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The Winning Duo Are...Vibration and Ultrasound Together

A combination of vibration and ultrasound techniques are in the same instrument. Problems are detected by static measurements while dynamic measurements are used when a further diagnosis is required.

More than ever, production cost control affects the operations of the Maintenance Department. While its role still consists in knowing the health condition of the machines, its associated costs must remain as low as possible.

For the operator, the ideal situation is then to have versatile tools that can deliver fast and reliable diagnosis: simple instruments to solve 80 percent of the drifts and more advanced instruments for the remaining 20 percent.

The current economic situation imposes maximum production cost control to companies. To this aim, they use more and more performing, hence more and more complex, facilities. Their aim is obvious: produce more and better.

But that's not all. Companies also naturally tend to optimise the maintenance of their process, which has then a whole new dimension. Not only do they have to limit production losses, plan and optimise reconditioning, while limiting the stock of spare parts, but they also have to achieve this mission with reduced human and financial resources.

They can no longer waste workers' time doing time-consuming data collections and unnecessary analyses. Also, there is no question of systematically hiring specialists to do these tasks.

Towards Optimised Maintenance

Let's address the issue in a different way: why spend time to collect and analyse measurements or why entrust a specialist to do this work, if we finally find that the equipment is in good condition?

Quite ineffective, don't you think? Well, this is the case for 80 percent of the equipment under monitoring. The search for optimisation requires the use of a tool that is simple (hence available to the largest number of users), fast (to limit collection time) and efficient (to confirm the proper operating of 80 percent of the machinery).

The velocity spectrum of a fan can be used to detect unbalance through the presence of a peak at the rotation frequency of the machine, i.e., 29.11 Hz, and a welding problem on one of the 6 fan blades through a peak at harmonic 6 of the rotation frequency, i.e. 174.6 Hz.

But that's not all. Out of the 20 percent of suspicious machines, the cause of the drift is easy to solve for 80 percent of them. Once again, a simple tool used by an operator is the most suitable solution.

Finally, further diagnosis is required for 20 percent of the remaining 20 percent. Advanced measurements carried out by a specialist are then necessary. But, on average, only 4 percent of the machines are under regular monitoring.

Versatility Is Essential

Besides speed and ease of use, a versatile tool must be selected based on controlled investment costs. A measuring instrument that can address both rotating machinery and control of energy costs. The complementarity between ultrasound and vibration makes full sense here. Ultrasonic technology is the undisputed champion of energy savings: compressed air, vapour and, to some extent, partial discharges. For rotating machines, this is the simplest solution to detect friction phenomena (bearing lubrication), impact phenomena (bearing defects, including slow bearings, and gear defects) and bubble explosion phenomena in liquids (valve or pump cavitation). Vibration measurements are ideal to detect misalignment, unbalance, foundation, coupling and pulley problems, as well as aeraulic phenomena and defects affecting electric motors.

Static Measurements to Detect Problems

Static measurements (or overall measurements) are easy to implement because their result is a number. They are collected by the technician or operator in charge of lubrication and make up the basis of a maintenance programme. They allow creating trend curves and implementing alarm thresholds. They aim at making a quick decision on the condition of the machine: good or suspicious.

The time representation of the ultrasonic signal of a bearing shows the occurrence of repeated shocks every 0.138 seconds corresponding to the outer ring defect frequency.

In both examples, the technician performed on-site acquisition of the signals originating from the incriminated machines. They were analysed by a specialist who did not have to travel on site uselessly.

Dynamic Measurements for Further Diagnosis

The dynamic measurement consists of signal acquisition over a selected period. It allows generating time and frequency representations (FFT). With ultrasonic technology, time representation is preferred. For instance, it can be used to detect the presence of repeated shocks through simple visual interpretation. Based on bearing reference and rotation speed, the operator will be able to identify the defect origin. Finally, from shock amplitudes, he/she will assess the defect severity.

With vibration technology, frequency representation is more commonly used. Machine kinematics and knowledge of the defect vibration images will allow the specialist determining the cause of the breakdown. As previously, the severity of the defect and its development will be assessed based on the amplitudes of the spectrum lines.

Two Complementary Technologies

SDT270 relies on this approach, which combines vibration and ultrasound techniques: a basic tool for those searching for optimisation and efficiency of their maintenance programme. Based on combined ultrasound and vibration techniques, this versatile measuring instrument allows addressing both energy cost control and rotating machine monitoring. The technician can detect anomalies, both simply and quickly, using static measurements. It also benefits the advanced features for the specialist in charge of the search for breakdown causes using dynamic measurements.

This article was published in the November 2015 issue of Maintworld Magazine

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