Effect of Lubricant Supply Pressure on SFD Performance: Ends Sealed with O-rings and Piston Rings

A well-designed SFD must deliver enough damping to aid in decreasing rotor amplitudes of motion. Piston rings (PRs) and O-rings (ORs) are commonly used as end seals in dampers for commercial and military gas turbine engines, respectively. The paper details dynamic load tests conducted on a short length SFD (L/D = 0.2) sealed with either (a) PRs or (b) ORs and the experimentally estimated damping and inertia force coefficients. Lubricant (ISO VG2) flows thru one feedhole at the land middle plane with supply pressure increasing from 0.7 bar(g) to 6.2 bar(g). In the PR-SFD, oil leaves the film land through the rings’ abutted ends making a slit. The OR-SFD effectively seals any leakage; hence, lubricant flows out through a discharge hole at a location halfway of the film (upper) land length. Multiple sets of single frequency (10 Hz–100 Hz) dynamic loads produced circular centered orbits with amplitude (r) equal to 30% of the radial clearance. For both PR-SFD and OR-SFD, the viscous damping coefficient diminishes quickly as the lubricant supply pressure drops below 3 bar(g). The added mass coefficient, on the other hand, remains nearly constant for the PR-SFD and slightly increases for the OR-SFD. The OR-SFD delivers ~10% more viscous damping than the PR-SFD albeit it demands of a larger flow rate. Analysis of the recorded film dynamic pressures shows their peak-peak magnitude increases with whirl frequency. However, operation at the lowest oil supply pressure, 0.7 bar(g), generates film peak pressures not increasing as the excitation frequency rises, thus evidencing the presence of air ingestion and entrapment, as vividly shown by recorded film dynamic pressure waves, in particular for the PR-SFD.