Since fatigue life determination technology is a key technology in the field of helicopter design, foreign research on this aspect is rarely disclosed to the public. What we can understand is that with the requirements of helicopter reliability and high life, as well as the continuous improvement of design and technology level, the requirements for gear fatigue test have been raised to 10 ^ 7~10 ^ 8 cycles, but the basic method of fatigue life assessment still uses Miner criterion.
The fatigue test method of gear chain of domestic helicopter reducer is mainly based on the corresponding national military standards and the test experience of several types of helicopter reducer. For example, the fatigue test of the gear chain of the Zhijiu helicopter reducer is based on the fatigue life determination method in the French Aerospace CAL08.004. Under the required test load, the gear with the slowest speed in the transmission chain completes 5 × After 10 ^ 6 cycles, if the whole transmission chain has no fault, it can be considered that the reducer has infinite gear fatigue life under this reliable power.
The fatigue test of reducer generally adopts the test method of increasing the transmission power, and the power amplification coefficient changes with the number of test reducers. Generally, if there is one test piece, the amplification factor can be taken as 1.4, and if there are two test pieces, the amplification factor can be taken as 1.3. The power amplification factor can also be determined according to experience, and the number of test cycles can be determined according to the life requirements.
After completing the test according to the time specified in the test program, if the gear tooth does not have cracks or broken teeth, the test is passed. Based on the summary of previous test methods and relevant standards and specifications, the fatigue test methods of gear chain are summarized as follows:
(1) Determine whether the gear is designed according to infinite life or safe life. If it is designed according to infinite life, the test piece should work at a reliable power with a certain power amplification factor 5 × 10 ^ 6 cycles; If it is designed according to the safe life, it can be considered to reduce the power load of the tested gear or reduce the number of cycles, and then calculate the fatigue life under this load according to Miner’s criterion.
(2) Select the appropriate test load and evaluate the service life to meet the requirements. For gears designed according to the safe life, the test load can be reduced and adjusted. The reduced test load is assumed to be the reliable fatigue limit. According to Miner’s criterion, the damage accumulation calculation of the design load spectrum is carried out to evaluate the life. The test can be carried out according to this load. The number of test cycles is also required to be 5 × 10 ^ 6 times. If the test passes, it indicates that the gear has a certain service life under this design load condition. When the gear is damaged in advance during the test, it is necessary to analyze the specific problems. If it is not caused by defects, the fatigue life can be estimated according to the Miner criterion and the actual number of cycles according to the S-N curve.
(3) Optimize the test procedure, reduce the test risk and shorten the test cycle. Most of the helicopter reducers adopt the power split design, so the actual power split to each gear must be calculated. At the same time, the power on the entire transmission chain must be comprehensively considered, which not only meets the requirements of each gear assessment, but also avoids the damage of other gears caused by excessive power on the transmission chain. Reasonable design of the test and examination process and adoption of cross loading can reduce the test power on the transmission chain and shorten the total test time.
(4) For the case that there are two identical gears in a single reducer, if they bear the same load, the power amplification factor of the gear can be taken as 1.3. However, it should be noted that although the gears are the same and the transmission power is the same, they may also be affected by the transmission structure layout, and their actual loads are different. At this time, the power amplification factor must be taken as 1.4. For example, the helical bevel gears installed on both sides of the reducer are the same, but the resultant effect of the force acting on each gear is different.