Monitoring makes sense
Chris Hansford explains why it makes good business sense to protect automated production lines with vibration monitoring equipment.
Automation has become an essential tool for industry, delivering benefits that include increased productivity, consistent quality and cost reduction. Yet automation also brings a new set of challenges, as more and more companies become increasingly dependent on physical rather manual assets. In particular, machinery has to be constantly available – downtime costs money – which in turn places ever greater emphasis on the importance of effective maintenance.
Given the current economic climate, the challenges faced by manufacturers in 2012 – especially the pressures on operating costs – look set to continue for the foreseeable future, so an effective maintenance strategy is critical.
Vibration monitoring is one of the key tools for plant and equipment maintenance, providing a reliable tool with which to maximise machine uptime. In some circumstances where vibration is carefully monitored it is also possible to extend operating life beyond recommended maintenance intervals, while in others a rapid increase in vibration must be taken seriously if a catastrophic failure is to be avoided.
Automation, meanwhile, 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.
Mounted in a number of key positions on mechanical equipment, vibration sensors offer the potential for continuous monitoring and analysis, an inexpensive option when balanced against the potential cost of downtime on an automated line, and when condition monitoring measures are in place to detect factors such as vibration, machine downtime can virtually be eliminated.
Accelerometers are typically easy to install and use but a little knowledge goes a long way so it is well worth pausing to consider what an accelerometer is in order to understand how it works and ultimately achieve the best possible installation and management of your vibration monitoring equipment.
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.
The first thing to consider when specifying accelerometers is that 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. Hansford Sensors offers 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.
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.
Careful consideration must be given to issues such as the vibration level and frequency range to be measured, while environmental conditions, such as the temperature and presence of corrosive chemicals, will affect the specification. Once the most appropriate sensors have been selected it is important that advice is followed and care is taken during installation to ensure the maximum level of performance. For example, accelerometers should be located as close as possible to the source of vibration. Also, devices should be mounted onto a flat, smooth, unpainted surface, larger than the base of the accelerometer itself and this surface should be made free from grease and oil. Condition monitoring depends on stability and readings from a poorly mounted accelerometer may relate not only to a change in conditions but also to the instability of the sensor itself.
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.
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 when industry is coming under increasing pressure to boost productivity and cut operating costs.