1 Intended use
This part of ISO 6336, together with parts 2, 3 and 5, 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 teeth.
ISO 6336 includes procedures based on testing and theoretical studies such as those of Hirt [1], Strasser [2], and Brossman [3]. The results of rating calculations made by following this method are in good agreement with previously accepted gear calculations methods (see references [4] through [8]), for normal working pressure angles up to 25° and reference helix angles up to 30°
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 should be confirmed by experience.
2 Exceptions
The formulae in ISO 6336 are not applicable when any of the following conditions exist:
- spur gears with transverse contact ratios less than 1,0;
- spur or helical gears with transverse contact ratios greater than 2,5;
- where interference exists between tooth tips and root fillets;
- when teeth are pointed;
- when 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 patern
The procedures in ISO 6336 provide 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).
2.1 Scuffing
Formulae for scuffing resistance on cylindrical gear teeth are not included in ISO 6336. At the present time, there is insufficient agreement concerning the method for designing cylindrical gears to resist scuffing failure.
2.2 Wear
Very little attention and concern have been devoted to the study of gear tooth wear. This subject primarily concerns gear teeth with low surface hardness or gears with improper lubrication. No attempt has been made to cover the subject in ISO 6336.
2.3 Micropitting
ISO 6336 does not cover micropitting, which is an additional type of surface distress that may occur on gear teeth.
2.4 Plastic yielding
ISO 6336 does not extend to stress levels greater than those permissible at 10 cycles or less, since stresses in this range may exceed the elastic limit of the gear tooth in bending or in surface compressive stress. Depending on the material and the load imposed, a single stress cycle greater than the limit level at ＜ 10 cycles could result in plastic yielding of the gear tooth.