Internal gears can mesh with external pinions of the same module, but some tooth-count combinations create geometric interference. The interference type matters: involute interference can occur when the pinion has too few teeth, while trochoid and trimming interference impose upper tooth-count limits for the KHK stock combinations shown here. This table summarizes KHK’s 2025 catalog values for selected stock internal gears and derives the pinion range that satisfies all three listed limits.

Gear Tooth Interference Limits

How to read the tooth-count limits

Choose the row by the number of teeth on the internal gear. The manufacturer lists a lower pinion tooth-count threshold for involute interference and two upper thresholds, one each for trochoid and trimming interference. The derived allowable range uses the involute value as its minimum and the smaller of the two upper limits as its maximum. For an internal gear with 100 teeth, for example, the listed limits are 19 teeth minimum for involute interference, 92 maximum for trochoid interference and 84 maximum for trimming interference. The resulting range satisfying these listed thresholds is 19 through 84 pinion teeth.

These are integer tooth counts, not dimensional tolerances. The lower and upper values refer to different geometric mechanisms, so do not substitute one limit for another or average them. A combination outside the derived interval fails at least one of the catalog’s checks; a combination inside the interval only passes the checks represented in this specific table.

Three interference mechanisms

Involute interference occurs when contact extends outside the usable involute profiles. In an internal gear pair, the internal gear tooth tip can dig into the pinion root when the pinion has too few teeth. Trochoid interference involves the non-involute root curve near the tooth base; a small difference between internal gear and pinion tooth counts can bring the exiting pinion tooth into contact with the internal gear. Trimming interference is a manufacturing or assembly geometry condition in which a pinion may slide axially into position but cannot move radially as required.

Because each mechanism depends on the exact tooth forms and relationship between the mating members, a tooth-count rule from one catalog should not be generalized to another gear series. Module, pressure angle, addendum, profile shift, cutter geometry and the method used to generate the teeth can change the result. Special gears, nonstandard profiles and modifications need a dedicated gear calculation or confirmation from the manufacturer.

Verify a proposed gear pair

First identify the internal-gear catalog number and its specified tooth count. Confirm the pinion has the matching module and compatible pressure angle and tooth form. Then compare the pinion count with the lower involute threshold and both upper limits, rather than checking only the derived range. Use the exact current catalog for the gear series because a product revision or special geometry may change the permitted combinations.

Next check the complete assembly. Center distance, backlash, profile shift, tooth thickness, runout and manufacturing accuracy affect whether the gears assemble and operate as intended. Check contact ratio, lubrication, material, load capacity and service conditions separately. A catalog interference table does not establish torque capacity, fatigue life, noise, efficiency or safety, and it is not a substitute for a gear rating calculation.

Scope and limitations

The rows cover only the internal gear tooth counts listed by KHK and its stated stock-product assumptions. They do not provide an interpolation rule for an unlisted ring gear, nor do they qualify custom gears, helical or bevel sets, planetary arrangements with special shifts, or a production design. Do not infer a universal limit for every pinion simply because its tooth count falls within one of the ranges.

For a design that is safety-critical, heavily loaded, unusually small, or outside the listed products, request a detailed check from the gear supplier or a qualified gear designer. Resolve any conflict between a catalog selection table, a current standard and the actual drawing before manufacture.

Source

Thresholds come from KHK’s 2025 Internal Gears catalog, page 209, under allowable mating pinion tooth counts. The allowable range column is calculated from the published lower and upper thresholds; it is not a separately stated KHK rating.