Defining vibration

What is vibration? And what do we mean by levels of vibration? The dictionary defines vibration as “a periodic motion of the particles of an elastic body or medium in alternately opposite directions from the position of equilibrium, when that equilibrium has been disturbed or the state of being vibrated or in vibratory motion as in (1) oscillation or (2) a quivering or trembling motion”. The key elements to take away from this definition are, vibration is motion, and this motion is cyclic around a position of equilibrium.

How many times have you touched a machine to see if it was running? You are able to tell by touch if the motor is running because of vibration generated by motion of rotational machine components, and the transmittal of these forces to the machine housing. Many parts of the machine are rotating, and each one of these parts is generating its own distinctive pattern and level of vibration. The level and frequency of these vibrations are different, and the human touch is not sensitive enough to discern these differences. This is where Vibration Detection Instrumentation and Signature Analysis Software can provide us the necessary sensitivity.

Making sense of vibrations

Sensors are used to quantify the magnitude of vibration or how roughly or smoothly the machine is running. This is expressed as vibration amplitude. This magnitude of vibration is expressed as:

• Displacement – The total distance travelled by the vibrating part from one extreme limit of travel to the other extreme limit of travel. This distance is also called the “peak-to-peak displacement.”
• Velocity – A measurement of the speed at which a machine or machine component is moving as it undergoes oscillating motion.
• Acceleration – The rate of change of velocity. Recognising that vibrational forces are cyclic, both the magnitude of displacement and velocity change from a neutral or minimum value to some maximum. Acceleration is a value representing the maximum rate that velocity (speed of the displacement) is increasing.

Properly performed and evaluated Vibration Signature Analysis requires highly trained and skilled individuals, knowledgeable in both the technology and the equipment being tested

Various transducers are available that will sense and provide an electrical output reflective of the vibrational displacement, velocity or acceleration. The specific unit of measure to best evaluate the machine condition will be dependent on the machine speed and design. Several guidelines have been published to provide assistance in determination of the relative running condition of a machine. But my strong suggestion is that vibration data be sent to a qualified vibration specialist company for analysis. I must emphasise that it is impossible to establish absolute vibration limits. However, in setting up a predictive maintenance programme, it is necessary to establish some severity criteria or limits, above which action will be taken.

Such limits are not intended to be used for establishing vibration acceptance criteria for rebuilt or newly installed machines. They are to be used to evaluate the general or overall condition of machines that are already installed and operating in service. For those setting up a predictive maintenance programme, lacking experience or historical data, similar limits can serve as an excellent guide to get started, and the vibration specialist company will have all these limits in place for your equipment.

Tools for data crunching

Vibration Signature Analysis can be used in defining the machine location that is the source of the vibration, and in need of repair or replacement

As explained earlier, many vibration signals are generated at one time. Once the magnitude of vibration exceeds some predetermined value, Vibration Signature Analysis can be used in defining the machine location that is the source of the vibration, and in need of repair or replacement. By using analysis equipment and software, the individual vibration signals are separated and displayed in a manner that defines the magnitude of vibration and frequency. With the understanding of machine design and operation, an individual schooled in Vibration Signature Analysis can interpret this information to define the machine problem to a component level.

Depending on the application, a wide variety of hardware options exist in the world of vibration. Although not complicated, actual hardware requirements depend on several factors. The speed of the machine, on-line monitoring versus off-line data collection, analysis needs, signal output requirements, etc., will affect the type of equipment options available.

Regardless of the approach, any vibration program will require a sensing device (transducer) to measure the existing vibration and translate this information into some electronic signal. Transducers are relatively small in size, and can be permanently mounted or affixed to the monitoring location periodically during data collection. In some cases, the actual translation of the vibration to an electrical signal occurs in a handheld monitoring device. A metal probe attached to a handheld instrument is held against a point of interest, and the instrument translates the motions felt on the probe to some sort of electrical signal. Other portable devices use a transducer and handheld data collection device. Both styles will provide some sort of display where the vibration magnitude is defined.

Styles and equipment size vary greatly, but the equipment is designed to be portable. In addition to instruments designed to measure vibration magnitude, many manufacturers provide instrumentation that will perform signal analysis as well. Equipment that feature a stand-alone design performs analysis in the field, independent of computer interface. There are other equipment designs, however, that interface transducers directly with a PC, where analysis software is utilised to interpret the signal data.

Vibration monitoring and analysis can be used to discover and diagnose a wide variety of problems related to rotating equipment. The following list provides some generally accepted abnormal equipment conditions/faults, where predictive maintenance technology can be of use in defining existing problems:

• Unbalance
• Eccentric rotors
• Misalignment
• Resonance problems
• Mechanical looseness/weakness
• Rotor rub
• Sleeve-bearing problems
• Rolling element bearing problems
• Flow-induced vibration problems
• Gear problems
• Electrical problems
• Belt drive problems

Analysing equipment to determine the presence of these problems is not a simple and easily performed procedure. Properly performed and evaluated Vibration Signature Analysis requires highly trained and skilled individuals, knowledgeable in both the technology and the equipment being tested. Determination of some of the problems listed is less straightforward than other problems, and may require many hours of experience by the technician to properly diagnosis the condition.

I have often seen my engineers run into people who are convinced a chiller problem exists due to vibration of the chiller. They typically request the chiller compressor to be dismantled, only to find out that there were no issues with the chiller whatsoever, and that it was a flow vibration. This is why an experienced vibration specialist company must interpret the signatures.

How much and how

As indicated earlier, the styles, types and capabilities of vibration monitoring equipment vary greatly. Naturally, equipment cost follows this variance. Transducers can cost under USD 100. The expected cost for vibration metering devices capable of defining magnitude with no analysis capability is approximately USD 1,000. The cost goes up from there. A high-end vibration analyser with software and all the accessories can exceed USD 30,000. A typical industrial site can expect to recover the cost of the high-end equipment investment within two years. Sites with a minimal number of rotating equipment, low-cost equipment installations, and/or no production-related concerns may find it uneconomically advantageous to purchase a USD 30,000 vibration analysis system. These facilities may be wise to establish an internal programme of vibration monitoring using a low-cost vibration-metering device, and then employ the services of an outside specialist contractor to conduct periodic surveys. These services generally range in cost from USD 300 to USD 6,000 per equipment analysis.

Training for vibration analysis is available through a variety of system manufacturers and vendors. Additional training and certification is available through the Vibration Institute, from where certification for Levels I-IV is available.

In the final analysis, vibration analysis is a must. If you are only going to perform one Non-Destructive Testing – which we do not recommend – it would have to be vibration analysis.

Dan Mizesko is the Managing Partner of Al Shirawi US Chiller Services. He can be contacted at dan@uschillerservices.com

CPI Industry accepts no liability for the views or opinions expressed in this column, or for the consequences of any actions taken on the basis of the information provided here.