Detection of Impact Sources Using an Impulse Response Recognition Technique Applications on Rattle Sources in Gearboxes

Doctoral thesis, 1999

Noise and vibration properties of products have become important functions in the competition between different manufacturers and are seen today as indicators for the quality of the product. In this work, a new approach is presented for the localisation of impulse sources in complex structures, e.g. engines, driving units or complete vehicles. The approach is based on a correlation technique similar to methods used in radar techniques. The method is developed for an engineering application aiming on the detection of gear rattle in gearboxes. Gear rattle occurs in gearboxes and is caused by relative motion between acting gearwheels. Gear rattle depends on the play between the gearwheels, the loading and the overall dynamics of the system. Gear rattle can, under certain speed and loading conditions, dominate the total emitted noise from a truck. The proposed analysis method is based on the idea to measure impulse response functions (so called characteristic signals) from possible source locations to a receiving point on the surface of the structure, e.g. the casing of a gearbox. These characteristic signals are compared with vibration measurements when the analysis object is running and impulse sources are active (e.g. rattle impacts between cogwheels). The comparison is done by calculating the correlation coefficient between the characteristic signals and the measured vibrations in the receiving point. It is shown that this correlation coefficient corresponds to the maximum likelihood function and can, from defined perspectives, be seen as the best estimator to decide which of all possible sources is responsible for the vibrations in the receiving point. A substantial part of the work is devoted to the discussion of the quality of the decision based on the correlation coefficient. The problem is extended to the case of unknown arrival time and the presence of noise in the measured signals. From the theoretical background of the suggested methods, engineering solutions are suggested to handle the complex task to detect rattle source in gearbox under different driving conditions. The function of the method is demonstrated for both a modified gearbox under well-controlled conditions in a test rig and for a gearbox in a complete truck. It is shown that the method is a good tool for locating rattle sources in complex structures. It provides information concerning the location as well as the intensity of the impulse sources.