The PDM Bearing Section contains a mud-lubricated bearing stack that utilize a stacked ball-and-race to handle thrust loads, while carbide radial bearings handle side loads. An advantage of mud lubricated bearings is that there are no limitations on bit pressure drop compared to a sealed bearing assembly and temperature is not generally an issue. The bit pressure drop can be maximized for optimum penetration rate, without adversely affecting the life of the bearing assembly.