 Face gear

A face gear is a gear whose working teeth occupy an annular face and mesh with a mating cylindrical pinion. A common arrangement turns the drive through approximately a right angle; other shaft angles and tapered configurations are possible. Its working surface geometry must maintain compatible tooth contact, rather than simply placing ordinary cylindrical teeth on a flat disk.

Tooth counting still supplies an ideal speed relation. If the pinion has $N_p$ teeth and the face gear $N_f$, equality of tooth-passage rates givesAn original 20-tooth pinion driving a 100-tooth face gear at $1500$ revolutions per minute therefore gives $300$ revolutions per minute at the output. Rotation signs require a stated orientation of the two different shaft axes; the scalar minus sign for an external parallel-axis spur gear pair cannot be copied without that convention.

The annular layout also explains a geometric design issue. With $N$ tooth positions, angular spacing is $2\pi /N$, while arc spacing on a ring of radius $r$ isFor $N=80$, radii $40$ and $60\mathrm{mm}$ give arc spacings about $3.142$ and $4.712\mathrm{mm}$. This radial change does not prescribe two interchangeable gear modules; it shows why a generated face-tooth surface cannot be understood as a single extruded cylindrical tooth profile.

A primary 2002 program built a scaled-power split-torque transmission with two opposed face gears, two input pinions and two idlers.^[ref] Multiple meshes make the geometry useful for distributing and recombining load, but also make alignment and compatibility important.

A bench rotation check can test the tooth-count ratio. A gear contact marking test, dimensional inspection and load-sensitive measurements address different questions about the manufactured surfaces and their operation. Correct output speed alone does not establish an acceptable gear contact pattern, tooth stress or gear efficiency.