Page 226 - Improving Machinery Reliability

Machinery Reliability Audits and Reviews 197

Rated tangential load per helix:

8,200 x 1.5 = 6,150 Ibs
2

Tooth loading with maximum possible thrust:
(100) (5,140) = 84% of rated tangential load
6,150

This analysis shows that even with the maximum anticipated external force, the gear
teeth are safe for the particular train configuration shown.
For the drive arrangement shown in Figure 3-78, the external force FA acting on the
gear mesh can be as much as

This condition can exist because the low-speed shaft thrust bearing constrains the
axial movement of the input gear. Thus

We now have the following force equations (see Figure 3-79):

C
A = C tan q, = Altan q = 1.72A
B = D tan V, D = Bhan 1z'= 1.72B
C + D = FT = 8,200 lbs
A -k B =FT tan 9 = 4,730 Ibs
A-B+F,=O
A- B + 4,380 = 0
2A = 350 lbs, A = 175 lbs
B = 4,555 Ibs, C = 300 Ibs
D = 7,900 Ibs

Rated tangential load per helix:

8,200 x 1.5 = 6,150 lbs
2

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*Assuming a coupling-tooth pitch diameter of 7 in and a pressure angle of 14.5". Note that some gear
couplings have pressure angles as high as 40", which may cause a very significant increase in FA.