Transient Journal Bearing Orbits Using ANSYS
By Bill Kelly
Historically, analysts have made use of specialty rotordynamic codes to predict rotor shaft critical speeds and imbalance forced response of shafts for rotating machinery such as pumps, compressors, and turbines. Over recent years, rotordynamic functionality has been incorporated into general purpose finite element codes such as ANSYS. For example, plain and tilting pad journal bearings are characterized by their stiffness and damping. The ANSYS COMBI214 bearing element accepts the direct and cross-coupled bearing stiffness and damping terms as input and can solve static, modal, and forced response analyses.
I’d like to discuss some relatively new functionality of this COMBI214 element that many rotordynamicists may not be aware of. This element can now be used to predict static eccentricity and transient orbits of plain journal bearings (KEYOPT(1) ≠ 0). This feature was introduced in ANSYS Version 17.0 in 2016. It takes shaft rotational velocity, lubricant viscosity, bearing clearance, journal radius, and bearing length as inputs. It calculates the pressure distribution acting on the journal by solving the Reynolds Equation at each step. For static loading, it can therefore determine the rotor position where the pressure distribution balances out the external forces on the journal. For dynamic loading, such as from a rotating imbalance, it can predict the dynamic orbit of the journal. This is important because there are times when the orbit is large. A large orbit could result in significant changes in the instantaneous bearing stiffness and damping.
The figure shows orbit predictions using this element resulting from a steadily increasing imbalance load. It’s common practice in rotordynamic analysis to assume that dynamic journal motion is small relative to the static offset and that a constant/linearized stiffness/damping value is appropriate. This element allows for a more rigorous treatment of the bearing behavior, albeit limited to plain journal bearings.
A good example of where this feature is useful are reciprocating engine crankshaft bearings. Bearing loads vary dramatically in a reciprocating engine from inertial forces of connecting rods and combustion forces. This leads to large noncircular journal orbits that defy typical linearized rotordynamic calculations. The internal solution of the Reynolds Equation is an efficient means of accounting for the hydraulic pressures in the oil film as opposed to performing a CFD analysis at every time step and mapping those pressures onto a structural model in a stepwise manner.
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