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The pitch circle is the circle representing the original
cylinder which transmitted motion by friction, and its diameter the pitch circle
diameter.

The center distance of a pair of meshing spur gears is the
sum of their pitch circle radii. One of the advantages of the involute system is
that small variations in the center distance do not affect the correct the
correct working of the gears.

The addendum is the radial height of a tooth above the
pitch circle.

The dedendum is the radial depth below the pitch circle.

The clearance is the difference between the addendum and
the dedendum.

The whole depth of a tooth is the sum of the addendum and
the dedendum.

The working depth of a tooth is the maximum depth that the
tooth extends into the tooth space of a mating gear. It is the sum of the
addenda of the gear.

The addendum circle is that which contains the tops of the
teeth and its diameter is the outside or blank diameter.

The dedendum or root circle is that which contains the
bottoms of the tooth spaces and its diameter is the root diameter.

Circular tooth thickness is measured on the tooth around
the pitch circle, that is, it is the length of an arc.

Circular pitch is the distance from a point on one tooth
to the corresponding point on the next tooth, measured around the pitch circle.

The module is the pitch circle diameter divided by the
number of teeth.

The Diametrical pitch is the number of teeth per inch of
pitch circle diameter. This is a ratio.

The pitch point is the point of contact between the pitch
circles of two gears in mesh.

The line of action. Contact between the teeth of meshing
gears takes place along a line tangential to the two base circles. This line
passes through the pitch point and is called the line of action.

The pressure angle. The angle between the line of action
and the common tangent to the pitch circles at the pitch point is the pressure
angle.

The tooth face is the surface of a tooth above the pitch
circle, parallel to the axis of the gear.

The tooth flank is the tooth surface below the pitch
circle, parallel to the axis of the gear. If any part of the flank extends
inside the base circle it cannot have involute form. It may have ant other form,
which does not interfere with mating teeth, and is usually a straight radial
line.