Fig 14Forces at the mesh10 N·m · m 3 · 24 teeth · 20°helix 20°
Ft Fr Fa total Fn face overlap εβ
Ft: tangential, does the workFr: radial, pushes the shafts apartFa: axial, along the shaft10 N·m on a 72 mm pitch diameter makes 278 N of tangential force. That's the part that turns the other gear, and it doesn't depend on the tooth shape. The 20° pressure angle adds a radial push of Ft·tan 20° ≈ 101 N on a spur gear. Twist the teeth into a helix and an axial force Ft·tan β appears, so the bearings now have to take thrust. The payoff is face overlap εβ: above 1, a new tooth pair is always starting before the last one finishes, which is why helical gears are quieter.