Vibration Analysis

Vibration is the response of a system to an internal or external stimulus causing it to oscillate. While it is commonly thought that vibration itself damages machines, it does not. Instead it is the damage done by dynamic stress that causes fatigue of the materials: the dynamic stresses are caused by vibration. That is to say that different machines have a different toleration to vibration.

There are literally hundreds of specific problems that can cause a machine to exhibit excessive vibration. To locate the root cause of vibration an analysis of the vibration is required. The forces that cause vibration are usually generated through the rotating motion of the machine's parts or electrical supply frequency. When a fault condition occurs the problem will exhibit characteristics that are directly related to these frequencies.

As vibration is oscillating movement we could simply just measure the amount of movement in the system, this is called displacement.

The displacement is usually expressed as the distance from one extreme of travel to the other, or 'peak to peak displacement'. The units of displacement are micrometers, µm, (0.001mm=1 micrometer). However displacement is related to the frequency. For example 30µm at 1000Rpm is just as destructive as 300µm at 100Rpm. This makes the setting of alarms cumbersome to evaluate machinery condition. It must be remembered that each source of vibration contributes to the ultimate fatigue of the machine. The overall condition of the machine can only be determined by an overall measurement of vibration that takes into consideration all frequencies of vibration with relatively equal weighting. This is accomplished by using the first differential of displacement, called Vibration Velocity.

If any item is struck it will tend to vibrate or 'ring' at its natural frequency. If a flaw exists on a bearing this will act like a tiny hammer and cause the bearing to ring at its natural frequency and whatever the bearing is connected to mechanically will ring at its natural frequency. The same is true for a defective gear. To calculate all these natural frequencies would be a tremendous task. The vibration data collectors used by WYKO filter out all frequencies below a set value, (usually 300 000Cpm (5kHz), but can be varied) then take the overall amplitude of the remainder. This gives a reading that is directly proportional to the amount of metal-to-metal contact in a system. The unit given to this reading is gSE (g's SPIKE ENERGY). As it is a high frequency reading spike energy does not travel very far, thus wherever the highest spike energy reading is, is where the highest amount of metal-to-metal contact exists. If this unit is plotted with respect to time then a trend of the metal-to-metal contact is possible, thus giving bearing or gear condition.

In order to analyse a vibration it needs to be converted into an electrical signal. To do this a vibration transducer is used.

The first vibration transducers were a shaft stick with a magnet on the opposite end surrounded by a coil to produce an electrical signal. This is known as a displaceometer as its native units are displacement. The next generation of vibration transducers used spring-loaded magnets inside a coil to produce an electrical signal, these are known as velocometers. The most common sensors in use nowadays are based on piezoelectric technology, a weight inside the transducer is pressed against a series of piezoelectric discs and an electric signal is produced proportional to the vibration. These transducers are called accelerometers. All these transducers are still in use today, but the majority of data collectors use accelerometers.

Once a signal is obtained using a transducer it is just a matter of using signal processing 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 take the vibration reading. The accelerometers are usually portable mounted magnetically or 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 a vibration spectrum.

This enables all the individual frequencies to be analysed and alarmed to detect any changes that have occurred between readings.

If possible we take vertical, horizontal and axial velocity readings on each bearing in the system. Unfortunately access problems such as coupling and belt guards mean that it is not always possible to obtain all the readings.

To use vibration as a trending tool for condition monitoring a regular programme of data collection is necessary.

To set up a new vibration database an initial survey of the site is needed. Data is acquired from the machines as to the number of measurement points required, speeds of shafts and type of machine. A database would then be written on a computer with routes that are loaded into a data collector. An engineer then tours the plant with the data collector and captures the vibration data. Once all the data is collected it is then downloaded into the computer. The data is then analysed. More data may be taken to confirm analysis and recommendations submitted. The report is written and is usually with a customer within 5 working days.