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The periodicity of the measured signals can be exploited to isolate only the dynamics of interest. An effective means for minimizing the non-synchronous components in a mea-sured periodic signal is the Time Synchronous Average (TSA). This is the process of partitioning the data into in-dividual shaft cycles and averaging across the cycles. As a consequence, the non-synchronous parts of the signal act as random components with zero mean and fall off at a rate of
√

1/ N. McFadden calls this process the equivalent to pass-ing the vibration signal through a comb .lter which has pass bands at multiples of the rotation frequency of the gear of interest. Increased averaging has the effect of reducing the width of the comb teeth. (Ref. 21)
For shafts rotating at a constant velocity and data ac-quired at a constant sampling rate, there is a 1:1 corre-spondence between the shaft’s angular position and time. However, this is rarely the case because most shafts vary in speed, even slightly. Therefore, it is necessary to inter-polate the data to pre-determined angular points so that the time synchronous averaging is conducted at like angles.
A typical result for a vibration signal on the OH-58C is given in Figure 1. The x-axis is the frequency normalized

Fig. 1. Effects Of TSA -Baseline Case

by the planet gear mesh frequency. These are referred to as gear mesh orders and this convention make it easier to identify off-order components such as those due to the input pinion. The top plot is one cycle of data and the bottom is the result of 118 averages. The off-order components as well as the noise .oor have been reduced signi.cantly.
After TSA, waveform patterns associated with the mesh-ing teeth often emerge. In some cases, it is possible to iden-tify damage from inspection. However, if the damage is in its infancy, it may be dif.cult to isolate it from inspection. Thus, many techniques have been proposed to increase the detectability of damage in the TSA signal to make the early signs of gear failure easier to detect. The role of condition indicators is to provide a value that gives insight into the level of damage of a component.
　
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