The mark came from NASA report without modification. HCR tooth get a lower efficiency due to significant slippage between teeth.
The gearbox must reach a 99% efficiency, thus the efficiency of the tooth is really important but not critical for safety, they get a coefficient of 3. Tooth profile sound power level Power loss It is still interesting to notice that helical gear produce lower vibration (around 3 dB so twice lower) that there herringbones counterparts. Anyway for any frequencies, these results confirm the lowest vibration of helical gear compared to any king of spur (even High Contact Ratio)with around 10 dB lower for the double helical compared to the HCR spur gear. the results for this value are not as reliable than those at 30 rpm. In this test, the housing resonating frequency was exited for 5000rpm. The following table (from NASA) compare the average composite sound power level (dB) that is produced by different tooth type. This criteria will then get a coefficient of 3. All the none herringbone arrangements required extensive development and many persist with not particularly good reliability records in service. The study of different gearbox tell us that the message is anything can be made to work, but with a big however. This criteria is hardy quantifiable and there is no clear trigger value below which the gearbox is going to be safe. Vibration – NoiseĮxperience tell us that vibrations inside power aero gearbox has always been a issue. The rupture must be related to surface such as micro pitting. Lifeįor a given size and load the better there life time, the better the mark. This criteria get a coefficient of 2 as any type can be viable for a given lifetime but it will affect the size and the weight of the gearbox. This criteria is related to mechanical rupture such as ben ding (not related to surface). This solution offer the best compromise over the different criteria.įor a given size and load the better there life time, the better the mark. The table shows that the best tooth profile considering six different possibilities is the double helical or herringbone.