The basic parameters of gears cover all aspects of spur cylindrical gears, including the names of their components and the dimensional relationships between them. When the tooth direction of a cylindrical gear is parallel to the generatrix of the cylinder, it is referred to as a spur gear. Below, we take a closer look at the fundamental parameters of spur gears.

Basic Parameters of Spur Gears

Several key parameters are essential for understanding spur gears:

• Number of teeth (z)
  Indicates the total number of teeth on the gear.
• Module (m)
  The module is related to the circular pitch (p) and is defined by the following relationships:
  πd = zp
  d = zp / π
  where d is the pitch circle diameter.
• Addendum circle (da)
  The diameter of the circle passing through the tooth tips.
• Dedendum circle (df)
  The diameter of the circle passing through the tooth roots.
• Pitch circle (d)
  A reference circle on which the tooth thickness is equal to the space width.

Tooth Height Parameters

• Tooth height (h)
  The radial distance between the addendum circle and the dedendum circle, representing the overall height of the tooth.
• Addendum height (ha)
  The radial distance from the pitch circle to the addendum circle.
• Dedendum height (hf)
  The radial distance from the pitch circle to the dedendum circle.
• Circular pitch (p)
  The arc length between corresponding points of adjacent teeth on the pitch circle.
  It is equal to the tooth thickness plus the tooth space width.

Understanding these basic parameters is crucial for mastering gear design standards.

Additional Gear Terminology

• Tooth thickness (s)
  The arc length of one tooth measured along the pitch circle.
• Pitch circle (d′)
  In a pair of meshing gears, at the contact point C on the line of centers O₁O₂, the tangential velocities of both gears are equal. The circles with radii O₁C and O₂C are called the pitch circles of the respective gears.
• Pressure angle (α)
  The acute angle between the direction of the force acting on the tooth profile at point C (on the pitch circle) and the direction of motion.
  The standard pressure angle used in China is 20°.
• Angle of engagement (α′)
  The angle between the force direction and the direction of motion of the two gears at point C.

Module (m)

The module is a critical parameter in gear design and manufacturing. Gears with different modules require different cutting tools during production. To facilitate design and manufacturing, module values have been standardized.

Standard module series (first series):
1, 1.25, 1.5, 2, 2.5, 3, 4, 5, 6, 8, 10, 12, 16, 20, 25, 32, 40, 50

Second series:
1.75, 2.25, 2.75, (3.25), 3.5, (3.75), 4.5, 5.5, (6.5), 7, 9, (11), 14, 18, 22, 28, 36, 45

(Values in parentheses are less commonly used.)

Gear Parameter Formulas

Meshing Conditions

For a pair of meshing gears, both the module and the pressure angle must be the same.
For standard gears, the pressure angle (also known as the tooth profile angle) is 20°.