KEVOS® 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). |
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.
WORKOS tools are screening and decision-support utilities. Verify important financial, engineering, safety, legal or operational decisions using appropriate professional standards and your organisation’s approved data.
Open the Gearbox Selection Check (Service Factor, Thermal and Torque) calculator
