Tooth contact nomenclature Gear

point of contact 
any point @ 2 tooth profiles touch each other.
line of contact
a line or curve along 2 tooth surfaces tangent each other.
path of action 
the locus of successive contact points between pair of gear teeth, during phase of engagement. conjugate gear teeth, path of action passes through pitch point. trace of surface of action in plane of rotation.
line of action 
the path of action involute gears. straight line passing through pitch point , tangent both base circles.
surface of action 
the imaginary surface in contact occurs between 2 engaging tooth surfaces. summation of paths of action in sections of engaging teeth.
plane of action
the surface of action involute, parallel axis gears either spur or helical teeth. tangent base cylinders.
zone of action (contact zone) 
for involute, parallel-axis gears either spur or helical teeth, rectangular area in plane of action bounded length of action , effective face width.
path of contact
the curve on either tooth surface along theoretical single point contact occurs during engagement of gears crowned tooth surfaces or gears engage single point contact.
length of action
the distance on line of action through point of contact moves during action of tooth profile.
arc of action, qt 
the arc of pitch circle through tooth profile moves beginning end of contact mating profile.
arc of approach, qa 
the arc of pitch circle through tooth profile moves beginning of contact until point of contact arrives @ pitch point.
arc of recess, qr 
the arc of pitch circle through tooth profile moves contact @ pitch point until contact ends.
contact ratio, mc, ε 
the number of angular pitches through tooth surface rotates beginning end of contact. in simple way, can defined measure of average number of teeth in contact during period during tooth comes , goes out of contact mating gear.
transverse contact ratio, mp, εα 
the contact ratio in transverse plane. ratio of angle of action angular pitch. involute gears directly obtained ratio of length of action base pitch.
face contact ratio, mf, εβ 
the contact ratio in axial plane, or ratio of face width axial pitch. bevel , hypoid gears ratio of face advance circular pitch.
total contact ratio, mt, εγ 
the sum of transverse contact ratio , face contact ratio.





ϵ

γ


=

ϵ

α


+

ϵ

β




{\displaystyle \epsilon _{\gamma }=\epsilon _{\alpha }+\epsilon _{\beta }}







m


t



=

m


p



+

m


f





{\displaystyle m_{\rm {t}}=m_{\rm {p}}+m_{\rm {f}}}


modified contact ratio, mo 
for bevel gears, square root of sum of squares of transverse , face contact ratios.





m


o



=



m


p



2


+

m


f



2






{\displaystyle m_{\rm {o}}={\sqrt {m_{\rm {p}}^{2}+m_{\rm {f}}^{2}}}}


limit diameter 
diameter on gear @ line of action intersects maximum (or minimum internal pinion) addendum circle of mating gear. referred start of active profile, start of contact, end of contact, or end of active profile.
start of active profile (sap) 
intersection of limit diameter , involute profile.
face advance 
distance on pitch circle through helical or spiral tooth moves position @ contact begins @ 1 end of tooth trace on pitch surface position contact ceases @ other end.