Abstract: Water-lubricated bearings have attracted more and more interests in industry because of its advantages of low cost and environmental friendliness. However due to its low viscosity and poor load-carrying capability, water usually cannot work as well as oils under unexpected temporary harsh working conditions, for example, low operation speeds at start up/shut down, short-time dynamic impact and heavy load. Consequently excessive friction, wear and noise can be found. At present, many studies had been carried out to improve the performance of water-lubricated bearings, and most of them were focused on materials and mechanical structures of bearing bushings. In this paper it was demonstrated that a small quantity of emulsifying oil could be employed as a secondary lubricant to assist water lubrication under mixed/boundary lubrication regime and a critical speed was identified and investigated, at which there was no friction coefficient change with a secondary lubricating medium. A block-on-ring test rig (Model MRH-3), which was capable of measuring the coefficient of friction online under fixed loads and speeds, was used. In the experiments, a rubber block-on-steel ring contact was enclosed within the chamber filled with water. With a deliberately fabricated needle nozzle emulsifying oil, as a secondary lubricating medium, could be ejected to inlet of the water lubricated contact under mixed lubrication medium with prescribed flow rate and injection time. The loads applied were 175, 350 and 700 N, respectively. The flow rate of the emulsifying oil ejection was fixed at 10 μl/s. The total oil supply quantities were 25, 50, 75 and 100 μl. The emulsifying oil was supplied when the pre-running of the water-lubricated contact was finished and the friction force was stable. The results validated the feasibility of the injection of the small amount of emulsifying oil to enhance the water lubrication. Moreover it was found that there existed a critical ring rotational speed, at which the measured coefficient of friction did not change when the emulsifying oil was applied as the second lubricating medium to the water-lubricated rubber block-on-steel ring contact. When the ring speed was less/greater than the critical ring speed, injection of a small amount of emulsifying oil would decrease/increase the friction force. Experiments showed that the critical rotational speed increased with the decrease of the oil supply quantity, and increased with the increase in the applied load. The critical rotational speed did not change significantly when the oil supply quantity and the applied load were relatively small. Based on the Stribeck curve, the dependence of the critical rotational speed on the oil injection quantity and the applied load were clarified. At the critical rotational speed, if the quantity of the emulsifying oil was increased, there would be increase in the friction coefficient because more oil leaded to a higher apparent viscosity in the contact and then the Hersey number increased. To get an unchanged friction coefficient for this increased oil quantity, the rotational speed had to decline so that the Hersey numbers for the pure water lubrication and the assisted water lubrication with emulsifying oil would be decreased and a unchanged friction coefficient would be obtained, that was, increasing oil quantity presented lower critical rotational speed. When the load was renewed to a larger value, a higher critical rotational speed could be obtained considering the old critical speed presented a smaller friction coefficient under this renewed load. Surface measurement showed that surface wear could be reduced by the injection of a small amount of emulsified oil no matter the friction coefficient increased or decreased.