On the Controllability and Observability of Actively Lubricated Journal Bearings with Pads Featuring Different Nozzle-Pivot Configurations

The fundamental properties of an actively lubricated bearing (ALB) from a control viewpoint are investigated, i.e., the stability, controllability and observability. The ALB involves the addition of an oil injection system to the standard tilting-pad journal bearing (TPJB) to introduce constantly and/or actively high pressurized oil into the rotor-pad gap through, commonly, a single radial nozzle.

For the work goal, a four degrees-of-freedom (DOFs) ALB system linking the mechanical with the hydraulic dynamics is presented and studied, comprising: (i) the vertical journal movement, (ii) the pad tilt angle, (iii) the vertical pad movement-due to the pivot flexibility, and (iv) the controllable force as the hydraulic DOF.

The test rig consists of a rigid rotor supported by a single rocker-pivoted rigid pad. A thorough parametric study is carried out by investigating the effects of: (a) nozzle-pivot offset, (b) pivot flexibility, and (c) bearing loading on these control basics in order to determine the pad with the best control characteristics.

Different nozzle-pivot offsets can be set by varying the positioning of either the injection nozzle or the pivot line. The influence of the pivot compliance on the bearing dynamics is assessed by benchmarking the results obtained with the flexible pivot against the rigid pivot. Three different bearing loads are studied.

According to the results, the proposed configurations, especially the offset-pivot pad with slight offsets, improve the bearing control characteristics by introducing an extra mechanism to access the system states. The loading condition modifies the stability, controllability, and observability, while the pivot flexibility highly affects the ALB dynamics.