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17.1.4 Helix Angle Factor, Yb   Helix angle factor can be obtained from Equation(17-9). (17-9) 17.1.5 Life Factor, KL    We can choose the proper life factor, KL from Table 17-2. The number of cyclic repetitions means the total loaded meshings during its lifetime. 17.1.6 Dimension Factor of Root Stress, KFX    Generally, this factor is unity.    KFX = 1.00                                          (17-10) 17.1.7 Dynamic Load Factor, KV    Dynamic load factor can be obtained from Table 17-3 based on the precision of the gear and its pitch line linear speed.

17.1.8 Overload Factor, Ko    Overload factor, Ko, is the quotient of actual tangential force divided by nominal tangential force, Ft. If tangential force is unknown, Table 17-4 provides guiding values.    Ko =   Actual tangential force               (17-11)           Nominal tangential force, F1  17.1.9 Safety Factor for Bending Failure, SF    Safety factor, SF, is too complicated to be decided precisely. Usually, it is set to at least 1.2. 17.1.10 Allowable Bending Stress at Root, sFlim    For the unidirectionally loaded gear, the allowable bending stresses at the root are shown in Tables 17-5 to 17-8. In these tables, the value of aF,m is the quotient of the tensile fatigue limit divided by the stress concentration factor 1.4. If the load is bidirectional, and both sides of the tooth are equally loaded, the value of allowable bending stress should be taken as 2/3 of the given value in the table. The core hardness means hardness at the center region of the root.

Table 17-2     Life Factor, KL

Number of Cyclic Repetitions Hardness (1)  HB 120 ... 220 Hardness(2) Over HB 220 Gears with Carburizing Gears with Nitriding Under 10000 1.4 1.5 1.5 Approx. 105 1.2 1.4 1.5 Approx. 106 1.1 1.1 1.1 Approx. 107 1.0 1.0 1.0

NOTES: (1) Cast iron gears apply to this column.             (2) For induction hardened gears, use the core hardness.

Table 17-3 Dynamic Load Factor, Kv

Precision Grade of Gears form JIS B1702 Tangential Speed at Pitch Line (m/s) Tooth Profile Under 1 1 to less than 3 3 to less than 5 5 to less than 8 8 to less than 18 12 to less than 18 18 to less than 25 Unmodified Modified   1 - - 1.0 1.0 1.1 1.2 1.3 1 2 - 1.0 1.05 1.1 1.2 1.3 1.5 2 3 1.0 1.1 1.15 1.2 1.3 1.5   3 4 1.0 1.2 1.3 1.4 1.5     4 - 1.0 1.3 1.4 1.5       5 - 1.1 1.4 1.5         6 - 1.2 1.5

Table 17-4 Overload Factor, Ko

Impact from Prime Mover Impact from Load Side of Machine Uniform Load Medium Impact load Heavy Impact load Uniform Load  (Motor, Turbine, Hydraulic Motor) 1.0 1.25 1.75 Light Impact Load (Multicylinder Engine) 1.25 1.5 2.0 Medium Impact Load (Single Cylinder Engine) 1.5 1.75 2.25

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