing: Helical gears come in left-hand and right-hand versions, referring to which way the teeth lean. Meshing gears on parallel shafts must have opposite hands – one left, one right. Each gear experiences thrust in the opposite direction, so while the system is balanced overall, each individual gear still needs thrust support. Reversing the rotation direction also reverses the thrust direction, which matters for bidirectional applications.
- Bearing Selection and Gear Mounting: The axial thrust from helical gears demands special bearing arrangements. You’ll need thrust bearings or angular-contact bearings specifically designed to handle axial loads. These bearings are typically larger and more expensive than those needed for spur gears. The shaft design might also include shoulders or flanges to help absorb thrust forces, especially in high-load applications.
FAQs
Can you completely eliminate axial thrust in helical gears?
Yes, by using double helical or herringbone gears. These designs feature two sets of teeth angled in opposite directions on the same gear, creating equal and opposite thrust forces that cancel each other out completely.
How does load affect axial thrust?
Axial thrust increases proportionally with the transmitted load. If you double the torque through the gears, you double the axial thrust. The thrust ratio stays constant (determined by the helix angle), but the absolute force grows with load.
What happens if you reverse a helical gear’s rotation?
Reversing the rotation direction flips the axial thrust to the opposite direction. A gear that was being pushed to the right will now be pushed to the left, which is why bidirectional applications need bearings that can handle thrust in both directions.
