I was recently collaborating with a member here who was putting together his own Yamaha engine and prop reduction drive. So not the Skytrax "integrated" gearbox.
Because of the high engine RPM (8900 rpm) he was electing to use a smaller diameter giubo with 78mm bolt circle and 12mm pins. I initially did the FEA in the same way I had done it with the Viking gearbox (each pin carries 1/6 of the giubo mass for the purpose of centrifugal load + torque transfer is accomplished by 3 pins.
This was the result
Then it occurred to me that if the giubo was still in 1 piece (a full circular part) that this approach was probably not correct because the giubo contains a considerable amount of cordage which was most likely capable of containing the centrifugal forces, thus not loading the flange pins in the way I had studied (which is the correct approach for a giubo cut into 3 segments, like Viking is using).
So I went back and re-ran the calculation without the added mass for the 1/6 guibo segment, thus the load was only from the peak cylinder pressure (nominal torque*2.5). So one gets this result. Max principle stress of 528MPa in the pin. The flange which has an 11mm thick web is not at all stressed, unlike the <6mm web on the Viking engine.
That got me to thinking about that stress level on the pins or bolts, because it could certainly get that high in the automotive application. Perhaps peak BMEP does not make it to the gearbox output flange in the car, given it has to pass through the torque converter and various clutches first. But that lead me to thinking about the bolts and how strong bolts really can be, so I looked up the properties of grade 12.9 bolts.
Based on the data above, it seem that proof stress is 970MPa which would give us a nearly 2x factor of safety in the application?
Does this sound right or is the assumption that the load case in the vehicle is actually lower and the bolts never get stressed this much ?
Thoughts ?
Because of the high engine RPM (8900 rpm) he was electing to use a smaller diameter giubo with 78mm bolt circle and 12mm pins. I initially did the FEA in the same way I had done it with the Viking gearbox (each pin carries 1/6 of the giubo mass for the purpose of centrifugal load + torque transfer is accomplished by 3 pins.
This was the result
Then it occurred to me that if the giubo was still in 1 piece (a full circular part) that this approach was probably not correct because the giubo contains a considerable amount of cordage which was most likely capable of containing the centrifugal forces, thus not loading the flange pins in the way I had studied (which is the correct approach for a giubo cut into 3 segments, like Viking is using).
So I went back and re-ran the calculation without the added mass for the 1/6 guibo segment, thus the load was only from the peak cylinder pressure (nominal torque*2.5). So one gets this result. Max principle stress of 528MPa in the pin. The flange which has an 11mm thick web is not at all stressed, unlike the <6mm web on the Viking engine.
That got me to thinking about that stress level on the pins or bolts, because it could certainly get that high in the automotive application. Perhaps peak BMEP does not make it to the gearbox output flange in the car, given it has to pass through the torque converter and various clutches first. But that lead me to thinking about the bolts and how strong bolts really can be, so I looked up the properties of grade 12.9 bolts.
Based on the data above, it seem that proof stress is 970MPa which would give us a nearly 2x factor of safety in the application?
Does this sound right or is the assumption that the load case in the vehicle is actually lower and the bolts never get stressed this much ?
Thoughts ?
