“What ratio do I need?” is the most-asked question when specifying a worm gearbox — and one of the most consequential. The ratio affects not just output speed, but efficiency, self-locking capability, thermal loading, and service life. Choosing 50:1 vs 40:1 vs 60:1 is not just about hitting the right rpm — it also determines whether the gearbox self-locks, how much energy it wastes, and whether the thermal rating becomes a constraint. This guide covers every standard ratio from 5:1 to 100:1, what each delivers technically, which applications each suits, and the decision rules for common scenarios where multiple ratios could technically work.

The Master Ratio Reference Table

The table below summarises what each standard ratio delivers across the dimensions that matter for real-world selection. Values assume 1,400 rpm motor input, PAO synthetic lubricant, run-in unit:

The Self-Locking Threshold — Why 30:1 Is the Key Ratio

The most operationally significant ratio decision is whether to specify at or above 30:1 to achieve self-locking. Self-locking allows the output shaft to hold its position when the motor de-energizes — without any additional brake mechanism. The physics: at 30:1 and above, the worm lead angle falls below the friction threshold (approximately 6° for standard bronze-on-steel pairs), preventing back-drive from the output side.

At ratios 15:1 and 20:1, the self-locking condition is borderline — it may or may not hold depending on the lubricant condition, operating temperature, and worm-wheel wear state. These ratios should never be relied upon for safety-critical load-hold without a supplementary brake. At 25:1, most units self-lock under normal conditions but the margin is thin. At 30:1 and above, self-locking is reliable and consistent over the service life. The decision rule: if self-locking is required, specify 30:1 minimum, 40:1 preferred for reliable margin.

Ratio Selection by Application Category

Below are ratio selection guidelines for the most common worm gearbox application categories, derived from standard output speed requirements and application constraints:

• Belt conveyors (head pulley, typical belt speed 0.5–1.5 m/s, drum 200–400 mm diameter): Required output 24–90 rpm → ratios 16:1 to 58:1 → standard choices: 20:1, 25:1, 30:1, 40:1, 50:1 depending on specific belt speed.
• Screw conveyors (grain, mineral powder, cement, 30–120 rpm typical): Ratios 12:1 to 47:1 → standard choices: 15:1, 20:1, 25:1, 30:1. Use 30:1+ if anti-runback on inclined sections is required.
• Agitators and mixers (50–180 rpm typical): Ratios 8:1 to 28:1 → standard choices: 10:1, 15:1, 20:1, 25:1. Self-locking not usually required for horizontal agitators.
• Gate openers and door drives (2–10 rpm at drive shaft): Ratios 140:1 to 700:1 — requires double-stage worm gearbox (single-stage max 100:1). Alternatively, 40:1–80:1 with mechanical linkage multiplication.
• Solar trackers (single-axis: 0.05–0.25 rpm at drive shaft): Ratios 5,600:1 to 28,000:1 — double-reduction plus additional drive chain, or worm gearbox + slewing ring combination. Standard worm range provides the final gear stage.
• Packaging machinery indexing (30–90 rpm): Ratios 16:1 to 47:1 → standard choices: 20:1, 25:1, 30:1.
• Food processing (10–60 rpm): Ratios 23:1 to 140:1 → standard choices: 25:1, 30:1, 40:1, 50:1, 60:1. Prefer 30:1+ for anti-runback safety in inclined or vertical applications.

When Multiple Ratios Could Work — How to Choose

Calculated ratio: 38:1 — which falls between standard 30:1 and 40:1. Both could work from a speed standpoint. Apply these tie-breaking criteria in order:

1. Does self-locking matter? If yes, both 30:1 and 40:1 self-lock → go to criterion 2. If borderline: 40:1 has more self-locking margin.
2. Which output speed is more acceptable? 30:1 gives 46.7 rpm (3.6% faster than 38:1 target); 40:1 gives 35 rpm (7.9% slower). If a ±5% speed tolerance applies, 30:1 passes and 40:1 does not — choose 30:1.
3. Thermal budget: 30:1 runs at ~76% efficiency; 40:1 at ~73% — slightly more heat from 40:1. If the unit is near its thermal limit, 30:1 has a small advantage.
4. Energy cost: For continuous-duty applications, 30:1 wastes slightly less energy than 40:1 — generally too small a difference to govern the choice unless annual hours are very high.

