Fluid Film Bearing Operating Theory, Part 1

Alexander M. Tomsick, P.E. - Director of Engineering

This video is part of a series of videos being produced to talk about the Voyager FDS sensor. This video provides an introduction to the operating theory of hydrodynamic journal bearings.

Figure 1: Plain Cylindrical Journal Bearing

The bearing shown in Figure 1 represents a plain cylindrical journal bearing which will be the example case for this overview. The dark gray section in the Figure represents the bearing, the light gray section represents the shaft, and the orange section represents the oil that circulates through the bearing.

As the shaft rotates, viscous forces between the shaft and the oil draws oil beneath the shaft into the converging oil flow section. As the oil clearance in the converging section decreases, the oil pressure increases below the shaft and develops a high-pressure oil wedge that supports the shaft as it rotates, much like a tire hydroplaning on water.

As the circulating oil reaches the minimum oil film thickness region, the oil moves into the diverging oil flow section. This leads to a drop in oil pressure and an increase in oil velocity as it circulates through the bearing into the converging section. This oil flow spirals circumferentially from the center of the bearing before leaking out of the ends of the bearing and reaching ambient pressure as it exits the bearing. Since oil is lost from the edges of the bearing, new oil must be replenished from the oil supply system.

There are two important pieces of nomenclature that must be discussed that relate to the operation of fluid film bearings.

Eccentricity Ratio

The first term to discuss is the eccentricity ratio of the bearing. Eccentricity ratio is the ratio of the distance between the shaft center and the bearing center in a straight line (r) divided by the bearing's total diametral clearance (2Cb), where (Cb) is the bearings, total radial clearance calculated by subtracting the shaft radius from the bearing radius. Equation 1 shows the calculation of the eccentricity ratio.

The eccentricity ratio can vary from a value of 0 to a value of 1. A value of 0 occurs when the shaft is centered in the bearing bore, while a value of 1 occurs when the bearing clearance is taken up and the shaft touches off on the inside diameter of the bearing.A typical target value of the eccentricity ratio for stable bearing operation is 0.7.

Attitude Angle

The second term to discuss is attitude angle. Attitude angle is the angle from vertical to the line between the bearing and shaft centers, shown by the dotted line in Figure 1. Attitude angle exists due to cross coupled forces differing between the horizontal and vertical direction causing the rotor to move in the direction of rotation until reaching an equilibrium position during a startup event.

Bearing Differences

After discussing the basic operating theory, it is beneficial to discuss the advantages and disadvantages of fluid film bearings compared to rolling element bearings.

Fluid film bearing:

• Have a high load carrying capacity
• Are capable of operating at high speeds
• Have a theoretically infinite life while operating
• Can provide high stiffness
• Can provide high damping
• Have a short time to failure following the loss of oil feed

Rolling element bearings:

• Can support radial and axial loads
• Have a limited operating speed range
• Have a shorter theoretical life
• Have high stiffness
• Have low damping
• Have a long time to failure once the initial point of failure occurs

Oil Film Development

Figure 2: Stribeck Curve

Figure 2 shows the Stribeck curve, which plots friction against the ratio of fluid viscosity and shaft speed divided by load. This ratio is also known as the ZN/P value. If we first assume that the oil viscosity and bearing load are held constant while speed is varied, we can see that as the shaft first starts rotating, the bearing operates in the thin film region of the Stribeck curve. While in the thin film region, surface imperfections on the shaft and bearing surfaces are not fully separated and high friction exists. This high friction that occurs during the initial startup causes wear of the bearing babbitt. As speed is increased the operating point on the curve will move to the right and the friction decreases. The increasing shaft speed pushes the operating point into the partial lubrication region where the minimum friction occurs.

As speed is increased further, the operating point moves further to the right into the thick film lubrication region and friction increases in increasing speed. In this region, the oil film fully separates the shaft and bearing surfaces. While the friction is higher in the thick film lubrication region, it is more advantageous to operate in this region to give a larger buffer from the thin film lubrication region. If the bearing operates in the thick film lubrication region, speed, viscosity or load can increase or decrease while the shaft is still supported by the oil wedge. If viscosity increases or load decreases at a constant speed, the operating point on the Stribeck curve would shift to the right. Alternatively, if viscosity decreases or load increases the operating point would shift to the left. This allows the bearing to remain stable following variability in these operating parameters, allowing the bearing to remain in the thick film or hydrodynamic lubrication regime.

Figure 3: Equilibrium Locus Plot

Figure 3 shows the equilibrium locus plot of a plain cylindrical journal bearing. The X- and Y-axes in the Figure are plotted using eccentricity in the X- and Y-directions. This plot shows the theoretical equilibrium operating point of the shaft in the bearing from shaft speeds of 100 rpm to 3,600 rpm. As the shaft speed increases the minimum film thickness increases. Given this increase, the eccentricity ratio decreases while the attitude angle increases.

Topic of Next Video

Thank you very much for watching this video. The next video in this series will discuss fluid film bearings in further detail, including a discussion on other bearing designs and the expected oil pressure distribution in these bearings.