A simple analytical model of bit whirl is presented. In contrast to the previous works, which consider BHA mass imbalance to model bit whirl, the model here takes into account the history-dependent boundary conditions at the bit-rock interface. Within this model, the geometry of the bit is characterized by three dimensionless parameters. The results have shown that, depending on the value of these parameters, the system can be stable or undergo forward or backward whirl. As a result, the model has potential application as a tool that assists bit design.

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