this paper first discusses the functionalrelationship between dimensionless amplitude parameters and amplitude probability density of random signals then presents the results of computer simulationanalysis and experimental study of four states （for example normal

crack

pitting

fractue of gearbox

Meanwhile） the law governing the variation of dimen-tionless amplitude parameters with faults is expounded. A comparison of sensibilities and stabilities of parameters in the presence of practical signals is madethere fore the suitable diagnostic parameters is identified. On this basis

this paper defines kurtosis variation factor and makes trend analysis of the practicalsignals of gearbox

which will aid the surveillance andmaintenance of gearbox.