COMSOL 6.4 - Step Thrust Bearing

In this example the pressure distribution in a step thrust bearing is analyzed. A step thrust bearing consists of a stepped bearing surface on which the end of the shaft rotates. The entire assembly is submerged in a lubricant. A six step thrust bearing is considered in this example. The shaft collar is assumed to be spinning without any axial motion in the bearing. The simulation is performed using the Rotordynamics Module’s Hydrodynamic Bearing interface. This interface solves the Reynolds equation to compute the pressure developed in a thin fluid film.

Model Definition

A six-pad step thrust bearing is considered. The inner and outer pad diameters are 0.1 m and 0.2 m, respectively. The grooves span 15° and have a depth of 200 μm.

The bearing is analyzed under various conditions. This includes a parametric study of the angular speed and the film thickness. Here the angular speed and film thickness are varied in the intervals Ω ∈ [500,1000] rad/s and hfilm ∈ [60,160] μm, respectively.

The bearing geometry is shown in Figure 1 below.

Figure 1: Step thrust bearing geometry.

The effect of cavitation is also included in the computation of the fluid-film pressure distribution. The following fluid properties are needed — the dynamic viscosity, the density at cavitation pressure, and the compressibility. The utilized fluid parameters are summarized in Table 1. The selected values are close to those of lubricating oils used in real bearings.

Table 1: Fluid properties.
Property	Value
Density ρ	866 kg/m3
Dynamic viscosityμ	0.072 Pa·s
Compressibility β	10-7 Pa-1

Results and Discussion

Figure 2 shows the fluid pressure profile in the bearing for the highest values of angular speed and film thickness.

Figure 2: Fluid-film pressure profile at Ω = 1000 rad/s with a film thickness of 160 μm.

The resulting vertical force on the collar associated with the pressure distribution shown above is 7500 N. This is the load carrying capacity of the bearing at the specified speed.

The mass fraction of the lubricant, which is a measure of the cavitation, is shown in Figure 3. It is clear that amount of cavitation is very small and is localized near the trailing edge of the pads.

Figure 3: Mass fraction at Ω = 1000 rad/s with a film thickness of 160 μm.

The pressure distribution in radial and circumferential directions on a single pad are shown in Figure 4 and Figure 5, respectively. The pressure at the inner and outer radius locations are zero with the distribution marginally biased toward the outer edge. The velocity of the collar varies linearly from the inner to the outer radius. Since the pressure is proportional to the shear velocity in the film, it is expected to increase linearly from the inner point to the outer point. However, along the inner and outer edge the film pressure is set to zero. Therefore, the maximum pressure occurs toward the outer side relative to the mid position. In the circumferential direction, the pressure suddenly rises at the step location, that is, the leading edge of the pad, and reduces slowly toward the trailing edge.