The types of contact fatigue of machine transmission gears are considered. It is shown that the main criterion for the workability of gear wheels of transmissions of tractors and heavy-duty vehicles is the deep contact fatigue of the tooth surfaces. It is established that the calculation of the strength of gears according to GOST 21354—87 gives significant errors, which lead to significant uncertainty in the calculation. A calculation of the life of gears is proposed, taking into account the fatigue resistance of the tooth material and based on the dependence of the limit of deep contact endurance on the hardness and structure of the hardened layer. An example of calculating the life of a gear is considered, comparing the results with bench test data. Keywords:gear, transmission, contact fatigue, pitting, deep contact spalling, calculation, life sprud.47@mail.ru, sand_work@mail.ru, valcoalex5@gmail.com