For helical gear pair transmission, the number of rotation cycles of the pinion is more than that of the big gear, and the tooth thickness is smaller, and the load bearing capacity is also weaker. Its gear teeth are more likely to produce fatigue fracture under cyclic variable stress compared with the big gear, so it is usually only necessary to check the strength of the pinion.
For the helical gear failure caused by surface pitting and tooth fracture, the tooth contact fatigue strength coefficient SHmin and tooth root bending fatigue strength coefficient SFmin are often used to measure. Under general reliability conditions, the allowable safety factors SHmin and SFmin of helical gears with modulus greater than 2 shall be greater than 1 and 1.4 respectively, which means they are qualified.
| Driving wheel | Passive wheel | |
| Tooth contact fatigue strength coefficient SHmin | 1.76 | 1,79 |
| Tooth root bending fatigue strength coefficient SFmin | 5.1 | 5.23 |
For example, the table shows the contact fatigue strength coefficient of the tooth surface and the bending fatigue strength coefficient of the tooth root before the modification of the helical gear pair. Obviously, the contact fatigue strength coefficient SHmin of the pinion, that is, the driving gear, is 1.76, greater than 1, and the bending fatigue strength coefficient SFmin of the tooth root is 5.1, much greater than 1.4. Therefore, the strength of the helical gear is qualified.