A Balancing Act, Part 1 – Vibration Analysis of Machinery
We’ve all been there – started a load of laundry, the washer gets past the agitation cycle and is in the middle of the spin cycle when – Bang! Bang! Bang! And the washer is jumping around as if possessed.
Well, in a way, it is possessed – by an off-balanced load! Rearranging the laundry in the drum and restarting usually tames that wild beast and life returns to normal. The off-balanced load of laundry is a simple example of rotational imbalance – the change of an evenly balanced spinning assembly to a spinning assembly with a shift in the center of mass.
Many different rotating components you use on a daily basis have ways to let you know when there is an imbalance – such as the noisy and jumpy washing machine or the wobble in your steering from an unbalanced wheel. The operation of many types of machinery are also sensitive to balance – ventilation fans, pumps, refrigeration compressors, generators, and mixers (actually, any machine that rotates) are all susceptible to becoming imbalanced. An imbalanced rotating machine can then suffer a multitude of additional problems – worn shaft, premature bearing failure, higher operating current or increased fuel consumption, overheating, or decreased output – which stem from rotational imbalance. Specifically, a worn shaft or premature bearing failure is a direct consequence of imbalance; the imbalance creates forces that are off-center of the shaft – referred to as the axis of rotation – which these components are not intended to carry.
The best way to determine the root cause of these problems is to contract for a vibration analysis. Vibration is the motion resulting from off-axis forces. Recall from introductory physics, force equals a mass times acceleration. The professional conducting a vibration analysis will place one or more accelerometers – sensors that measure acceleration – on static parts of the machinery. From these sensors, the professional will get a time history of accelerations. The professional knows what the orientation of each sensor is – he placed them there – either up/down, side-to-side, or parallel to the axis of rotation. The professional uses a software program to convert the time history into a plot of acceleration as a function of frequency (referred to as a histogram).
Frequency is a count of occurrences per unit of time – normally either counts per second or counts per minute. RPM – rotations per minute – is a specific count of the number of times a shaft turns a full revolution in one minute. When the professional looks at the histogram, he compares the peaks at each frequency to the operational speed (RPM) of the machine.
There are two types of frequency peaks which are of interest – synchronous and asynchronous.
Synchronous peaks are at frequencies which are a multiple of the operational speed and are indicative of imbalance.
Asynchronous peaks are at frequencies which are at fractions of the operational speed and indicate specific bearing failure modes.
Machines with failing bearings will exhibit both synchronous and asynchronous acceleration frequencies. The professional then can use his knowledge of the relationship of each synchronous or asynchronous peak at the different sensor orientations to predict what needs to be changed to alleviate the imbalance and restore order to the machine.
The next blog on vibrational analysis will be a discussion of how vibration analysis can detect resonance in structures to identify vibrations that affect machinery or equipment mounted in or on those structures.
Other associated articles:
Part 2: A Balancing Act, Part 2 – Vibration Analysis of Structures
About the Author
Steven M. Lindholm, P.E., P.M.P. is a consulting engineer with our Oakland Office. As an approved Third Party Organization (TPO), EDT can audit the Safety Management System of towing vessel operators, issue Towing vessel Safety Management System (TSMS) certificates, survey towing vessels, and issue survey reports.
Beyond his capacity as the TPO lead surveyor, Mr. Lindholm consults on inspection, evaluation, and design analysis of ship construction; stability; propulsion and auxiliaries condition assessment; ballast water treatment systems; vibrational analyses; and ship motion. He interprets and applies international (International Maritime Organization (IMO), class society, and flag state), United States Coast Guard (USCG/CFR), and regional regulations/guidelines to maritime casualties. Mr. Lindholm explores root cause investigation and analysis of mechanical damage to equipment, components, and materials, including fracture analysis and failure analysis, and prepares repair and replace cost estimates for marine, industrial, commercial, and residential systems. You may contact Steve for your forensic engineering needs at stevelindholm@edtengineers.com or (925)674-8010.
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