Protection for automation

Published:  11 May, 2012

In order to provide the lower prices and shorter lead times that customers demand, manufacturing businesses have increasingly turned to automated production systems and backed their decision with significant investment.  It can take a good deal of time and money both to design and install the right automation system and to maintain it effectively, so it is therefore crucial that the system is protected as powerfully as possible.  The cost of providing internal engineering teams is not always viable for some organisations and even when it is there is still a pressure to manage maintenance as cost-effectively as possible.  Whatever the budget, one of the best ways to reduce demands on maintenance is to apply a solid, reliable vibration monitoring programme.

A series of sensor and detection technologies are now available that can maximise machine uptime on a regular basis by extending operating life beyond recommended maintenance intervals.  The demands of many different industries have driven the development of this technology to such a degree that custom vibration sensors are not always needed because such a wide range of solutions can already be found off-the-shelf.

Though the applications served by vibration monitoring equipment are wide, the causes of vibration are essentially the result of misalignment due to poor installation, or as a consequence of wear and tear.  With vibration monitoring tools, the source and cause of vibration can be swiftly identified, enabling problems to be prevented before they develop. 

In an automated manufacturing facility an undetected change in, for example, the alignment of machinery could compromise product quality, leading to a series of problems, the worst of which are likely to be costly downtime and dissatisfied customers.  Avoiding these problems can be a challenge when machinery is difficult to maintain and check; in some cases, regular access may not be possible because of risks to health and safety.  The solution is to employ vibration monitoring, using accelerometers that can be used as either in-line or hand-held instruments.

Here’s how an accelerometer works: 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.

There are two main categories of accelerometer: AC accelerometers, which are typically used with data collectors for monitoring the condition of higher value assets such as turbines, and 4-20mA accelerometers, which 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. 

Sensors are also available that provide a dual output, enabling hand-held equipment to corroborate the readings of the online system. Hansford Sensors recently provided a vibration monitoring solution to a customer that was concerned about high levels of wear in a conveyor pulley bearing.  The maintenance team purchased our HS-423 sensor to gain dual output - 4-20mA acceleration into the PLC and AC output via a data collector.  The HS-423 sensor triggered a pre-set alarm level in the PLC, alerting engineers that acceleration levels had increased significantly and the results were confirmed by acceleration readings from a hand-held data collector.  This potential to gain a ‘second opinion’ can be highly valuable to engineers though, in this case, the management chose to ignore the warnings in order to maintain production. The result was a catastrophic bearing failure, causing a fire that damaged equipment and, worse still, put lives at risk.

While this story underlines the potential of vibration monitoring to not only optimise plant performance but avoid a major incident, it also introduces another key point that must be made about vibration monitoring tools: they are only as good as the engineers or managers that install and use them.  Just as plant management can choose to ignore a warning, engineers can choose to ignore the installation guidelines and thus reduce the potential of the vibration monitoring solution. 

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.  Following this advice will enable you to achieve repeatable and consistent measurements.

Considering the massive potential of vibration sensors, it is well worth taking the time to achieve the right installation.  Vibration sensors can measure both high and low frequencies, with low hysteresis characteristics and high levels of accuracy over a wide temperature range.  Packaged within compact stainless steel sensor housings, these devices can withstand high levels of moisture and contamination, enabling their use in a whole range of aggressive conditions. 

Investment in a vibration monitoring solution is highly cost-effective when balanced against the potential cost of a shutdown.  Indeed, machine downtime can be virtually eliminated when condition monitoring measures are in place.  In the current economic climate, effective maintenance is essential in order to optimise plant performance and vibration monitoring is a powerful way of providing that maintenance.



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