For very high-torque applications in metallurgy, steel mills, and foundry equipment where ratios above 50:1 are needed with large-frame units, our heavy-duty worm gearbox range covers ratios up to 100:1 in frames rated to 78,000 Nm. For the full standard NMRV range, our NMRV worm gearbox series covers 5:1 to 100:1 from NMRV030 to NMRV150. For additional application-specific ratio guidance across industrial sectors, see the industrial worm reducer application guide.

Frequently Asked Questions

How Ratio Affects Thermal Loading — The Practical Engineering Implication

One practical consequence of the efficiency-ratio relationship that surprises many engineers: for a given frame size, the thermal input-power rating (not the mechanical rating) often limits the permissible continuous input power, and this thermal limit becomes more restrictive as ratio increases.

For an NMRV090 as a concrete example:

• At 10:1 (η = 86%): heat generated = 14% of input. Thermal P₁ rating: approximately 4.5 kW continuous.
• At 50:1 (η = 72%): heat generated = 28% of input. Thermal P₁ rating: approximately 1.9 kW continuous.
• At 100:1 (η = 63%): heat generated = 37% of input. Thermal P₁ rating: approximately 1.2 kW continuous.

The same NMRV090 housing can handle 4.5 kW input at 10:1 but only 1.2 kW at 100:1 — not because the gears are weaker at 100:1, but because the heat generation rate per kW of input is 2.6× higher. This is why ratio selection and thermal budget analysis must be done together for any continuous-duty application above 4 hours per day.

Output Speed Quick-Reference for Standard Motor Speeds

For quick field or design-room reference — output speed for each standard ratio at common motor input speeds:

Note that a 50:1 worm gearbox on a 2-pole 2,800 rpm motor delivers the same output speed (56 rpm) as a 20:1 on a 4-pole 1,400 rpm motor. When the required output speed can be achieved at either a lower ratio with a higher-speed motor, or a higher ratio with a standard 4-pole motor, the lower-ratio option is almost always more efficient and thermally easier to manage.

Using a Variable-Frequency Drive to Fine-Tune Ratio — When It Makes Sense

When the application requires an output speed that falls exactly between two standard ratio steps — and output speed tolerance is tighter than ±5% — specifying a variable-frequency drive (VFD) on the motor is almost always more economical than ordering a custom non-standard gearbox ratio. The VFD adjusts the motor speed to dial in the exact output speed using the nearest standard catalog ratio.

Example: required output speed 33 rpm from a 1,400 rpm motor. Calculated ratio: 1,400 / 33 = 42.4:1. Nearest standards: 40:1 (35 rpm, 5.7% high) or 50:1 (28 rpm, 15.2% low). Output speed tolerance of ±3% is too tight for either standard ratio. Options:

• Custom 42:1 ratio: Lead time 8–12 weeks, setup cost $200–$400, per-unit cost premium 15–25% vs standard.
• Standard 40:1 + VFD set to 1,320 rpm input: Standard gearbox at catalog price, VFD cost $80–$200 for this power range, 1-week lead time, exact 33 rpm output at 1,320 rpm × 1/40 = 33 rpm. Total system cost almost always lower than custom ratio option. Additionally, VFD enables soft-start (reducing motor current at start-up) and speed adjustment for future process changes.

The VFD+standard-ratio approach is not suitable for all applications: VFDs should not be used with motors that are not VFD-rated (check motor insulation class), and very low frequency operation (<20 Hz) at high torque can cause motor thermal problems without additional cooling. For most industrial OEM applications at 0.37–22 kW, VFD + standard-catalog worm gearbox is the preferred solution for non-standard speed requirements.