Selection of fatigue crack evolution test method for heavy load transmission gear

The bending fatigue characteristic curve and bending fatigue limit stress of the test gear shall be combined according to different test points according to different test purposes.

When using the group method to calculate the Reliability stress life curve (P-S-N curve) of the gear, the test results should include 4-5 stress levels, and in all stress levels, at least 5 test points (except the out of range point) should be included. The stress interval between the highest stress level and the second highest stress level is 0.4-0.5 of the total range, and the number of bending stress cycles at each test point shall be greater than or equal to 0.5×103, which is under the highest stress level. At least one test point in the lowest stress level is exceeded. The bending fatigue limit stress of gear can also be obtained by group method.

The method of few test points (usually 7-16 points) can be used to measure the S-N curve. If the limit stress can be determined by the same method, it can also be determined by the lifting method. When measuring S-N curve, select 4-10 stress levels and 1-4 test points under each level. During the contrast test, the orthogonal method with no less than 3 test points shall be selected for each contrast factor.

It is not suitable to use the method of few test pieces, because the number of tests is small, so its error is large; If the lifting method is adopted, the S-N curves of several samples need to be tested. This process should lay a certain foundation for the fatigue limit value when the fatigue life is known before the fatigue bending test. This process is time-consuming, laborious and costly. Therefore, the conventional group method is adopted.

The failure criteria of bending fatigue test of 42CrMo steel heavy equipment transmission gear are as follows:

1) Visible fatigue cracks appear at the tooth root of the gear.

2) 5% load or frequency drop? 10%。

3) Break the tooth along the tooth root.