The purpose of maintenance is to discover, to diagnose, to foresee, to prevent and to eliminate damages, with the aim to maintain the technical system (gear unit) in the most suitable working condition. The purpose of modern maintenance is not only to eliminate failures; when carrying out maintenance according to the condition, the stage when there is a potential danger of a sudden failure of system operation is being defined. The procedure is referred to as diagnostics; it includes the definition of the current condition of the system and the location, shape and the reason of the damage formation. Incorrect operation, the possibility and the location of damages and the possibility of elimination of those damages are defined by means of diagnostic values, such as different signals, condition parametres and other indirect signs. The form of a damage is defined by establishing deviations from the expected or wished for values. On the basis of current data concerning the damage and on the basis of knowledge and experience, a hypothesis, i.e. a possible explanation of the damage formation and condition, is made. Afterwards, a suitable hypothesis is selected, the selection is based on the knowledge and experience. If the selected hypothesis is the correct one, a suitable activity is carried out. Otherwise the procedure of a suitable hypothesis creation and selection is carried out again. In this paper, the influence of various lengths of notches in a pinion tooth root of a single-stage gear-unit upon the noise it produces are dealt with. Time signals obtained by experiments are analysed by means of different statistical methods. In addition to that, frequency analysis has been made, which helps predict the general state of a gear.
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