The characteristics feature of spur gears is that their axes are parallel. The gear teeth are positioned around the circumference of the pitch circles which are equivalent to the circumferences of the friction rollers in Fig. 24.13 below.
The teeth are of involute form, the involute being described as the locus traced by a point on a taut string as it unwinds from a circle, known as the base circle. For an involute rack, the base-circle radius is of infinite length, and the tooth flank is therefore straight.
Gear centre i
Base circle radius
True involute profile
Fig. 24.14 Involute construction. The distance along the tangent from each point is equal to the distance around the circumference from point 0
Base circle radius
Tangents
True involute profile
Fig. 24.14 Involute construction. The distance along the tangent from each point is equal to the distance around the circumference from point 0
The construction for the involute profile is shown in Fig. 24.14. The application of this profile to an engineering drawing of a gear tooth can be rather a tedious exercise, and approximate methods are used, as described later.
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