DIN 3974 Precision
Worm Gears & Worm Wheels

A self-locking worm gear cannot be driven in reverse by torque applied to the worm wheel. The worm can rotate the wheel, but the wheel cannot rotate the worm — the gear set acts as a brake when unpowered. Self-locking requires a low lead angle (typically single-start, ratio ≥ ~20:1) and is used in lifting equipment, door actuators, and stage machinery. Note: self-locking is not guaranteed under shock loading — a mechanical brake should still be added to safety-critical lifts.

A multi-start worm has 2, 3, or 4 separate helical threads. Each additional start multiplies linear advance per revolution, increasing the lead angle. Gear ratio = Wheel teeth ÷ Starts. So a 40-tooth wheel + 2-start worm → 20:1; + 4-start → 10:1. Multi-start worms are back-drivable, more efficient (70–85%), and used in robot joints, servo gearheads, and valve actuators.

Efficiency depends on lead angle: single-start self-locking (γ ~3–5°): 40–55%; double-start (γ ~6–10°): 55–70%; four-start (γ ~12–18°): 70–85%; high-lead (γ > 20°): 85–92%. Spur/helical gears achieve 97–99% per mesh. Efficiency is improved by ground worm flanks (Ra ≤ 0.4 µm), softer bronze wheels, correct EP oil viscosity.

Phosphor bronze (PB2 / CuSn10P) is the industry standard — excellent anti-friction and anti-seizure against hardened steel. Aluminium bronze (AB2) is stronger, used for heavy-load or shock applications. Cast iron suits large slow drives. POM / Nylon for light-duty quiet drives. PEEK for medical and high-temperature environments. The worm is always harder than the wheel.

DIN 3975 defines four worm thread forms: ZA Archimedes — straight axial flanks; simple to turn. ZN convolute — straight normal flanks; good for multi-start. ZI involute — involute transverse profile; most accurate and grinds well — HXC's preferred form for precision worms. ZK Klingelnberg — conical wheel ground; precision instruments.

Worm gears require an extreme-pressure (EP) gear oil, typically ISO VG 220 or VG 320. Sliding contact generates high interface temperatures and pressure; standard oils without EP additives cause bronze wheel pitting. For food-grade: NSF H1 food-safe synthetic oils. For PEEK or POM wheels — consult HXC: some EP additives attack certain plastics.

Provide: module · starts · wheel tooth count · centre distance · worm pitch circle diameter · pressure angle · thread form · materials · DIN quality class · finish · quantity. A STEP file or detailed engineering drawing is ideal — but a ratio and input torque is enough to start a conversation. Our engineers can compute the full geometry from a duty-cycle specification.

Prototype (any module, any material): 5–10 days. Small batch (10–50 sets): 10–16 days. Ground precision (DIN Q6–7, post-hardened): add 5–8 days. Multi-start grinding: add 2–3 days vs single-start.