The gear ratio is the ratio of the number of teeth in the gear to the number of teeth in the pinion, the pinion being the smaller of the two gears in mesh.
The pitch-circle diameters of a pair of gears are the diameters of cylinders co-axial with the gears which will roll together without slip. The pitch circles are imaginary friction discs, and they touch at the pitch point.
The base circle is the circle from which the involute is generated.
The root diameter is the diameter at the base of the tooth.
The centre distance is the sum of the pitch-circle radii of the two gears in mesh.
The addendum is the radial depth of the tooth from the pitch circle to the tooth tip.
The dedendum is the radial depth of the tooth from the pitch circle to the root of the tooth.
The clearance is the algebraic difference between the addendum and the dedendum.
The whole depth of the tooth is the sum of the addendum and the dedendum.
The circular pitch is the distance from a point on one tooth to the corresponding point on the next tooth, measured round the pitch-circle circumference.
The tooth width is the length of arc from one side of the tooth to the other, measured round the pitch-circle circumference.
The module is the pitch-circle diameter divided by the number of teeth.
Fig. 24.15 Spur-gear terms
Fig. 24.16 Gear conventions
Fig. 24.15 Spur-gear terms
Fig. 24.16 Gear conventions
The diametral pitch is the reciprocal of the module, i.e. the number of teeth divided by the pitch-circle diameter.
The line of action is the common tangent to the base circles, and the path of contact is that part of the line of action where contact takes place between the teeth.
The pressure angle is the angle formed between the common tangent and the line of action.
The fillet is the rounded portion at the bottom of the tooth space.
The various terms are illustrated in Fig. 24.15.
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