Gear
Terminology & Design
A gear is a kind of machine element in which
teeth are cut around cylindrical or cone
shaped surfaces with equal spacing.
Pitch Circle
A pitch circle is an imaginary circle that
represents the outer circumference of a
gear's friction wheel and is used to determine
the size of gear teeth
Teeth
The tip diameter (da) and root diameter (df)
correspond to the top and bottom of the
teeth. The reference diameter is used in gear
designing and calculation.
Module/Diametral Pitch
Diametral pitch: The number of teeth per inch of
pitch diameter.
Fillet: The small radius that connects the tooth
profile to the root circle.
Module: Teeth per millimeter of pitch diameter.
Outside diameter: The major diameter of the gear.
Addendum
In gear terminology, the addendum is the
distance between the pitch circle and the
tooth tip circle of a gear. It's also the radial
distance between the pitch diameter and the
outside diameter.
Dedendum
In gear terminology, the dedendum is the
distance between the pitch circle and the
root circle of a gear tooth.
It's also known as hf.
Meshing
Meshing refers to the overlapping contact
between gear teeth, chains and sprockets, or
belts and pulleys. A proper mesh is necessary
for maximum torque transmission.
Gear design involves creating gears—
mechanical components used to transmit
power and motion between rotating shafts.
Gears change speed, torque (rotational force),
and the direction of motion in machinery.
Basic Terminology
Gear: A toothed wheel used for transmitting motion.
Pinion: Smaller gear in a gear pair.
Gear Ratio: Ratio of number of teeth in driven gear to
driver gear, controlling speed and torque.
Pitch Circle: Imaginary circle where gear teeth mesh
together.
Module: Measurement that defines tooth size (ratio
of pitch circle diameter to number of teeth).
Addendum: Tooth height above the pitch circle.
Dedendum: Tooth depth below the pitch circle.
Spur Gears
Simplest, most common type.
Straight teeth parallel to the axis.
Used for simple speed changes at low-medium
speeds.
Helical Gears
Teeth cut at an angle.
Smooth, quiet operation, handles high speeds.
Used in car transmissions, heavy machinery.
Bevel Gears
Conical-shaped, used for changing direction of
shafts (typically 90°).
Found in automobile differentials.
Worm Gears
Worm (screw-like gear) meshes with worm wheel (spur-
type).
High reduction ratios, low speed, high torque.
Used in lifts, conveyors, steering systems.
Rack and Pinion
Linear gear (rack) and pinion (circular gear).
Converts rotational motion to linear motion, like
steering in cars.
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