THE OXFORD English Dictionary describes automation as, “Automatic control of a manufacturing or other process through a number of successive stages”. Manufacturers might add that automation is a powerful means by which machinery can carry out the work of a human being to achieve a significant economic advantage.

Automation has become increasingly affordable for SMEs as well as larger operations but the investment still justifies a solid vibration-monitoring regime to support it. Vibration is a common problem in machines across applications, sometimes resulting from misalignment of rotating equipment due to poor installation, sometimes the consequence of natural wear and tear. However, it is increasingly possible to reliably identify sources of wear with the use of vibration monitoring equipment.

Vibration sensors and accelerometers

Vibration sensors can be mounted in a number of key positions on mechanical equipment, offering the potential for continuous monitoring and analysis, an inexpensive option when balanced against the potential cost of a shutdown; when condition-monitoring measures are in place to detect factors such as vibration, machine downtime can virtually be eliminated.

The continuing enhancements in vibration monitoring sensors and measuring instrumentation have given engineers some powerful options and, when used as part of a planned predictive maintenance programme, they can make a major improvement to plant performance and profitability.

There are two main categories: AC accelerometers and 4-20mA accelerometers. AC accelerometers are typically used with data collectors for monitoring the condition of higher value assets such as turbines, while 4-20mA components are commonly used with PLCs to measure lower value assets, such as motors, fans and pumps.

Both AC and 4-20mA accelerometers can identify misalignment, bearing condition and imbalance, while AC versions offer the additional capability to detect gear defects, belt problems, looseness and cavitation. Manufacture should offer AC and 4-20mA accelerometers that are intrinsically safe, being ATEX and IEC Ex certified, and can be used to monitor vibration levels on pumps, motors, fans and all other types of rotating machinery.

Accelerometers contain a piezoelectric crystal element, which is bonded to a mass. When subjected to an accelerative force, the mass compresses the crystal, and this causes the crystal to produce an electrical signal that is proportional to the force applied. This output is then amplified and conditioned by inbuilt electronics to produce a signal that can be used by higher level data acquisition or control systems either ‘online’ or ‘offline’.

An online system is one that measures and analyses the output from sensors that interface directly with a PLC. An offline system is created by mounting sensors onto machinery and connecting them to a switch box; engineers can then use a hand-held data collector to collect readings.

Monitoring of typical applications

Once you have specified the right equipment and installed carefully in order to yield the most repeatable and consistent measurements, machine reliability data can easily be analysed to predict potential problems before they occur. Increases in vibration indicate deteriorating operating conditions, such as wear or misalignment, and vibration sensors can identify these changes swiftly and with exceptional reliability.

The massive potential for these tools to benefit the engineering industry has dramatically increased demand, which, in turn, has driven the manufacturers of vibration monitoring devices to enhance and adapt their products to suit a broadening range of industries and specifications, resulting in accelerometers that are increasingly easy to install and use.

For rotating machinery, vibration analysis has proved a convenient and highly effective method of measurement with which to assess machine condition. Accelerometers can be easily mounted on casings to measure the vibrations of the casing and/or the radial and axial vibration of rotating shafts.

A typical technique in vibration monitoring has been to examine the individual frequencies within the signal that correspond to certain mechanical components or types of malfunction, such as shaft imbalance or misalignment, so that analysis of this data can identify the location and nature of a given problem. A typical example would be a rolling-element bearing that exhibits increasing vibration signals at specific frequencies as wear increases.

Installation secures effective information

Care must be taken during the installation of vibration sensors to ensure the maximum level of performance. Condition monitoring depends on stability; a poorly mounted accelerometer may give readings that relate not only to a change in conditions but also to the instability of the sensor itself. Accelerometers should be mounted as close as possible to the source of vibration onto a surface than has been made free from grease and oil. The surface should be smooth, unpainted and larger than the base of the accelerometer itself. It should also be flat and this may require the creation of a flat surface using spot facing tools to eliminate instability.

A good spot facing kit will give you all the necessary tools needed to accurately mount a vibration sensor onto the rotating machine, including a tapping drill, taps, tap wrench and a spot facing tool. These kits are now available to allow for different mounting threads: ¼, M6 and M8. Correct mounting of the sensor is vital to ensure true readings and, where possible, mounting a sensor via a drilled and tapped hole directly to the machine housing will give the best results. However, if the housing is not flat, a spot facing installation kit allows the creation of a flat surface.

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Click Image to Enlarge

Fig. 1: Typical selection of different accelerometers.

Fig. 2: Vibration sensors for fans.

Automation has the potential to boost plant productivity and efficiency, not to mention product quality, but it can only do so if the engineering system is adequately monitored and protected. Effective maintenance is essential to protect the plant from the failure of automated equipment and the many benefits it brings to a manufacturing operation. Far from being an expensive option, the use of vibration monitoring can enable companies to operate with enhanced performance and increased flexibility, both vital attributes at a time industry is coming under increasing pressure to boost productivity and cut operating costs.