An experimental investigation of the peak film temperatures for 13 different oils has been performed with a precision journal bearing test rig. This study examined the differences between mineral, synthetic and viscosity index enhanced oils with regards to peak oil film temperatures. The oils examined include: three mineral oils (1-ISO 68 and 2-ISO 32), eight synthetic oils (1-ISO 22, 6-ISO 32, 1-ISO 68) and two viscosity index enhanced oils (1-ISO 22 synthetic, 1-ISO 32 mineral). The synthetic oils included both Diester base and Polyalphaolefin base products. Data for each oil were obtained at 6000, 8000, 10800, 12000, and 14000 RPM (100, 133, 180, 200 and 233 Hz); for loads of 670, 1330, 2000, 2670 and 3340 N. These speeds and loads correspond to surface speeds between 16 m/s to 37 m/s and unit loading between 0.5 MPa and 2.6 MPa. A 50.8 mm diameter, 25.4 mm long bearing with the upper portion relieved was used for all tests.
The data obtained tend to confirm the initial expectation that the bulk fluid characteristics are much more important than the lubricant composition with regards to static performance characteristics (i.e., load capacity and peak film temperature) in the thick film operating regime characteristic of a moderately loaded journal bearing. The results showed a 5 to 10 °C decrease in peak film temperature can be obtained by changing from an ISO 68 to an ISO 32 oil. It was also found that viscosity index enhancers can allow a relatively inexpensive mineral base oil to show the same general level of thermal performance as a synthetic oil. The reduction in peak temperatures was seen in both an enhanced ISO 32 mineral base oil and an enhanced ISO 22 synthetic base oil. These observations suggest that the effect of shear thinning on polymer thickened oils may not be as great as the published analytical investigations would suggest. The reductions in temperature observed should also be of interest with regards to thrust bearings which often experience severe thermal crowning.