# Cost-Effective Design Modification of a Sleeve Bearing with Large Bearing Clearance
###### Abstract  
In 2015, a 45 MW vertical hydropower machine exhibited excessive vibration after refurbishment. Measurements revealed a substantial bearing clearance at the lower generator guide bearing. Consequently, the bearing was unable to generate sufficient opposing force to drive the rotor toward the bearing center, resulting in more pronounced overall system vibration. Addressing this challenge required a cost-effective and feasible solution for mitigating the vibration problem. To this end, a design modification was implemented wherein the lower generator guide bearing (originally a sleeve bearing) was modified to a four-lobe bearing by offsetting the two halves of the bearing twice in two axes. Numerical simulations and experimentations were conducted, and the dynamics of the machine before and after the design modification were investigated. Both the simulation and experimental results showed that the machine with the four-lobe bearing improved the system stability and reduced the vibration amplitudes. The numerical simulation result demonstrated that, due to the design modification, the first and second critical speeds were effectively eliminated for a speed range of up to three times the nominal speed. Furthermore, for nominal operation with unbalanced magnetic pull, the four-lobe bearing provided a stability advantage in terms of the modal parameters relative to the original sleeve bearing. hydropower; four-lobe bearing; sleeve bearing; journal bearing; vertical rotors; rotordynamics; bearing coefficient 2024 14 1  
performance compared with the symmetric bearing. [PERSON] [12] presented an extensive literature review of non-circular bearings and discussed the results from theoretical and experimental investigations. In addition, the potential research gaps were identified and suggested for valuable insights and further investigations. [PERSON] and [PERSON] [13] studied the stability of a submerged four-lobe bearing and compared the results with those of plain cylindrical bearings under similar conditions. The authors reported that the trajectory of the journal center was damped out in the case of the four-lobe bearing. On the contrary, the motion exhibited significant excursion before reaching stability for the plain cylindrical journal bearing. Vertically oriented rotors operate under light or no radial static load conditions because the weight acts axially. This typical characteristic of vertical machines makes them susceptible to oil-whirl instability. [PERSON] et al. [14] studied the rotordynamic characteristics of a vertical twelve-stage multiphase pump, considering both small and large bearing clearances. The results indicated that the large bearing clearance caused more pronounced vibration due to sub-synchronous instability and critical speeds within the operation range. [PERSON] and [PERSON] [15] studied the field vibration measurements of several large vertical pumps experiencing significant wear in the impellers, wear rings, and seals. This wear was attributed to the operating speed closely aligning with the natural frequency of the pump-rotor system. To address the vibration problem, the authors proposed some potential solutions. [PERSON] et al. [16] investigated the rotodynamic instability in a vertical pump employed in the nuclear waste processing industry. The bearings, which were originally plain cylindrical bearings, were replaced by tilting pad journal bearings to resolve the vibration problem due to whirling. [PERSON] et al. [17] examined the stability of a vertical sulfur pump undergoing sub-synchronous vibrations caused by \"sulfur whirl\" at nearly 0.5 times the operation speed. The authors reported that replacing the existing plain bushings with the three-lobe bearing improved the system's dynamics, reducing the sub-synchronous vibration and increasing the onset speed of instability. Experimental tests were later conducted by [PERSON] and [PERSON] [18] on bearings with a geometry similar to that described in [17].