How to Specify a Worm Gear — The Complete Engineer’s Checklist

Start with your motor speed (n₁) and the required output shaft speed (n₂). The ratio i = n₁ ÷ n₂ is the primary design input — everything else follows from it. A 4-pole motor at 1450 RPM driving a shaft that must turn at 29 RPM requires i = 50:1. Always calculate the exact required ratio first, then select the nearest standard catalog ratio or specify a custom ratio. Standard ratios (10, 15, 20, 25, 30, 40, 50, 60, 80, 100:1) may not match your requirement exactly. Non-standard ratios are available at Level 3 semi-custom specification without new tooling. The gear ratio also determines whether self-locking is achievable: at high ratios (≥ 30:1 with single-start worm), self-locking is typically achievable; at low ratios, it requires verification.

The start count determines two properties simultaneously: self-locking capability and efficiency. Single-start (z1=1): lead angle shallow → self-locking at most ratios → efficiency 50–75%. Double-start (z1=2): efficiency improves to 72–82% → self-locking marginal. Four-start (z1=4): efficiency 83–90% → self-locking not achievable. Specify z1=1 whenever load-holding (safety self-locking) is required — for inclined conveyors, hoists, and cobot joints. Verify self-locking at maximum operating temperature, not ambient: friction coefficient drops with temperature, potentially eliminating self-locking behaviour in a drive that self-locks at 20°C but not at 70°C housing temperature.

Module is selected from the required output torque, not from the ratio. The torque-module relationship for tin bronze wheel: T₂_rated ≈ 0.9 × m³ × z₂ × 120 MPa (approximate for ZCuSn10Pb1 at moderate speed). For a required T₂ of 300 Nm at 50:1 (z₂=50): m³ ≥ 300 / (0.9 × 50 × 0.12) → m³ ≥ 55.6 → m ≥ 3.82 → select M4. Standard modules: M1, 1.25, 1.5, 2, 2.5, 3, 4, 5, 6, 8, 10. Non-standard modules (M3.5, M4.5, M7) require Level 4 custom tooling. Always select one standard module step above the minimum calculated value to provide service factor margin.

Calculated torque from the application: T₂ = F × r for linear mechanisms (F = load force, r = moment arm), or T₂ = P/ω for rotary mechanisms. Apply a service factor: 1.0–1.25 for smooth constant load (fans, pumps); 1.5 for moderate shock (conveyors starting under load); 2.0–2.5 for heavy shock (material handling with potential jams, start-stop high-cycle). The design torque T₂_design = T₂_load × SF. Motor torque at output shaft ≠ design torque: T₂_motor = T_motor × i × η — the efficiency reduction means the motor must supply more input torque than the load torque divided by ratio.

Once module, start count, and tooth count are fixed, centre distance is determined: a = m(q + z₂)/2 where q is the diameter quotient (typically 8–16, often chosen as q=12 or q=10). For M4, q=12, z₂=50: a = 4(12+50)/2 = 124 mm. Centre distance is not a free variable. The machine housing must accommodate the calculated centre distance within the tolerance required for the precision class (typically ±0.10 mm for standard, ±0.05 mm for precision drives). Housing design or selection follows from centre distance — do not design the housing first and fit the gear set to it.

The bore is manufactured to H7 tolerance (standard hole basis). Shaft fit type: H7/k6 — transition fit, removable for maintenance; H7/n6 — light interference, standard medium-duty permanent assembly; H7/p6 — medium interference, heavy-duty shock applications (requires hydraulic press or heating to assemble). Non-standard bore diameters (any value, not just catalog steps) are available as Level 2 custom with 2–4 week lead time and no tooling cost. Specify bore diameter to 0.1 mm and fit type explicitly. Duplex worm shafts (adjustable backlash) require a different shaft fit — H7/g6 clearance fit to allow axial adjustment.

Keyway dimensions follow DIN 6885A as a function of bore diameter. A 30 mm bore: 8×7 mm key (8 wide × 7 high). A 50 mm bore: 14×9 mm key. Specify: (1) keyway standard (DIN 6885A metric default), (2) keyway width tolerance (JS9 for normal clearance; P9 for interference key fit), (3) whether a set screw hole is required. If no keyway is required, state this explicitly — without instruction, a keyway will be machined on all bores above 10 mm as standard. If two keyways are needed (90° apart for balancing or redundancy), this must be specified at order placement.

Shaft material governs hardness and hardenability; wheel material governs anti-scuffing and strength. These are a pairing — the correct combination depends on duty class and environment. D1 light: C45 induction-hardened + ZCuSn10Pb1. D2 medium: 40Cr through-hardened + ZCuSn10Pb1. D3 heavy: SCM415 carburized + ZCuAl10Fe3. Food/marine: SS316 + SS316 or SS316 + ZCuSn10Pb1. Stating only the shaft grade (‘I need a 40Cr shaft’) is insufficient — the wheel alloy must be specified too. A 40Cr shaft against ZCuAl10Fe3 wheel has inadequate hardness differential in some conditions; see the material selection guide for pairing rules.

DIN precision class specifies the allowable tolerance on lead deviation, profile deviation, pitch error, and tooth thickness. DIN 12: commercial (hobbed only, general industrial); DIN 9–10: standard industrial (hobbed + possible touch-grind); DIN 7–8: precision (thread-ground); DIN 5–6: high precision (ground and lapped, for robotic and positioning drives). Each step tighter roughly doubles the manufacturing cost. Specify the minimum class your application requires. Over-specifying DIN 6 for a warehouse conveyor drive adds cost with no operational benefit; under-specifying DIN 9 for an indexing robot produces position errors. State the required precision class alongside the application type so Korea Ever-Power can confirm the specification is appropriate.

Documentation level must match your quality system requirement. Standard supply: material certificate (heat number traceable) + CMM dimensional inspection report. Food / HACCP: add surface roughness report (Ra measurement) + NSF H1 lubricant compatibility confirmation + HACCP zone statement. Marine / offshore: add 500h ASTM B117 salt spray test certificate. Medical device (ISO 13485): add ISO 10993-1 biocompatibility reference + heat treatment record + mill test certificate. Automotive OEM (PPAP): specify PPAP Level 1, 2, or 3. Documentation requirements cannot always be fulfilled retrospectively from a shipped order — state them at order placement, and Korea Ever-Power will confirm availability before accepting the order.