1.Base circle -: It us the circle from which involute form is generated. only the base circle on a gear is fixed and unalterable.
2.pitch circle -: It us an imaginary circle most use-ful in calculations. It my be noted that an infinite number of pitch circle can be chosen,each asso-ciated with its own pressure angle.
3.Module -: It is defined as the length of the pitch circle diameter per tooth. Thus, if pitch circle diameter of gear be D and number of teeth n, then module (m) = D/n .
4.Pitch circle diameter-: It is the diameter of a circle which by pure rolling action would produce the same motion as the toothed gear wheel. this the most important diameter in gears.
5.Circular pitch -: It the arc distance measured around the pitch circle from the flank of one tooth to a similar flank in the next tooth.
6.Clearance -: This is the radial distance from the tip of a tooth to the bottom of a mating tooth space when the teeth are symmetrically engaged.
7. Diameteral pitch -: The diameteral pitch can be defined as the number of teeth per inch of the pitch circle diameter.
8. Addendum -: Addendum is the radial distance from the pitch circle to the tip of the tooth.
9. Dedendum -: This is the radial distance from the pitch circle to the bottom of tooth space.
Dedendum = Addendum + Clearance
= 1 m + 0.157 m. = 1.157 m
10. Blank diameter -: This is the diameter of the blank from which gear is cut . It is equal to pitch circle diameter plus twice the addenda.
11. Tooth thickness -: The tooth thickness is the arc distance measured along the pitch circle from its intercept with one flank to it's intercept with the other flank of the same tooth.
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