d. By using our knowledge of trigonometry,

7. Pitch diameter (PD)--This is the diameter of

the gear blank one addendum down at the large

we can obtain the PCR by using the cosec

end of the gear.

and one-half the pitch diameter.

of

8. Outside diameter (OD)

a. This is the maximum diameter of the gear.

× 0.5*PD.*

b. The gear blank is machined to this outside

13. Pitch cone radius small (PCRs). This is the

diameter.

difference between the pitch cone radius and the face

c. The outside diameter is obtained by adding

width. *PCR**s = PCR - FW.*

the pitch diameter and twice the angular

addendum.

14. Face width ratio (FWR)

9. Angular addendum (ANG ADD)

a. This is the ratio of the pitch cone radius and

a. This is one-half the difference between the

the face width. *FWR =*

pitch diameter and the outside diameter.

b. In the triangle shown in figure 14-16, view

b. The small tooth dimensions are calculated

C, the hypotenuse is the addendum and the

from this ratio.

side adjacent to the angle (BCA) is known

15. Proportional tooth factor (PTF). This is the

as the angular addendum.

ratio between the pitch cone radius small and the pitch

C. To obtain the angular addendum (ANG

cone radius. *FTF*

ADD), simply multiply the addendum of

the gear by the cosine of angle BCA.

16. Small tooth dimensions. Multiply any large

tooth dimension by the proportional tooth faction to

find the dimension of the small tooth of the gear or

10. Tooth dimensions (TD)

pinion.

a. All tooth dimensions at the large end are

17. Number of teeth for cutter selection (NTCS)

the same as a spur gear of the same DP.

a. In the triangle shown in figure 14-16, view

b. All tooth dimensions at the small end are a

E, the NTCS is the hypotenuse and the side

percentage of the large end, depending of

adjacent is the number of teeth of the gear.

the face width ratio.

b. The known angle in this case is the pitch

11. Face width (FW) (fig. 14-15)

cone angle, or the back cone angle.

a. This is the length of the tooth.

c. To obtain the NTCS, simply multiply the

b. The gear blank is machined to this dimen-

sion.

by the NT.

secant of

12. Pitch cone radius (PCR)

a. This is the length of the side of a cone

formed by the bevel gear.

d. The NTCS is taken from the number of

b. This radius is used extensively in cal-

teeth on an imaginary spur gear that has a

culations.

different pitch diameter (PD) than the pitch

c. In the triangle shown in figure 14-16, view

diameter (PD) of a bevel gear.

D, the hypotenuse is the pitch cone radius

e. When your computation for the NTCS

and the side opposite the pitch cone angle

contains a decimal number, round the

is equal to one-half the pitch

computation to the next higher whole

number.

diameter (0.5 PD).

14-18

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