Unlocking the Mystery of Vibration used for Equipment Condition Assessment

John Mitchell

There is no real mystery about vibration and vibration analysis. Even the most technically inept recognize when their car develops a strange sound or vibration. All recognize when a washing machine spin cycle is out of balance. In both cases most know what to do.

Chances are builders and maintainers of ancient war chariots knew they had to have round and reasonably balanced wheels. If excessive vibration caused the charioteer to misaim a spear or arrow the builder and /or maintainer may not be in for a happy or lengthy life! Early Dutch artisans assuredly knew that windmill blades had to be of reasonably the same weight. Otherwise the resulting vibration could shake down the mill. Early in the industrial revolution, millwrights undoubtedly gauged machine condition by touch and sound. It didn’t take much intuition to conclude a noisy machine vibrating heavily wasn’t going to run much longer. High vibration is not a comforting sensation that would lead anyone to a conclusion of well being—personally or for the machine. Especially if the vibration is fracturing pipes, destroying gauges and causing the machine to shed parts! Judging machine health primarily with physical senses, touch and sound lasted into the 1930’s and practically much longer.

The first qualitative criteria to judge machine condition from vibration measurements was published in 1938 by T.C. Rathbone. The famous Rathbone chart introduced a number of profound ideas including a set of frequency versus amplitude severity curves that approximated constant velocity around the rotating frequency of typical steam turbine generators. Severity criteria were based on observations and experience. They linked measured vibration amplitude with condition—and by implication, service life and risk of failure. It is amazing to recognize that the basic concepts and severity criteria developed in 1938 continue to serve us well today; over 80years later!

 Beginning of vibration analysis

The modern era of vibration analysis likely began in 1950 when the late Art Crawford, then a graduate student, accepted a challenge to developa means to reliably balance high speed machine tool spindles. Art’s success led to his forming a company directed to developing and extending the technology. Knowledge that began with dynamic balancing ultimately evolved into frequency analysis and condition assessment.

It should be recognized that vibration measurement and frequency analysis is nothing more than measuring and trending amplitude and frequency values over time. Amplitudes at frequencies that, for the most part can be felt and/or heard. With knowledge gained from a variety of sources and numerous creative people over many years, today’s vibration analyst can link both the presence of a specific frequency or frequencies to a defined malfunction. Measuring amplitude and change in amplitude over time identifies its threat to continued successful operation.

Evolution of instrumentation

Today’s practitioners have little comprehension of the time and difficulty involved with gaining insight into the frequency content of a complex vibration signal when the best, and only, tool at the time was a manually tuned filter. Similar to searching on a dial for a radio station (many probably don’t remember that!) the manually tuned filter opened the window to identifying specific frequencies and frequency patterns associated with common mechanical problems. It was not unusual to spend an hour in a very hot,noisy and generally unpleasant environment to identify sufficient frequency content to assess condition.

Early instruments for recording and detailed vibration analysis were heavy, bulky and cumbersome to use. Many consultants at the time traveled with “portable” instrumentation weighing several hundred pounds, required an SUV for transport to the job site and had to be schlepped by hand to an often remote machine. Then there was the suspicious “old timer,” often verbally doubting what this youngster could determine with his magic instrumentation than he couldn’t with a screwdriver held to the ear as a stethoscope (hopefully pointy end down!). When the “youngster” could point to what he was hearing on a visual display, particularly if there was some periodic variation, the “old timer” came away a believer!

There were humorous stories during the early days when dynamic instrumentation was bulky and weighed a ton. Salespeople showing up in suit and tie to demonstrate their latest gadget were asked to lug a 75 pound analyzer up two floors in a very hot and noisy environment to “demonstrate its portability” and effectiveness! Would anyone offer help or let the sales person know there was an elevator—of course not!

Enter the computer

In the early days of computerized analysis it wasn’t unusual to demonstrate machinery analysis to highly experienced “old timers” who had never before seen a computer! People involved had to develop techniques to bridge between “old timers” knowledge of machine characteristics and failure modes presented with totally new, very mysterious and even threatening technology! The job was made a bit less challenging by the fact that the principles were long known, well understood and provided the basis for successful job performance. Only the technology was new. Technology that provided a wider look, accurate, quantitative values and was fast. Little like a close up video of a tree stump that suddenly expands to a wide and beautiful forest scene! Not much has changed except a lot more information is available in a lot more detail.

In about six decades the technology has advanced from essentially a hand tuned radio that required hours to accomplish a frequency analysis. A frequency analysis that today can be performed with greater range, definition and accuracy with a handheld device. Same process, same information as available to the industrial age millwright; performed instantly and in far greater detail and with a much larger knowledge base. Ain’t technology wonderful?

Contributors to the successful vibration analysis technology utilized today

In closing this brief explanation of the genesis of today’s vibration analysis technology, the contribution of many individuals, most no longer with us, must be acknowledged. Without them, and their perseverance, ideas and drive, today’s technology and practice wouldn’t exist. We owe them our success.