The most common and costly oversight in custom gear specifications is not an incorrect pressure angle or module – it is the isolation of individual tolerances without considering the assembled system. Designers often meticulously specify tooth profile and helix modifications to achieve AGMA Q10 or even Q12, yet they completely omit radial runout control (total indicator reading of the pitch circle relative to the gear bore). A gear can have a near‑perfect involute – every flank within 2 µm of theoretical – but if the tooth center wobbles by 0.05 mm relative to its mounting axis, that runout translates into a cyclic velocity variation. At meshing frequencies, this generates side‑band noise that is unmistakable: a high‑pitched whine or siren that intensifies with speed. Beyond acoustics, runout causes alternating tooth‑to‑tooth load sharing, drastically reducing fatigue life. The fix is simple: add a runout requirement (e.g., AGMA Q10 includes runout class, but many buyers overlook it) and specify that runout be measured on the finished gear after all heat treatment and grinding.
Equally overlooked is backlash allocation. Backlash is not a single number – it is a system property that depends on center distance, tooth thickness variations, and thermal expansion. When you order a custom gear to mesh with an existing mating gear, you must provide that mating gear’s tooth thickness (or its master gear data). Otherwise, the shop will cut your gear to a standard “zero‑backlash” master, and you will discover that at room temperature the assembly binds, or worse, at operating temperature it jams because the steel expands while the housing does not. Conversely, too much backlash – often chosen “to be safe” – turns a precision index drive into a sloppy mechanism with lost motion and poor repeatability.
The comprehensive solution is a three‑part data package:
Mating gear’s existing data – tooth thickness over pins, active profile diameter, lead crown, and any modification (tip relief, root relief).
Operating temperature range – not just the oil temperature, but also the expected gradient between gear body and housing. This allows the shop to calculate thermal growth of pitch diameter and adjust tooth thinning accordingly.
Expected shaft deflection under full load – radial and angular misalignment at the mesh. Without this, the gear designer cannot specify adequate lead crowning or tip relief to avoid edge loading.
Finally, remember that many shops treat these as “optional” inputs. Explicitly state that all three must be on your purchase order. A good gear manufacturer will thank you – because they would rather machine a correct gear once than argue over warranty claims after delivery. In short: profile alone is not a gear; the gear lives in a system, and your specification must reflect that reality.