A worm gearbox cannot be selected reliably from motor power alone. A sound selection begins with the required machine speed and load torque, then checks operating hours, starts per hour, shock loading, mounting position, ambient conditions and external shaft loads. The formulas below are useful for preliminary sizing; the final model must be verified against the manufacturer's selection tables.
1. How to calculate worm gearbox ratio and output speed
The nominal reduction ratio is the input speed divided by the output speed:
i = gear ratio, n1 = input speed in rpm, n2 = output speed in rpm
If a motor runs at 1400 rpm and the machine needs approximately 47 rpm, the required ratio is 1400 ÷ 47 ≈ 29.8. A nominal 30:1 gearbox is the logical starting point. Conversely, output speed can be estimated as:
Always check the exact catalog ratio. The actual output speed can differ slightly from a rounded nominal ratio, and motor speed varies with load and motor design.
2. How to estimate output torque
For metric units, preliminary output torque can be estimated from input power, output speed and gearbox efficiency:
M2 = output torque in Nm, P1 = input power in kW, η = gearbox efficiency as a decimal
Efficiency must not be guessed for final sizing. Worm gearbox efficiency changes with ratio, geometry, lubrication, speed, temperature and running condition. Use the efficiency or output-torque data supplied for the specific gearbox. The gearbox's permitted output torque—not the calculated ideal torque—is the controlling value.
3. Worked example
Assume the preliminary application data are:
| Input | Example value |
|---|---|
| Motor power | 0.75 kW |
| Motor speed | 1400 rpm |
| Nominal ratio | 30:1 |
| Example efficiency for calculation | 0.75 (verify with selected model) |
| Required service factor | 1.5 (example only) |
Step 1 — Output speed: n2 = 1400 ÷ 30 = 46.7 rpm.
Step 2 — Estimated output torque: M2 = 9550 × 0.75 × 0.75 ÷ 46.7 ≈ 115 Nm.
Step 3 — Preliminary rated-torque target: 115 × 1.5 ≈ 173 Nm.
This result does not automatically identify a gearbox size. The selected model must provide adequate permitted continuous and peak torque at the relevant input speed and ratio. It must also pass thermal and shaft-load checks. If the actual machine requires 115 Nm at the load—not merely the estimated motor-derived torque—apply the appropriate service factor to that measured or calculated load requirement.
4. Why service factor matters
Service factor represents additional gearbox capacity relative to the application demand. A conveyor running eight hours with uniform loading does not impose the same duty as a mixer with repeated starts and shock loads, even if their average power is similar.
Consider at least:
- operating hours per day and duty cycle;
- starts, stops, reversals and braking frequency;
- uniform, moderate-shock or heavy-shock loading;
- load inertia and acceleration torque;
- ambient temperature and ventilation;
- personnel safety and whether a separate brake is required.
Service-factor definitions are not fully interchangeable between manufacturers. Use the tables and calculation method belonging to the gearbox being selected rather than comparing a single service-factor number across unrelated catalogs.
5. Final worm gearbox selection checklist
| Check | Information required | Why it matters |
|---|---|---|
| Speed | Input rpm, required output rpm and ratio | Defines the operating point |
| Torque | Continuous, starting and peak torque | Prevents mechanical overload |
| Duty | Hours/day, starts/hour and load pattern | Determines service requirement |
| Thermal capacity | Ambient temperature, ventilation and duty cycle | Limits heat-related performance |
| Shaft loading | Sprocket, pulley or coupling forces and position | Protects bearings and output shaft |
| Installation | Mounting position, flange, hollow/solid shaft | Affects fit and lubrication |
| Environment | Dust, washdown, corrosion, altitude and temperature | Determines seals, coating and derating |
What information should you send a gearbox manufacturer?
For a faster and more accurate recommendation, provide motor power and speed, target output speed or ratio, continuous and peak torque, operating hours, starts per hour, mounting position, shaft dimensions, quantity and application conditions. Photographs or a dimensional drawing are useful when replacing an existing unit.
View SMK NMRV worm gear reducers or send your application data to SMK Transmission for model-selection support.
Frequently asked questions
How do you calculate worm gearbox ratio?
Divide input speed by required output speed: i = n1 ÷ n2. A 1400 rpm motor and an output speed near 47 rpm indicate a ratio of about 30:1.
How do you estimate worm gearbox output torque?
Use M2 = 9550 × P1 × efficiency ÷ n2 for metric units. Final selection must use the chosen manufacturer's efficiency and permitted output-torque data.
Should torque be multiplied by a service factor?
Yes. Account for duty, starts, shock, inertia and operating conditions, but use the service-factor method published for the selected gearbox.
Is motor power enough to select a worm gearbox?
No. Speed, continuous and peak torque, thermal capacity, mounting, shaft loads and environmental conditions must also be checked.