Downloaded from https://www.everyspec.com ADS-79D-HDBK The alarm limit is a damage severity level where it is recommended that the machine be shut down for inspection and servicing because continued operation may result in secondary damage to the machine. In order to quantify bearing degradation severity in terms of a suitable alarm limit, it is necessary to represent severity in terms of an equivalent angle of spall. An angle of spall of concern is considered to be the point where the supported shaft begins to experience some loss of position when two rolling elements have begun to simultaneously roll over the spalled area.
This is equivalent to a spall angle of approximately 360 degrees divided by the number of rolling elements as shown in Figure M-6. This criterion for setting the alarm limit has been found to be a conservative limit. The formulas account for bearing size in calculation of the limits. Formulas define alarm limits for rolling element bearings in terms of accumulated mass or accumulated counts of metallic wear debris. The formulas correspond to a bearing spall wear scar size equivalent to the length between two rolling elements (Figure M-6). FIGURE M-6 Bearing wear scar equal to two rolling elements spall length MALARM = Km (360/N) D w (1) where:
M = Mass detected by sensor (mg) Km = Calibration constant relating sensor detected debris mass for a specific bore size sensor to bearing spall geometry characteristics (mg/deg mm 2) N = Number of rolling elements D = Bearing pitch diameter (mm) w = Rolling element width (mm) CALARM = Kc (360/N) D w (2) where: C = Counts detected by sensor (counts) Kc = Calibration constant relating sensor detected debris counts for a specific bore size sensor to bearing spall geometry characteristics (counts/deg mm 2) N = Number of rolling elements D = Bearing pitch diameter (mm) w = Rolling element width (mm) 258