Detuning and Proving Resonance - Where to Measure Plotting Points

Practical Solutions to Machinery and Maintenance Vibration Problems

Chapter 3, Detuning and Proving Resonance

Section 2, Where to Measure Plotting Points

The following diagrams show typical sections that may be resonant (but not all resonant at the same time, or at the same frequency).

There are instruments and special computer programs for determining mode shapes of various parts in order to identify possible resonance problems. They not only plot the mode shape, but animate it. As most of those viewing this course work on machinery vibration from a maintenance or field service perspective and do not yet have such instrumentation in their plant, the following procedure requires only the most basic vibration instruments.

Procedure:

If the vibration is large in one direction and relatively small in the other direction, and if time is short, take readings only in the direction of the high amplitudes.
If vibration is high in two directions (such as often occurs in pipes), make two sets of plots--one plot for each direction.
Divide the suspected part into approximately 10 to 12 roughly equal parts. Example: for parts about 10 inches long, have spaces about one inch apart. For longer parts such as lengths of pipe, papermachine rolls, or building structural members of about 10 feet long, use spaces of approximately one foot. The spacing does not have to be exact. You do not have to use a tape measure. Approximation is close enough. Accuracy within about 10 percent is acceptable. Using this spacing for a suspect part will result in a similarly shaped resonance curve for a resonant part whether it is relatively small or large. For example: a 10 inch long pipe nipple that is resonant at its first resonance frequency will give somewhat the same characteristic mode shape (curl) that will be given by a 20 foot length of pipe that is also resonant at its first resonance frequency.
If amplitude is measured in displacement units on the vertical axis, use a scale of approximately ½" = 1 mil. For metric, use a scale of 10 mm = 20 or 25 micrometers. For velocity units, use a scale of approximately ½" = 0.1 in/sec or approximately 10 mm = 3 mm/sec.
Amplitude is plotted on the vertical scale. The positions where the measurements were taken are plotted on the horizontal scale. It is suggested you use alphabet letters for indicating position where the measurements were obtained so as to not be confused with instrument readings which are usually given in numerical form.
Put the horizontal "zero" amplitude line at about mid-page.
On the graph paper's horizontal axis, the distances between positions where the measurements were taken should be approximately one inch (or 20 to 25 mm).
Before plotting, divide each amplitude reading by two, and plot a positive and negative point for each (half plotted above the line and half plotted below the line). The result will be an enlarged view of the vibration mode shape.
Resonant parts will exhibit much more pronounced curling than non-resonant parts. In general, the plotted mode shapes of resonant parts will be similar to the mode shapes in the diagrams.

However, the more flexible the bearing support system, the less pronounced the curling will be, and the less closed the nodes at the end will be.

Caution: For the person new in vibration control, it's easy to suspect that any part that vibrates badly is resonant. It may be vibrating badly, though not be resonant, due to high flexibility, or more probably, due to high source vibration.

Unfortunately, using knowledge about nodes and antinodes to determine a resonant condition is not always familiar to others with whom you are working. For example, supervisors, other engineers, or perhaps a vendor's machine serviceman may have good knowledge of many facets of vibration and yet not be familiar with this part. Therefore, proving that the part is resonant (or not) may require making temporary changes that show that the vibration is actually lowered. A subsequent section on "Confirming a Parts Resonance Through Quick and Temporary Means" provides several means to prove your answer to others, as well as yourself, before making permanent corrections.

Using Phase to Help Determine Mode Shape

Put the pickup at a section at or near the high vibration at one of the antinodes, and obtain the vibration's phase. Repeat for other antinodes, and compare phase angles to determine how one part is vibrating with respect to the other.

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