The shape of the tooth profile affects the stress-strain state in the gear. In the case of a fatigue failure the stress state is a decisive criterion for the lifetime of the gear. The shape of the tooth flank affects the magnitude of the contact pressure in the contact of the meshing teeth. The consequence of which are surface cracks and pitting. The shape of the tooth root influences the magnitude of the root stress, which, when limit is exceeded, leads to root cracks and teeth breakage. Many different types of gearing are known, but in the practice most widely used is the involute one. Other types of gearing become interesting especially when polymer materials are being used. If the gears are injection molded the type of gearing does not affect the cost of the tool. In the case of metal gears, standardized tools for involute gearing make the use of other special types of gearing economically unjustified. Our research is focused on the S-gears, which got their name from the S-shaped path of contact. The paper presents the research of how the defining parameters of S-gears impact the stress-strain state in the gear. This was done using a numerical model which simulates gear meshing. The stress state of two different types of S-gears was compared with the stress state in an involute gear of the same dimensions (same module, number of teeth and width). It was found that with a proper choice of gearing type we can improve the load bearing capability of the gear pair. With use of our numerical model we have also analyzed the impact of the tip relief on the stress state. The numerical model was validated for the case of meshing steel involute gears, where we can compare the results of the model with the results according the ISO 6336 calculation. A good match between the results of the model and results according the standard was obtained. After validation the same numerical model was used for the calculation of the stress-strain state in S-gears. This was calculated then for metal and polymer (POM/PA) gear pairs.