Plain Cylindrical Journal Bearing

Figure 1: Plain Cylindrical Journal Bearing

Figure 1 shows the plain cylindrical bearing, which was used as an example in the previous video. The plain cylindrical bearing is the simplest design of a fixed geometry fluid film bearing. While there are many different designs of fixed geometry bearings, all are susceptible to instability due to cross-coupling. Cross-coupling causes displacement of the shaft in the direction of the supporting wedge from the oil wedge and displacement perpendicular to the direction of the supporting load in the direction of rotation. Cross-coupling leads to the attitude angle of the rotor during operation.

Figure 2: Circumferential Oil Film Pressure Distribution at (a) 100 rpm and (b) 3,600 rpm

Figure 2 shows the circumferential oil film pressure distribution in a plain cylindrical journal bearing at 100 rpm and 3,600 rpm. Initially, at 100 rpm, the oil film thickness is thin, and the bearing operates at a high eccentricity ratio. In this condition, the oil wedge supports the weight of the rotor over a relatively small area. This results in the narrow pressure distribution shown in Figure 2.a. As the rotor’s speed increases, the eccentricity ratio decreases while the attitude angle increases. This results in a wider area of oil, which supports the shaft leading to the higher pressure distribution seen in Figure 2.b.

Two important observations can be made based on the oil pressure distribution. First, the highest pressure always occurs at the bottom dead center location below the shaft. This high pressure transmits the load from the rotor to the bearing housing. Given this, it is recommended that an FDS sensor be mounted at the bottom dead center location on a plain cylindrical location. Another observation that can be made is that the bearing oil pressure decreases rapidly in the direction of rotation following the minimum film thickness. Given the low expected oil pressure after the minimum film thickness, it is recommended to install a second FDS sensor at 45° in the direction of rotation. This sensor can be used for further troubleshooting to infer that the bearing oil pressure distribution follows the expected distribution. Figure 3 shows the recommended sensor mounting locations based on the bearing pressure distribution.

Figure 4: Plain Cylindrical Bearing 3D - Pressure Profile

Figure 4 shows the three-dimensional pressure profile of the plain cylindrical bearing. From this pressure profile, the maximum oil pressure occurs in the middle of the bearing's width, which is denoted as the Z-direction in the image. It can also be observed that the bearing pressure is not constant across the bearing width but decreases rapidly down to atmospheric pressure at the edge of the bearing.Based on this, the best location to monitor the housing strain would be at the bottom dead center location at the middle of the bearing’s width. However, this is not feasible in practice, so instead, the sensor is mounted on the face of the bearing housing at the bottom dead center location.

Tilt Pad Bearing

Another type of hydrodynamic bearing is a tilt pad bearing. Figure 5 shows two examples of tilt pad bearings. Figure 5.a shows an example of a load over pad bearing, while Figure 5.b shows an example of a load between pad bearing.

Figure 5: (a) Load Over Pad and (b) Load Between Pad Tilt Pad Bearings

Tilt pad bearings utilize pads that have a small amount of rotational freedom about a pivot to support the load from the shaft. Tilt pad bearings are designed with many different arrangements, including different numbers of pads, different preloads, and different pivot designs. All of these different arrangements share one major benefit over fixed geometry bearings. The ability of the pad to tilt provides a reaction force in line with the shaft center, which eliminates cross-coupling. This means tilt pad bearings can operate with zero attitude angle during operation. The lack of attitude angle improves the bearing stability and allows the bearing to operate at a lower eccentricity ratio than a fixed geometry bearing without becoming unstable. This allows tilt pad bearings to offer high damping compared to fixed geometry bearings.

Figure 6: Oil Pressure Distribution of (a) Load Over Pad and (b) Load Between Pad Tilt Pad Bearings

Figure 6 shows the oil pressure distribution of a load over pad and load between pad bearings. Shown in Figure 6.a, the film pressure is highest on the bottom pad, which shows that the majority of the load is supported by the lower pad. As can be seen in Figure 6.b, the lower-left and lower-right pads of the load between pad bearing are where the high film pressure occurs. Based on these pressure distributions, it is recommended that a single FDS sensor be installed at the bottom dead center location of a load over pad bearing. Two FDS sensors are recommended for a load between pad bearing; one below the lower-left pad and a second below the lower-right pad. Figure 7 shows the recommended FDS sensor mounting locations for each style of tilt pad bearing.

Figure 7: Recommended FDS Sensor Mounting Locations on (a) Load Over Pad and (b) Load Between Pad Bearings

Topic of Next Video

Thank you very much for watching this video. The next series of videos will review the recommended installation procedures for the Voyager FDS sensor.