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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.

A gear contact pattern is the observed region of tooth-flank contact during a stated mesh test. Its position and extent reveal how the installed tooth surfaces meet. A gear contact marking test uses a thin marking layer and controlled rotation to make that region visible; the mounting position, transmission direction and applied torque belong in the test description. Manufacturer guidance treats surface-contact checks as evidence about the assembled mesh, including shaft alignment.^[ref]

A useful bench comparison holds the gear pair and test conditions fixed, records the initial mark, then changes one alignment setting and repeats the test. A mark moving toward one face edge indicates a change in where the flanks meet. This is an observation of contact location, rather than an automatic measurement of operating pressure or a universal diagnosis of the alignment error.

For example, a mark occupying the full20mm gear effective face width is compatible with a hypothetical uniform line load $w=50N/mm$, but also with the tilted positive distributionBoth integrate to1000N, yet their peak line loads are50 and80N/mm. Merely observing full-width contact cannot choose between those gear face load distributions. The gear face load factor or local stress needs additional load/deformation information.

Loaded and lightly loaded patterns can also differ because shaft, housing and tooth deformation change the gap. A quantitative loaded assessment may use strain measurements or a validated contact model alongside the pattern; the2006 gear-rating standard explicitly distinguishes comprehensive load assessment from the limited information in a bearing pattern.^[ref]

A gear marking-layer gap threshold is an effective separation below which a stated marking process produces a visible transfer or removal of material. It is a model of the observation in a gear contact marking test, rather than a universal thickness or a material constant. Layer thickness, pressure, rolling history and the marking material can all matter; a visible gear contact pattern does not directly give local operating pressure.

For an original idealization, let the unloaded separation across a face beand stipulate a visible mark where $g(z)\le h$. Here $h$ is the effective threshold, $g_0$ the minimum gap and $a>0$ its slope magnitude. For a centered pattern on face width $b$, the predicted marked width isChoose $b=20\mathrm{mm}$, $g_0=0.5\mu \mathrm{m}$ and $a=0.4\mu m/mm$. Thresholds $h=1.5$ and $2.5\mu \mathrm{m}$ give marked widths $5$ and $10\mathrm{mm}$. The geometry has not changed; the observation threshold has.

Away from the onset and saturation corners,Here a threshold change $0.2\mu \mathrm{m}$ changes the indicated width by $1\mathrm{mm}$. In the shallow-gap limit, small changes of the marking process can therefore cause conspicuous changes in the visible pattern.

A controlled comparison should hold the coating method, applied torque and number/direction of roll-throughs fixed, then vary one geometric setting. A separate fixture with known separations can test whether a proposed threshold model describes the marking process, without treating nominal coating thickness as an exact gap measurement.

Manufacturer contact guidance describes patterns as evidence about assembled tooth surfaces, and the historical rating standard limits the load information inferred from a bearing pattern.^[ref][ref] The threshold equation is a deliberately specified illustrative model; actual deformation and transfer behavior need validation before converting a mark into a quantitative gap or gear face load distribution.