It can be complicated to compute the NTCS with

the formula, so table 14-1 provides the data for a

simplified method to cube the secant of the helix angle.

To use it, multiply the NT by a factor (K) you obtain

from the table.

To determine the constant K, locate on table 14-l

the helix angle you plan to cut. If the angle is other than

a whole number, such as 15°6´, select the next highest

whole number of degrees, in this case 16°. The factor

for 15°6´is 1.127.

The following section will show you how to use the

numerical value of the NTCS to compute corrected

chordal addenda and chordal thicknesses.

As in spur gearing, you must determine corrected

chordal addenda and chordal thicknesses since you will

be measuring circular distances with a gear tooth

vernier caliper that was designed to measure only

straight distances.

In helical gearing, use the NTCS rather than the

actual NT to select the constant needed to determine the

number, go to the next highest whole number.

chordal addendum (CA) and the chordal thickness

From table 14-2, an NTCS of 23 provides the

(CT). Table 14-2 provides these constants. Remember,

following:

the numbers listed in the Number of Teeth column are

not actual numbers of teeth, but are NTCS values. After

you have determined the chordal addendum and chordal

thickness constants, you can calculate the corrected

Therefore, *CADD =*

= 0.102680 and

chordal addendum by using the following formula:

= 0.156958

and the corrected chordal thicknesss by using this

formula:

The backlash allowance for helical gears is the same

as that for spur gears. Backlash is obtained by

decreasing the thickness of the tooth at the pitch line

and should be indicated by a chordal dimension. Table

As an example, calculate the corrected chordal

14-3 gives maximum allowable backlash in inches

addendum and the corrected chordal thickness for a

between the teeth of the mating gears.

helical gear with a DP of 10, a helix angle of 15°, and

20 teeth.

To determine the proper amount of backlash,

multiply the maximum allowable amount of backlash

found in table 14-3, part A, by 2 and add the result to

= 20 × 1.11 (constant from table 14-l)

the calculated whole depth. In this case the maximum

= 23

backlash allowance is a constant.

14-11