The apparatus consists of a plain steel shaft encased in a bearing and directly driven by a small electric motor. The bearing is freely supported on the shaft and sealed at motor end. The motor speed is precisely controlled by control unit and can be run in both directions. The bearing contains twelve equi-spaced pressure tappings around circumference and four along the axis. All are connected by light flexible plastic tubes to the manometer so that the pressure head of all sixteen points can be observed at a time. The bearing can be loaded by attaching weights to the arm supported beneath it.