A face gear is a disc with teeth cut on its flat face (not its cylindrical edge) that meshes with a standard spur or helical pinion whose axis is perpendicular to the disc. Visually, it looks like a crown of teeth pointing along the shaft axis. Functionally, it does what a bevel gear does — transmit power between shafts at 90° — but with one critical manufacturing advantage: the mating pinion is a plain cylindrical gear that can be cut on ordinary hobbing equipment. Only the face gear itself requires specialized tooling.

Why they exist: Bevel gears (especially spiral bevel and hypoid) require expensive dedicated machines — Gleason or Klingelnberg cutters — and their pinions are conical and difficult to inspect. Face gear sets let you replace the bevel pinion with a cheap, off-the-shelf spur pinion. This matters enormously when you need many pinions and few face gears, or when the pinion must be replaceable in service.

Key geometric properties:

• Axial insensitivity of the pinion: the spur pinion can slide along its axis without changing mesh quality. This tolerates thermal growth and assembly stack-up that would destroy a bevel mesh.
• Radial sensitivity of the face gear: the face gear must be located precisely in the radial direction — the opposite of a bevel set, where the pinion location is critical.
• Variable tooth profile: the tooth thickness and pressure angle change across the face width. This is why they need dedicated shaping cutters.
• Limited ratio range: practical ratios run roughly 1.5:1 to 8:1. Above that, undercutting at the inner radius becomes severe.

Real-world example: The Sikorsky/Boeing RAH-66 Comanche and later Boeing AH-64 Apache upgrade programs evaluated face gear main rotor drives specifically because a helicopter transmission carries torque through many parallel pinion paths into one central gear. Replacing spiral bevel pinions with spur pinions removed the most expensive gears from the maintenance chain and allowed torque-splitting geometries that spiral bevels can't achieve.

Rule of thumb — face width: keep the active face width b under about 0.3× the mean pitch diameter of the face gear. Beyond that, the inner-radius teeth undercut and the outer-radius teeth become pointed. So for a 200 mm face gear, target a face width no greater than ~60 mm. If you need more load capacity, increase the diameter — don't widen the face.

Where you'll see them: helicopter main rotor gearboxes, aerospace accessory drives, some high-end printing press drives, and specialty robotic joints where a replaceable spur pinion drives a large, precisely-mounted face gear.