Worm Gear Standards — DIN, ISO, and AGMA: What Each Measures and How They Compare

AGMA 10 corresponds approximately to DIN 7 for typical worm gear module and diameter combinations. Korea Ever-Power manufactures AGMA 10-equivalent gears at DIN 7 precision class (ground worm thread, DIN 3976 tolerances measured and reported). The exact relationship depends on gear size — for very small modules (M1-M2), DIN 7 may be slightly tighter than AGMA 10; for large modules (M8+), the relationship may differ. When an AGMA quality class is specified on a customer drawing, Korea Ever-Power provides the DIN-measured values alongside an AGMA class compliance statement confirming the actual measured deviations are within the AGMA 10 tolerance limits for the specific gear dimensions.

They refer to the same concept with different terminology by standard. DIN 3976 uses Qualitaetsstufe (quality grade), translated as precision class in English, numbered 3-12 with lower numbers being more precise. AGMA uses Quality Number (Q) numbered 3-14 with higher numbers being more precise — the numbering is inverted relative to DIN, which causes frequent confusion. ISO 10064-6 uses accuracy grade with a similar system to DIN. When specifying internationally, always confirm which numbering system is in use: DIN 8 and AGMA 8 have the same number but represent different precision levels — DIN 8 is significantly higher precision than AGMA 8.

Standard CMM reports are in DIN 3976 format, reporting measured values against DIN tolerance limits. When a US customer requires AGMA-format compliance documentation, Korea Ever-Power prepares an AGMA class compliance statement from the DIN-measured values — confirming that the measured profile deviation, lead deviation, and pitch variation satisfy the AGMA tolerance limits for the specified quality number at the specific module and pitch diameter. This service is available at no additional charge when AGMA documentation is requested at order placement. For orders where AGMA documentation is requested after shipment, allow 3-5 working days.

Worm gear backlash is measured as the angular movement of the output shaft (worm wheel shaft) in degrees when the input shaft is held stationary and the output shaft is rotated alternately in both directions under a specified reference torque. The measurement is performed on a gear mesh test rig at theoretical centre distance. Backlash in angular units converts to linear at the wheel pitch radius. Korea Ever-Power measures backlash on the assembly test rig as part of the contact pattern test for precision class DIN 7 and better — the contact pattern photograph and backlash measurement are both included in delivery documentation. Standard supply (DIN 8-9) includes contact pattern documentation; backlash measurement available on request.

Total profile deviation Fa is the peak-to-valley amplitude of the profile error in the normal section — the algebraic difference between the highest and lowest points on the actual profile relative to the theoretical involute. Small Fa means the thread profile closely matches the theoretical and load distributes uniformly across the engaged contact arc. Large Fa means local high and low points cause load concentration at high points, increasing Hertz contact stress, raising friction, and producing transmission error pulses at the mesh frequency. Fa is the primary source of worm gear mesh noise — reducing Fa through thread grinding (DIN 7 vs DIN 9) is the most effective single action for reducing worm gear acoustic output.

Tooth thickness of a worm is measured by the measurement over balls (or pins) method as specified in DIN 3960 applied to worm gears via DIN 3975. A calibrated ball of specified diameter is placed in the thread space; the measurement over the ball with another ball diametrically opposite gives a dimension from which tooth thickness is calculated. The ball diameter is selected per the normal module and pressure angle to ensure contact at the pitch cylinder. Korea Ever-Power uses CNC gear measurement centres with touch-probe styli for thread flank profile and lead measurement, supplemented by pin measurement for tooth thickness verification at both the blank stage and after thread grinding. Both measurements appear in the CMM inspection report.

Define the specification to international standard tolerances, not proprietary callouts. Specify: module (exact value), tooth count, helix direction, pressure angle (20 deg standard), precision class (DIN class number), bore to H7 (exact diameter), keyway to DIN 6885A, and material to a composition standard with heat number traceability. Both suppliers must demonstrate compliance with the same CMM inspection report format. For initial qualification, compare actual measured values from both suppliers — not just pass/fail reports — to confirm they achieve equivalent geometry. A first-article dimensional correlation review between two suppliers CMM reports is the most reliable way to confirm interchangeability before production quantities are committed.

To issue a CMM report to DIN 3976, Korea Ever-Power needs: module m; number of starts z1; number of wheel teeth z2; normal pressure angle an; diameter quotient q (or pitch diameter d1); specified precision class (DIN grade number); and bore diameter with tolerance. With these parameters, the DIN 3976 tolerance values for each measured parameter can be calculated, and the measured deviations evaluated for compliance. If any parameter is missing from the order specification, Korea Ever-Power requests it before issuing the report — a report without confirmed specifications is not a valid quality document.