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As machinery has become more complex, mankind has demanded greater productivity from each mechanical system. In turn, there has been a corresponding growth in the need for state-of-the- art accelerometers that can maximize the performance of many engineering processes.
Accelerometers, also known as vibration sensors, contain a piezoelectric crystal element, which is bonded to a mass (piezoelectric means generating a voltage when mechanical force is applied, or to producing a mechanical force when a voltage is applied). 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 in-built electronics to produce a signal that can be used by higher level data acquisition or control systems either online or offline.
An online system measures and analyses the output from sensors that interface directly with a Programmable Logic Controller (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 also two main categories of accelerometer: AC accelerometers, typically used with data collectors for monitoring the condition of higher value assets such as turbines, and 4-20mA accelerometers, 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.
In applications such as wind farms where it has been difficult to justify the expenditure of a full on-line system, some operators have applied a lower-cost option using fixed sensors, with a local junction box and multicore connected to a switchbox on the ground. They then have either a contract vibration analysis company to take readings or, alternatively, the wind farm owner employs a vibration analysis team to take regular readings.
The latter method, selected when budgets are tight, is limited in its effectiveness since the data is only as good as the last reading that was taken. However, this is still a useful means of applying preventative maintenance.
If a bigger budget becomes available, this approach can be expanded into an online system by replacing the switch box with a monitoring station that is capable of providing in-depth vibration analysis using FFT (Fast Fourier Transform) with remote web access. When specifying a vibration monitoring solution, therefore, it pays not only to select one according to the available budget, but to also be aware of its potential to serve future needs in terms of upgrades.
Additional factors to take into account with in the selection of a vibration monitoring system include unbalance, misalignment, bad bearings, mechanical looseness, hydraulic forces (cavitation and resonance) and rubbing. To detect these faults, vibration sensors should be located to ensure that horizontal, vertical and axial movement are measured effectively.
For horizontal measurement, mount vibration sensors on the two motor bearings and two pump bearings. This measures velocity mm/sec (Peak or RMS) to detect unbalance, and problems with structural rigidity and foundation. For vertical measurement, locate sensors on the motor and pump drive end bearings. This measures velocity mm/sec (peak or RMS) to detect looseness and problems with structural rigidity and foundation. For axial measurement, attach sensors to motor and pump drive end bearings. This measures velocity mm/sec (Peak or RMS) to detect misalignment between the motor and the fan.
Depending upon the criticality of the application and the budget available a greater number of sensors can be installed to give additional information but the key components to measure using vibration monitoring are the motor and gearbox assemblies.
Having understood where to locate the sensors, the next priority is to ensure selection of the correct sensor. A wide range of vibration sensors, including specific models that have been designed to withstand harsh conditions, are certified for use in areas where Intrinsically safe products are required.
Care must be taken during the installation of vibration sensors to ensure the maximum level of performance. Effective 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 provide 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, including 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 creation of a flat surface.
Finally, vibration sensors can measure both high and low frequencies, with low hysteresis (the lag in response exhibited by a body in reacting to changes in the forces) characteristics and high levels of accuracy over a range of temperatures. Packaged within stainless steel sensor housings, these devices can withstand high levels of moisture and contamination, enabling their use in aggressive conditions.
Written by: Chris Hansford, Managing Director at Hansford Sensors, a company specialising in the design and manufacture of accelerometers for monitoring vibration and temperature levels of industrial machinery. For more information visit www.hansfordsensors.com .