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Gear Tooth Profile

3. Gear Tooth Profile / One of the most popular tooth profiles is the Involute Tooth Profile

The majority of gears used in industrial machinery are gears with an involute tooth profile. The popularity of the involute tooth profile is derived from many of it’s advantages, such as simplicity in design and ease of use.

Characteristics of Involute Tooth Profiles

  1. Easy machining for production (Straight rack tooth profile)
  2. Rotation is smooth despite changes occurring in the center distance.
  3. Gears with different number of teeth can be modified by machining if they have the same module and the pressure angle.

What is an involute Tooth Profile ?

Involute tooth profile (Involute curve) is a curve made by a base circle (db). The definition of an involute is the spiraling curve traced by the end of an imaginary taut string unwinding itself from that stationary circle called the base circle. The resultant is the Involute curve A – b – c – d – e, and so on.
Figure 3.1 shows the state of the curve when the straight line is rolled down by 90 degrees (1/4 of the circle).

Fig. 3.1 Involute Curve

What is Base Circle ?

The base circle is the base circle of the involute, and its value is determined depending on the sizes of the pressure angle (α) and the reference circle (d).

db = d cos α

The Base circle is also the base circle of the involutes.
The Reference circle is the actual reference that determines the size of the gear.
Dimensions of both the base circle / reference circle are crucial for gearing.
An Involute tooth profile is the curving line created external to the side of the base circle.
The value of the pressure angle becomes O (zero) on the base circle.

Meshing of Involute Gear

A pair of standard involute gears mesh together between the reference Center Distance, making contact at a point on the reference circle of each other.
This state resembles friction wheels with reference diameter’s, d1 and d2. However, involute gears actually mesh based on the base circle, instead of the reference circle, resembling a rotation/power transmission by using a cross-coupled belt placed around the 2 base circles.
In case of a belt drive, slippage tends to occur when frictional force gets smaller than power transmission. However, in case of gears, they can transmit rotation/power without slippage as they have teeth. The common tangent of the two base circles (A and B) is called the line of contact, or line of action.

Fig. 3.2 Power Transmission

(Important Gear Terminology and Gear Nomenclature in Fig 3.2)

The contact point P1 -> P2 -> P3 of the two involutes slide along the common tangent of these two base circles. Look at the yellow teeth of the drive gear in the figure on the right, after the tooth begins to mesh, two of the teeth are kept in mesh (P1/P3). Then, when the contact point on the two base circles moves to point P2, it meshes with one more tooth. With further movement of the drive gear, the contact point moves to P3 and the next tooth P1 starts meshing, so two teeth mesh again. Therefore, the gear transmits rotation by meshing two teeth and one tooth, alternately and repeatedly.

Fig. 3.3 Meshing of Involute Gear

(Important Gear Terminology and Gear Nomenclature in Fig 3.3)

Related links :
Spur Gears - A detailed description of Spur Gears
Involute Tooth Profile - A detailed description of Involute Tooth Profile
Pressure Angle - A detailed description of Pressure Angle
Involute Gear Profile - A detailed description of Involute Gear Profile

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