Technical Learning

Thermal conductivity is the rate of transfer of heat for a given temperature gradient through a material. The rate of transfer of heat from a hot spot to a cooler area in a bearing is a factor in controlling overheating. Therefore, an oil with a high thermal conductivity would be expected to lower the temperature of a bearing. Thermal conductivity of a lubricating oil is used in equations to calculate heat transfer in bearings. The unit of thermal conductivity is heat over a distance per degree of temperature. Most mineral oils have approximately the same thermal conductivity of near 0.1 W m-1 degree C-1. (For reference, the thermal conductivity of water and ethylene glycol are about 3 times higher than oils ).

Thermal conductivity is measured using ASTM D 2717. This method consists of a test vessel containing oil and a platinum resistance thermocouple. Thermal conductivity is determined by measuring the temperature gradient produced across the liquid sample by a known amount of heat introduced into the cell by an electrically heated platinum element. Thermal conductivity increases slightly with higher temperatures.