Mechanical Backlash Case

Mechanical backlash is usually caused by improper adjustment between component parts for dynamic rotor force.
Type C is often caused by a linear backlash of the housing in its cover, excessive backlash in a bushing or a rotating element of a bearing or a loose rotor with backlash relative to the shaft. The phase is often unstable and can vary widely from one medication to the next, particularly if the rotor changes position on its axis with each start. Mechanical clearance is generally highly directional and can cause very different readings compared to 30⁰ level increments in the radial direction all the way around a bearing housing. Also note that backlash will often cause multiples of sub-harmonics at exactly 1/2 or 1/3 RPM (5x, 1.5, 2.5x, etc.)