ERIKS Maintenance and Condition Monitoring Technical ... - Eriks UK

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example 30µm at 1000Rpm is just as destructive as 300µm at 100Rpm. This makesthe setting of alarms cumbersome to evaluate machinery condition. It must beremembered that each source ofvibration contributes to the ultimatefatigue of the machine. The overallcondition of the machine can only bedetermined by an overall measurementof vibration that takes into considerationall frequencies of vibration withrelatively equal weighting. This isaccomplished by using the firstdifferential of vibration with relativelyequal weighting. This is accomplishedby using the first differential ofdisplacement, called vibration Velocity.If any item is struck it will tend to vibrateor ‘ring’ at its natural frequency. If a flaw exists on a bearing this will act like a tinyhammer and cause the bearing to ring at its natural frequency and whatever thebearing is connected to mechanically will ring at its’ natural frequency. The same istrue for a defective gear. To calculate all these natural frequencies would be atremendous task. The vibration data collectors used by ERIKS filter out all frequenciesbelow a set value, (usually 300 000Cpm (5kHz), but can be varied) then take theoverall amplitude of the remainder. This gives a reading that is directly proportional tothe amount of metal-to-metal contact in a system. The unit given to this reading is gSE,gE peak-vue (dependant upon equipment manufacturer As it is a high frequencyreading spike energy does not travel very far, thus wherever the highest spike energyreading is, is where the highest amount of metal-to-metal contact exists. If this unit isplotted with respect to time then a trend of the metal-to-metal contact is possible, thusgiving bearing or gear condition.In order to analyse a vibration it needs to be converted into an electrical signal. To dothis a vibration transducer is used. The first vibration transducers were shaft-ridingsticks with a magnet on the opposite end surrounded by a coil to produce an electricalsignal. This is known as a ‘displaceometer’ as its native units are displacement. Thenext generation of vibration transducers used spring-loaded magnets inside a coil toproduce an electrical signal, these are known as ‘velocometers’. The most commonsensors in use nowadays are based on piezoelectric technology, a weight inside thetransducer is pressed against a series of piezoelectric discs and an electric signal isproduced proportional to the vibration. These transducers are called accelerometers.All these transducers are still in use today, but the majority of data collectors useaccelerometers.The transducers can be fitted to machines and have their output monitored either byprotection systems or if specified before installation the machine’s own PLC. Theoutputs can be 4-20mA or 0-10 Volts and there are transducers that exist that will domulti-dimensional data, that produce a signal respective to velocity and another for theenveloped (bearing condition) reading. The cost difference between a standard (aconly) transducer and a conditioned (4-20mA, 0-10 V) is minimal and should always beconsidered when fitting fixed transducers.Once a signal is obtained using a transducer it is just a matter of using signalprocessing techniques to store, filter and break down the signal into its component
frequencies. There is a need to get as close to the bearing housing as possible to takethe vibration reading. The accelerometers are usually portable mounted magneticallyor they can be installed permanently if guarding does not permit access.Once a signal is obtained it is converted using a Fast Fourier’s Transform into avibration spectrum→This enables all the individual frequencies to be analysed and alarmed to detect anychanges that have occurred between readings. The original vibration waveform signal,if possible, will be stored without averages, as there is a huge value in the datacollected when looking for transient defects or non-synchronous eventsIf possible we take vertical, horizontal and axial velocity readings on each bearing inthe system. Unfortunately access problems such as coupling and belt guards meanthat it is not always possible to obtainall the readings.To use vibration as a trending tool forcondition monitoring a regularprogramme of data collection isnecessary, but vibration analysis canbe useful at locating problems on amachine, some data collectors can beconnected to strobe lights to ‘stop’ thecomponent that is vibrating, otherscan have an input from a lasertachometer to carry out start up andcoast down tests to locate momentsof resonance and critical speeds.These are classed as analytical toolsand usually do not form part of routinevibration data collection and analysis.To set up a new vibration database an initial survey of the site is needed. Data isacquired from the machines as to the number of measurement points required, speedsof shafts and type of machine. A database would then be written on a computer withroutes that are loaded into a data collector. An engineer then tours the plant with thedata collector and captures the vibration data. Once all the data is collected it is thendownloaded into the computer. The data is then analysed. More data may be takento confirm analysis and recommendations submitted. The report is written and isusually with a customer within 5 working days.
Page 1 and 2: Introduction into maintenance pract
Page 3 and 4: One can see that the percentage of
Page 5 and 6: If we knew that the critical alarm
Page 7: the same point under the same condi
Page 11 and 12: Benefits of having an inspection:
Page 13 and 14: The efficiency of hydraulic oil is
Page 15: ERIKS condition monitoring business

ERIKS Maintenance and Condition Monitoring Technical ... - Eriks UK

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