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Vibration Analysis for Optimum Screen Performance

DeisterData Mining Can Help Achieve Optimum Machine Performance.

By Carol Wasson

Most producers will agree that vibrating screens must be properly specified and designed, or they will be the biggest bottleneck within an operation. The key to optimum screening is maximizing capacity without losing efficiency – and that means that there are many operating parameters that one must consider.

The art of screening lies in the meticulous fine tuning, tweaking and synchronizing of screen setups within a limitless number of applications. When optimum machine performance is achieved, the results are higher production capacities, greater product quality, reduced maintenance requirements, and lower costs per ton.

One of the tools involved in ensuring optimum vibrating screen performance is the use of a technology called vibration analysis. According to Dale Loshe, vice president of engineering for Deister Machine Co. Inc., vibration analysis is defined as the acquisition and analysis of data regarding the vibrational characteristics of the machine. “Our goal is ensuring long-term performance and reliability. Vibration analysis helps us to predict or identify any detrimental operating conditions, while providing insight into what modifications are needed,” he said.

Loshe explains that vibration analysis collects data on parameters such as natural frequencies, displacements and stroke amplitude, and the operation of bearings and gears. It typically involves the use of a hand-held analyzer that’s connected to a series of accelerometers. Vibrational data is recorded electronically into the analyzer. “We can examine the data immediately on the analyzer, or we can download the data onto a computer for a more detailed analysis,” he said.

Tests are conducted both at the factory and in the field. Baseline readings are taken at the factory on every machine while they are on the test stand for Q.C. running; with subsequent field readings being compared to the factory readings.

Readings in the field can be collected by the end user; however, Loshe said that it’s best to have them taken by the factory service technician. “If the end user has had proper experience, it is possible for them to take good readings with their own hand-held analyzer. But many times there are complicating factors – and readings alone do not tell the whole story. The qualified factory technician has the ability to recognize all the possible contributing factors, while examining the condition of the machine and the quality of readings,” he said.

As to when readings should ideally be taken once machines are operational in the field, Loshe said that the first time should be shortly after startup. “At that point, readings should be taken while the machine is empty and also when it is fully under load,” he said. “Readings should also be taken any time a speed or stroke change is made to the machine; or when significant screen media changes occur; or when applications change; and importantly, when and if there are any major support tower upgrades or rebuilds,” he added.

Deister testingTesting in the Field
An actual case study illustrates the value of vibration analysis. A high-tonnage producer was having some issues with erratic strokes and material feed on a screen that had been in operation for approximately five years without incident. A Deister Vibration Technician was sent onsite to evaluate the situation in detail.

Upon inspection, the vibrating screen showed no obvious or visual signs of conditions such as bottoming, impacting or broken springs, for example. A field vibration study was started to help diagnose any issues. The typical study starts by taking all of the exact data points used by the Quality Control Department at the factory. After comparing the field readings to the factory baseline readings, the field readings were found to be well out of specification for Deister Q.C. standards. Consequently, the next steps are troubleshooting, making a correct diagnosis, and establishing a plan of action.
In the end, the problem turned out to be excessive wear, which could not be seen because of chute work and/or easy visual access to inspect for wear. To verify the diagnosis, a more advanced vibration analysis technique was used to demonstrate the erratic movements.

This technique involves collecting vibration data at various points on the screen side plates (see photo) while the screen is in operation. This data is then used in conjunction with a simple computer-generated model, and is subsequently animated. As they say, a picture is worth a thousand words – once the end user saw the animation, the problem was clearly understood.

The wear had ultimately weakened the structure of the screen deck bracing, which in turn was causing the erratic movement. Additionally, the problem was also confirmed with a more intensive visual inspection from within the chute work. The worn parts were replaced and the screen is delivering reliable day-to-day operation without any further issues.

Complete Analysis Solutions
“Deister recommends vibration analysis beyond simple orbits and bearing readings,” said Loshe. Impact Testing and Operating Deflection Shape (ODS) analysis in one of the advanced tools available.

Impact Testing is used to determine natural frequencies that could cause issues at run speeds, or would require structural changes. A baseline reading is taken on each machine at the factory and is used to confirm the accuracy of engineering models. ODS analysis is used to animate and check new equipment and new concepts, while also confirming engineering models for accuracy. ODS identifies how a machine moves in actual operation and at specific frequencies. The analysis compares mode shapes to determine the most effective structural modifications to the machine.

Deister has been conducting vibration analysis testing since the late 1990s. “More and more end users are recognizing the value of the technology to their operational efficiency. But some may think that it’s a magic black box that’s going to solve every problem. In truth, it’s simply a tool, albeit a powerful tool,” said Loshe.

Importantly, Loshe notes that Deister takes a “holistic” approach to performing a machine analysis. “Deister vibrational technicians and engineers are trained to look at the big picture. In other words, not every potential problem is apparent on the digital readout display of the vibration analyzer. There is still no substitute for the experienced set of eyes and ears,” he stressed.

About the Author
Carol Wasson is a Fort Wayne, Ind.-based freelancer.