This article introduces the technological aspects and recent trends of research on greases for rolling bearings. In particular, the present paper describes the investigations on elongating lubrication life because the subject is the eternal problem in improving the performance of rolling bearings and is also the mainstream of studies on lubricating greases. The topics include the activities of JAST Technical Committee on Grease, the recent outcomes on lubrication life prediction, and some individual works regarding lubrication life extension. Further, this article illustrates the features of research and development regarding the grease technologies which contribute to the reduction of rolling bearing torque.

Lubricating greases are a type of solid or semi-solid lubricants composed of base oil, thickener and additives which are added to improve certain properties. In particular, the thickener is the most important material with regards to basic grease performance. The history of grease thickener development is very long, starting with calcium thickeners which were developed and used in lubricating greases and continue as one of the basic grease types today. Since then however, new thickeners have been developed with excellent heat resistance and longer lubricating life via steady advancement of grease technology. In North America, lithium complex and calcium sulfonate complex greases have been actively developed. On the other hand, in Japan polyurea greases are most actively studied, having the second largest share in the total grease manufacturing volume after lithium greases. In recent years, the study of grease thickeners with stronger environmental considerations and further improvement of heat resistance has also been carried out. In this report, recent trends of these grease thickeners for rolling bearings are explained.

Grease lubricated rolling bearings are used in a wide range of fields such as automobiles, home appliances and steel mills. Grease differs from lubricating oil in that it has semi-solid characteristics due to dispersion of thickener. It is generally accepted that thickener in base oil forms a network structure which causes complicated rheological behaviors. Therefore, it is necessary to clarify the structure of thickener for a better understanding of grease lubrication. There have been many studies to investigate the structure of thickener through various instrumental analyses. One of the most important among them is microscopic observation of thickener. This article outlines microscopic observation methods including not only conventional methods such as TEM and SEM but also FE-SEM with a higher resolution and CLFM which enables observing grease as it is, without removing base oil.

Grease for rolling bearing needs to have 'Low noise' property. To achieve good low noise property, grease thickener need to have fine structure, and to be homogenized evenly. Due to the increasing demand for quietness of rolling bearing from the expansion of vehicle electrification, superior grease for rolling bearing is required. Grease manufacturers have been developing several way by optimizing thickener type, additive and special homogenization process to achieve such a demand. Especially urea grease, it is challenging to homogenize very finely due to the nature of urea thickener and reaction of isocyanate and amine. Several methods are reported to overcome this challenging mainly by optimizing production process. Urea grease, which is made by special high-share reactor, shows superior characteristics such as smooth texture, low noise and low fretting-wear. It is proposed that these superior characteristics is due to the unique rheological property of the grease.

For grease lubricated rolling bearings, it is necessary to reduce its friction torque. The behavior of grease inside rolling bearings has a significant effect on the energy loss. However, it is difficult to predict the grease behavior inside the bearings due to the complex mechanical properties of grease. This article summarizes the latest visualization techniques of grease behavior. The visualization techniques in the article include methods using X-rays, neutron beams, fluorescent agents, infrared spectroscopy, and colorants. The principle and results of a unique visualization technique using colorants is described in detail.

The oxidation of lubricating grease brings about decomposition or polymerization of base oil and generation of degradation products such as oxidation products and sludge. These degradation in composition cause the change of grease properties such as the rheology and oil separation, which leads starved lubrication and failure of rolling bearings. To improve bearing performances, the mechanism of the oxidation has been studied using detective methods of oxidation. In addition, detections of the oxidation could contribute for supporting increasing demands for condition monitoring of machines. This article describes the mechanism of oxidation of hydrocarbon oil and recent detective technologies for oxidation using color and gas. The gas method could be powerful tool for analyzing the generation of the oxidation on a specific bearing, since it can detect oxidation during a bearing rotation.

The cause of white structure flaking have been investigated since the premature failure of rolling bearings became apparent in the early 1980s. However, the mechanism of the failure has not been completely elucidated. One of the challenges of the study is analysis hydrogen, generate or permeate on steel surfaces. In addition, establishing reproductive / evaluation test method is also considered to be hard when taking account of combination of the vibration and impact load. In this report, we will introduce the trends in the field and examples of our efforts regarding the elucidation of the white structure flaking mechanism and countermeasure technology under such circumstances.

In recent years, reduction of frictional energy loss at sliding surfaces is required for mechanical seals to prevent leakage of seal liquid. However, it was not easy for the complicated frictional phenomena of mechanical seal because of difficulty of removing the wear products from the sliding surface and supplying lubricant to the sliding surface. In this paper, SiC (silicon carbide) plane contact frictional tests were carried out with mechanical seal type and bearing type in water. Low friction coefficient of 0.05 or less was achieved under boundary lubrication with mechanical seal type, and the behavior of the friction coefficient during the running-in process was significantly different from that of the bearing type. Compared to the bearing type, the low friction of the mechanical seal type generated under more limited friction conditions. The low-friction sliding surface was smooth surface with a 4 nm thick nano-interface and the deposition of wear products composed of carbon, oxygen and silicon in the pores. It was found that the wettability of low friction sliding surface was better than that of sliding surface of running-in, and good wettability leads to a lower friction coefficient.

Tilting pad journal bearings are widely used in turbine generators to support rotating shaft stably. However, reduction of the bearing loss is required for increasing generator efficiency because the bearing loss accounts for approx. 10% of the total loss in the generator. Generally, the bearing loss is caused by shear resistance of oil around rotating shaft and increases in proportion to amount of lubrication oil. In recent years, the reduction of the supply oil flow rate has been promoted in order to reduce bearing losses. However, if the oil supplied to the gap (oil film) between the rotating shaft and the pad are insufficient, oil film pressure is decreased due to insufficient lubrication, which may cause abnormal vibration of the rotating shaft and the pad, and the seizure due to contact between the rotating shaft and the pad. In this study, the effects of lubrication oil on vibration and oil film thickness are evaluated experimentally, and the appropriate supply oil flow rate is discussed. As a result, it is confirmed that when the supply oil flow rate is below a certain threshold, the sub-synchronous vibration of the upstream pad increases and the difference between the maximum oil film thickness and the minimum oil film thickness approaches zero, which causes the seizure due to contact between the shaft and pad.

Autoxidation is a chain reaction that occurs in oil when it is in the presence of free radicals (oxidants) that can be stopped by adding another substance to trap the radicals. To accomplish this, radical scavengers called antioxidants are commonly added to lubricating oils. Zinc dialkyl dithiophosphate (ZnDTP), which provides both good antioxidation performance and wear resistance, has been used for that purpose in the past, but because it contains phosphorus, zinc and sulfur, which cause catalyst poisoning, alternative additives are currently being sought. Fullerenes, which have recently attracted significant interest due to their ability to reduce friction and wear, are now being explored due to their oil antioxidant capabilities. Furthermore, some studies have indicated that static electricity spark discharges resulting from electrification flows could promote oil oxidation. In the present study, we conducted experiments on oil containing 0.1 wt% fullerenes by attempting to promote oxidation of the oil using an in-house fabricated electrostatic discharge generator. These samples were then heated at 80°C and maintained at that temperature for four weeks during which they were analyzed by FT-IR once each week. Since our obtained results found that no oxidation occurred in the oil to which fullerenes had been added, we conclude that fullerenes effectively suppress oil autoxidation by trapping both alkyl and peroxy radicals, while conventional antioxidants target only peroxy radicals.