I. The Three Core Elements of Gear Precision

(I) Tooth Profile Accuracy: The "Lifeline" of Meshing Smoothness

1. Definition: The deviation of the actual tooth profile shape from the ideal involute on the transverse section, describing the accuracy of the "longitudinal curve" of a single gear tooth.
2. Key Parameters:
   • Total tooth profile deviation (Fα): The distance between two design tooth profile traces that enclose the actual tooth profile trace within the evaluation range;
   • Tooth profile form deviation (ffα): The total tooth profile deviation after excluding the tilt component (e.g., convexity, concavity);
   • Tooth profile slope deviation (fHα): The deviation of the line connecting the two endpoints of the actual tooth profile trace from the line connecting the two endpoints of the design tooth profile trace within the evaluation range.
    
3. Core Function: Determines the smoothness and continuity of transmission speed; an ideal involute ensures a constant transmission ratio.
4. Accuracy Grades and Applications (Refer to ISO/GB Standards):
   • Grades 4-5: Ultra-precision scenarios (high-speed turbines, precision indexing mechanisms, gear gauges);
   • Grades 6-7: Mid-to-high-end core scenarios (aero gears, machine tool spindle gears, key automotive gears);
   • Grades 8-9: General industrial scenarios (tractors, reducers, agricultural machinery gears).
    
5. Adaptable Circumferential Speed: Grade 4 (spur gears < 35 m/s, helical gears < 70 m/s); Grade 9 (spur gears < 6 m/s, helical gears < 10 m/s).

(II) Tooth Alignment Accuracy: The "Balancer" of Load Distribution

1. Definition: The deviation of the actual contact trace in the tooth width direction (tooth line direction) from the ideal contact line on the reference cylinder, describing the accuracy of the "transverse contour" of the gear tooth.
2. Key Parameters:
   • Total tooth alignment deviation (Fβ): The maximum enclosing distance between the actual tooth line and the design tooth line;
   • Helix form deviation (ffβ): The total tooth alignment deviation after excluding the tilt component;
   • Helix slope deviation (fHβ): The deviation of the line connecting the two endpoints of the actual tooth line from the line connecting the two endpoints of the design tooth line within the evaluation range.
    
3. Core Function: Ensures uniform load distribution along the tooth width, with full tooth width contact synchronously under ideal conditions.
4. Accuracy Corresponding Relations Across Multiple Standards:

   Standard	High Precision Grade	Medium Precision Grade	General Precision Grade
   ISO/GB	Grades 3-5	Grades 6-7	Grades 8-10
   DIN (Germany)	Grades 4-5	Grades 6-8	Grades 9-11
   JIS (Japan)	Grades 0-1	Grades 2-3	Grades 4-6
   AGMA (USA)	Grades 14-15	Grades 11-13	Grades 7-10

    

(III) Cumulative Accuracy: The "Metronome" of Rotational Uniformity

1. Definition: The maximum difference between the actual cumulative angle and the theoretical cumulative angle of each gear tooth's reference circle position within one revolution of the gear, reflecting the equal division of gear teeth distribution along the circumference.
2. Key Parameters:
   • Total pitch cumulative deviation (Fp): The maximum pitch cumulative deviation within any arc segment;
   • Single pitch deviation (fpt): The difference between the actual pitch of a single gear tooth and the theoretical pitch.
    
3. Core Function: Ensures the "rhythm" of gear rotation, similar to how the equal division of clock gears determines the uniformity of pointer movement.

II. Key Means of Precision Optimization: Micro-Modification Design