ABSTRACT
Conventional industrial gears tend to use the standard 20° pressure angle and standard proportions and thus encounter undercutting problems when the number of pinion teeth falls below about 18. If gears are highly stressed they will normally be carburised and the standard AGMA2001 or ISO 6336 calculations will typically give a so-called “balanced” design at about 27 teeth. This means that there is an equal likelihood of failure by flank pitting or by root cracking. In practice as root failure would be disastrous, it is normal to have considerably less than 27 pinion teeth to make sure that root breakage is ruled out. This leads to most standard spur designs having between 18 and 25 pinion teeth and typically having a nominal contact ratio about 1.6.
