For an ordinary Spiral Bevel Gear, the teeth are inclined along the face width direction, with a spiral angle of 20°–35°. Therefore, when a pair of teeth engage, they decompose a portion of the force into an axial component → the shaft is pushed in one direction → thrust bearings must be arranged.
The approach of Zero Degree / Zerol is: The tooth trace is still made into a curved arc shape (this comes from the Gleason face milling process). But the spiral angle is symmetrically brought back to 0° (the tangent at the midpoint of the tooth trace is perpendicular to the radial direction).
Result: The contact behavior of the teeth gains the gradual/progressive nature of a curved tooth, but the axial projection of the forces is approximately at the level of a straight bevel gear.
Essentially, it is a bevel gear that is “bent in shape, but not twisted in angle.”
