When choosing a gearbox for a conveyor, screw, feeder or agitator, the questions to answer are: Which ratio is needed? How much torque is produced at the output shaft? Is the gearbox capacity sufficient for the application? Which oil should be used? In this guide we explain the basic formulas of gearbox calculation, the service factor, a worked example for a conveyor pulley and gear oil selection.
For gearbox types, maintenance and fault diagnosis see our what is a gearbox guide.
1. Motor Speed
The speed of induction motors depends on the number of poles and the supply frequency. Approximate values on a 50 Hz supply:
| Poles | Synchronous Speed | Typical Full-Load Speed |
|---|---|---|
| 2-pole | 3000 rpm | ~2900–2970 rpm |
| 4-pole | 1500 rpm | ~1450–1485 rpm |
| 6-pole | 1000 rpm | ~960–990 rpm |
| 8-pole | 750 rpm | ~720–740 rpm |
The 4-pole motor is the most common choice for geared drives. For motor selection and maintenance see our electric motors guide.
2. Ratio Calculation
i = n₁ / n₂
- n₁: Gearbox input speed (motor speed, or speed after a belt drive),
- n₂: Required output speed.
The standard gearbox ratio closest to the calculated ratio is chosen and the actual output speed is recalculated. If a belt drive is used as an intermediate stage, total ratio = pulley ratio × gearbox ratio (belt and pulley calculation).
3. Output Torque Calculation
T₂ (Nm) = 9550 × P (kW) × η / n₂ (rpm)
Here η is gearbox efficiency. Helical and bevel-helical gearboxes are highly efficient (roughly 97–98% per stage); worm gearboxes are markedly less efficient depending on ratio. For rough calculations, a total efficiency of about 0.94–0.96 can be assumed for helical gearboxes.
4. Service Factor
A gearbox’s catalogue capacity is given for uniform load and a specified operating time. In real applications, shock, frequent starts and long operating hours load the gearbox more. Therefore:
Required gearbox capacity ≥ Application torque (or power) × Service factor (fs)
| Load Character | Example | Service Factor (approx., 8–24 h/day) |
|---|---|---|
| Uniform | Evenly fed conveyors, agitators | ~1.0–1.5 |
| Moderate shock | Heavy conveyors, screw washers, feeders | ~1.25–1.75 |
| Heavy shock | Crusher drives, heavy feeders, irregular loads | ~1.75–2.0 and above |
The exact service factor should be taken from the gearbox manufacturer’s application tables according to daily operating time and starts per hour.
Worked Example: Conveyor Pulley Gearbox
Let belt speed be 2 m/s, lagged pulley diameter 500 mm, and the motor 30 kW at 1480 rpm (for conveyor power calculation see belt conveyor calculation):
| Step | Calculation | Result |
|---|---|---|
| Pulley speed | n₂ = 60 × v / (π × D) = 60 × 2 / (π × 0.5) | ≈ 76.4 rpm |
| Required ratio | i = 1480 / 76.4 | ≈ 19.4 |
| Standard ratio | Nearest standard ratio | i = 20 |
| Actual pulley speed | 1480 / 20 | 74 rpm |
| Actual belt speed | π × 0.5 × 74 / 60 | ≈ 1.94 m/s |
| Output torque | 9550 × 30 × 0.95 / 74 | ≈ 3,680 Nm |
| Required capacity with service factor | 3,680 × 1.5 | ≈ 5,520 Nm |
The gearbox is chosen with a nominal output torque of at least this value. The output shaft’s radial load capacity, mounting arrangement and thermal capacity are also checked; a backstop is added on inclined conveyors.
5. Thermal Capacity
In continuously running gearboxes, the limiting factor is often heat rather than mechanical strength. The thermal power given in the catalogue is corrected for ambient temperature, ventilation and operating time. If thermal capacity is insufficient, a fan, oil cooler or larger gearbox is needed. Dust building up on the housing reduces heat dissipation; keep the housing clean.
Which Gear Oil Grade Should Be Used?
