A gear reducer that tripped on thermal overload after 3 hours of continuous operation. The reducer was a helical inline unit, 5.5 kW input, ratio 10:1, running a mixer at 150 RPM output. The ambient temperature was 35°C. The reducer was rated for 7.5 kW at ambient 40°C. It should have had margin. But the surface temperature reached 95°C. The thermal switch tripped. The issue was the lubricating oil viscosity — the wrong ISO VG grade for the operating temperature. This is about gear reducer thermal rating and oil selection.

The oil viscosity trap

Gear reducer manufacturers specify oil viscosity based on the ambient temperature at startup, not the operating temperature. A reducer in a 35°C ambient is typically filled with ISO VG 220 gear oil. At startup (35°C), the oil is about 500 cSt. As the reducer warms to 80°C, the viscosity drops to 50 cSt. That’s in the correct range for the gear mesh (30-100 cSt). But if the ambient is 35°C and the reducer is running in a sealed enclosure, the internal temperature climbs to 95°C. At 95°C, the VG 220 oil is at 25 cSt — too thin. The oil film between the gear teeth breaks down. Metal-to-metal contact. Wear. Heat. A vicious cycle.

The viscosity index

The viscosity index (VI) measures how much the oil thins as it heats. A standard mineral gear oil has VI of 90. A synthetic gear oil (PAO) has VI of 140-160. At 95°C, the synthetic oil stays at 50 cSt while the mineral oil drops to 25 cSt. The synthetic maintains the oil film. The reducer runs 15°C cooler.

For the 5.5 kW mixer, the oil was changed from mineral VG 220 to synthetic PAO VG 220. The running temperature dropped from 95°C to 72°C. The thermal switch stopped tripping. The synthetic costs 4x more but the reducer didn’t need to be replaced.

The thermal rating calculation

A gear reducer’s thermal rating (how much power it can dissipate as heat) is lower than its mechanical rating (how much torque the gears can take). The thermal rating depends on:

  • Ambient temperature
  • Mounting orientation (horizontal vs vertical)
  • Ventilation (free air vs enclosed)
  • Oil type (mineral vs synthetic)
  • Duty cycle (continuous vs intermittent)

For a 5.5 kW reducer in 35°C ambient, enclosed, continuous duty, the thermal rating might be 4.5 kW — below the 5.5 kW input. The reducer overheats. The mechanical rating (7.5 kW) is irrelevant if the thermal rating (4.5 kW) limits the application. The solution: either upsizing the reducer (higher thermal rating), improving ventilation (add a fan), or switching to synthetic oil (better thermal rating).

What to check

When a reducer overheats, the sequence is: verify oil level, check oil viscosity for the operating temperature, verify ventilation, then check duty cycle. Most overheating is solved by changing the oil grade, not upsizing the reducer. A VG 320 oil in a high-temperature application stays thicker at operating temperature but too thick at startup. The sweet spot: VG 220 synthetic (stays fluid at low temp, stays thick at high temp).

The oil I specify: synthetic PAO VG 220 for reducers running over 80°C surface. The overheating mixer wasn’t undersized — it was mineral oil thinning out at 95°C. Check the thermal rating, not just the mechanical rating. Synthetic oil raises the thermal rating by 20%. Add a fan if the enclosure restricts airflow.