An accuracy class that sets tolerances on pitch, profile, helix, runout and composite errors. On the Q-number scale, common on US drawings, numbers run roughly from Q3 to Q15 and higher means more accurate. On the A-number scale, aligned with ISO 1328, numbers run from A2 to A11 and lower means more accurate. Typical industrial gears sit around Q8 to Q10, with ground aerospace and precision gears at Q12 and above.
Print says AGMA quality number Q11, the vendor quoted A6, check which scale they're on before anybody argues about who's tighter.
The two scales run in opposite directions, so a bigger Q number and a smaller A number both mean a better gear.
Also heard as
- Q number
- A number
- gear quality class
Related terms
Profile deviation
How far a tooth's actual shape from root to tip departs from the intended involute and modifications.
Helix deviation
How far a tooth's actual path across the face width departs from the intended helix, including crowning.
Pitch deviation
Errors in tooth spacing, either from one tooth to the next or accumulated around the gear.
Runout
How much the teeth wobble radially relative to the gear's axis, measured with a probe in each tooth space.