When a piece of machinery is designed and built, dimensional variations are allowed in its manufacture. These cause the machine to vibrate. (From this information we can ascertain what is acceptable vibration.) If we were to take a series of vibration readings, we would end up with the machines inherent vibration "signature." Each signature is unique with similar machines showing similar signatures. This signature is used as a baseline measurement with which all consequent readings are compared.

With consequent vibration readings, taken under identical conditions, we are able to conclude if any deterioration has taken place in the machine’s condition, with the defect affecting the vibration signature. Because defects change the vibration signature in different ways, we are not only alerted to a possible problem, but we are also able to establish problem source. There are less than 20 likely causes of excess machine vibration, with the majority falling into three major areas: unbalanced machinery, misalignment and defective anti-friction bearings. Other common causes of vibration are, eccentric journals, defective gears, mechanical looseness, bad drive belts, electrical forces, reciprocating forces, rubbing, resonance, oil whirl, and hydraulic pulsation.

In order to analyse a vibration signal, we first of all have to collect it. This is achieved with vibration pickup devices, which are usually placed on the machinery bearing caps. The caps are an ideal place for pick-ups because of their accessibility, and because they are the points through which vibration transmits the most readily. For each point, the vibration signal picked up is then recorded in a vibration meter or vibration analyzer. A vibration meter will allow the operator to detect defect, but in order to analyze the cause, we need to use a vibration analyzer.

There are three basic types of vibration pick-ups in common use:
1.Velocity pick-up. These devices are commonly used for general measurement. They are easy to use and can be hand held. They are generally used for measuring in the 600 - 60,000 cpm frequency range. The unit uses a moving coil supported by a spring. A magnetic field is set up around the coil by a permanent magnet attached to the outside case. When the pick-up is placed against a vibrating object, the magnet moves back and forth over the stationary coil generating voltage in the wire. This voltage is then transmitted and decoded by the meter or analyzer.

2. Accelerometer. This small, lightweight pick-up is very sensitive to vibration occurring at high frequencies up to 600,000 cpm. Because it is less sensitive to stray magnetic fields, it is ideally suited for the monitoring of large AC motors and alternators. Piezolectric crystals are sandwiched between the accelerometer case and a small weight. When the unit is attached to a machine, the vibration compresses the piezoelectric crystals against the weight. Because these crystals generate an electrical charge proportional to the amount they are compressed, an output signal proportional to the vibrations acceleration is sent to the analyzer.

3. Non-contact pick-up. This kind of pick-up is generally used in permanent installations for continuous monitoring. It is commonly found on high speed machinery such as turbines, centrifugal pumps and compressors, where lightweight rotor shafts are mounted in massive bearings. Because of its installation requirements it does not readily adapt to portable situations. They are designed to measure the relative vibration between a shaft and its bearings through proximity sensing.

This technology demands structured work methods, proper set-up, proper training, dedication and commitment. With these in place, the maintenance department will make good use of what is probably the most powerful diagnostic tool available to them. The obvious benefit of vibration analysis is reduced downtime, but non-intrusive vibration monitoring is also used for new equipment inspections, balancing, noise control, shaft alignment, and more.