Nuclear Power Plant case study of how a combination of electrical and vibration trending programs saved them tremendous costs and allowed for a planned maintenance instead of an overtime catostrophic reaction to a sudden failure. (Baker Instruments supplied this case study)
Using any vibration measuring tool that provides vibration amplitudes (how much vibration) and at what frequency (how often over time) that this amplitude of vibration occurs, these basic rules can be used to take action to fix what specifically is causing the destructive vibration.
Whether your vibration meter or data collector gives you graphical readouts showing vibration amplitude on the Y-axis (the vertical axis) and frequency on the X-axis (horizontal), or your vibration meter merely provides numerical readouts of vibration amplitude at a specific frequency, using the basic rules of thumbs for rotating equipment gives you a good starting point of what to correct.
One important and easily known piece of the puzzle to keep in mind is the rotating speed of the equipment you are measuring/analyzing. With electric motors and equipment driven by electric motors, you can usually work backwards and tell the operating rotational speed of the motor from the vibration readouts. Standard AC electric motors, due to the magnetic poles that make them run, generally run close to one of the following Rotations Per Minute (RPM):
3600 RPM (More Common)
1800 RPM (More Common)
1200 RPM
900 RPM
Electric motors have Nameplates installed that provide the approximate fully-loaded running speed including what is called "slip". When it comes to using vibration analysis to fix problems, all you care about is knowing the actual real running RPM.
Usually (sorry to keep using that same qualifier), usually your first vibration amplitude peak will be showing up at one times the actual rotating speed of the electric motor. For example, if you have an electric motor that is running at 1,750 RPM; your first noticeable vibration amplitude peak will show up at a frequency of 1,750 Cycles Per Minute (CPM) - or 29.16 Hertz (Hz) (Hz is a unit that means 60 times per second - so to convert something happening at 1,750 times per minute to Hz - you would divide 1750RPM by 60 to get the equivalent Hz).
The many different vibration meters use various units to denote frequency. Many higher-end vibration meters can vary what type unit (CPM, Hz, or Orders of RPM) you want to use for frequency display. What you need to know is how whatever unit of frequency your meter is displaying relates to the actual rotating speed of the machine you are measuring. As in our example, if our meter is showing a high vibration is occurring at the same frequency as the motor's rotating speed, we know we most likely have to balance the rotating part of the machine to fix it.
A high vibration at a frequency that coincides with the running speed of the electric motor is almost always related to an out of balance condition. To explain further, the rotating component of the machine has a heavy spot. Because of centrifugal force, each time this heavy spot passes by your vibration meter's pickup (transducer) it reads a vibration caused by the centrifugal force pulling the "heavy spot" away from the machine's centerline. This heavy spot passes by the meter's vibration pickup one times the running speed of the motor.
So using our example motor that is running at 1,750 RPM, if the rotating part of this machine had enough of a problem imbalance (heavy spot), we would see an excessive vibration amplitude at a 1,750 CPM, (or 29.16Hz) ... which is one times rotating speed of our example machine. If we had a vibration amplitude (how much vibration force) reading of say .15 inches/second occurring at a frequency of 1,750 CPM then our first conclusion is that we probably need to fix a mechanical out of balance problem. We need to either add weight equal to the heavy spot exactly opposite from the heavy spot or remove weight from the heavy spot.
To visualize the concept, imagine a big cylinder sitting on 2 rollers. Now take a big clump of clay and slap it onto the top of the cylinder - what happens - the cylinder rolls until the clump clay is hanging at the bottom. Even if you roll the cylinder so the clump clay is back at the top, the cylinder will roll back around until it comes to rest with the clay at the bottom. But if you take another clump of clay of the same weight and attach it on the opposite side of the cylinder, no matter where you move and stop the cylinder, it stays put because you have balanced the cylinder.
With our real world motor, exactly where to make our corrective weight addition or subtraction is determined by stopping the machine and adding or subtracting a trial run weight and then restart the machine to see what effect this trial weight had on increasing or lowering the one times running speed vibration amplitude. There are many balancing software programs available to reduce the number of subsequent trial balance runs that you need to make after the initial trial run. You feed the results of your trial balance run into the software and the program calculates how much weight and in relation to the first trial weight where to place the corrective weight.
This method of balancing assumes you have access to the rotating part of the machine to add or subtract weight while the machine is assembled. If no access to the rotating component is available you would then have to dismantle the machine and have the rotating component balanced in a balancing machine (which usually can provide a better balance).
Another rare but frustrating event regarding rotating machine balance is that in some machines and applications the rotating component physically changes under load and/or temperature. An example of this are some of the modern large 3600RPM electric motors have what are called "flexible rotors". In an effort to increase energy efficiency the rotating component (rotor) is designed longer with a smaller diameter. At full loaded running speed and heat from their load these "flexible rotors" may physically change moving a part of themselves away from the centerline of the machine and develop an imbalance despite their having achieved an excellent balance in the dynamic balancing stand. Different balancing methods are employed for "flexible rotors" such as full rated speed runs and heat application while under full speed in the balancing stand. If the problem develops after the machine is installed, "hot balancing" can sometimes solve a 'flexible rotor" problem. "Hot Balancing" is using the methods of trial weight balancing described above after the machine has been running at rated speed and load which heats the rotor causing the excursion or mechanical movement resulting in a heavy spot or imbalance under running load.
Finally, imbalance can sometimes exacerbate or influence other machine or mounting conditions or defects which can make the vibration problem solving confusing - one problem masking another. For example, a motor with an imbalance problem can be installed on a mounting base that just happens to have it's natural resonant frequency excited by the motor's imbalance problem. All physical masses (such as equipment bases, railings, attached driven equipment, etc) have an inherent natural resonant frequency - or "reed critical frequency". When an outside influence such as an electric motor or other nearby equipment shakes that physical mass (like a motor mounting base) at it's resonate frequency then it begins to actively and destructively increase vibration - just like when a tuning fork is struck - or when a singer hits a note that begins vibrating a glass at it's reed critical frequency until is shatters. The point being, the mounting base may be vibrating and taking out bearings or pump seals but the real culprit could be the out of balance motor rotor which is influencing the base's reed critical frequency.
But getting back to the routine usual basics of vibration - or rule of thumb - an objectionable amplitude (amount or level) of vibration showing up at one times the rotating speed of the machine usually means the rotating part(s) are out of balance.
If you found this technical piece useful, check back on this website as we will post a series of articles about how to use vibration meters and basic analysis to detect bent shafts, misalignment, and bearings that are going bad BEFORE they seize and destroy your machine. The goal of this and upcoming articles on vibration is to keep things in a basic real world understandable environment.
