Measurements of the Static and Dynamic Force Performance on a Five-Pad, Spherical-Pivot Tilting-Pad Journal Bearing: Influence of Oil Flow Rate
Abstract
Tilting-pad journal bearings (TPJB) are commonly utilized to support rotors in high-speed rotating machinery. They can provide significant load capacity, reduced drag power loss, and a stable high-speed operation. Over the years, the design of TPJBs has developed to satisfy various performance needs. However, bearing pad high metal temperatures and drag power loss are a matter of concern to support the growing industry demand for high power capacity and efficient machines. For certain conditions involving lightly loaded bearing and/or evacuated housing configuration, TPJBs operate with a reduced oil flow rate which may cause subsynchronous rotor vibration (SSV) hash. In the past, many investigated the influence of supplied oil flow rate on the bearing steady-state characteristics; however, test data for the impact on its dynamic forced performance are scarce.
This thesis presents and discusses the measurements of the static and dynamic forced performance of a TPJB under load-between-pad (LBP) orientation in the floating bearing test rig. In addition, it also discusses the influence of reduced supply oil flow rate (27% and 50% of nominal) and increased supply oil flow rate (150% of nominal) on the bearing static and dynamic forced properties. The test bearing has the following design characteristics: five pads, ball-in-socket (spherical) pivots, L/D = 0.4, pivot offset = 50%, clearance to radius ratio (Cᵣ/R) ≈ 0.0013, preload = 0.42, and pressurized flooded housing (with end seals). The operating test conditions include six shaft surface speeds (15-85 m/s) and seven specific loads (0.17 to 2.1 MPa). ISO VG 32 lubricates the test bearing with speed-dependent flow rate (100% nominal flow = 3.65 to 19.45 Lit/m) and at supply temperature of 49°C.
The bearing steady-state performance parameters include the journal static equilibrium position, attitude angle, oil temperature rise, pad temperature rise, and drag power loss. The experimental drag power loss is obtained by two methods: (1) a direct measurement of drag torque and shaft angular speed, and (2) an estimation based on the oil flow rate and the oil temperature rise at the bearing exit plane. The dynamic forced performance parameters include rotordynamic force coefficients (stiffness, damping, and virtual-mass) obtained by a multi-frequency dynamic force excitation. Some of the test results are in a dimensionless form to characterize a general trend for TPJBs.
The direct drag power loss is lower by ≈ 20% as compared to the estimated drag power loss for most operating conditions. Interestingly, the measured drag power loss is less dependent on the supplied oil flow rate as compared to the estimated drag power loss. A reduction in oil flow rate up to 50% of nominal magnitude causes an increase in the measured shaft eccentricity, pad temperature up to 6℃ and direct stiffness coefficient up to 5%, and a decrease in measured drag power loss up to 15% and direct damping coefficient up to 7%. For a single case of operation with 27% of nominal flow rate, the pad temperature significantly increases up to 15℃ and direct damping drastically reduces up to 16%; however, the supplied oil temperature increases by 13℃ which falls outside of standard operating conditions. The test bearing operates safely (no significant pad temperature rise) at reduced oil flow rate (by 50% and 73% of nominal) for operation up to 14 krpm (74m/s). Thus, a low oil flow rate only causes a slight degradation in the bearing’s static and dynamic forced performance. Furthermore, an increase in supplied oil flow rate causes a slight increase in drag power loss and direct damping up to 10%, a slight decrease in direct stiffness up to 5%, an insignificant increase in shaft eccentricity up to 9% and pad temperatures up to 3℃.
Lastly, this thesis presents a comparison of the experimental test data with predictions from the XLTPJB® model. The model under predicts the journal eccentricity, over predicts the drag power loss and predicts well the maximum pad temperature (for > 12 krpm). For dynamic forced performance, the model predicts well the stiffness and damping coefficients at low Sommerfeld number (S < 2) and under predicts the force coefficients for S > 2.
Citation
Jani, Hardik Yomesh (2018). Measurements of the Static and Dynamic Force Performance on a Five-Pad, Spherical-Pivot Tilting-Pad Journal Bearing: Influence of Oil Flow Rate. Master's thesis, Texas A&M University. Available electronically from https : / /hdl .handle .net /1969 .1 /188940.