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Practical Solutions to Machinery and Maintenance Vibration Problems

Chapter 3, Detuning and Proving Resonance

Section 22, Use of Dynamic Absorbers

For almost all non-rotating parts that are resonating, the cure has almost always been to either detune the part by changing its natural frequency, reduce the vibration at the source, such as by better balance, alignment and so on, or to change the source frequency, such as rpm or number of vanes, etc. However, sometimes none of these methods are very practical for the specific situation, due either to the excessive cost of shutdown or the cost of making the necessary changes. There is still another way to reduce the vibration that has been magnified by resonance. It is through the use of what are called "dynamic absorbers." As these are resonance devices that are tuned to the offending resonance frequency, Update often refers to them as "tuned absorbers."

Although tuned absorbers are very helpful in many resonance situations, their use is not always preferred over the usual resonance correction techniques. In one sense, they are a "band-aid" solution. However, for certain vibration situations described later, they will probably be the easiest to accomplish and least expensive solution, often without requiring machine shutdown.

A word of caution: absorbers work only for resonance situations where the source vibration frequency is not going to change, such as for constant speed machines. For variable speed machines, a simple absorber as described will absorb the frequency for which it is designed, but will also magnify the vibration of frequencies just below or above that frequency. For variable speed machines, there are special, more complicated designs of tuned absorbers. Such designs are usually found in relatively technical engineering textbooks and are not considered here. For variable speed machines, either the usual methods for curing resonance should be used or perhaps the resonating part could be corrected by additional damping. (Damping is very effective on a resonating part. For a non-resonant part, damping has very little effect.)

As shown in Fig. 1, the absorber is actually very simple in principle and design. Usually it is simply a length of flat steel bar that is tuned to the desired resonance frequency by a sliding weight.

Determining bar dimensions and sliding weight used to appear complicated. However, the simple algebraic formulas are now readily available in more technical vibration textbooks and are easily programmed into most vibration computers. However, to get an idea of the range of sizes, usually the steel bar's thickness can be anywhere from 1/8" (approximately 10 mm) to 3/4" (approximately 20 mm) and widths of 1" (approximately 30 mm) to 4" (100 mm). Sliding weights vary from approximately a fraction of a pound to 4 or 5 pounds. Tuning is also determined by the bar's length which can vary anywhere from a few inches to 3' (1M). (Approximations can be used, but computer guidance is faster and recommended.)

These ranges are not specific guidelines but give an easy reference for the range of sizes. A small absorber, for example 1" wide by 1/4" deep and 10" long with an appropriate sliding weight, can be tuned to a specific frequency. A much larger absorber, such as 3" wide by 3/8" deep and 2 ½' long, and an appropriate sliding weight, can be tuned to the same frequency. For a specific situation, the size of the absorber is determined by the size of the resonating part.

For many maintenance departments, a quick solution is needed before a computer program is available. Therefore, a simple method (without calculations) is as follows:

1. Choose what seems to be an appropriate bar of steel with a reasonable length, considering the size of the resonating part to which it will be attached.

2. Choose an amount of sliding weight consisting of a few parts of scrap steel, clamped to the bar with a small clamp.

3. Cantilever the bar from a relatively sturdy shop bench vise.

4. Add the sliding weight, clamping at approximately 1/3 the distance from the tip.

5. Apply vibration pickup to the bar, relatively near the vise.

6. Use normal resonance bump test procedure to determine the bar's resonance frequency.

7. Adjust bar's cantilevered length until the resonance frequency is within about 20 percent of the desired frequency.

8. Adjust amount and/or position of the sliding weight until the desired resonance frequency is obtained; mark that position on the bar.

9. If the desired results are not obtained, try another combination of bar sizes and weight. Mark the bar's cantilevered length and the amount of weight. Convert these numbers to a finished bar that can be clamped to the resonating part. Fabricate a proper sliding weight and clamping procedure that will approximately equal the previous system.

The absorber can now be attached to the resonating part, positioned at an antinode or worst amplitude of vibration. It must be oriented so that the tuned absorber will resonate in the same direction as the original resonating part.

If the machine is shut down, use the bump test to tune the absorber by positioning the sliding weight. If the machine is running, adjust the sliding weight's position until the maximum amplitude is obtained on the absorber. If the absorber is not too large or too small, the absorber should be resonating and the part to which it is attached should not.

Absorbers may be the only fast and easy solution for a vertical pump that has its total framework resonating as a cantilevered structure. In such pumps, changing the total structure's cantilevered resonance frequency may be difficult, and a tuned absorber may be the only practical solution. The same applies for absorbing the resonance vibration from a resonating bearing housing. Bearing housings are usually relatively rigid and, therefore, have higher resonance frequencies. They usually resonate to high rpms or higher frequencies, such as vanepass, gearmesh, etc. Sometimes bearing housings are resonant to one of their own bearing's defect frequencies. Absorbers work well on pipes, beams and columns. It is still usually better to cure the resonant situations through the regular procedures.

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