The oil film sliding bearing used in the turbo compressor belongs to the dynamic pressure bearing category, which relies on the rotation of the journal (or thrust disc) itself to bring lubricating oil between the journal (or thrust disc) and the bearing shell, forming a wedge-shaped oil film. Under the compression of the load, the oil film pressure is established to withstand the load, as shown in Figure 1.
Figure 1 Principle of oil film sliding bearing
To illustrate the load-bearing capacity of the wedge-shaped oil film, taking the thrust bearing oil wedge as an example, first assume that the tile is not tilted and remains parallel to the thrust disc. Due to the rotation of the thrust disc, oil will naturally be carried into the gap between the tile and the thrust disc, forming an oil film. The amount of oil brought in by the rotation of the thrust disc will also be affected by how much oil is taken out. If pressure is applied to such an oil film with the help of a thrust disc, the oil will be squeezed out of the gap. Of course, after being squeezed, the oil will not only be squeezed out from the outlet, but also from the inlet. As a result, compared with when no pressure is applied, the average speed of the oil flowing in from the inlet is smaller than that flowing out from the outlet, and the amount of oil entering the gap is inversely proportional to the flow rate flowing out, so this oil film cannot last long. If the area of the oil inlet is appropriately increased, it will converge with the flow direction. Under the action of the load, the amount of oil entering and exiting will be kept equal, which can maintain a stable oil film, generate sustained oil film pressure, and bear the load of the bearing.
From this simple analysis, it can be seen that in order to make the oil film stable and have load-bearing capacity, the oil gap must be wedge-shaped, with a large inlet and a small outlet; Secondly, the thrust disc (or journal) has a relative velocity to the tile block; Thirdly, oil has a certain viscosity. The oil film of the thrust bearing and the supporting bearing mentioned earlier both have these characteristics and can therefore function as bearings. The pressure in the bearing oil film is not consistent at all locations, as shown in Figure 1 (b). The pressure gradually increases along the lower half of the bearing oil film from the oil wedge inlet to the Z large pressure pmax, and then gradually decreases. The task of supporting bearings is to form an oil film pressure under certain load P, speed, and oil supply conditions, bear the load P, maintain a certain Z small gap hmin between the journal and the bearing shell, and the oil temperature should not be too high.
The formation of bearing oil film and the magnitude of oil film pressure are influenced by factors such as shaft speed, oil viscosity, bearing clearance, bearing load, and bearing structure. Generally, the higher the rotational speed, the higher the viscosity of the oil, and the more oil is carried in. The greater the oil film pressure, the greater the load it can bear. However, excessive viscosity of oil can cause uneven distribution, increase friction loss, and fail to maintain good lubrication effect. Excessive bearing clearance is detrimental to the formation of oil film and increases oil consumption; Being too small will result in insufficient oil and fail to meet the cooling requirements of the bearings. Excessive load can make it difficult to form an oil film, and when the bearing capacity is exceeded, the bearing shell will burn out. For supporting bearings, the ratio of the length L to the diameter d of the bearing also has a significant impact on the bearing capacity of the bearing, The larger the L/d, the greater the carrying capacity. But if the L/d is too large, the lubricating oil is not easy to flow away from the shaft end, causing the bearing temperature to rise. Moreover, due to manufacturing and installation errors, the inevitable shaft deviation causes excessive edge pressure at the bearing end, resulting in serious wear and fatigue damage. Therefore, an excessive L/d is not beneficial. Generally, the L/d of round pad bearings and tilting pad bearings are 0.6~1.0 and 0.4~0.6, respectively.
Turbocompressors commonly use round pad bearings, and later gradually adopt elliptical pad bearings, multi oil wedge bearings, and tilting pad bearings.
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