This paper is focused on analysis of tooth-root strength of internal gears. Methods of calculation of the tooth-root stress of internal gears according to the ISO 6336-3:1996 and DIN 39903:1987 standards are briefly described. Reasons for potential errors in the strength calculations of internal gears performed in accordance with the ISO 6336-3:1996 and DIN 3990-3:1987 standards are discussed. Determination of the precise values of the parameters of the critical section and introducing them to use with the ISO 6336-3:1996 standard, proposed in the paper and called the modified ISO 6336-3:1996 method, is presented. Finite element analysis of various accurate models of internal gears is performed and location of critical section as well as its parameters are determined for all considered models of gears. On the grounds of results of precise finite element computations made for a large number of models of gears the influences of several main parameters of the machined gear and applied tool on the tooth-root strength of internal gears are investigated. General conclusions concerning the tooth-root strength of internal gears are formulated. They concentrate on the influences of several main parameters of the machined gear and tool on the tooth-root stresses of gear. They are useful for correct determination of the parameters of the critical section of the internal spur gear and for proper selection of the main parameters of the machined gear and applied tool.
