Abstract: The present situation of low-noise sealed bearing greases mainly used in the bearing industry is reviewed, and on this basis, dynamic centrifugal oil separation tester, apparent viscosity tester, flow pressure tester, velocity type and acceleration type bearing vibration measurement are used The influence factors and mechanism of grease on the vibration and noise performance of sealed bearings were further analyzed.
1. Status of domestic grease for sealed bearings
Grease is called the fifth largest component of rolling bearing except inner ring, outer ring, rolling body and cage. When the machining accuracy of other components of the bearing reaches a certain level, the role of grease becomes more and more obvious. Grease can relatively reduce the vibration noise of sealed bearings [1]. Technological progress in the bearing industry has driven the upgrade of bearing greases. Especially since the 1990s, some domestic bearing manufacturers have paid more and more attention to the selection of grease. At present, the products of foreign companies in the domestic bearing grease market have evolved from the original MP-DX-NO.2 and Alvania2 to the current Beacon325, CheveronSRI, and MutertempSRL. The world-renowned grease companies have come up with their own high-quality special bearing greases to enter the Chinese market. As early as the 1970s, Japan Synergy Grease Company successfully developed MutertempPS2 grease, which well met the requirements of bearing grease at that time. In the mid-1980s, MutertempSRL was successfully developed, which greatly improved the life and reliability of precision sealed bearings. Later, it cooperated with NSK Seiko Bearing Company to develop a series of low-noise bearing grease, the most typical of which is polyurea high-temperature, long-life and low-noise bearing grease. SHELL has specially developed Alvania RL2 and Alvania RLQ2 for bearing customers in China. MOBILE company improved HP to HP222 and XHP222 to meet the needs of domestic bearing companies. CHEVERON companies changed their CheveronSRI to CheveronSRIOEM to secure the company’s market share. Compared with foreign grease companies, although domestic manufacturers have been involved in this field earlier, the research on this technology has not been in-depth, but the cleanliness of grease has been improved [2]. Table 1 shows the recommended greases for sealed bearings by several major companies in the world.
Each company has a wide variety of products, but through the analysis of the inspection and analysis data of more than a dozen products, the existing domestic and foreign low-noise sealed bearing greases can be roughly divided into three categories, namely: general low-noise motor bearing grease, low temperature and high speed grease. Bearing grease, high temperature, long life and low noise bearing grease [3].
1.1 General purpose low noise motor bearing grease
This type of grease is the most widely used in the bearing industry, accounting for about 70% of the total grease used. Because this type of grease uses soft fiber and easy-to-filter lithium soap as thickener, and uses mineral oil as base oil, it is easy to meet the requirements of low vibration value for medium and low-grade bearings. Good pumpability is also one of its advantages, whether it is mechanical greasing or manual greasing, it is convenient and easy to operate. Low price is the biggest advantage of this type of grease. In the current situation of the bearing industry’s capital and profits, it can reduce costs, which is also its vitality. However, the application of mineral base oil makes this kind of grease greatly reduce some physical and chemical indicators except anti-rust performance. If the bearing life can only barely reach about 200h (ASTMD3336), it is difficult to test the starting and running torque at -20 °C, so the power consumption and temperature rise of the motor are also an unavoidable problem. The operating temperature of this type of grease is generally – 20 ~ 120 ℃, when used at high temperature above 150 ℃, the base oil will evaporate too quickly and the loss will be serious, which will greatly shorten the service life of the bearing.
