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Screens & Feeders, Tips & Advice













Managing underperforming vibrating screens

Opinions and emphasis placed on vibrating screens varies from person to person and from site to site. However, given they are the last machines to process material, their importance is clear. What is not clear is how to get a screen running well and keeping it that way. Darren Toth reports.

The focus of this article is the aspects of vibrational behaviour that affect the life of a screen, rather than the motion relating to its application.

It should also be noted that where most screens appear to run poorly, there are a small number of sites where all screens run well.

Figure 1. Counterweight plates oxy-cut by hand are quite inconsistent.
Figure 1. Counterweight plates oxy-cut by hand are quite inconsistent.

The sites where these observations have been made are mainly (but not limited to) quarries in the eastern states of Australia.

Given the cyclical nature of vibrating screens, their components experience fatigue. This is unavoidable and so their service life is finite. As the screen box is rapidly pulled through its motion (circular, linear, oval/elliptical), stresses are generated simply due to the acceleration of its components.

Add feed material to the decks and stress levels increase. It is fair to expect that the designer of each piece of equipment has allowed for this, and if these stress levels are not exceeded the screen is likely to be reliable and long-lasting.

So why do some screens run for many years and simply wear out, yet others can show signs of structural failure within months of being put into service?

It’s because, in the screens that fail prematurely, additional stresses are caused by internal flexing within the screen box. The flexing is caused by the screen vibrating differently on one side compared with the other.

When these additional stresses are combined with those that are typical and unavoidable, peak fatigue stresses within the screen box are significantly higher. This internal flexing will drastically shorten the life of the screen.

As a rule of thumb, a 50 per cent increase in fatigue stress levels will decrease a component’s life by five times.

Poor motion – the difference in vibration from one side to the other – can be caused by any of the following:

  • Unequal excitation force between the left and right side of the machine. This can be caused by inequality of the mass of the counterweights (from one side of the screen to the other) and by a difference in their position from the centre of rotation (difference in shape or simply misalignment). Figure 1 shows counterweight plates that have been oxy-cut by hand that are quite inconsistent.

  • A bias of mass of the screen toward one side. If one side of a screen has a greater mass than the other, the two sides will vibrate with different acceleration and amplitude (throw). A difference in the mass of screen box components from one side to the other or a bias of mass of feed material to one side will result in uneven motion. Figure 2 shows a significant variation in feed material across the deck.

  • A difference in spring compression/heights. Springs allow free movement and help to isolate vibration from their support structure, but they also influence the motion of the screen by magnifying the vibration through “dynamic response”. Significant differences in spring heights (greater than 10mm) from one side of the screen to the other, or within a spring set, will have an adverse effect on screen motion. Figures 3 and 4 show vast differences in spring heights within a spring set. Variation in spring heights between the left and right sides of the screen is a more common issue and is difficult to highlight in a photo.

  • Screen resonance. Resonance occurs when an object is subjected to vibration at a similar frequency to that at which it would naturally vibrate. If you carry out a search on the internet for videos relating to “resonance”, you will find some spectacular footage of wine glasses smashing, helicopters crashing and bridges falling. Screen failure due to resonance is not quite as sudden or visible as these examples, but it follows the same principle.

Figure 2. A significant variation in feed material across the deck.
Figure 2. A significant variation in feed material across the deck.

There are many ways (modes) that a screen box can vibrate internally, but there are two main modes where the natural frequencies are often close to typical screen operating speeds. One of these modes is a torsional (twisting) vibration through the length of the screen and the other is a racking type of motion (see Figure 5).

If screen resonance is not detected and rectified, it will result in structural failure of the screen box, usually in the deck frames. In extreme cases the variation in motion, around the screen, can be seen from a distance. In such cases, feed material behaviour becomes quite erratic, with differing bed depths from one side to the other.

For example, a customer significantly changed the mesh sizes on a screen and it started behaving erratically. Their description of the issue was that the screen was very lively in two diagonally opposite corners and “dead” in the other two. It sounded a lot like resonance and some testing on site confirmed that to be the case. By changing from lighter meshes to heavy ones, the customer unknowingly changed the natural frequency of the screen to something very close to running speed. Following an analysis of the natural frequencies and the screening application, the speed of the screen was reduced and the erratic behaviour ceased.

Figure 6 shows the results from testing over separate site visits. The middle set of data shows the stark difference in vertical motion (Y) between the left and right sides of the screen. The third set of data shows improved motion after speed change.

Figures 3 and 4. Vast differences in spring heights within a spring set: rubber springs (left) and coil springs (right).

Tips for improvement

New screens should be properly commissioned and running well. If you are not confident your equipment supplier will do this for you, get someone independent to test your screen. If it is running badly from the start it will cost in repairs and downtime.

Carry out regular inspections on major components (deck frames, side plates, etc), looking for signs of cracking, abnormal wear and materials build-up.

Frequently check bearing temperatures.

Frequently measure and record static laden spring heights (ie when the screens aren’t running). Make sure the spring heights are within the manufacturer’s guidelines. If these are not available, consider adding packers if the difference is more than 5mm and then aim to get the spring heights within 3mm from one side to the other.

Remember, you shim the taller springs to take the load off the ones that are compressed more. Don’t be concerned if the feed end springs are different heights from those at the discharge end. It is left to right that is important.

Figure 5. Torsional vibration (left) vs racking motion through the length of the screen.
Figure 5. Torsional vibration (left) vs racking motion through the length of the screen.

If spring heights have changed significantly from one check to the next, investigate why. If you replace a spring with a new one, also replace its equivalent on the opposite side.

Periodically check the motion of the screens by holding a piece of paper up against the side of the screen and plotting the motion by intermittently pushing the pen onto the paper. It takes a little bit of practice, but it is cheap and simple. If the motion looks different from one side to the other (as it does in Figure 7), there is an issue and it needs to be corrected.

If any changes are made to a screen (ie a meaningful change in size/weight/style of screening media or speed increase/decrease, etc) have someone test it to make sure it is still running acceptably.

If a counterweight adjustment is required, ensure these changes are carried out with precision. Small inaccuracies in counterweight mass and/or alignment will negatively impact the screen.

If weight is added to one side of a screen (liners, material added in a repair, etc), add the same mass to the opposite side in the same location. Whatever you do to one side, repeat it on the opposite side.

Get your screens checked at least once a year (but more frequently is beneficial) by someone with a vibration meter who can tell you how your screens are operating and advise on how to get them running better.

Figure 6. Testing results of a vibrating screen on separate site visits.
Figure 6. Testing results of a vibrating screen on separate site visits.

Image courtesy: IFE-Bulk











ABOUT THE AUTHOR
Darren Toth

Darren Toth is the director of ToThinkEngineering. Email: darren.toth@tothink.com.au









Wednesday, 24 October, 2018 04:14pm
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