A gear reducer that ate its own oil. The reducer was a worm gearbox, 5 kW, ratio 20:1, running a conveyor. After 6 months, the oil was black and thick. The customer changed the oil. It blackened again in 2 months. The reducer ran at 75°C surface. The oil was ISO VG 68 hydraulic oil (the customer used what was on the shelf). Worm gearboxes need extreme pressure (EP) gear oil, not hydraulic oil. The wrong oil couldn’t handle the sliding contact in the worm mesh. This is about gear oil selection and why viscosity alone isn’t enough.
Worm gear sliding contact
A worm gear doesn’t roll like a helical gear. The worm slides across the gear teeth. The sliding velocity is about 2-3 m/s. The contact pressure is high (1000 MPa). The oil must form a boundary film that prevents metal-to-metal contact. Regular hydraulic oil (anti-wear, AW) has zinc-based anti-wear additives. These protect steel-on-steel rolling contact. But the worm gear sliding contact needs extreme pressure (EP) additives — sulfur-phosphorus compounds that react with the metal surface under high pressure.
Without EP additives, the worm mesh runs metal-to-metal. The oil heats up. The additives in the oil break down. The oil turns black (oxidation). The worm and wheel wear. The reducer loses efficiency. After 6 months, the worm is scored.
What was changed
1. Drained the hydraulic oil, filled with EP gear oil. ISO VG 220 EP gear oil (AGMA 4 EP) was used. The EP additives (sulfur-phosphorus) protected the worm mesh. The running temperature dropped from 75°C to 55°C. The oil stayed clear for 2 years. The worm and wheel showed no wear. The change cost $50 for the oil. The worm gearbox would have cost $1500 to replace.
2. Checked the oil viscosity for the operating temperature. At 55°C, VG 220 is about 150 cSt. The worm mesh needs 100-200 cSt. Correct. In winter (ambient 5°C), the cold oil is 1000 cSt. The reducer needs 10 minutes to warm up. The customer installed a heater pad for cold starts. The worm didn’t run dry during warm-up.
3. Changed the oil every 2 years. Worm gear oil doesn’t need frequent changes. Every 5000 hours or 2 years is standard. The customer had been changing it every 6 months because it went black. With the correct EP oil, the interval extends to 2 years. The drain interval is printed on the reducer nameplate.
The oil selection guide
| Gear type | Oil type | Typical viscosity |
|---|---|---|
| Spur/helical, low speed | EP gear oil | VG 150-220 |
| Spur/helical, high speed | EP gear oil | VG 68-150 |
| Worm gear (steel on bronze) | EP gear oil (no lead) | VG 220-460 |
| Bevel gear | EP gear oil | VG 220-320 |
| Bearing, circulating | Anti-wear hydraulic | VG 32-68 |
The bronze compatibility note
Worm gears use a bronze wheel. EP oils with sulfur additives can corrode bronze at high temperatures. For worm gears running over 80°C, use a lead-based EP oil (not sulfur-based). For temperatures under 70°C, sulfur-phosphorus EP is fine. The reducer manufacturer specifies the correct oil. Don’t substitute. A wrong oil that attacks bronze destroys the wheel in months. The oil change interval doubles if the correct oil is used.
The oil rule: use the manufacturer’s specified gear oil, not whatever is on the shelf. The blackened oil wasn’t contaminated — it was AW hydraulic oil in a worm gear mesh that needed EP additives. VG 220 EP for most worm gearboxes. Check the nameplate. For high-temperature worm gears, verify bronze compatibility. A $50 oil change saves a $1500 gearbox.
