<div class="htmlview paragraph">The most widely used method of measuring bearing oil film thickness (BOFT) is the “capacitance technique”. It has two implicit assumptions that need to be carefully considered. These are: <ol class="list nostyle"> <li class="list-item"><span class="li-label">(1)</span> <div class="htmlview paragraph">The bearing surfaces are smooth down to the molecular level.</div></li> <li class="list-item"><span class="li-label">(2)</span> <div class="htmlview paragraph">The dielectric constant is uniform across the interface.</div></li> </ol></div> <div class="htmlview paragraph">This paper examines each of these assumptions and finds: <ol class="list nostyle"> <li class="list-item"><span class="li-label">(1)</span> <div class="htmlview paragraph">The real MOFT is smaller for a rough surface than the reported MOFT</div></li> <li class="list-item"><span class="li-label">(2)</span> <div class="htmlview paragraph">The real MOFT is larger if the VI improver concentrates at the bearing surface than the reported MOFT</div></li> <li class="list-item"><span class="li-label">(3)</span> <div class="htmlview paragraph">The magnitude of the first effect is larger than the second.</div></li> </ol></div> <div class="htmlview paragraph">For values of MOFT greater than one micron, the reported MOFT values are sufficiently independent of surface roughness and possible adsorption effects that the standard MOFT equation can be used.</div>