1.2 Low temperature high speed bearing grease
Bearings with dn>106 in the bearing industry are named high-speed bearings. Since this type of bearing can only be lubricated by oil and gas, the high speed mentioned here is only a concept relative to grease lubrication, and its dn value is about 5 × 105 ~ 10 × 105. For precision miniature bearings, the use of general-purpose grease is far from meeting some performance requirements. Due to the highly refined use of precision bearing equipment, the grease used in the bearing must have the following properties: 1. It has the same operating life as the bearing, prolongs the maintenance period as much as possible, shortens the downtime, and improves labor productivity. ②At the high speed of dn>5×105, it is necessary to ensure that the temperature rise of the motor is low, and the oil is not thrown away, thereby reducing power consumption and protecting the motor. ③ Its impurity content is controlled under a certain standard to minimize the pollution of equipment noise to the environment. ④Ensure the flexibility of starting and running of the bearing under ultra-low temperature working conditions, and ensure the minimum output power. In order to solve the above four problems, domestic and foreign lubricating grease companies have learned from the achievements of military industrial grease, and have developed and produced low-temperature high-speed high-speed products in high-clean environment and equipment by selecting high-purity chemical synthetic oil and some special additives. Bearing grease. However, the shortcomings of this type of grease are also very prominent. When used above 150 °C, its service life decreases sharply with the increase of temperature, and its resistance to load and impact is relatively weak. It should be avoided to use on medium and large bearings to prevent boundary lubrication due to boundary lubrication. resulting in sintering. In addition, its price is generally about ten times that of general-purpose grease, which cannot be afforded by the cost of general motor bearings.
1.3 High temperature, long life and low noise bearing grease
In both NSK and SKF bearing technical manuals, a class of greases with a temperature exceeding 150 ℃ is mentioned, which is determined by the specific working conditions of the bearing. When the bearing operates in a high temperature and high speed environment, the internal grease has to undergo a severe test. First of all, it is not lost at high temperature, that is, this kind of grease can maintain a certain consistency at a temperature of about 180 °C without softening. Secondly, there is less grease leakage, that is, when the grease is repeatedly sheared by high frequency and subjected to a large centrifugal force in the bearing, the grease can return to the raceway in time without being thrown out. Third, such greases must have a certain high-temperature service life and have good anti-oxidation ability at high temperatures, thereby extending the life of vitron o-rings and related equipment. Finally, this type of grease has a certain quiet effect. Since the fibers of high-temperature grease are generally harder, the quiet performance in the bearing noise test is far less than that of lithium-based grease, which requires the processing of this type of grease. In the process, a special process is used to miniaturize the soap fibers.
High temperature and long life grease is generally divided into two types, one is lithium complex grease, which has better high temperature performance, but due to the addition of low molecular acid in the thickener crystal, the hardness of the fiber increases greatly. The vibration performance is poor in the bearing. Grease product developers have been inspired by the polymerization principle of polymer chemistry, using a variety of non-metallic organic compounds to obtain a kind of product with uniform molecular weight distribution by controlling the degree of polymerization, and dispersing this product with a special method Another grease with equally good noise performance and high temperature performance is made in the base oil. This type of product has an average dropping point greater than 250 °C, and the operating temperature can reach about 200 °C. The thickener crystal does not contain metal ions, the fiber hardness is small, and the noise performance is good. At the same time, due to the thermal coupling effect between its basic chemical components, it can reduce high-temperature grease rejection. This high-temperature, high-speed, low-noise grease does not contain any metal ions that can cause catalytic oxidation. In addition, the thickening agent rich in nitrogen and oxygen atoms is also an excellent antioxidant, so the bearing life and sound quality are good.
2. Properties of typical low noise sealed bearing greases
In the research, several kinds of lubricating greases commonly used by domestic sealed bearing production companies were selected, and their apparent viscosity, centrifugal oil separation and other general physical and chemical indicators were tested respectively. 1 Bearing noise analyzer and acceleration type S0910-1 bearing vibration measuring instrument for analysis. It is worth noting that the data we have selected are all obtained from the joint research project of postgraduate thesis by the National Testing Laboratory of Hangzhou Bearing Test and Research Center aided by the United Nations and Yanhua Group Tianjin Lubricating Grease Co., Ltd., among which for several foreign products , there may be cases where the final data of the same test standard is different from the original index of its product manufacturing company, in this case, our test data shall prevail.