Condition monitoring based on vibration measurement and analysis of planetary gear transmissions has not provided the same good results observed in conventional fixed-shaft gear transmissions. One of the causes being the improper interpretation of the vibration spectrum. The structure of the lines present in the spectrum of the vibrations measured on a non-faulty planetary gear transmission, with a sensor mounted on the outer part of the ring gear, is strictly related to the geometry of the transmission. Hence, different spectral patterns can be found for different planetary gear transmissions. In this work, a vibration model for a single-stage planetary gear transmission is presented to explain this phenomenon. The model is developed in the time domain and analyzed in the frequency domain by using the Fourier transform. It is shown that some planetary gear transmissions, with different geometries, present similar spectral structures. Based on this, a classification of planetary gear transmissions in four groups is proposed. The spectral characteristics of the vibrations of each group are presented. The classification allows for a fast estimation of the expected spectral pattern of the vibrations of any single-stage planetary gear transmission. Through this, contribution is expected in order to increase the effectiveness of vibration-based condition monitoring in planetary gear transmissions.