Enhancing Water Lubrication by Secondary Assistant Lubricant in Small Quantity

Water lubrication has the advantage of no pollution and low cost. However, due to its poor load-bearing capacity, a water-lubricated bearing is often under the boundary/mixed lubrication regime during the service, where excessive wear can be found. Previous research has improved the performance of water lubricated bearings. However, the bearing is still prone to lubrication failure when it is subjected to severe working conditions, such as the stern bearing of ships due to low speed, heavy load, short-time load impact.　In the present paper, an enhanced water lubrication scheme by small quantity secondary assistant lubricant was proposed to deal with the water lubrication failure under short-time severe conditions. A block-on- ring test rig was used to validate the idea. The material of the block was NBR3606, and the ring was made of 316 stainless steel. The secondary lubricant used was commercial emulsion oil. To facilitate the supply of the small quantity secondary lubricant, a nozzle was placed near the entrance of the contact region, and its direction was arranged in such a way to avoid the revers flow.　The block-on-ring contact was immersed totally in water. The friction coefficient of the contact at different speeds was measured to find the boundary/mixed lubrication regime, where small quantity secondary lubricant was applied. The friction coefficient of the block-on-ring was further measured when small quantity emulsion oil was injected into the entry zone of the contact under boundary/mixed lubrication regime. The injection speed was 10 µL/s and the injection time was 10 s. During the experiment, the running-in time for the block-on-ring contact was 600 s, so that the friction coefficient can become stable. Emulsion oil was then supplied to the contact area. The total test time was 1 800 s. The applied loads were 350 N and 1 kN.　The results showed that the wear of the block can be reduced by short-time supply of small quantity secondary lubricant (emulsion oil) under the conditions of boundary/mixed lubrication. Furthermore, there was a critical rotation speed of the ring for the block-on-ring contact. When the rotation speed of the ring was less than the critical rotation speed, the friction coefficient was decreased by a short-time supply of small quantity emulsion oil and can also be maintained for a long time. When the rotation speed of the ring was higher than the critical rotation speed, the friction coefficient was increased by a short-time supply of small quantity emulsion oil and then decreases again due to the dilution and removal of emulsion oil in the contact area. The critical rotation speed increased with the increase of the load. In addition, the line contact light interference technology revealed that the emulsion oil formed a plate-out lubricating film in the water environment.　In conclusion, a preliminary study had been carried out on the enhancement of water lubrication by supply of small quantity secondary lubricant. The experimental results showed conceptually that the supply of small quantity secondary lubricant can be used to improve the water lubrication performance, and this proposed idea can be applied when water-lubricated rubber bearings encounter with short-time severe conditions. In practical application, a short-time supply of small quantity secondary lubricant can be realized by a lubricant inlet hole at the entrance of water-lubricated rubber bearings, and environment-friendly lubricant is employed to prevent the pollution of water environment.