DIAGNOSTICS OF SELF-ALIGNING ROLLING-ELEMENT BEARING

Abstract

Rolling element bearings are the essential components of every rotating machinery. They are primarily used to support the rotating components carrying static or dynamic loads and thus find their application in areas like power plants, automotive industry, chemical industry, agriculture equipment’s, aerospace industry etc. The idea of these bearings was conceived long back and since then a lot of research and development have taken place to improve their performance and life. Depending upon the requirements different kind of rolling element bearings were designed e.g. deep groove ball bearings, spherical rolling element bearings, angular contact bearings, roller bearings etc. The rolling element bearings play a vital role in characterizing the dynamic behaviour of the system as they themselves act a prime source of vibrations, due to any imperfection. Rolling element bearings can generate vibration even when they are free from any imperfections. These vibrations are parametric in nature and are referred to as varying compliance (VC) vibrations. These VC vibrations are time varying in nature and occur as a result of to the contact deformation due to the finite number of equally spaced loaded roiling elements passing over the raceways. These deformations result in stiffness variation at the contact spots and thus the whole bearing suffers a time varying stiffness variations. The time varying stiffness variations are responsible for VC vibrations which are parametrically excited in nature. The other sources of vibration in rolling element bearings are internal clearance, unbalanced rotor, misalignment and surface imperfections. The imperfections can be either distributed or localized in nature To analyze the dynamic behaviour of a rolling element bearing a mathematical model is developed incorporating the effect of various sources of vibrations. The model is based on the Hertzian contact deformation theory accounting the effect of rotor unbalance, misalignment of raceways and the surface imperfections. The contact between the rolling elements and the raceways is modelled using the nonlinear spring and dampers. Further the governing equations of motion were developed and were solved numerically, using the 4th order Range-Kutta method. The results showing the dynamic behaviour of the bearing were analyzed using the Fast Fourier transform, phase trajectory, orbit plots and Poincare maps. Other advanced signal processing techniques like spectral kurtosis, Hilbert spectrum and parameter optimized variational mode decomposition (VMD) method were also used during the analysis. The rolling element bearing considered in this study is SKF’s 1205 ETN9 which is a double-row self-aligning ball bearing (DRSABB). In order to validate the results from numerical simulations, an experimental viii investigation was also performed using two test-rigs, where the healthy and defective DR-SABB was tested for varying rotor speed, radial load and misaligned conditions. The vibration signals from the test-rig were acquired using two uniaxial contact type accelerometers and a data acquisition (DAQ) system. For solving and analyzing the mathematical model’s governing equations of motion as well as for analyzing the experimental data, the MATLAB environment is used. From the observation made during the analysis it was observed that the periodicity of the response increased with the increase in rotor speed and radial load values. Also there were observed certain values of speed and loads where the response of system changed. A similar behaviour was also observed from the experimental analysis. Secondly, the amplitude vibrations associated with the waviness of the raceway surfaces and having the wave pass frequency (WPF) was observed to be maximum at the waviness order value of 24 i.e. equal to the number of rolling elements. Also their amplitude was significant at waviness order of 12 and 36 i.e. at the multiples of number of rolling elements in each row. Apart from all these values of waviness order the amplitude of vibration was very less. The response of the system was observed to be periodic with predominance of WPF at waviness order values of 12, 24 and 36, while at other vales the system response lost its periodicity and was quasi-periodic in nature. Thirdly, in case of misalignment of the raceways, it was observed the during the misalignment one of the two rows of rolling elements carried mores load that the other row and thus the contact load variations were also observed among the two rows. Also due to the misaligned orientation of the raceways, the trajectory of the rolling element while passing over a rectangular spall also varied. Finally, an attempt was made to develop an autonomous system for diagnosing the localized defects in the inner and outer raceways along with their misalignment. For this harmonic product spectrum along with its improved version was used and the proposed system was found to accurately predict the localized defects and misalignment in the DR-SABB.