A motor and gearbox should be selected as one drive system. A motor can have the correct kW rating but still produce the wrong output speed, insufficient starting torque, excessive gearbox heat or an incompatible mechanical interface.
1. Define the machine requirements
| Input | Data required | Selection purpose |
|---|---|---|
| Application | Conveyor, mixer, packaging machine, pump or other equipment | Identifies load behavior |
| Output speed | Required rpm at the machine shaft | Determines gearbox ratio |
| Output torque | Running, starting and peak Nm | Determines gearbox and motor capacity |
| Electrical supply | Voltage, frequency and phase | Defines motor winding |
| Duty | Hours/day, starts/hour and reversing | Affects service factor and heat |
| Installation | Mounting, shaft, flange and available space | Ensures mechanical fit |
2. Calculate the required gearbox ratio
A four-pole 50 Hz motor may have a rated speed near 1450 rpm. If the machine requires 48 rpm, the calculated ratio is approximately 30.2:1. Select the nearest available ratio and verify the actual output speed using the motor's rated full-load speed and the gearbox's exact ratio.
If precise adjustable speed is required, a variable-frequency drive may be suitable. Check low-speed motor cooling, permitted frequency range and gearbox lubrication before approving continuous low-speed operation.
3. Verify output torque and motor power
This formula is useful for preliminary selection. Actual gearbox efficiency varies by gear type, ratio, speed, load, lubricant and temperature. The gearbox's catalog torque and thermal rating—not the theoretical result—must be used for final approval.
Apply a service factor for shock, frequent starts, reversing, high inertia, long operating hours or elevated ambient temperature. High starting torque may require a larger motor, a larger gearbox or controlled acceleration.
4. Choose a suitable gearbox type
- Worm gearbox: compact right-angle arrangement, broad ratios and economical construction.
- Hypoid gearbox: compact right-angle transmission with rolling-sliding tooth contact and generally higher efficiency than many worm designs.
- Helical gearbox: suitable for efficient parallel-shaft or inline transmission.
- Double-stage reducer: used when very low output speed or a high total ratio is required.
The best choice depends on ratio, efficiency, space, noise, duty, maintenance and total cost—not only purchase price.
5. Match the IEC motor and gearbox interface
Confirm the IEC frame size and complete flange geometry. Typical motor mounting forms include B5, B14, B35 and B34. Check flange outside diameter, pilot diameter, bolt-circle diameter, motor shaft diameter, shaft length and keyway.
Two motors with the same power may use different frame sizes. Never approve compatibility from kW alone. For a replacement, send the existing motor nameplate and dimensional drawing.
6. Check duty, mounting and operating environment
Confirm continuous or intermittent duty, starts per hour, reversing frequency, ambient temperature, altitude, dust, moisture and washdown conditions. Gearbox mounting position affects lubrication, oil quantity and breather placement. Motor enclosure and IP rating must match the installation environment.
For vertical lifting or safety-critical equipment, use an engineered brake or holding device. Do not rely on assumed worm-gear self-locking.
7. Worked selection example
Suppose a conveyor requires about 60 rpm and 160 Nm at the drive shaft. The available motor speed is 1450 rpm.
- Required ratio: 1450 ÷ 60 ≈ 24.2, so a standard ratio near 25:1 may be considered.
- Preliminary motor power at 75% estimated transmission efficiency: 160 × 60 ÷ (9550 × 0.75) ≈ 1.34 kW.
- A nearby standard motor rating may be evaluated, but the supplier must verify starting torque, service factor, gearbox rating and thermal capacity.
This example demonstrates the method only. Final selection depends on verified application and manufacturer data.
8. Gear motor RFQ checklist
Send: application; required output rpm and torque; motor voltage, frequency and power; starts/hour; daily operating time; load type; gearbox type or preferred ratio; mounting position; output shaft or bore; IEC frame and flange; ambient conditions; brake or VFD requirements; sample quantity and annual quantity.
Related resources: three-phase motor selection, worm gear reducer selection, SMK gear motors and selection support.
Frequently asked questions
How do I calculate the gearbox ratio for a motor?
Divide motor rated speed by required output speed, choose the nearest standard ratio and verify the actual loaded speed.
How do I estimate gearbox output torque?
Use 9550 × motor kW × estimated efficiency ÷ output rpm, then check the result against the manufacturer's permitted torque and thermal ratings.
Does a motor with the correct power always fit?
No. Frame, flange, pilot, bolt pattern, shaft diameter and shaft length must also match.
What should I send for a gear motor quotation?
Provide application, power, speed, torque, voltage, frequency, duty, service factor, mounting, shaft or flange requirements and quantity.
Conclusion
A reliable gear motor is selected by treating the motor, gearbox and machine as one system. SMK Transmission supports motor and reducer matching for conveyors, packaging machinery and other industrial equipment.