An industrial gearbox can carry the torque a conveyor needs and still run too hot during a full shift. Its mechanical rating tells you what load the drive can transmit at a given speed. The thermal power rating of a gearbox tells you how much power it can transmit continuously under stated conditions while keeping the oil within the manufacturer’s temperature limit.
That distinction matters when a quotation gives both ratings in kW. A gearbox rated to transmit 100 kW continuously does not release 100 kW of heat. Most of the input power leaves through the output shaft and drives the machine. Only the losses generate heat. To decide whether the gearbox needs an oil cooler, first establish which rating limits the application.
What Do Mechanical and Thermal Ratings Measure?
| Rating | What the figure means | What to compare it with |
|---|---|---|
| Mechanical rating | Permissible torque, or the corresponding power at a specified shaft speed | The load requirement after applying the manufacturer’s service factor and checking peak loads |
| Thermal rating | Power the drive can transmit continuously within its oil temperature limit | The operating power for the duty, using a thermal rating adjusted for the site and cooling arrangement |
Consider a cooling tower fan. Bringing a large fan up to speed can impose a substantial starting torque. The mechanical check accounts for loads on the gears, shafts and bearings. Once the fan reaches operating speed, hours of steady running pose a different question. Can heat from the gear meshes, bearings and oil churning escape as fast as it builds up?
Why the Same Gearbox Has Different Thermal Ratings
Read the conditions printed above the catalog table. For its X..e series, SEW-EURODRIVE gives reference conditions that include 20°C ambient air, a large hall with air moving at least 1.4 m/s, and an installation altitude below 1,000 m. The table also distinguishes between self-cooled and fan-cooled configurations. A buyer using that table for a drive in a confined room needs the rating for the actual installation.
The air temperature that matters is the temperature around the gearbox. A plant’s outdoor weather record may tell you little about a drive behind a guard beside a hot process line. Record the expected local temperature, airflow and hours under load. Check whether the quoted gearbox thermal capacity already includes a fan. Otherwise, two offers for the same transmitted power may assume different equipment.
How to Check Both Ratings in a Gearbox Catalog
The Falk Drive One catalog gives a worked example on page 15. A draw bench runs ten hours a day and transmits 100 hp. The example makes the two checks easy to follow because it uses a different comparison for each.
- Check mechanical capacity. The catalog assigns this duty a service factor of 1.25. Multiplying 100 hp by 1.25 gives a mechanical selection requirement of 125 hp. At the specified speed and ratio, the selected drive is rated at 143 hp, so it passes.
- Adjust the thermal rating. At the selected ratio and input speed, the drive has a basic thermal rating of 201 hp without auxiliary cooling. Air movement in the small room is limited. The catalog applies an airflow factor of 0.75, bringing the site-adjusted rating to about 151 hp.
- Compare with operating power. Falk compares 151 hp with the 100 hp actually transmitted. The 125 hp figure belongs to the mechanical check. Under the conditions in this example, the drive meets both requirements without an oil cooler.
When Does a Thermal Limit Call for Auxiliary Cooling?
If the mechanical rating falls short, the supplier needs to reconsider the drive selection. Cooling the oil cannot increase the torque the gear teeth or shafts can carry. If the mechanical check passes but the adjusted thermal rating falls below the required operating power, ask the gearbox supplier to compare approved options. Depending on the design, industrial gearbox cooling methods can include a casing fan or an external oil circuit. A larger drive may also meet the duty.
Suppose a drive must transmit 100 kW and its site-adjusted thermal rating is 90 kW. The 10 kW difference is a shortfall in transmitted power. It does not specify a 10 kW oil cooler. To size the exchanger, ask for the heat that the oil circuit must remove at the design load, accounting for gearbox losses and the heat already released through the housing.
Request a Custom Oil Cooler for Your Gearbox
Once the gearbox supplier has confirmed the cooling duty, JEDHeatExchanger can review a custom unit for your oil circuit. For a shell and tube oil cooler, provide the available water flow and inlet temperature. For an air-cooled oil cooler, specify the design air temperature and space available for the fan.
To request a gearbox oil cooler quote, send the required heat duty, oil grade, flow, inlet and target outlet temperatures, operating pressure and allowable pressure drop. Include the connection sizes and mounting dimensions on the drawing so we can check the fit before quoting.
Gearbox Rating FAQ
Can I use a thermal rating for another gear ratio in the same housing?
Use the table entry for the selected ratio and input speed. The same housing can contain different gear stages, and the manufacturer may list different thermal capacities. If the catalog groups several ratios together, check that yours falls within that group.
Which operating points matter for a variable-speed drive?
Give the supplier the speeds, loads and time spent at each sustained operating point. Identify frequent starts and peak torque separately. A shaft-driven fan also changes speed with the shaft, so a slow, heavily loaded period deserves its own review.
What should I request if a quotation lists only motor power?
Ask for the gearbox model and a selection sheet showing the mechanical service factor, site-adjusted thermal rating and cooling equipment included in the price. This gives you a common basis for comparing two suppliers’ offers.
