Techniques for the Early Detection of Gear and Bearing Failures in Helicopter Drive Trains

This paper presents a technology which can be used to determine the condition of the rolling element bearings, and in many cases the gears, in both simple and complex gearboxes. The technique is not dependent upon any previous measurement taken on the gearbox under test and does not require the collection of data over a lengthy time period for trending analysis. The technology is based upon measurements of the vibrational energy in the frequency band between 15 kHz and 35 kHz. It has been found that minor spalls in rolling element bearings and gears produce transient ringing pulses which are directly related to the defect repetition rate of faults within the assembly. A spall on the outer race of a ball bearing assembly will cause the bearing to emit ringing pulses at a frequency equal to that which the balls encounter the defect. If the geometry of the gearbox and its rotational speed are known, processing techniques similar to those employed in modern radar signal processing allows the detection and identification of a fault which otherwise would be buried in the noise and clutter. The electronic hardware necessary for implementing these processing techniques has previously been both voluminous and extremely expensive. However, the recent introduction of VLSI single chip digital signal processing circuits, such as Texas Instrument's TMS-320, has allowed the use of modern detection techniques to be employed in small inexpensive systems. in fact, the processing electronics built for this system is on two five by seven inch printed circuit cards which could be built in production very inexpensively.