Implications of Axial Displacement on the Stacked Ring Configuration of Permanent Magnetic Bearings (PMB) in a Horizontal Overhung System

Abstract

A stacked ring configuration of Permanent Magnetic Bearings (PMB) in a horizontal overhung system demonstrates axial instability and exhibits sensitivity to geometric imperfections. Backers's research model investigates the effects of maximum radial displacement; however, it is limited in its capacity to address deficiencies related to determining stiffness for axial displacement. Since then, extensive research has employed various models to bridge the gaps in understanding axial stiffness. Despite advancements in the study of stacked ring configurations of PMBs for horizontal overhung systems, current designs frequently suffer from low axial stiffness, resulting in excessive rotor vibration, inherent instability, and restricted application potential. Most studies indicate that axial instability can be mitigated through the use of bearing supports, such as electro-mechanical servo mechanisms; however, further knowledge is required to optimize design parameters for effective axial displacement control. Therefore, there is a critical need to develop a novel PMB configuration that maximizes axial stiffness while minimizing reliance on bearing supports for axial displacement management. A rotor system is constructed incorporates an N35 grade axially magnetized neodymium magnet within a cartridge assembly, along with a rotating shaft sleeve and setting plates designed for a non-metallic steel forging step shaft. A complete hybrid overhung rotor design features a marine-grade thrust bearing housing with a split industrial Plummer block housing for a set of C3 angular contact ball bearings. Results from experiments reveled a stable linearity of air gaps for the range of ring thickness-wavelength ratio d/l size up to 1.06. Conversely, reverse effect to the ring thickness-wavelength ratio (d/l) below than 1 enhances the threshold of axial displacement. It can be concluded that axial displacement induces a sixfold reduction in the magnetic wavelength (λ), influences the size of the PMB according to the ring thickness-to-wavelength ratio (d/λ), and preserves stable PMB stiffness up to a ratio of 1.06. This stability offers an advantage in achieving an improved axial displacement threshold when the PMB thickness-to-wavelength ratio (d/λ) is less than 1.