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The value of vibrations: part two

World Cement,

Installation considerations

Care should be taken with installation to ensure that each sensor produces accurate and consistent results. Ideally, accelerometers should be mounted directly onto a flat, smooth and unpainted surface, which is free of grease or oil and slightly larger than the base of the sensor. The mounting surface should be as close as possible to the source of vibration and perpendicular to the axis of rotation.

Accurate sensor mounting is essential to ensure true readings. Mounting a sensor via a drilled and tapped hole direct to the machine housing is likely to produce the best results. A good way of ensuring that the vibration sensor is mounted into the rotating machine correctly – especially if the housing is not flat – is by using a spot facing kit that includes a tapping drill, taps, tap wrench and spot facing tool.

Cement plant

In order to correctly determine imbalance, misalignment, bad bearings, mechanical looseness, hydraulic forces (cavitation, resonance) and rubbing, the sensor needs to be in a position that guarantees the horizontal, vertical and axial movements are measured effectively.

When reading horizontal displacement in, for example, a pump motor, the sensors should be mounted on the two motor or pump bearings. This arrangement measures velocity in mm/sec. (peak or RMS) to detect imbalance and issues with structural rigidity and/or foundation. For vertical measurements, the sensors should be located on the motor pump or pump drive end bearings. This measures velocity in mm/sec. (peak or RMS) to identify looseness and problems with structural rigidity and/or foundation. Axial measurement uses the same measurement criteria to detect misalignment between the components, such as the motor and fan.

For a typical crusher motor, accelerometers are mounted radially on the drive end and non-drive end, in order to monitor the motor bearing condition. Within a gearbox, a sensor will be mounted radially on input and output shafts. This enables the condition of the bearings (g) and fan (velocity) to be monitored. An optional axial accelerometer on the input shaft can give a good indication of the thrust on the shaft.

Cement manufacturing applications typically use 4 – 20 mA accelerometers connected to PLC systems. Each sensor is connected to a local junction box; multicore (screened twisted pair) cable is then connected to the PLC system for data trending and alarming.

Depending on accessibility, a local junction box can be installed close to each device being monitored. Multicore cable is used to connect the junction box back to a main switch/connection enclosure. In some applications the sensor cable is connected directly to the switch enclosure. This is a sealed industrial enclosure, which has been designed to withstand harsh conditions. It is available in various forms, such as mild steel, stainless steel or polycarbonate, depending on environmental conditions.

This is the second of a three-part article written by Chris Hansford and featured in the July issue of World Cement. Subscribers can read the full issue by signing in, and can also catch up on-the-go via our new app for Apple and Android. Non-subscribers can access a preview of the July 2015 issue here.

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