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Mud motor bearings

Mud motor bearings, or ball bearings have conventionally been used to react thrust, or axial, loads in down hole mud motors. When low rpm power sections are used in the application such bearings provide sufficient life and reliability. However, bearing component fatigue causes life to decrease linearly as speed increases. This fatigue makes ball bearings unsuitable for high rpm motors. Standard engineering practice dictates that ball bearings are specified according to L10 life, or the number of revolutions a group of identical bearings is expected to sustain before 10% fail. As revolutions are related to speed, bearings operating in a higher speed motor will fail before those operating in a low speed motor, e.g. one could roughly expect a 90% reduction in bearing life if speed was increased from 100 to 1000 rpm. In addition to life, friction and horsepower losses are often of interest in drilling. Ball bearings are referred to as frictionless bearings due to the rolling nature of the elements, however in practice frictional losses exist due to rolling resistance and sliding. An order of magnitude approximation of the coefficient of friction (cof) for an angular contact bearing operating in ideal non-abrasive lubricant conditions is 0.0032.1 In drilling mud, the cof can be assumed to be significantly higher.