Bevel gears change the direction of rotation, and that extra job shows in their drawings. Where a spur gear needs only module and tooth count, a bevel gear needs pitch angles, a mounting distance and a cone distance before a factory can even estimate material. Buyers who send a sketch without these values usually get a list of questions instead of a price — the gap is not the supplier being difficult, it is that a bevel gear pair is defined by five parameters, not two.
The pitch angle is the angle of the cone, and it comes directly from the tooth ratio. For a 90-degree shaft pair, tan(δ1) = z1/z2 and the two pitch angles add to 90 degrees. A 20-tooth pinion driving a 40-tooth gear gives roughly 26.6 and 63.4 degrees. The pitch angle sets the whole geometry — tooth depth, face width and outer diameter all derive from it — so it is the first value a drawing should show.
Module for bevel gears is measured at the large end, not at the pitch circle like a spur gear. The same gear has a smaller module at the small end, which is why drawings must state "module at large end" and the face width. A face width beyond about one third of the cone distance makes the small-end teeth too thin to cut, so factories will flag that ratio even when the buyer does not.
Mounting distance is the dimension most often missing, and the one that causes the most rework. It is the distance from a locating face on the shaft or hub to the apex of the pitch cone, and it sets where the two gears sit relative to each other. Two bevel gears with the same tooth data but different mounting distances will not mesh correctly — the backlash opens on one side and the teeth bind on the other. Always mark the locating face and the mounting distance on the drawing.
Spiral angle and tooth type decide how the gear runs. Straight bevel gears are simpler to make and inspect, but they are noisier and carry less load. Spiral bevel gears with a 35-degree spiral angle run smoother and quieter, but they generate axial thrust and need the housing to hold it. Zerol gears sit in between — curved teeth but zero spiral angle, so no thrust. The choice should match speed, noise target and whether the housing can take thrust loads.
At our factory we cut straight, spiral and Zerol bevel gears on dedicated bevel gear machines, and we check tooth contact pattern on a rolling tester before dispatch. Send us a drawing with pitch angles, module at large end, mounting distance, spiral angle and tooth count marked, or just the ratio and shaft angle — we will confirm the geometry and quote within 24 hours.