In view of the blackening problem of some RCV pump couplings grease found during maintenance,relevant inspection and analysis were carried out.The results show that the primary cause of grease blackening is the presence of many metal wear particles in the grease,secondary cause is high temperature oxidation of grease.Through the relevant calculation and analysis,the reason for the temperature rise and abnormal of the coupling is found to be improperly selected lubricant.Specifically,the base oil viscosity of the currently using grease does not meet the minimum viscosity requirement of the CG-2 type gear coupling in the AGMA standard