Floating Ring Bearing
The floating ring bearing (FRB) is an extension of the plain journal bearing. An FRB typically consists of a journal, a bushing and a ring inserted between the journal and the bushing. As opposed to the plain journal bearing, in floating ring bearings there are two oil films formed in the bearing, the first between the journal and the ring (called inner film) and the second between the ring and the bushing (called outer film). The ring could be full-floating or semi-floating. In the full-floating case, the ring is free to rotate about the bearing axis due to the hydrodynamic shear forces on the ring. In semi-floating case, the axial rotation of the ring is suppressed, while it is free to perform in-plane motion between the journal and bushing. In the semi-floating case, the outer film does not have any significant spring action, but it provides damping to the ring motion.
Figure 7-16: Sketch of a floating ring bearing.
A complete description of the floating ring bearing requires the pressure distribution in both inner and outer films. It can be obtained by solving Reynolds equations for both films using the motion of journal, ring and bushing. However, the pressure distribution in these films cannot be treated independently. The inner and outer films are generally connected through oil channels in the ring. The location of the oil channels relative to the inner and outer film keeps changing due to axial rotation of the ring. Specifying this connection for the pressure distribution in both the films in the stationary reference frame is difficult due to the changing locations of the channels. Therefore, for time-dependent calculations in the FRBs, the pressure distribution equations are formulated in a reference frame fixed to the ring. The advantage of such a description is that locations of the oil channels do not change in this reference frame. In the ring reference frame, the bushing rotates with an angular velocity -Ωr, and the shaft rotates with an angular velocity Ω-Ωr. Ωr is the angular velocity of the ring and Ω is the angular velocity of the journal in the stationary reference frame.
Reynolds Equation for the floating ring bearing takes the following form in the ring reference frame:
where h is the film thickness and vavg is the average velocity in the film. Expressions for film thickness and average velocity in both films are given below.
Inner film:
Outer film:
ρ and μ are the density and viscosity of the lubricant, respectively. vj, vr and vb are the velocities of the journal, ring and bushing, respectively, measured in the ring reference frame and are given by
Similarly, the displacement of the journal, ring, and bushing has to be referred to the ring frame of reference. In general, the relation between the displacement in the stationary and ring frames of reference is given by
where subscript s denotes the quantity in the stationary frame.