10/14/2026
Oil analysis alone may not tell you a bearing is varnishing. Its temperature trace might.
In tilting pad journal and equalized (Kingsbury style) thrust bearings, varnish forms where the oil film is thinnest. That shrinks clearance, raises pressure on the film, and lowers load carrying capacity and critical speed. Shear-stress deposits, created by localized degradation at microscopic interfaces, can slip past traditional MPC analysis.
The evidence in the paper: in a centrifugal compressor case study (Shin et al., 2025), four years of varnish buildup drove rotor vibration up 300%, from 6 to 18 µm (0.24 to 0.71 mil). Replacing the oil and removing the varnish restored normal levels. In the field, bearing temperature showed a sawtooth pattern of rapid spikes and gradual cooling, with excursions of 7 to 17 °C (13 to 31 °F) above baseline.
In thermohydrodynamic modeling (Jang et al., 2024), varnish at just 20% of bearing clearance cut minimum film thickness, lowered critical speed by up to 17%, and pushed pressure to as high as 8.9 MPa (1,290 psi) and temperature past 112 °C (234 °F). For reference, typical alarms sit at 110 °C (230 °F), trip at 120 °C (248 °F), and babbitt begins to creep around 132 °C (270 °F).
The takeaway: pair temperature trending with viscosity, MPC and FTIR results and vibration monitoring. No single signal proves varnish, but together they can flag it early.
Read the whitepaper: "Correlation between Lube Oil Condition & Bearing Temperatures" by Greg Livingstone and Sanya Mathura
https://zurl.co/R78Ra