Selecting the correct industrial gearbox requires determining five critical parameters: (1) required output torque and speed, (2) service factor based on application and duty cycle, (3) gearbox ratio (input speed divided by output speed), (4) mounting configuration and shaft orientation, and (5) thermal capacity to ensure the gearbox can dissipate generated heat. An undersized gearbox is the leading cause of premature failure in industrial applications, accounting for approximately 30% of all gearbox warranty claims.
The fundamental gearbox selection equation: T_output = (9,550 x P) / n_output, where P is motor power in kW and n_output is required output speed in RPM.
| Motor Power | Output 30 RPM | Output 50 RPM | Output 75 RPM | Output 100 RPM |
|---|---|---|---|---|
| 15 kW | 4,775 Nm | 2,865 Nm | 1,910 Nm | 1,433 Nm |
| 30 kW | 9,550 Nm | 5,730 Nm | 3,820 Nm | 2,865 Nm |
| 55 kW | 17,508 Nm | 10,505 Nm | 7,003 Nm | 5,253 Nm |
| 90 kW | 28,650 Nm | 17,190 Nm | 11,460 Nm | 8,595 Nm |
| 160 kW | 50,933 Nm | 30,560 Nm | 20,373 Nm | 15,280 Nm |
The calculated torque must be multiplied by the application service factor (SF): T_required = T_output x SF
| Application | Duty Cycle | Service Factor |
|---|---|---|
| Belt conveyor (uniform load) | 8-16 h/day | 1.25 |
| Belt conveyor (mining, variable load) | 16-24 h/day | 1.5-1.75 |
| Bucket elevator | 8-24 h/day | 1.5-1.75 |
| Crane hoist (FEM M5) | Intermittent | 1.6-2.0 |
| Crane trolley/travel | Intermittent | 1.25-1.5 |
| Ball mill / Crusher | 16-24 h/day | 2.0-2.5 |
| Cement kiln drive | 24 h/day continuous | 2.0 |
| Steel rolling mill | Heavy shock | 2.0-2.5 |
| Agitator / Mixer | 8-24 h/day | 1.25-1.5 |
The required gearbox ratio = n_input / n_output. For a 4-pole motor (1,500 RPM at 50 Hz) driving a conveyor head pulley at 50 RPM, the ratio is 1,500 / 50 = 30:1. Standard gearbox ratios follow preferred number series: 5, 7.1, 10, 14, 20, 28, 40, 56, 80, 100, 140, 200. Select the nearest standard ratio. A slightly higher ratio (lower output speed) is generally preferable to a lower ratio that would require the motor to run above base speed.
Gearbox mounting must match the mechanical arrangement. Foot-mounted (horizontal shafts) is the most common. Flange-mounted suits direct motor attachment. Shaft-mounted (hollow bore with shrink disc) is used for conveyor head pulleys. The oil fill quantity and lubrication method depend on mounting orientation. Vertical shaft-down applications may require an oil pump for upper bearing lubrication.
Thermal capacity P_thermal must exceed the heat generated: P_heat = P_input x (1 - efficiency). For a 100 kW helical gearbox at 96% efficiency, P_heat = 4 kW. If the gearbox thermal rating at the installation ambient temperature is only 3.5 kW, the gearbox will overheat. Solutions: specify a larger gearbox with greater surface area, add a cooling fan (increases thermal capacity by 40-60%), or install an oil-to-water heat exchanger (increases capacity by 200-400%).
| Parameter | Helical | Planetary | Worm |
|---|---|---|---|
| Efficiency per stage | 94-98% | 97-98% | 50-85% |
| Torque density | Medium | High | Low |
| Back-driving | Yes | Yes | Self-locking possible |
| Noise level | Low | Low-Medium | Low |
| Cost (relative) | 1.0 | 1.5-2.5x | 0.5-0.8x |
| Best for | General industrial | Compact high-torque | Low power, intermittent |
A 500 t/h coal conveyor requires: belt speed 2.5 m/s, head pulley diameter 800mm, motor 90 kW at 1,480 RPM. Pulley RPM = (2.5 x 60) / (pi x 0.8) = 59.7 RPM. Ratio required = 1,480 / 59.7 = 24.8:1. Select 25:1. T_output = (9,550 x 90) / 59.7 = 14,397 Nm. With SF = 1.5 for mining: T_required = 21,596 Nm. Recommend: helical-bevel gearbox rated 25,000 Nm, with cooling fan for 35°C ambient.
A 20-ton overhead crane hoist requires: lifting speed 4 m/min, drum diameter 500mm, motor 37 kW at 980 RPM. Drum RPM = 4 / (pi x 0.5) = 2.55 RPM. Ratio = 980 / 2.55 = 384:1. Select 400:1 (two-stage helical + planetary stage). With SF = 1.8 for FEM M5: gearbox must handle shock torques to 2x continuous rating. Recommend: helical-planetary combination with integral brake and torque limiter.
BOYU BO provides free gearbox selection engineering for your specific application. Send us your motor power, required output speed, application type, and operating hours per day. Our engineering team returns a complete selection report including: recommended gearbox model, calculated service factor, thermal verification, mounting drawing, and delivery lead time. This service is provided at no cost and typically delivered within 24-48 hours.
Contact our engineering team for a free technical assessment and gearbox recommendation.
Request Engineering Support →Output torque (Nm) = (9,550 x Power in kW) / Output speed in RPM. For example, a 30 kW motor at 50 RPM output: (9,550 x 30) / 50 = 5,730 Nm. Multiply by service factor (1.25-2.0) for the minimum gearbox rating. Always use motor nameplate power, not operating power.
Uniform load (pump, fan): 1.0-1.25. Moderate shock (conveyor, mixer): 1.25-1.5. Heavy shock (crane hoist, crusher): 1.5-2.0. Extreme shock (reciprocating compressor): 2.0-2.5. For 24/7 operation, add 0.25 to the base factor.
Helical: 94-98% efficiency, best for medium-high power. Planetary: highest torque density, 97-98% efficient, compact. Worm: most economical under 15 kW, but 50-85% efficiency with significant heat generation.
Mounting position affects oil level and lubrication. Common types: M1 (foot-mounted, horizontal), M2 (vertical output down), M4 (flange-mounted, horizontal), M5 (vertical output up). Mounting must match the gearbox oil fill specifications exactly. Wrong position can starve bearings of lubrication.
Thermal capacity is the maximum power the gearbox can dissipate without exceeding 90°C oil temperature. A 100 kW gearbox at 96% efficiency generates 4 kW of heat. If the housing can not dissipate 4 kW, auxiliary cooling is required. Undersized thermal capacity causes oil degradation and premature bearing failure.