The “grade” is the oil’s ISO VG viscosity class (kinematic viscosity at 40 °C, mm²/s). Industrial enclosed gearboxes generally use gear oils with EP (extreme pressure) additives (e.g. CLP class to DIN 51517-3).
| ISO VG | Typical Use |
|---|---|
| VG 150 | Cold environments, high-speed and small gearboxes |
| VG 220 | The most common choice for general industrial gearboxes |
| VG 320 | Heavy load, low speed, hot environments |
| VG 460 and above | Very heavy load, very low speed, some worm gearboxes |
The exact viscosity should be set according to the gearbox manufacturer’s recommendation, ambient temperature, output speed and load. Mineral and synthetic oils must not be mixed, and oil level should be checked according to mounting position. The first oil change is after running-in, later changes at the manufacturer’s interval or according to oil analysis (predictive maintenance).
Gearbox Selection Checklist
- Motor power and speed,
- Required output speed and ratio,
- Output torque and service factor,
- Gearbox type (helical, bevel-helical, planetary, shaft-mounted),
- Mounting arrangement and position,
- Shaft dimensions and coupling / torque arm connection,
- Thermal capacity and ambient conditions,
- Need for a backstop,
- Oil type and quantity.
Common Mistakes
- Choosing a gearbox with capacity equal to motor power without a service factor.
- Not recalculating the actual output speed with the standard ratio.
- Not checking thermal capacity.
- Using oil of the wrong viscosity or mixed oils.
- Connecting the torque arm incorrectly on a shaft-mounted gearbox.
Frequently Asked Questions
How is gearbox ratio calculated?
Input speed is divided by output speed: i = n₁ / n₂.
How is gearbox output torque found?
With T₂ = 9550 × P × η / n₂ (P in kW, n₂ in rpm).
Which gear oil grade should be used?
General industrial gearboxes mostly use ISO VG 220, and VG 320 for heavy load and hot environments; the final choice follows the manufacturer’s recommendation.
Why is the service factor important?
Shock, frequent starts and long operating hours load the gearbox more than catalogue conditions; the service factor covers this difference.
Example: Screw Washer Drive
Suppose a screw washer shaft should turn at about 20 rpm, the motor is 22 kW at 1470 rpm, and a belt drive ratio of 1.5 is used in between:
| Step | Calculation | Result |
|---|---|---|
| Gearbox input speed | 1470 / 1.5 | 980 rpm |
| Required gearbox ratio | 980 / 20 | 49 |
| Standard ratio (example) | Nearest standard ratio | 50 |
| Actual screw speed | 980 / 50 | ≈ 19.6 rpm |
| Output torque | 9550 × 22 × 0.95 / 19.6 | ≈ 10,180 Nm |
| With service factor 1.5 | 10,180 × 1.5 | ≈ 15,270 Nm |
At this torque level, shaft-mounted or foot-mounted heavy-duty gearboxes are considered; the screw shaft’s lower bearing and the torque arm connection are also checked (sand screw washer).
Field Example
At one plant, a conveyor gearbox overheats and leaks oil in the summer months. Calculations show the gearbox’s mechanical capacity is sufficient but its thermal capacity is borderline for a hot environment and continuous operation. When the housing is cleaned, a fan is added, the breather is replaced with a filtered type and the oil viscosity is updated to the manufacturer’s recommendation, the problem is resolved.
Glossary
- Ratio (i): The ratio of input speed to output speed.
- Nominal torque: The continuous output torque capacity given in the gearbox catalogue.
- Service factor: A safety factor added to capacity for application conditions.
- Thermal power: The power a gearbox can transmit continuously without overheating.
- ISO VG: The viscosity class of an oil at 40 °C.
More Questions
How is a gearbox power calculation done?
Power is found from the torque and speed the machine requires (P = T × n / 9550); the motor and gearbox are selected after adding efficiency and the service factor.
Where can I find a gearbox ratio table?
Standard ratios are given in the gearbox manufacturer’s catalogue by type and size.
How much oil goes into a gearbox?
The oil quantity is given in the manufacturer’s table according to gearbox type, size and mounting position, and is checked at the level gauge.
CSP Mühendislik Support
CSP Mühendislik supports gearbox selection, installation, alignment and overhaul, and spare parts supply for conveyor, screw and feeder drives. See our belt conveyor spare parts page for conveyor drive parts, our screw washer spare parts page for screw washer drives and our service, maintenance and repair services for maintenance.
