Why forces and torque applied on a rotating body like shaft rotating and moving forward through a dense medium (ex. rotating turbine shaft or cutting tool,friction stir welding tool while passing through the weld line) plotted with respect to time exhibit a frequency pattern.Or in simple manner why torque and force fluctuations occur although the shaft is rigidly fixed?
Hi Abhijit
I believe most of your questions and their follow up questions are answered in this material
http://qringtech.com/learnmore/recommended-reading/
Sincerely
Claes
I think the simplest answer is, torque is not constant at all RPM!
Here is an example scenario. You start the machine, let's say it uses an electric motor. Initially, and at low RPM, torque is high. RPM increases. As RPM increases, torque decreases. Eventually, RPM will begin to decrease, because the torque won't be high enough to keep rotation constant through the dense medium.
So RPM begins to drop. Then torque goes up again, with reduced RPM.
The system should be definable as a differential equation. It may go into oscillation, as what you described.
Parenthetically, words alone are not always convincing. You might well ask, why doesn't the system stabilize? The answer is, it might. However here is one obvious example where it won't. Assume that the drop-off in torque, at a given RPM, is drastic. What will happen? Speed increases, suddenly very little torque from the motor, and the shaft cannot help but slow down. Then, lots of torque again at the slower RPM, and the shaft will speed up. So, whether the system oscillates depends on viscosity of the medium and on the torque/RPM curve.
Any other contributing factors may or may not exist. The shaft might be somewhat flexible, as you also posit, which would also tend to create oscillations.
So here's a fun exercise. One should be able to damp or eliminate oscillations, by tuning the flexibility of the shaft to the torque curve of the motor. In short, the shaft can store some torque by twisting, and then it can release that torque while the electric motor slows down due to lack of torque. This allows the electric motor to slow down, with no change to RPM at the other end of the shaft, because now the shaft is untwisting itself.
Then the torque of the electric motor rises again, as the RPM of the electric motor drops. Eventually, the increase torque of the electric motor begins to re-twist the shaft as the motor increases in speed again.
So, if perfectly tuned, the two effects will cancel one another, at the end of the shaft.
I think is depend on the some factors, the important is the friction forces that may be couses heated and sliping some infinite time then the tourque
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