What are the differences between sliding bearings and rolling bearings, and how should one choose?

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As an essential component in mechanical products, bearings play a crucial role in supporting rotating shafts. Based on the nature of friction within them, bearings are classified into rolling friction bearings (simply referred to as rolling bearings) and sliding friction bearings (simply referred to as sliding bearings). The two types of bearings have their own structural characteristics, as well as respective advantages and disadvantages in terms of performance. When making a selection, it is necessary to consider all factors based on actual circumstances. Comparison between rolling bearings and sliding bearings 1. Comparison of structure and mode of operation The most obvious difference between rolling bearings and sliding bearings lies in whether there are rolling elements or not. Rolling bearings contain rolling elements (balls, cylindrical rollers, tapered rollers, needle rollers), which rotate to support the shaft; as a result, the point of contact is a single point. The more rolling elements there are, the more contact points there will be. A sliding bearing has no rolling elements; it relies on smooth surfaces to support the rotating shaft, so the contact area is a surface. The difference in their structures determines that rolling bearings move by rolling, while sliding bearings move by sliding; as a result, their friction patterns are also completely different. 2. Comparison of load-carrying capacity: Generally speaking, due to their larger bearing area, sliding bearings have a higher load-carrying capacity than rolling bearings. Rolling bearings also lack the ability to withstand impact loads. However, fully liquid-lubricated bearings can handle greater impact loads, as the lubricant film acts as a buffer and absorbs vibrations. At high rotational speeds, the centrifugal force on the rolling elements in rolling bearings increases, which reduces their load-carrying capacity (noise is likely to occur at high speeds). For dynamic pressure sliding bearings, their load-carrying capacity increases as the rotational speed rises. 3. Comparison of friction coefficient and starting frictional resistance: Under normal operating conditions, the friction coefficient of rolling bearings is lower than that of sliding bearings, and this value remains relatively stable. The lubrication of sliding bearings is susceptible to external factors such as speed and vibration, resulting in a large range of variation in the friction coefficient. At startup, since a stable oil film has not yet been formed in sliding bearings, the resistance is greater than that in rolling bearings; however, hydrostatic sliding bearings have very low starting friction resistance and operating friction coefficients. 4. Comparison of applicable operating speeds: Due to the limitations imposed by the centrifugal force of the rolling elements and the increase in bearing temperature, the speed of rolling bearings cannot be too high; they are generally suitable for medium and low-speed operating conditions. Due to heating and wear of the bearings, the operating speed of incompletely liquid-lubricated bearings cannot be too high. Fully liquid-lubricated bearings exhibit excellent high-speed performance; in particular, when air is used as the lubricant in hydrostatic sliding bearings, their rotational speed can reach 100,000 rpm. 5. Comparison of power loss: Due to the low friction coefficient of rolling bearings, their power loss is generally minimal, being less than that of bearings with incomplete fluid lubrication. However, it can increase significantly when lubrication and installation are improper. Fully liquid-lubricated bearings have lower frictional power losses, but for hydrostatic sliding bearings, the overall power loss can be higher than that of hydrodynamic sliding bearings due to the power losses associated with the oil pump. 6. Comparison of service life: Due to the effects of material pitting and fatigue, the design lifespan of rolling bearings is generally 5 to 10 years, or they are replaced during major overhauls. The bearing bushes of partially liquid-lubricated bearings suffer severe wear and need to be replaced regularly. The lifespan of a fully liquid-lubricated bearing is theoretically infinite; in practice, however, due to stress cycles, particularly in hydrodynamic sliding bearings, the bearing bush material may suffer from fatigue failure. 7. Comparison of rotational accuracy: Due to their small radial clearance, rolling bearings generally have a higher rotational accuracy. Bearings with incomplete liquid lubrication are in a state of boundary lubrication or mixed lubrication; they operate unstably, experience significant wear, and have low precision. Fully liquid-lubricated bearings have a high degree of precision due to the oil film, which provides cushioning and vibration absorption. Hydrostatic sliding bearings offer higher rotational precision. 8. Comparison in other aspects: Rolling bearings use oil, grease, or solid lubricants in small quantities; more lubricant is required at high speeds. High cleanliness standards are imposed on the oil, which necessitates the use of seals. However, the bearings are easy to replace, and it is generally not necessary to repair the shaft journals. For sliding bearings, except for those operating under imperfect fluid lubrication conditions, the lubricant is generally a liquid or gas. Large quantities of lubricant are required, and its cleanliness must meet very strict standards. The bearing bushes need to be replaced frequently; sometimes, the shaft journal also has to be repaired. Selection of rolling bearings and sliding bearings: Due to the complex and varied actual operating conditions, there are no unified standards for selecting rolling bearings and sliding bearings. Rolling bearings have a low friction coefficient, resulting in low starting resistance, high sensitivity, and high efficiency. They are also standardized, offering excellent interchangeability and versatility; their use, lubrication, and maintenance are all straightforward. For these reasons, they are given priority when making choices, and as such they are widely used in various machines. Sliding bearings have some unique advantages, and they are generally used in situations where roller bearings cannot be used, are not suitable, or do not offer any advantages. Such situations include the following: 1. Cases where radial space is limited, or where the bearing must be installed in separate parts. Roller bearings, due to their structure consisting of an inner ring, an outer ring, rolling elements, and a cage, have a relatively large radial dimension, which imposes certain limitations on their use. When strict radial dimension requirements are present, needle bearings can be used; sliding bearings are necessary in such cases. For components where bearings are inconvenient to use, cannot be installed axially, or must be installed in sections, split-type sliding bearings should be chosen. 2. High-precision applications: When the bearings used require high precision, sliding bearings are generally chosen, as the lubrication film in sliding bearings can absorb vibrations. When extremely high precision is required, hydrostatic sliding bearings are the only option. Sliding bearings are widely used in precision and high-precision grinders, various precision instruments, etc. 3. Heavy-load applications: Whether they are ball bearings or roller bearings, rolling bearings tend to generate heat and suffer from fatigue in heavy-load situations. Therefore, when the load is relatively large, sliding bearings are often chosen; examples include steel rolling mills, steam turbines, aircraft engine accessories, and mining machinery. 4. In other situations, such as when the operating speed is extremely high, there are severe shocks and vibrations, or when operation is required in water or corrosive media, sliding bearings can also be appropriately selected. For a piece of machinery, rolling bearings and sliding bearings each have their advantages and disadvantages; a reasonable choice should be made based on practical engineering considerations. In the past, large and medium-sized crushers generally used sliding bearings lined with babbitt metal, as they could withstand relatively high impact loads, were quite wear-resistant, and operated smoothly. Small jaw crushers mostly use rolling bearings, which offer high transmission efficiency, greater sensitivity, and ease of maintenance. With the improvement of rolling bearing manufacturing technology, rolling bearings are now also widely used in large jaw crushers.