Monitor vibration to cut costs

Chris Hansford explains how vibration monitoring can maximise machine uptime.

Downtime costs money.  Although planned downtime for routine maintenance is not generally an issue, it’s unexpected machine or component failures that cause most disruption.  Additionally, and especially in the current economic conditions where every saving in operating costs is crucial, the ability to keep machines running for longer between routine maintenance checks is of ever greater importance. 

Minimising downtime depends on efficient condition management and monitoring.  There are many techniques available to engineers, but one of the most effective is the ability to detect often minute changes in the operation of rotating components by monitoring levels of vibration.

Vibration is an excellent indicator of problems such as wear or misalignment and is easily monitored using accelerometers and either in-line or hand held instruments. Accelerometers such as those manufactured by Hansford Sensors are easy to install and use, operate over a wide temperature range, and measure high and low frequencies, with low hysteresis characteristics and excellent levels of accuracy.  These devices are also robust and reliable, with stainless steel sensor housings that prevent the ingress of moisture, dust, oils and other contaminants.

There are two main categories: AC accelerometers, used with data collectors for monitoring the condition of higher value assets such as turbines, and 4-20mA accelerometers, which are used with PLCs to measure lower value assets, typically motors, fans and pumps.  Both types of accelerometer are capable of detecting imbalance, bearing condition and misalignment but AC versions can also identify cavitation, looseness, gear defects and belt problems. 

Accelerometers contain a piezoelectric crystal element bonded to a mass.  When the sensor is subject to an accelerative force, the mass compresses the crystal, causing it to produce an electrical signal, proportional to the force applied.  The signal is then amplified and conditioned using inbuilt electronics that create an output signal, suitable for use by higher level data acquisition or control systems.  Output data from accelerometers mounted in key locations can be read periodically using hand-held data collectors, for immediate analysis or subsequent downloading to a PC, or routed via switch boxes to a centralised or higher level system for continuous monitoring.

To specify an accelerometer correctly, engineers need to consider the vibration level and frequency range to be measured, weight or fitting restrictions and environmental conditions, such as ambient temperatures, the presence of moisture, chemicals or potentially explosive atmospheres.  The best approach is to work closely with a supplier that has appropriate industry experience and knowledge.

To correctly install an accelerometer, engineers should mount each device directly onto a machine surface that is flat, smooth, unpainted and larger than the base of the accelerometer.  The installer should ensure that the surface is free from grease and oil, as close as possible to the source of vibration and at right angles to the axis of rotation.  By following these guidelines, you will have already supported your accelerometer, and thus your maintenance regime, by enabling the device to give the most accurate and consistent measurements.

Once data has been collected in the most appropriate and efficient manner, machine reliability data can easily be analysed to enable engineers to predict wear rates and identify potential problems before they occur.  The result is improved machine performance, greater uptime and lower costs.

Date: 08/02/2012