Breve Tutorial De Analisis De Vibracion En Maquinaria Rotativa

A BRIEF TUTORIAL ON MACHINE VIBRATION
by Victor Wowk, P.E.

MACHINE DYNAMICS, INC.
The purpose of this tutorial is to provide sufficient knowledge to understand machine vibration diagnosis. You may be tasked with solving a vibration problem, or you may be overseeing someone else and you need to understand the process. This tutorial discusses the symptoms, taking measurements, analyzing thedata, additional testing, understanding the physics, and finally, fixing the problem. It provides practical information that has proven useful over several decades in correcting all types of machine vibration problems, with a few tenacious exceptions. There are usually several technical solutions to any vibration problem, and this tutorial will guide you through the choices. The first step is tounderstand the problem from its outward symptoms. This means observation and measurements to quantify the symptoms, and then analysis to interpret the data. It is usually best to proceed from a position of knowledge while deflating opinions that are not supported by the data. The diagnosis is the shorter part of the journey, taking one person several hours to accomplish, but is the most importantpart because recommendations here will commit many persons and many hours to remedial work. We would like the correction to be successful on the first attempt. Strategy All vibration is not bad. Machines produce some oscillatory motion as part of their normal operation and these are nothing to be concerned about. I call these benign vibrations and here are some examples: • 120 Hz motor hum • bladepassing frequency • pure tones from motors, especially those driven by VFD’s • broadband turbulence from fluid handling machines, like fans and pumps • gearmesh frequencies These benign vibrations are characteristic of regular operation of a machine doing what it is supposed to do. The amplitudes will vary from machine to machine and are a measure of the quality of manufacturing and load condition.The presence of these benign vibrations at “normal” levels provides a comfortable feeling that the machine is still alive. A change above normal levels not explained by a corresponding load change is reason for investigation, but not alarm. Serious vibration are: • 1xRPM amplitudes above the balance limits in Table 1. • shock pulses • large shaking motion • abnormal noise These serious vibrationswill cause accelerated wear and premature failure. They should be acknowledged as damaging and addressed with some corrective action. The first task for the vibration analyst is to obtain frequency selective amplitude data to identify the source of vibration. The frequency is the key information that establishes the possible causes and then the amplitude is used to judge the severity. This meanshaving a vibration analyzer at one’s disposal. More on this in the section “Instruments and Methods.” This takes us to the first fork in the analysis flowchart, Fig. 1. The appropriate question to ask when armed with the frequency and amplitude data is whether the vibration is benign or serious. If it is benign, then we can safely ignore it. It may, however, cause concern for a sensitive occupantnearby by virtue of transmitting to that space and interfering with that process. An example is a rooftop fan that shakes a delicate optical microscope below. In this case the fan may be O.K. but we need to treat the path of vibration. The corrective action to take for benign vibrations that are not bothering anyone is to explain this
- Page 1 of 10 -

Vibration

Serious

Benign

Reportnothing to be concerned about

Forced

Natural (resonance)

Fixes

Diagnostic Journey

• Change Bad Parts • Balancing • Alignment

Remedial Journey • Change Speed • Change fn (by stiffening) • Add Dampening • Reduce Source Input • Dynamic Absorber

Figure 1 Vibration Flowchart

to those concerned, and perhaps monitor this vibration for a time to verify that it is not trending...