Page 218 - Improving Machinery Reliability

Machinery Reliability Audits and Reviews 189

Table 3-21
Material Index Numbers and Maximum Ud’s
Maximum Pinion
Lld

Gear Hardness Pinion Hardness Material Double Single
Minimum Minimum Index Number Helical Helical
223 Bhn 265 Bhn 130 2.4 I .7
262 Bhn 302 Bhn 160 2.3 1.6
302 Bhn 341 Bhn 200 2.2 1.5
352 Bhn 50R, (Nitrided) 260 2.0 1.4s
50Rc (Nitrided) 50R, (Nitrided) 300 1.9 I .4
SSR, (Carburized) 55Rc (Carburized) 410 1.7 1.35
58Rc (Carburized) 58Rc (Carburized) 440 1.6 1 .3
Note:
L = Net jbce width plus gap
d = Pinion pitch diariieter

The sample calculation on page 190 indicates that both gears are satisfactory,
since the maximum actual bending stresses do not exceed the maximum allowable
bending stresses. As long as the final ratio calculated in Step 5 equals or exceeds 1,
the gear incorporates an adequate safety margin for the intended service. On the
other hand, the higher this ratio, the stronger the gear. Everything else being equal,
Vendor B made the better offer.

Checking for Scoring Susceptibility. The susceptibility of a given design to
scoring damage in service can be checked by calculating a so-called scoring index:

500
400
300
200
150

F 00
220 240 260 280 900 320 340 360 50 52 54 56 58
BRINELL AOCKWELL C
GEARHARDNESS
Figure 3-75. Material index numbers.