Dynamic and thermal modeling of touch-down bearings considering bearing non-idealities

Abstract

In an active magnetic bearing (AMB) supported rotor, the touchdown bearings support the rotor in case of an electromagnetic field failure or a fault in the control system. In this work, a dynamic and thermal model of touchdown bearings was studied, and non-idealities in the bearing were taken into account. The effect of off-sized ball/balls on the stress in the touchdown bearing was investigated. The Hertzian contact model was applied to study the stress in the touchdown bearing. The results show that off-sized balls affect the load distribution in the bearing. Therefore, the contact forces and stresses change. The outcome indicates that stress in the touchdown bearing is affected by the dimension and location of off-sized ball.

Furthermore, heat generation due to the impact and collision of the rotor and touchdown bearing, in addition to the internal friction in the bearing, can raise the temperature of the touchdown bearing. The effect of different orders and amplitude of surface waviness in an angular contact ball bearing type of touchdown bearing were investigated. An equivalent electrical circuit model was used for the thermal model of the touchdown bearing. It was found that the surface waviness of the bearing alters the contact force and friction loss in the touchdown bearing. The effect of a higher amplitude of surface waviness on the thermal expansion of the touchdown bearing was also studied.

In addition, an experimental and numerical study on the dropdown of a rotor is presented. The recorded behavior of the rotor in the sudden failure of the electromagnetic field is demonstrated. A comparison between simulation results and dropdown test data can help the further development of the model. This was achieved in this study by considering the dropdown test data to determine the initial position of the rotor at the start of dropdown as well as the support properties. The machine was not equipped with force sensors for the touchdown bearing. Non-contact displacement sensors were provided for monitoring the displacement of the rotor, which make it difficult to evaluate a safe dropdown. The model was verified by comparing the rotor orbits in the simulation and measurements. The Fast Fourier Transform (FFT) was applied to the measured displacement of the rotor and revealed the harmonics of the operation speed, rub-impact frequencies, pendulum motion of the rotor, as well as the first two bending frequencies of the rotor as it settled on the touchdown bearings. A critical speed map justified the bending frequencies of the rotor and identified the support properties. The study enables the comparison of a single dropdown in relation to a previous drop.