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Defects occur at specific frequencies in relation to the running speed of the equipment. Most vibration analysis software will allow these specific frequencies (bands) to be measured and trended over time. Trending this information will help identify problems as they occur in your equipment. This results in more accurate analysis of equipment problems that will help determine the severity and repair urgency of the problems identified.

For example, if the vibration trend is increasing slowly, then the failure may not be progressing rapidly. However, a sharp increase in a specific vibration trend over time indicates that a defect may have developed and failure is more imminent.­

We often hear about the need to acquire vibration measurements at precisely the same location each time we measure a point. Why is this important? In order for vibration measurements to be trendable they must be closely repeatable, and we need to eliminate measurement error. With high-frequency measurements, the vibration attenuates rapidly as it travels away from its source. The author has seen readings vary by as much as 50% when the collection transducer was moved by as little as ¼ inch.  If the transducer is not placed in the same location, the trended data will show an error that may be mistaken for a change in machine condition. When the collection point is different, the transmission path is either longer or shorter. This affects the amount of energy perceived by the transducer. Standing waves also exist in vibrating machinery. The transducer may sometimes be located at a nodal point of one of these waves; and if care isn’t taken in transducer placement, the next measurement may be at an anti-node. This is more apparent in larger machines because of the amount of surface area available for standing wave formation. There are several ways to precisely mark data points for measurement with a magnet-mounted transducer. Paint, glue-on-pads, stud-mounted pads, machined surfaces, and dimples made by a small drill bit are all used with success. Best of all are permanently mounted coded attachment studs (such as VIBCODE®) that guarantee precise re-placement of the transducer every time, at the same location, angle, and pressure. Regardless of the method employed, it is important to always precisely identify data collection points.

Many vibration programs fail because they become too complicated. Too much data can sometimes become more confusing than too little data. Many potential machinery problems can be eliminated with the analysis if one keeps in mind several simple concepts:

1. Vibration units such as acceleration are more sensitive to high frequencies than low frequencies.

2. Vibration units such as displacement are more sensitive to low frequencies than high frequencies.

3. Velocity units are evenly sensitive between about 60 CPM to 60, 000 CPM.

4. High-frequency vibration does not travel far and degrades rapidly through metal seams.

5. In general the closer your measurement is to the source of the vibration the higher the amplitude will be.
These differences can be used to zero in on machine faults.

Example: Take a generic 100 HP motor. If an outboard rolling element bearing begins to fail because of a lack of lubrication the first indicator is high-frequency ringing from the bearing. This is characterized by a large increase in acceleration amplitude and a small to no increase in velocity or displacement. Now you have identified that there is a high-frequency problem and not a low-frequency mechanical problem. You can eliminate low-frequency sources such as looseness, unbalance, or misalignment. What is the most likely source of high-frequency vibration on the back end of a motor? Probably a bearing or shaft or rotor rub. Now you can apply a simple test. Grease the bearing and see if the acceleration returns to normal. If it does, you have nailed the problem without knowing the bearing frequencies or even taking a spectrum. Come back the next day and see if the acceleration is back up. If it is, you either have a lubrication problem with contamination or a loss of grease, a damaged bearing, or both.

While not perfect, understanding the behavior of vibration units combined with a mechanical understanding of machinery can help you quickly identify machinery problems.

How fast is the data collection speed of your vibration data collector? You may feel that this is not an important characteristic. Does it really matter if a data collector is one or two seconds faster-acquiring data versus another? Data collection speed is very important and should be taken into consideration by your facility management and Condition Monitoring Group. For example, consider a vibration monitoring program that monitors 1000 machine trains per month with 10 measurement points per machine train. If a vibration data collector requires 8 seconds to acquire data for each measurement point, then it would necessitate 22 hours of real data collection time (this does not include time moving between each machine, reporting time or etc). Does a second really make a difference? A data collector that is one second faster acquiring the same data will result in 3 hours per month saved or almost one full man week per year. If the data collector is 3 seconds faster in acquiring the same data, then the time savings are more substantial. A savings of 3 seconds per measurement point will result in a savings of 2.5 man-weeks per year. Multiply the time savings by your labor cost per hour and the savings could be very important to your facility. The savings may surprise you!

Take a look at our new VIBXPERT II analyzer featuring a crisp color display, fast data acquisition, and powerful vibration diagnostics tools.

All laser shaft alignment systems should be able to take two sets of readings on well-built machines and find agreement at the coupling within 0.5 mils offset and 0.1 mils/inch angularity. It is important to always take two sets of readings immediately after setting up the brackets and compare the coupling results for repeatability. A good laser shaft system should have the capability of automatically recording the misalignment information for the user to compare for repeatability. If the readings do not repeat the problem may lie within the machines rather than in the bracketing or the shaft alignment tool itself.

Watch our Webinar: Laser Shaft Alignment: Repeatability & Accuracy Issues. Troubleshooting Your Readings.

Fault frequencies are very important in vibration analysis because they allow the analyst to correlate vibration data to specific components in the equipment that may be in some stage of failure (equipment faults). Fault frequencies change with any adjustment in the speed of the equipment being monitored. Most modern vibration data collectors and vibration software will automatically re-calculate the displayed fault frequency information as the rotational speed of the equipment changes. Component information (bearing information, gear information, etc.) is required to calculate and display the fault frequencies of specific components in machinery. It is important to create fault frequency setups at the beginning of a vibration analysis program. Not doing so will affect the overall success of the vibration analysis program.