This part of ISO 6336 presents the basic principles of, an introduction to, and the general influence factors for, the calculation of the load capacity of spur and helical gears. Together with ISO 6336-2, ISO 6336-3, ISO 6336-5 and ISO 6336-6, it provides a method by which different gear designs can be compared. It is not intended to assure the performance of assembled drive gear systems. It is not intended for use by the general engineering public. Instead, it is intended for use by the experienced gear designer who is capable of selecting reasonable values for the factors in these formulae based on knowledge of similar designs and awareness of the effects of the items discussed. The formulae in ISO 6336 are intended to establish a uniformly acceptable method for calculating the pitting resistance and bending strength capacity of cylindrical gears with straight or helical involute teeth. ISO 6336 includes procedures based on testing and theoretical studies such as those of Hirt [1], Strasser [2] and Brossmann [3]. The results of rating calculations made by following this method are in good agreement with previously accepted gear calculations methods (see References [4] to [8]) for normal working pressure angles up to 25° and reference helix angles up to 25°). For larger pressure angles and larger helix angles, the trends of products YF YS Yβ and, respectively, ZH Zε Zβ are not the same as those of some earlier methods. The user of ISO 6336 is cautioned that when the methods in ISO 6336 are used for other helix angles and pressure angles, the calculated results will need to be confirmed by experience. The formulae in ISO 6336 are not applicable when any of the following conditions exist: — spur or helical gears with transverse contact ratios less than 1,0; — spur or helical gears with transverse contact ratios greater than 2,5; — interference between tooth tips and root fillets; — teeth are pointed; — backlash is zero. The rating formulae in ISO 6336 are not applicable to other types of gear tooth deterioration such as plastic yielding, scuffing, case crushing, welding and wear, and are not applicable under vibratory conditions where there may be an unpredictable profile breakdown. The bending strength formulae are applicable to fractures at the tooth fillet, but are not applicable to fractures on the tooth working surfaces, failure of the gear rim, or failures of the gear blank through web and hub. ISO 6336 does not apply to teeth finished by forging or sintering. It is not applicable to gears which have a poor contact pattern. The procedures in ISO 6336 provide rating formulae for the calculation of load capacity, based on pitting and tooth root breakage. At pitch line velocities below 1 m/s the gear load capacity is often limited by abrasive wear (see other literature for information on the calculation for this).