Analysis of Oil Lubricated Thrust Collars for Application in Integrally Geared Compressors
MetadataShow full item record
Integrally geared compressors (IGCs) comprise of single stage impellers installed on the ends of pinion shafts, all driven by a main bull gear (BG) and shaft system. When compared to single shaft multistage centrifugal compressors, the benefits of IGCs include better thermal efficiency, reduced footprint and simple foundation, dispensing with a high speed coupling, as well as better access for maintenance and overhauls. In IGCs the compression of the process gas induces axial loads on the pinion shafts that are transmitted via thrust collars (TCs) to the main drive shaft and balanced by a single thrust bearing. The TCs, located on either side of pinion gears, slightly overlap with the BG outer diameter to form lentil-shaped lubricant-wetted regions. A numerical model, based on classical thin film lubrication theory, predicts the force response of lubricated thrust collars for use in integrally geared compressors. The predictive model determines performance parameters such as lubricant flow rate, mechanical power loss, peak pressure, lubricant temperature rise, as well as rotordynamic stiffness and damping coefficients for a lubricated TC and bull gear pair. A Newton-Raphson based iterative procedure determines an equilibrium operating position for a given set of TC and BG operating conditions and geometry. Periodic, dynamic displacements from the equilibrium position renders complex dynamic stiffnesses (H = K+iωC), from which the fluid film force and moment stiffness and damping coefficients are determined. In a lubricated thrust collar and bull gear, a hydrodynamic pressure builds in the lower half of the lubricated zone and lubricant cavitation occurs in the upper half. The minimum film thickness and peak pressure in the lubricated zone shift location due to the difference in taper angles between the TC and BG surfaces. For a given applied load, a study on the differences in taper angle between the TC and BG surfaces reveals that current angular tolerances of ±0.1 produce TC/BG pairs with similar performance parameters (power loss, lubricant temperature rise, etc.) and dynamic force and moment stiffness and damping coefficients. Increasing the taper angles of both the TC and BG decreases mechanical power loss and lubricant temperature rise, but also decreases the fluid film axial stiffness and damping coefficients. In addition, static angular misalignments of the TC and BG about the horizontal (x) axis joining the BG and TC centers alters the shape and extent of the lubricant cavitation region. This change alters the load carrying capacity and mechanical power losses of the lubricated element. For the specific TC/BG pair investigated herein, there exists a maximum difference between the two taper angles for which the mechanical element can support an imposed thrust load. Increasing the speed of the BG (and proportionally the TC speed) increases the mechanical power loss and lubricant temperature rise and decreases the fluid film axial stiffness and damping coefficients. As with most fluid film bearings, increasing the applied load increases the power loss, lubricant temperature rise, and axial stiffness and damping coefficients. The thesis delivers a predictive tool, yet to be benchmarked against experimental data, that provides insight to both the static and dynamic force performance of a lubricated thrust collar, not currently in the archived literature.
Cable, Travis Alexander (2015). Analysis of Oil Lubricated Thrust Collars for Application in Integrally Geared Compressors. Master's thesis, Texas A & M University. Available electronically from