Worm gearbox running hot with the wrong top-up oil
A 15 kW, 1470 rpm squirrel-cage motor drives a single-start worm gearbox through a flexible coupling. The gearbox has a 4.5 litre oil-bath sump and is filled with ISO VG 220 mineral EP oil, splash lubricated, with a shaft-mounted fan. After nine months the sump runs at 78 °C with a 30 °C ambient; at commissioning it ran 55 °C at 25 °C ambient. The fitter has been topping up about 0.3 litre every week with ISO VG 460 that was available in the store. Oil analysis gives iron 92 ppm, copper 18 ppm, water 0.12 %, viscosity 268 cSt at 40 °C and TAN 2.8 mg KOH/g (new oil 0.4). Vibration shows 1× running speed at 1.2 mm/s and a new broadband high-frequency component at the greased input bearing, which is regreased every 500 h with a hand gun until grease escapes the relief. The worm wheel shows a bright polished band on the loaded flank near the pitch line; the oil is dark and smells hot.
1. Interpret the oil analysis and temperature trend. What is the most likely chain of causes, and what immediate and medium-term actions follow?
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Interpretation:
- Viscosity 268 cSt against 220 cSt nominal (about +22 %) is too large to be normal in-service thickening. It matches the repeated addition of VG 460 to a VG 220 sump, plus oxidation.
- TAN 2.8 mg KOH/g against 0.4 new confirms oxidation, consistent with 78 °C sump temperature. Every 10 K above about 70 °C roughly halves mineral-oil life, so the service interval must be shortened until the temperature is fixed.
- Iron 92 ppm and copper 18 ppm indicate wear of steel worm/wheel and bronze wheel or bearing cages, but are not yet catastrophic. Water at 0.12 % is below the 0.2–0.5 % concern band for this oil type.
- The polished band near the pitch line is the normal loaded contact pattern; discoloration and the hot smell are the real signals.
Actions:
- Stop the VG 460 top-ups, label the store bin and correct the lubrication chart.
- Drain, flush and refill with the specified VG 220 EP oil; change the filter or clean the breather.
- Investigate the temperature rise: oil level too high causing churn, fan damaged or reversed, cooling airflow blocked, overhung load, or the gearbox selected too small after a duty change.
- Re-sample at 250 h and 500 h; trend viscosity, TAN, iron and copper.
2. Evaluate the greasing practice at the input bearing and explain how the high-frequency vibration should be investigated rather than assumed to be grease starvation.
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The instruction 'until grease escapes the relief' is a symptom of over-greasing, not proof of correct lubrication. A filled bearing cavity churns grease, generating heat that raises bearing temperature and can soften or degrade the grease; excess pressure can also displace seals and push grease toward the gearbox.
Correct practice:
- Use only the specified lithium EP2 grease; never mix thickener types.
- Calculate quantity from the bearing geometry: approximate fill grams = 0.005 × bearing outside diameter (mm) × width (mm); for a typical 6208 bearing, about 0.005 × 80 × 18 ≈ 7 g per application, not a gun until relief.
- Set interval from speed and temperature (the 500 h figure is a starting point, not a law) and clean the nipple before coupling.
Diagnosing the high-frequency energy:
- Take a spectrum and envelope (acceleration) measurement at the same load and speed, compare with the commissioning baseline, and check for bearing defect frequencies rather than assuming lubrication.
- Feel and measure housing temperature; check axial location and preload, coupling alignment and belt or fan imbalance.
- If enveloped peaks are absent and temperature is high, over-greasing is likely; if defect frequencies or a rising noise floor appear, plan bearing replacement and inspect the shaft seat and housing fit.