Axial thrust in helical gears creates unnecessary stress on bearings, reduces efficiency, and can lead to premature failure—but you can eliminate it entirely with the right design approach. This sideways force happens because helical gear teeth are cut at an angle, causing part of the driving torque to push the gear along its shaft instead of just rotating it.
Double Helical and Herringbone Gears
Double helical gears eliminate axial thrust by using two mirrored sets of teeth that form a V-shape, with each half generating equal and opposite forces that cancel each other out. Think of it like two helical gears back-to-back—one with left-hand teeth and one with right-hand teeth—machined onto the same gear hub.
The opposing halves push against each other internally, so the gear transmits torque without creating any net sideways force on the bearings. This means you can use standard radial bearings instead of expensive thrust bearings, and you can even increase the helix angle for smoother operation without worrying about thrust loads.
Opposed Helical Gear Pair Arrangements
You can cancel axial thrust without expensive double helical gears by cleverly arranging pairs of standard single helical gears with opposite helix directions. This approach uses the thrust from one gear to counteract the thrust from another, achieving balance through system design rather than specialized components.
- Back-to-Back Gear Sets: Mount two single helical gears of opposite hand on the same shaft, with one left-handed and one right-handed gear transmitting equal torque. The left-hand gear pushes the shaft one way while the right-hand gear pushes it the opposite way, resulting in zero net thrust. This setup works like a double helical gear but uses standard single helical components that are cheaper and easier to manufacture.
- Opposed Helix in Multistage Gearboxes: Alternate the helix direction between stages so the thrust from one reduction stage cancels the thrust from the next. An intermediate shaft might carry a left-hand gear for the first stage and a right-hand gear for the second stage, allowing the shaft to “float” with minimal axial load. This method is particularly effective in multi-stage compressors and turbine gearboxes where you’re already using multiple gear sets anyway.
Using Thrust Bearings and Angular Contact Bearings
The simplest solution is often to accept the axial thrust and handle it with proper bearings—thrust bearings and angular contact bearings are specifically designed to absorb these sideways forces. Instead of eliminating the thrust at the gear level, you install bearings that can safely carry the axial loads throughout the gearbox’s lifetime.
FAQs
Which method is most cost-effective for eliminating axial thrust?
For most applications, using thrust bearings is the most economical solution. Double helical gears offer the best performance but cost significantly more, while opposed arrangements work well in multi-stage gearboxes where you’re already using multiple gear sets.
Can I completely eliminate axial thrust in a single helical gear?
No, a single helical gear will always produce axial thrust due to its angled teeth. You must either use double helical gears, pair it with an opposing gear, or install thrust bearings to handle the force.
How much thrust force do helical gears generate?
The axial thrust equals the tangential force multiplied by the tangent of the helix angle (F_a = F_t × tan β). A 30-degree helix angle generates thrust equal to about 58% of the tangential force.
Why not just use spur gears to avoid thrust altogether?
Spur gears eliminate thrust but run much noisier and handle less load than helical gears. Helical gears provide smoother power transmission, higher load capacity, and quieter operation—benefits that usually outweigh the thrust management challenges.
