Basic Parameters Of Gears

• Module

Using the symbol m (module) and the number (millimeter) to indicate the size of the teeth, the bigger the number, the bigger the teeth.

Module times PI to get the pitch (p), pitch is the length between two adjacent teeth.

Formula: p= PI x module = πm

• Pressure angle

The pressure angle is at a point on the teeth surface (generally refers to nodes), the angle between the radius line and the tangent line of the teeth profile. The pressure angle (α) is generally 20°.

Module (m), pressure angle (α) and number of teeth (z) are the three basic parameters of gear.

• Teeth height and teeth thickness

The height of the teeth is determined by the modulus (m).

Whole depth: h =2.25m (= dedendum + addendum)

Addendum (ha) is the height from the teeth tip to the reference line. ha=1m

Dedendum (hf) is the height from the teeth root to the reference line. hf=1.25m

The datum for teeth thickness (s) is half of the pitch. s=πm/2

• Diameter of gear

Reference diameter of gear determine the gear size. Taking the reference circle as the reference, to determine the teeth pitch, teeth thickness, teeth height, addendum, dedendum.

reference diameter: d=zm

addendum circle diameter: da=d+2m

dedendum circle diameter: df=d-2.5m

Reference circle cannot be seen directly in actual gears because they are assumed circles to determine the size of the gears.

• Center distance and back lash

When the reference circle of a pair of gears meshes tangentially, the center distance is half the sum of the two reference circles diameters.

Centre-to-centre distance: a=（d1+d2）/2

In the meshing of gears, backlash is an important factor to get a smooth meshing effect. Backlash is the gap between the teeth surfaces when a pair of gears mesh.