A gearbox has two completely separate ratings and they fail in completely different ways. The strength rating is about whether the teeth and bearings can carry the load — exceed it and something eventually breaks. The thermal rating is about how much heat the housing can get rid of. Exceed that and nothing breaks at all. The oil just runs hot, ages fast, stops protecting the gears, and a few years later the unit is worn out long before its time.
Where the heat comes from
Every gear mesh and every seal turns a small fraction of the transmitted power into heat, and the housing is the only radiator the unit has. Efficient gear types shed little heat and rarely have a problem. Less efficient types — worm drives in particular — turn several times more of the power into heat, usually inside a smaller housing. That is why worm units are the ones that catch people out: the torque rating looks generous, the price looks attractive, and then the unit runs far too hot in a plant room with no air movement.
How to tell which rating is governing
Compare the power the drive transmits continuously against the thermal figure printed in the specification table for the frame — it is listed alongside the torque for every unit on this site. If the continuous power is getting close to the thermal figure, treat the selection as heat-limited and check it properly rather than trusting the headline torque number.
What actually moves the number
- Duty cycle. Thermal ratings assume continuous running. A drive that works intermittently has time to cool between cycles, so its effective capacity rises.
- Ambient temperature. Ratings assume a normal workshop temperature. In a hot environment a large part of the capacity disappears, because the housing can no longer get rid of heat as easily.
- Air movement. A cooling fan on the input shaft is the cheapest capacity you will ever buy. A drive tucked inside a sealed guard with no airflow loses just as much.
- Oil choice. A synthetic oil runs cooler than a mineral oil at the same load and tolerates higher temperatures before it starts to degrade.
What we do about it
Every selection that leaves our applications desk carries a heat check as well as a strength check. If the heat check is the tight one, the quote says so and offers the fix — a fan, the next frame up, or an oil cooler — rather than leaving it as a footnote for someone to find after commissioning.