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Conventional rotordynamic analyses generally treat the rotor as a continuous body without considering effect of clamped joints. However, in modern rotating machines, rotors are often assembled with multiple complex-shaped parts and joints, which may significantly affect rotodynamic behavior. Several authors have proposed methods for implementing contact effects in rotordynamic analysis, but a more general modeling method for handling arbitrary contact geometries with various levels of surface roughness is needed. The present paper suggests a new contact model for rotordynamic analysis of an assembled rotor-bearing system with multiple parts connected by multiple joints. A contact element formulation is presented using solid finite elements and statistics-based contact theories. A test arrangement was developed to validate the proposed contact model for varying interface surface roughness and preloads. An iterative computation algorithm is introduced to solve the implicit relation between contact stiffness and stress distribution. Prediction results, using the contact model, are compared with measured natural frequencies for multiple configurations of a test rotor assembly. A case study is performed for an overhung type rotor-bearing system to investigate the effect of contact interfaces, between an overhung impeller and a rotor shaft, on critical speeds.

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Hirth coupling transmits high torques in the rotating assemblies of compressors and turbines. Their mating surface contacts cause local changes in lateral shaft stiffness. This is affected by the teeth geometry, contact surface area, coupling preload, and surface finish at the contact faces. Industry practice ignores localized lateral flexibility from the Hirth coupling, or is guided by limited experience-based rules of thumb. The authors provide a novel modeling approach utilizing 3D solid finite elements which accounts for contact deformations, intricate interface teeth geometries, stress concentration, and surface finish. This provides an increased accuracy localized stiffness model for the Hirth coupling, to improve rotordynamic response predictions. Free-free natural frequencies of a test rotor including a Hirth coupling are experimentally measured. The rotor is instrumented with strain gauges for preload force measurements, and the Hirth coupling contacting surface profiles are measured with a stylus type surface profiler. A GW contact model is obtained from the measured surface profiles. An iterative computation algorithm is utilized to calculate Hirth coupling contact stiffness and contact pressure at the complex-shaped contact surfaces. Predicted and measured natural frequencies are compared vs. preload.

Drillstring vibration can be highly detrimental to its mechanical integrity, and significantly reduce overall operational efficiency. Vibrations often arise due to the contact force and moment arising at the bit and formation (rock). These occur due to cutting and friction related actions. The literature has various bit-rock interaction BRI models, which may have time delay and nonlinear terms. The time delay term arises from modulation of the depth of cut per revolution by the vertical vibrations. A major inertia participating in the vibrations is the bottom hole assembly BHA, consisting of the bit, instrumentation and power subs, drill collar and stabilizers. Control of the BHA vibrations is imperative to prevent destructive vibration that may break the pipe, dull the bit and diminish hole trajectory and rate of penetration. The most severe vibration types include stick-slip, bit-bounce and lateral whirl. Stick-slip is caused by the alternate stopping of the bit due to friction and its release when the drillpipes produces a sufficient torque as it winds up. Bit-bounce occurs due to time delay in the torque and axial force due to modulation of the cutting force and torque by axial vibration. Finally, lateral whirl results from friction occurring at lateral contact points of the BHA and wellbore. Modelling of these complex, nonlinear, self-excited vibrations is a challenge given the large order models involved and nature of the BRI forces and moments. The paper provides a systematic means to accurately simulate drillstring vibrations with high fidelity and efficiency. This is achieved using a Timoshenko beam based finite element model FEM, and is illustrated with an example containing the Detournay BRI model. High accuracy codes need user friendly interfaces to be effective for field and design use. The paper also provides algorithms and methods for programming the solution-modelling component of the code, and the user interface component.