Electrical Methods of Detecting Film Failure

ELECTRICAL METHODS OF DETECTING FLUID FILM FAILURE 

If the fluid film separating two metallic bounding surfaces is an electrical insulator, then loss of insulating behavior and appearance of conductance can be interpreted as contact between these surfaces. This is the premise behind the electrical conductance method of detecting lubricant film failure. A practical limit on the applicability of this premise is electrical breakdown of the fluid as thinning of the film concentrates the field intensity. Another complication arises from the fact that for the structured surfaces of every day experience first contact is at the highest asperities rough surfaces may therefore come into initial contact sooner than indicated by the overall trend of the resistance measurements. 

Some work carried out by C. M. Allen on the electrical measurement of oil film behavior in plain bearings is illuminating [21]. The study of a plain bearing has certain innate advantages because of the relative ease with which the theoretical oil film thickness can be calculated. The basic circuitry employed is illustrated in Fig. 6-13. The input is 

When the oil film between the ball and the ring is intact, the oscilloscope trace indicates infinity. In actual practice the ratio 2/ adjusted to give a convenient working midscale resistance by means of a 

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