Purpose -The purpose of this paper is to propose a method of qualitative ferrographic analysis by quantitative parameters of wear debris characteristics. Design/methodology/approach -The amount of the wear debris needed for analysis on the ferrogram made by rotary ferrograph is discussed based on the theory of debris group. Quantitative parameters are constituted to express the characteristics of wear debris group, and correlation coefficients are employed to establish the relationship between wear debris and wear condition. The reliability of the method was verified by wear test experiments and ferrographic analysis. Findings -The wear condition of machines should be determined by studying all the debris together as a group rather than by focusing on individual debris. In the proposed method, the qualitative analysis result is obtained by synthetic analysis of quantitative parameters of wear debris characteristics using a computer program, which makes the judgment of the wear system condition more objective and precise.Research limitations/implications -In the procedure of wear condition monitoring by the proposed method, because the weight factors and correlation coefficients introduced in this paper are determined according to the experiences deriving from practice among mining machinery, further rectifications may be needed if they are applied to other industrial field. Originality/value -The paper illustrates a more objective and precise ferrographic analysis method for wear condition monitoring. Nomenclature A¼ total area of wear particles A 1 ¼ total area of certain severe wear particles C 1-4 ¼ parameters of debris group characteristic C ¼ quantitative parameters set of wear debris group D L ¼ concentration of large wear particles D S ¼ concentration of small wear particles D ¼ transition set for synthetic analysis I ¼ severity of wear k ¼ proportional constant n ¼ number of debris categories in the debris group N ¼ total quantity of wear particles in a debris group N 1 ¼ total quantity of certain severe wear particles N 2 ¼ total quantity of certain severe wear particles larger than a 1 R ¼ correlation coefficients set U 1-12 ¼ debris categories W P ¼ weight factors set of characteristic parameters W D ¼ weight factors set of debris categories S ¼ qualitative judgment set of wear condition a ¼ criterion of severe wear particles a 1 ¼ average size of certain severe wear particles a 2 ¼ the largest size of certain severe wear particles

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