Abstract
Rolling‐element bearing forces vary nonlinearly with bearing deflection. Thus, an accurate rotordynamic analysis requires that bearing forces corresponding to the actual bearing deflection be utilized. For this work, bearing forces were calculated by COBRA‐AHS, a recently developed rolling‐element bearing analysis code. Bearing stiffness was found to be a strong function of bearing deflection, with higher deflection producing markedly higher stiffness. Curves fitted to the bearing data for a range of speeds and loads were supplied to a flexible rotor unbalance response analysis. The rotordynamic analysis showed that vibration response varied nonlinearly with the amount of rotor imbalance. Moreover, the increase in stiffness as critical speeds were approached caused a large increase in rotor and bearing vibration amplitude over part of the speed range compared to the case of constant‐stiffness bearings. Regions of bistable operation were possible, in which the amplitude at a given speed was much larger during rotor acceleration than during deceleration. A moderate amount of damping will eliminate the bistable region, but this damping is not inherent in ball bearings.
Highlights
Rotordynamic response of all but very flexible rotors depends strongly on bearing properties
Unbalance response data was presented for a rotor supported on ball bearings with accurate bearing stiffness calculated as a function of speed and load
Bearing stiffness was found to be a strong function of bearing deflection, with higher deflection producing markedly higher stiffness
Summary
Rotordynamic response of all but very flexible rotors depends strongly on bearing properties. Up-to-date stress/life data, accurate stress calculations considering bearing installation press fits, and an interactive front end are some of its features It accounts for 5 degrees of freedom in the bearing, and can calculate load-displacement data for high-speed radial and angular contact ball bearings, and for cylindrical and tapered roller bearings. Nonlinear bearing data from COBRA-AHS is used with a steady-state unbalance response code to calculate. One of the early and still viable codes was formulated by Lund [5] It uses the transfer-matrix method to calculate unbalance response of a flexible rotor in asymmetric bearings. This code was simplified for the case of symmetric bearings by Kirk [6]. Kirk’s code was further modified for the present work to enable the use of nonlinear bearings by iterating on the rotor amplitude
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