= Gear contact marking test
A \b[gear contact marking test] makes a <gear contact pattern> visible by using a thin contrasting marking layer and rolling the mating <gears> under stated conditions. The observed transfer or removal of the layer locates the region where the flanks meet closely enough to mark. A useful test record includes the marked flank, rotation direction, mounting position, <backlash> and applied <torque>. Manufacturer guidance discusses tooth-surface contact as an assembly and alignment check.<Elements of Metric Gear Technology by SDP-SI>[Section 19 and Section 19.1, Features of Tooth Surface Contact, printed T-221]{ref}
A controlled comparison changes one alignment setting while keeping the same marking method and loading condition. A shift of the mark toward an edge then records a change in the mesh geometry. Changing layer thickness at the same time would confound that comparison: the visible pattern is the response of the surfaces and the marking process together.
An original threshold model illustrates this limitation without claiming a universal paint law. Suppose an unloaded gap over a $20\,\mathrm{mm}$ face is
$$
g(z)=\bigl(0.5\,\mathrm{\mu m/mm}\bigr)|z-10\,\mathrm{mm}|,\qquad 0\le z\le20\,\mathrm{mm}.
$$
Stipulate that a mark appears wherever $g(z)\le h$, with <gear marking-layer gap threshold> $h$. At $h=2\,\mathrm{\mu m}$ the marked width is $8\,\mathrm{mm}$; at $h=4\,\mathrm{\mu m}$ it is $16\,\mathrm{mm}$. The indicated coverage doubles even though the underlying gap is unchanged. Real transfer also depends on deformation and the marking material.
Nor does a broad mark uniquely determine the <gear face load distribution>. Operating <torque> deforms teeth, shafts and supports, so a lightly loaded pattern can differ from a loaded one. The historical gear-rating standard distinguishes comprehensive loaded assessment from the limited information in a bearing pattern.<ISO 6336-1:2006>[Section 7.3.1, Method A, load assessment and interpretation of bearing patterns]{ref}
A meaningful validation pairs the pattern with an independent load-sensitive observation, such as tooth-root strain, or a checked deformation model. The question is whether the setup places contact where intended under the specified condition, rather than whether one visually attractive mark proves uniform pressure at every operating load.
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