T-151
It should be noted that the greatest bending stress is at the root of the flank or base
of the dedendum. Thus, it can be stated:
sF
= actual stress on dedendum at root
sFtlim= allowable stress
Then Equation (17-4) becomes Equation (17-5)
sF £ sFlim
(17-5)
Equation (17-6) presents the calculation of Ft lim:
mnb KLKFX 1
Ft lim = sF lim ––––––– (––––––) ––– (kgf)
(17-6)
YFYeYb KVKO SF
Equation (17-6) can be converted into stress by Equation (17-7):
YFYeYb KVKO
sF = Ft ––––––
(––––––) SF (kgf/mm2)
(17-7)
mn b KLKFX
17.1.1 Determination of Factors in the Bending Strength Equation
If the gears in a pair have different blank widths, let the wider one be bw and the
narrower one be bs .
And if:
bw – bs £ mn, bw and bs can be put directly into Equation (17-6).
bw – bs > mn, the wider one would be changed to bs + mn and the narrower one, bs ,
would be unchanged.
17.1.2 Tooth Profile Factor, YF
The factor YF is obtainable from Figure 17-1 based on the equivalent number of teeth,
zv, and coefficient of profile shift, x, if the gear has a standard tooth profile with 20°
pressure angle, per JIS B 1701. The theoretical limit of undercut is shown. Also, for
profile shifted gears the limit of too narrow (sharp) a tooth top land is given. For internal
gears, obtain the factor by considering the equivalent racks.
17.1.3 Load Distribution Factor, Ye
Load distribution factor is the reciprocal of radial contact ratio.
1
Ye =–––
(17-8)
ea
Table 17-1 shows the radial contact ratio of a standard spur gear.
