plotted as functions of pinion revolutions. Some previous development test revealed that as wear severity  and friction increase, sensor outputs became more erratic even though their mean value remain constant.  For example, when friction on the gear teeth increases, the range of the separation force increases even  though the mean may still be constant. This phenomenon is the result of the tooth friction force reversing  direction as the tooth contact passes through the pitch point. Thereby, the friction force first adds to, and  then subtracts from, the magnitude of the separating force during the tooth mesh cycle. With higher tooth  friction the excursions from the mean become larger. These observations and experience in health  monitoring of geared machines led to the definition and use of “condition indicators” as a means to  monitor the overall capability of the MoS 2 films to provide low friction. Condition indicators were defined  as follows. Data records were collected for 1 second at 1-kHz sampling rate. For each data record, the  standard deviation was calculated as the “condition indicator”, stored, and plotted as a function of  accumulated pinion revolutions. Such condition indicators were reliable indicators of a change in the  MoS 2 performance. Figure 4(a) provides a trend plot from test MOS2 1B of the condition indicator for the  LVDT sensor that measures gear center distance changes. Marked on the plot is the indication where the  MoS 2 functioning compromise has started. Also marked are three regions: Region I being the smooth  running regime, Region II being near the start of MoS 2 compromise, and Region III being a significant  friction regime. An example of a data record from which a “condition indicator” was calculated is shown in  Figure 4(b), for the thrust force sensor, during operation in Region III. In plot of 4(b) there was relatively  high tooth friction during the last 400 samples of this particular data record causing a varying thrust force.  Film durability was determined using the condition indicator trend plots. The film durability was defined as  the number of pinion revolutions until the film compromise started, such as indicated on Figure 4. The film  compromise was defined as the very beginning of a steady degradation of the film’s performance  regarding friction. A mechanism may continue to perform its intended function for some time after such  film degradation begins. The intent was to assess a relative measure of the film durability, with and  without exposure to humid air.  Figure 4. Typical trend and features of a condition indicator of MoS 2 film function. (a) Trend of condition  indicator for the center distance (LVDT sensor), for test 1B. (b) Typical data record for calculation of  standard-deviation-based condition indicator.  Experiment Results  First will be discussed the results of quantitative measures of film durability. As was described in the  previous section, “condition indicators” were calculated from sensor data. A set of condition indicators  were calculated every second. Figure 4(a) is an example condition indicator data trend plot from test 1B.  The number of pinion revolutions corresponding to the start of film compromise, as marked on Figure 4,  was determined for each test by visual inspection of such trend plots. This film durability measure was  determined from each of three sensors, the gear center distance, the gear thrust force, and the gear input shaft revolutions 0 10000 20000 30000 40000standard-deviation-based condition indicator 0.0000.0010.0020.0030.0040.0050.0060.0070.008 sample number0 500 1000signal (volts) -0.08-0.040.000.040.08 (a) (b)IIIIII MoS2compromised NASA/CP—2018-219887 133