Why do most mining machinery choose rolling bearings instead of sliding bearings?

As an indispensable and important component in mechanical products, bearings play an important role in supporting rotating shafts. According to the different friction properties in the bearing, the bearing is divided into rolling friction bearing (referred to as rolling bearing) and sliding friction bearing (referred to as sliding bearing). The two types of bearings have their own characteristics in structure, and each has its own advantages and disadvantages in performance.

Comparison of rolling and plain bearings

1. Comparison of structure and movement mode

The most obvious difference between rolling bearings and plain bearings is the presence or absence of rolling elements.

Rolling bearings have rolling elements (balls, cylindrical rollers, tapered rollers, needle rollers) that rely on their rotation to support the rotating shaft, so the contact part is a point, and the more rolling elements, the more contact points.

Plain bearings have no rolling elements and rely on smooth surfaces to support the rotating shaft, so the contact part is a surface.

 

The difference in the structure of the two determines that the movement mode of the rolling bearing is rolling, and the movement mode of the sliding bearing is sliding, so the friction situation is completely different.

 

2. Comparison of carrying capacity

In general, due to the large bearing area of the sliding bearing, its bearing capacity is generally higher than that of the rolling bearing, and the rolling bearing's ability to bear the impact load is not high, but the completely liquid-lubricated bearing can bear a large impact load due to the role of cushioning and vibration absorption due to the lubricating oil film. When the rotational speed is high, the centrifugal force of the rolling elements in the rolling bearing increases, and its load-bearing capacity is reduced (noise is prone to occur at high speeds). In the case of dynamic plain bearings, their load-bearing capacity increases with higher speeds.

 

3. Comparison of friction coefficient and starting friction resistance

Under normal working conditions, the friction coefficient of rolling bearings is lower than that of plain bearings, and the value is more stable. The lubrication of sliding bearings is easily affected by external factors such as speed and vibration, and the friction coefficient varies widely.

 

At start-up, the resistance is greater than that of the rolling bearing because the sliding bearing has not yet formed a stable oil film, but the starting friction resistance and working friction coefficient of the hydrostatic sliding bearing are very small.

 

4. Comparison of applicable working speeds

Due to the limitation of the centrifugal force of the rolling element and the temperature rise of the bearing, the speed of the rolling bearing cannot be too high, and it is generally suitable for medium and low speed working conditions. Incomplete liquid lubricated bearings due to the heating and wear of the bearing, the working speed should not be too high. The high-speed performance of fully liquid-lubricated bearings is very good, especially when the hydrostatic plain bearings are lubricated with air, and their rotational speeds can reach 100,000 r/min.

 

5. Comparison of power loss

Due to the small friction coefficient of rolling bearings, their power loss is generally not large, which is less than that of incomplete liquid lubricated bearings, but it will increase dramatically when lubricated and installed properly. The frictional power loss of fully liquid-lubricated bearings is low, but for hydrostatic plain bearings, the total power loss may be higher than that of the hydrostatic plain bearings due to the loss of oil pump power.

 

6. Comparison of service life

Due to the influence of material pitting and fatigue, rolling bearings are generally designed for 5~10 years, or replaced during overhaul. The pads of incomplete liquid-lubricated bearings are severely worn and need to be replaced regularly. The life of fully liquid-lubricated bearings is theoretically unlimited, but in practice fatigue failure of the bearing material can occur due to stress cycling, especially for dynamic plain bearings.

 

7. Comparison of rotation accuracy

Rolling bearings generally have a high rotation accuracy due to the small radial clearance. The incomplete liquid lubricated bearing is in the state of boundary lubrication or mixed lubrication, and the operation is unstable, and the wear is serious, and the accuracy is low. Due to the presence of oil film, the fully liquid-lubricated bearing cushions and absorbs vibration with high accuracy. Hydrostatic plain bearings have higher rotation accuracy.

 

8. Comparison of other aspects

Rolling bearings use oil, grease or solid lubricant, the amount is very small, the amount is large at high speed, the cleanliness of the oil is required to be high, so it is required to be sealed, but the bearing is easy to replace, and generally does not need to repair the journal. For plain bearings, in addition to incomplete liquid lubrication bearings, the lubricant is generally liquid or gas, the amount is very large, the oil cleanliness requirements are also very high, the bearing pads need to be replaced frequently, and sometimes the journal is repaired.

 

Selection of rolling bearings and plain bearings

Due to the complex and diverse actual working conditions, there is no unified standard for the selection of rolling bearings and sliding bearings. Due to the small friction coefficient, small starting resistance, sensitivity, high efficiency, and standardization, rolling bearings have excellent interchangeability and versatility, and are convenient to use, lubricate and maintain, and are generally given priority in selection, so they are widely used in general machines. Plain bearings themselves have some unique advantages, which are generally used in some occasions where rolling bearings cannot be used, inconvenient or without advantages, such as the following occasions:

 

1. The radial space size is limited, or the installation must be divided

Due to the inner ring, outer ring, rolling element and cage in the structure, the radial size of the rolling bearing is large, and the application is limited to a certain extent. Needle roller bearings are available when radial dimensions are strict, and if necessary, plain bearings are required. For parts that are inconvenient to have bearings, or cannot be mounted from the axial direction, or where parts must be split into parts, split plain bearings are used.

 

2. High-precision occasions

When the bearing used has high precision requirements, the sliding bearing is generally selected, because the lubricating oil film of the sliding bearing can buffer vibration absorption, and when the accuracy is extremely high, only the hydrostatic sliding bearing can be selected. For precision and high-precision grinding machines, various precision instruments, etc., sliding bearings are widely used.

 

3. Heavy load occasions

Rolling bearings, whether ball bearings or roller bearings, are prone to heat and fatigue in heavy-duty situations. Therefore, when the load is large, sliding bearings are mostly used, such as rolling mills, steam turbines, aero engine accessories and mining machinery.

 

4. Other occasions

For example, the working speed is particularly high, the shock and vibration are extraordinarily large, and the need to work in water or corrosive media, etc., sliding bearings can also be reasonably selected.

 

For a kind of machinery and equipment, the application of rolling bearings and sliding bearings, each has its own advantages and disadvantages, and should be reasonably selected in combination with the actual project. In the past, large and medium-sized crushers generally used sliding bearings cast with babbitt, because they could withstand large impact loads, and were more wear-resistant and stable. The small jaw crusher is mostly used with rolling bearings, which has high transmission efficiency, is more sensitive and easy to maintain. With the improvement of the technical level of rolling bearing manufacturing, most of the large jaw breakers are also used in rolling bearings.