Sunday, September 30, 2007

Doing Vibration Research

At the suggestion of Paul J, I started doing research on vibration analysis today. There are several online tutorials regarding vibration analysis. I'm currently reading this one:

http://www.dliengineering.com/vibman.htm

This paragraph taken from the "Natural Frequencies" section is somewhat noteworthy:

'The multitude of spring-mass-damper systems that make up a mechanical system are called "degrees of freedom", and the vibration energy put into a machine will distribute itself among the degrees of freedom in amounts depending on their natural frequencies and damping, and on the frequency of the energy source. For this reason, the vibration will not be uniformly distributed in the machine. For instance, in a machine driven by an electric motor, a major source of vibration energy is residual imbalance in the motor rotor. This will result in a measurable vibration at the motor bearings. But if the machine has a degree of freedom with a natural frequency close to the RPM of the rotor, its vibration level can be very high, even though it may be a long distance from the motor. It is important to be aware of this fact when evaluating the vibration of a machine -- the location of the maximum vibration level may not be close to the source of the vibration energy. Vibration energy frequently travels great distances along pipes, and can wreak havoc when it encounters a remote structure with a natural frequency near that of its source.'


Based on the paragraph above, I suspect that the AC motor is providing a small excitation around 5500 RPM and that is causing major vibrations out in the extremities of the transmission. The transmission mounts are original and cracked, so replacing those might help a bit.

One of my coworkers said he would let me borrow an accelerometer to attach to my oscilloscope. I'll probably take him up on the offer and see what I can find.

4 comments:

Roger Daisley said...

Tim: I'm sure, for an engineer, all this analysis is very fun and rewarding. My gut instinct tells me that your technical evaluation are right on track. Probably you'll find the solution in several places, including firming up the motor and tranny mounts (not rigid, but firm) and also perhaps replacing some of the bearings in the tranny ... especially the input shaft bearing.

Carlo said...

Tim, for my day job, I work with boilers and pumps (with an emphasis on controls). As I mentioned before, I work a lot with 3-phase variable frequency drives on pumps. When we get vibrations, they are often at specific hertz values or RPM values as you have noticed. Unfortunately, your controller doesn't allow you to program skip frequencies or skip RPM's. The bad news is that it is very difficult (in my experience) to eliminate these without that feature built in to the VFD. If the pump impeller is changed (akin to changing your transmission), usually the problem either goes away or changes to another frequency.

I really don't know why they didn't add that parameter. It would have been handy to have in there even if it was only used 5% of the time.

Next time I go to Portland I would love to see your car (I live in Seattle).

http://www.austinev.org/evalbum/preview.php?vid=1333

TimK said...

Hi Roger, thanks for the kind words. During these experiments, I'm not even spinning the tranny, just adding the weight of it to the system. I actually replaced all the bearings including the input gasket during the rebuild several months ago. I have the new tranny mounts on order from Pelican Parts, but I'm not sure those will be enough.

Hi Carlo, you're right that the DMOC doesn't support RPM skipping. I'm curious, how does one accelerate through a particular RPM if the controller skips it? I've heard other people using the transmission to maneuver the engine around a particular RPM, but the motor still has to pass through it.
Let me know if you're in the area and I'd be happy to give you a tour (and a ride if the car is drivable).

Cheers,
Tim

Carlo said...

The VFD just skips right over it. It seems a little strange (and not always smooth) to go from 39Hz right to 45Hz, but if the harmonic imbalance is gone, then it is better overall.