BIZOMAT MAKEOS · Power Transmission
Gearbox Selection Check (Service Factor, Thermal and Torque)
Is this catalogue gearbox right for my motor and load once the service factor and thermal rating are applied?
Selection check of a catalogue gearbox: output speed and torque from motor power, ratio and efficiency, the mechanical rating needed with the service factor against the catalogue rating, the thermal rating against the actual power (thermal ratings are not multiplied by the service factor), and output torque with service factor against the catalogue torque.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Motor power | kW | Rated power of the driving motor (or the actual absorbed power if known and limited). |
| Input speed | rpm | Input shaft speed. |
| Gear ratio | Gear ratio. | |
| Gearbox efficiency | % | Gearbox efficiency. |
| Service factor for the duty | Service factor for the driven machine, hours per day and starts per hour (from the gearbox catalogue). | |
| Catalogue mechanical rating at this input speed | kW | Mechanical power rating of the gearbox at this input speed and ratio (catalogue). |
| Catalogue thermal rating | kW | Thermal power rating for the mounting and ambient (catalogue). |
| Catalogue output torque rating | N·m | Rated output torque (catalogue). |
What each input means
Open an input to see what it is, its units, the worked example and the common mistakes, with a small diagram.
What this means: Motor power
- What it is
- The power the driving motor supplies to the gearbox input, normally its rated power, or the absorbed power when that is known and limited.
- Units and format
- Enter a number in kW: kilowatts. The form accepts 0.000001 kW to 100000 kW. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 7.5 kW as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Output torque, the mechanical rating needed and the thermal check all rise with power.
- Get it right
- Using absorbed power is only safe if the motor cannot deliver more, for example through a torque limit.
What this means: Input speed
- What it is
- The speed of the gearbox input shaft, usually the motor speed when the motor drives it directly.
- Units and format
- Enter a number in rpm: revolutions per minute. The form accepts 0.000001 rpm to 100000 rpm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1450 rpm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A higher input speed raises output speed and lowers output torque for the same power.
- Get it right
- Catalogue ratings depend on input speed; read the ratings at this same speed.
What this means: Gear ratio
- What it is
- The reduction ratio of the gearbox: input speed divided by output speed.
- Units and format
- Enter a plain number with no unit. The form accepts 1 to 10000. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 14.5 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Output torque rises in proportion to the ratio; output speed falls.
- Get it right
- Use the exact catalogue ratio, not a rounded nominal, when torque matters.
What this means: Gearbox efficiency
- What it is
- The share of input power that reaches the output shaft, the rest being lost as heat in gears, bearings and seals.
- Units and format
- Enter a number in %: percent: the number is read as a percentage, so type 8 for 8 %. The form accepts 30% to 100%. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 94% as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Output torque is directly proportional to efficiency.
- Get it right
- Type the percentage itself. Worm gearboxes are much less efficient than helical ones; use the maker’s figure for this ratio.
What this means: Service factor for the duty
- What it is
- The multiplier for the duty: type of driven machine, hours per day and starts per hour. It raises the mechanical rating and torque the gearbox must have.
- Units and format
- Enter a plain number with no unit. The form accepts 0.8 to 4. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1.4 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Raises the mechanical rating and torque needed; it is not applied to the thermal check.
- Get it right
- Take it from the same gearbox maker’s catalogue as the ratings; factors from different makers are not interchangeable.
What this means: Catalogue mechanical rating at this input speed
- What it is
- The power the gearbox can transmit continuously without gear or bearing failure at this ratio and input speed, from the catalogue.
- Units and format
- Enter a number in kW: kilowatts. The form accepts 0.000001 kW to 100000 kW. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 11 kW as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- It does not change the calculated values, only the mechanical check.
- Get it right
- Read the rating at this input speed and ratio; ratings at other speeds differ.
What this means: Catalogue thermal rating
- What it is
- The power the gearbox can transmit continuously without overheating for its mounting and ambient conditions, from the catalogue.
- Units and format
- Enter a number in kW: kilowatts. The form accepts 0.000001 kW to 100000 kW. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 9 kW as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Compared with actual motor power, without the service factor.
- Get it right
- Thermal ratings depend on ambient temperature, mounting and cooling; use the figure for your installation.
What this means: Catalogue output torque rating
- What it is
- The output torque the gearbox is rated to carry at its output shaft, from the catalogue.
- Units and format
- Enter a number in N·m: newton-metres. The form accepts 0.000001 N·m to 100000000 N·m. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1000 N·m as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- It does not change the calculated torque, only the torque check.
- Get it right
- Read the rating for the same ratio and input speed as the other catalogue figures.
Method and formulas
- Output speed = n₁ ÷ i; output torque T₂ = P η ÷ ω₂.
- Mechanical rating needed = P × SF against the catalogue mechanical rating.
- Thermal: the actual power against the thermal rating (no service factor).
- Torque: T₂ × SF against the catalogue output torque.
- Example: 7.5 kW through 14.5:1 gives 673.23 N·m at 100 rpm; it needs a 10.5 kW mechanical rating.
Limitations
- Catalogue ratings and service factors come from the gearbox maker; they differ between makers.
- Overhung and thrust loads on the shafts are separate checks.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Check a gearbox chosen from a catalogue
- Explain why a gearbox overheats while mechanically adequate
- Compare gearbox sizes for a duty
Who it is for
- Design engineers
- Maintenance engineers
- Power transmission engineers
What you get
- Output speed and torque
- Mechanical rating needed
- Thermal and torque checks
Interpreting the result
- A gearbox can pass mechanically and still overheat: the thermal check often governs at high input speed.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 3339–3340, 3304–3313. Topic reference; no source tables or recommended settings are reproduced.
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Open the Gearbox Selection Check (Service Factor, Thermal and Torque) calculator