Downloaded from https://www.everyspec.com ADS-79D-HDBK individual fault, and identification of the exact nature of the fault is not necessary, so long as the HI is able to reliably provide the correct maintenance recommendation. For the AH-64 MSPU, two thresholds are commonly established for each CI. The lower threshold, the caution or yellow threshold, indicates that a component‘s functionality is reduced. Maintainers should maintain such a component as soon as practical. The higher threshold, the exceedance or red threshold, indicates that the component has a lack of functionality. Maintainers should immediately perform maintenance on such a component.
The initial thresholds are set using engineering judgment and statistical analysis. As more data is collected and faulted components are found and confirmed through teardown analysis, the thresholds are revised to more accurately convey the condition of the component. To determine the accuracy of an HI‘s maintenance recommendation and to evaluate a CI‘s contribution to it, one must first identify the correct maintenance recommendation for a number of data sets. Two methods are used to make this identification: teardowns and seeded fault testing. Teardowns are used to determine the actual condition of components for which values of a CI have been calculated.
In seeded fault testing, a component with a known fault is placed on a test stand to determine how its CI values differ from those of healthy components. Often, at least one of two additional assumptions are made for the determination of HI and CI performance. The first is to assume that the vast majority of components are healthy. The second is to assume that components are healthy at the point they are installed. Neither assumption is completely accurate, but it is not feasible to tear down every component. A good CI should provide distinct – more than 10% separation between healthy, degrading, and faulted component populations.
This allows thresholds to be selected such that the known faulted components are above the healthy and degraded component thresholds and healthy components are below the degraded and faulted components. D.7.2 Fault Frequency Calculation. Due to the design of a bearing, the various components (rolling elements (RE), races, and cage) of the bearing come in contact with each other at various frequencies. These frequencies are known as fault frequencies because a fault or defect in one of these components will produce an impulse response at that frequency as it comes in contact with the other elements of the bearing.
The four fault frequencies are the cage fault frequency (CFF), the ball spin frequency (BSF), the outer race ball pass frequency (BPFO), and the inner race ball pass frequency (BPFI). The actual frequency of a vibration produced by a fault may differ somewhat from the nominal value due to rolling elements slipping slightly rather than purely rolling. The first step in developing an aft hanger bearing CI is to calculate the bearing fault frequencies for the bearing of interest. These frequencies can be calculated based on the geometry of the bearing and the rotational speed of the bearing.
Unless the bearing separates two rotating components, the rotational speed of the bearing is simply the rotational speed of the shaft or gear to which it is attached. 93