Effects of friction modifiers on Pb-free plain bearing lubrication

Abstract In order to improve fuel efficiency for automotive engines, decreasing oil viscosity and using friction modifiers (FMs) for engine oil have been widely used. On the other hand, the reduction of lead, which has been used for plain bearings in engines, is also advanced from the perspective of reducing environmental pollutants. Therefore, whether FMs contained in engine oil were effective even for lead-free plain bearings was investigated. In total, four kinds of bearing materials were tested. Two of them are Cu alloy on which Bi overlay was applied. Others are Al alloy on which Bi overlay was also applied. Two kinds of ashless FMs with and without sulphur were used as representatives of ashless ester-type FMs. As a result, the FMs with and without sulphur are effective, respectively, for Al alloy and Cu alloy plain bearings. They were also effective in preventing metal contact between the shaft and the plain bearing. This suggests that the interaction between the FM and the plain bearing material increased the oil film thickness between the shaft and the plain bearing.