To　achieve　the　expected　purpose　monitoring　the　gearbox　condition　and　preventing　the　occurrence　of　dangerous　situations,　it　is　important　to　improve　the　accuracy　of　the　vibration　response,　which　is　primarily　caused　by　the　time-varying　mesh　stiffness.　Thus,　a　universal　gear　profile　equation　that　references　the　actual　manufacturing　process　was　applied　to　calculate　the　meshing　stiffness.　Based　on　the　universal　gear　profile　equation,　two　methods　of　calculating　tooth　thickness　decrements　(method　1:　straight-affecting　line;　method　2:　parabolic-affecting　line)　in　cracked　gears　were　compared,　and　their　different　effects　on　the　vibration　response　were　investigated　with　fault　detection　indicators,　including　RMS　and　kurtosis.　The　results　revealed　that　the　fault　detection　indicators　of　two　affecting　lines　intersect　when　the　crack　size　reaches　a　special　point　at　which　the　two　affecting　lines　overlap,　and　this　result　contrasts　with　those　of　previous　research.　The　double　pulse　observed　in　the　residual　signals　was　analyzed,　and　the　results　revealed　that　the　double　pulse　was　generated　by　the　two　step-points　of　the　residual　stiffness.　The　behaviors　of　the　fault　detection　indicators　of　the　two　affecting　lines　were　investigated,　and　the　results　revealed　that　the　parabolic-affecting　method　was　appropriate　for　the　calculating　the　decrement　in　tooth　thickness.　(C)　2015　Elsevier　Ltd.　All　rights　reserved.