Vibration of Machinery

1Q. What one thing is most important when measuring a machines vibration?

1A. There are many great tutorials available from vendors of vibration monitoring equipment. However most will agree that it is important to have established a baseline on your equipment, meaning what was its vibration level and frequency when the machine was operating well. A good history base will make changes quite obvious. The allowable vibration level of a rock crusher or gas engine compressor is not the same as that of an induction motor. Likewise taking a velocity reading on the outside of a 50 ton gearbox may not tell you what is happening deep within the gearbox on a single bearing. The signal may be highly attenuated. Then again you may be able to learn something if you had historical and trending data and a Specialist who had the experience to make sense out of it.

2Q. You mention analytical modeling on machinery. Is it important to model vibration?

2A. Good question. It's very important. Vibration related problems can be very costly to solve, especially in a new piece of equipment. I have been on ship systems that have been snapping its propeller shaft and the proposal was to go to a bigger diameter shaft (60 ft long and 8 inches in diameter). This was exactly the wrong way to go as the shaft needed to be smaller to get a critical frequency out of the operating range. A simple analytical model on the back of an envelope proved this.

3Q. I've heard the term sympathetic vibration used. What does it me?

3A. It hasn't been used in a long time, but I believe it means the same as resonant frequency.

4Q. What is a single degree of freedom problem and why is it important?

4A. If you hung a weight on a vertical spring pulled it and let it go it would vibrate at it's natural frequency and this is a single degree of freedom. Now if you had a flat plate and hit it, it would vibrate at many frequencies but one would be called the first or fundamental frequency, much like the single degree of freedom spring. However the total motion is the vectorially sum of all the modes. The first mode is usually the one that has the most motion and can be excited with the least amount of energy and in many cases is the most important. It is also simple to model and to understand both the frequency and amplitude.