crankshaft stress calculation 2

[elogesh]

Hello,
This is regarding automotive compressor crankshaft stress calculation. We have compressor crankshaft driven by engine through belt drive. It was decided to calculate the stresses for the newly developed compressor crankshaft. We decided to start simple and then later on to add complexities one by one.

We felt the crankshaft is subjected to following forces,

Load from connecting load during compression of air (act as bending on crankshaft eccentric pin) - Fc
Load from belt as belt tension load - Fb
Imbalance load - Fi
Vibration load - Fv
Torsional load due to torque transmission - Ft

Load on eccentric pin fluctuates from 0 to positive Fc

Load from belt tension remains constant, Fb

Imbalance load (I feel the system is partially well balanced even though it is single cylinder reciprocating compressor and can be ignored)

Vibration load: Entire compressor vibration may transfer to the crankshaft. Not quite sure of load transfer path mechanism and consider including later.

Torsional load: Torque transmitted from engine through belt drive-Ft

Then assumed hydrodynmaic-bearing supports as simply supported.

It is decided to consider compression load as fluctuating and belt tension load and torsional load as non-fluctuating.

Then the stresses evaluated independently for each load case, at the eccentric pin location, including stress concentration factors for the fillets.

Then using Good Mann diagram with belt tension load, torsional load and ½ of air load as mean stress and ½ of air load as alternating stress, factor of safety is evaluated for fatigue.

Whether the approach, mentioned above is appropriate? If not please comment.
I look forward for your suggestions.


We felt we have not considered following aspects,

1) We have not considered multiple load combination of many different cycles. For evaluation of various load combination we need kinetostatic simulation using package like Adams, LMS-Motion simulation, etc. Then we have to use Miner’s rule to determine the cumulative fatigue damage…
2) We have not considered inertia effects…


Usually how the crankshaft is evaluated for strength using hand calculations before the evolution of computer based numerical techniques?



Regards,
Logesh.E

 

[ivymike]

It sounds like you're working in the right direction. Using simple supports at the main bearings is reasonable. I don't like the way you've addressed the gas loading. A more conventional approach would be to calculate bending moments on the crankshaft due to pressure and inertia loads at a variety of points in the cycle, and use the max and min moments in your calculation of mean and alternating stress. You should be able to arrive at max and min bending moments relatively easily even if working by hand. I would also allow the torsional load to vary, and calculate its maximum and minimum. For simplicity I would assume that max torque happens at the same time as max bending, and that the min of each happen at the same time as each other. It would be good to include the effects of reciprocating assembly inertia in your calculation as well, at the same places where you evaluate bending due to pressure.