It is a common practice in high power gear design to apply relieves to tooth flanks. They are meant to prevent
stress concentration near the tooth edges. Gears with crownings have point contact without load. When load
is applied, instantaneous contact turns from point into a Hertzian contact ellipse. The contact area grows and
changes location as load increases. To prevent edge contact, gear designer has to choose suitable relieves
considering contact indentations as well as relative displacements of gear members.
In the majority of spiral bevel gears spherical crownings are used. The contact pattern is set to the center of
active tooth flank and the extent of crownings is determined by experience. Feedback from service, as well as
from full torque bench tests of complete gear drives have shown that this conventional design practice leads to
loaded contact patterns, which are rarely optimal in location and extent. Too large relieves lead to small
contact area and increased stresses and noise; whereas too small relieves result in a too sensitive tooth
contact.