Can gears with undercut teeth still be used?

Can gears with undercut teeth still be used?

Undercut gear teeth: sometimes usable, never a default assumption

Gears with undercut teeth can sometimes be used, but the tooth form must be checked rather than accepted by default. Undercut can reduce root thickness and affect contact ratio, strength and fatigue margin, especially on a small pinion. PairGears should evaluate the actual drawing, mating gear, load and life target; a positive profile shift or different tooth count may be the better solution.

Comparison of negative and positive profile shifts in a spur gear tooth
Negative and positive profile shifts visibly change the tooth-root geometry and undercut risk.

Check why the undercut occurred before choosing a remedy

Undercut is a geometry and manufacturing issue. In a generated involute gear, a small tooth count, pressure angle, addendum and profile-shift choice can cause the cutting tool to remove material near the root. A visible root relief is not automatically a failure, but it must be assessed against bending stress, contact ratio, interference risk, mating geometry and the specified process route.

Do not repair the decision by changing one dimension in isolation. The PairGears profile-shift guide explains how a positive shift can help a small-tooth-count pinion while also changing tooth thickness and mesh behavior. Confirm module or DP, pressure angle, tooth counts, profile shifts, center distance, face width, material, heat treatment and duty before release.

Gear profile-shift comparison showing changes in tooth thickness
The profile-shift comparison shows why tooth thickness and mating geometry must be assessed together.
Positive profile shift generating a gear tooth with a rack cutter
The rack-cutter diagram directly shows how positive profile shift can avoid undercut during generation.

Example or decision check

A compact stage may require a 12-tooth pinion to achieve a ratio in one mesh. Instead of assuming the standard tooth form is acceptable, compare the generated profile with the root-strength and contact-ratio targets, then review a profile-shifted pair or a second stage. The correct choice is the one that meets the complete mesh requirement, not simply the smallest diameter.

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