2026-07-20
Spur gears have teeth cut parallel to the shaft axis. Each tooth engages all at once across its full face width — that sudden load transfer is what makes spur gears whine at high speed and hammer themselves apart under shock loads. Helical gears solve this by cutting the teeth at an angle (the helix angle, typically 15° to 30°) so that engagement rolls smoothly across the face instead of slamming into it.
What you gain:
What you pay: the angled teeth generate an axial thrust force that spur gears don't. That thrust has to go somewhere — usually into a thrust bearing, a shoulder, or (in high-end designs) a matched pair of opposite-hand helical gears (double helical or herringbone gears) that cancel each other's thrust internally.
The thrust calculation rule of thumb: the axial thrust force equals the tangential (torque-producing) force multiplied by the tangent of the helix angle:
Faxial = Ftangential × tan(ψ)
where ψ is the helix angle. For a 20° helix gear transmitting 1,000 N of tangential force, you get about 364 N of thrust load into your bearings. Double the helix angle to 30°, and the thrust jumps to 577 N — nearly 60% of the working load. This is why helix angles above 30° are rare outside of double-helical designs.
Concrete example: your car's manual transmission. Every forward gear is helical — that's why they run silently. Reverse is often spur-cut because it's used briefly, doesn't need to be quiet, and avoiding the axial thrust simplifies the synchronizer design. That characteristic "reverse whine" you hear backing up? That's the acoustic signature of spur teeth engaging.
Design implications: if you're specifying a helical gearbox, always check that the input and output shaft bearings are rated for the axial load — not just the radial load. Angular contact bearings, tapered roller bearings, or a dedicated thrust washer are typical. A deep-groove ball bearing sized only for radial load will fail early from unhandled thrust.