Vibration sensor, using centripetal accelerometers
I'm having a hard time visualizing your setup.  The RPM/centripetal acceleration calculation makes sense, but I can't see what you're trying to measure with the second equation.  

Since you're using a 5g sensor, I'm guessing your rotational rate is slow enough that the acceleration due to gravity is going to be a significant error term in your calculations. The RPM equation will average this out, but the second will not (Although I'm not sure what it will do).  Another characteristic of your sensors is that they're limited to 400Hz signals - How fast is your shaft spinning?  

Can you further explain what the maths you're using are supposed to be doing or how they were derived?