Do you know what speed and stroke your vibrating screens are running at? By far the most common answer I hear when I ask this question on sites is “no”. Before I continue, I want to point out that my customers are predominantly, but not restricted to, quarries. Although there are exceptions within this industry, most sites are not aware of how their screens are running. I should also add that testing screens is not a new practice and there are several businesses that carry out this type of work to varying extents.
It is important to know how your screens are running. From a screen “health” perspective it is important to know that each screen is behaving the same on both sides (left and right) and that they are not running too hard, with excessive acceleration. Both lead to higher internal stress levels which means a significantly reduced service life.
It is also important to know how your screens are running from a productivity and screening efficiency standpoint. Several parameters affect screen productivity and efficiency: stroke, running speed (frequency of vibration), acceleration (Gs), deck inclination angle, stroke angle (oval and linear motion machines), direction of rotation (circular motion screens), type of screening media, etc.
If you don’t know the stroke, speed and acceleration of your screens then you don’t really know how they’re running. I will concentrate on these three.
To maximise screening efficiency the aim is to encourage under-size particles (within the feed material) to contact with the screening surface as many times as possible. Therefore it is logical to minimise the stroke of the machine so the material is being thrown only a short distance with each rotation of the machine.
So, what drives the “minimum” value of the stroke? It’s the need to avoid “pegging”. Pegging occurs when near sized particles fall into (but not through) apertures and the machine doesn’t have enough stroke to throw them back out. The bigger the aperture (and therefore the near size particles) the greater the stroke required to avoid pegging.
This is why screens in coarse applications need more stroke and why machines in fine screening applications can run with less stroke.
Stroke also plays a key role in conveying the material along the screen decks. More stroke leads to an increase in the transport velocity of the material. For a given material throughput, lower transport velocity means higher material bed depth.
Excessive bed depths prevent effective stratification. This is where the bulk material loosens (opens up) and fine material can fall between the larger particles.
Excessive stroke leads to a reduction in the number of times under-size particles present themselves to apertures. Too little stroke leads to inadequate stratification resulting in many of the under-size particles not falling down to the screening surface anyway.
Strokes typically range from about four millimetres up to 20mm. Note that high frequency screens run outside the range of typical operating parameters.
Vibrating screens are designed to operate within a specific range of acceleration. Exceeding these limits will significantly shorten the service life of the machine.
Acceleration is also a significant factor in the dynamics at (and above) the screening surface. The relationship between the acceleration of the screen mesh and gravity influences how the bulk material and individual particles behave. Gravity doesn’t change, which is why screen manufacturers recommend that their screens run at specific Gs or within a range of accelerations. The general rule of “more throw, less speed” comes into play. Recommended accelerations vary between screens with different shapes of motion (ie circular, oval and linear) and between inclined and horizontal machines. Every screen manufacturer should be providing a recommended acceleration (or speed/stroke combination) for its machine (most do) and ensure it is supplied as such. This provides a benchmark to compare with when the equipment is tested over its service life.
Another consideration around acceleration is “blinding”, which occurs when the adhesive force between fine material and the screening media exceeds the force applied to these particles by the screening surface. It’s a bit like getting something sticky on your hand and not being able to flick it off! As the fine material builds up around the apertures it eventually blocks the mesh. This has a “snowball” effect as the load increases on the screen and the acceleration then decreases, making blinding even more likely. Sometimes blinding occurs simply because of unusually wet conditions or a change in other properties of the feed material. Quite often though, the machine not running as intended causes it.
This is simply the rotational speed of the mechanism that is causing a screen to shake. In typical applications, it can range from 700 revolutions per minute (rpm) to 1200 rpm (or 11.7 Hz to 20 Hz).
The laws of physics have stroke, speed and acceleration (Gs) tied together in the following way: Gs = stroke x speed2/1,789,000.
There are other methods and formulas for calculating Gs. You may already know (or notice from the formula) that Gs and stroke are directly proportional. Increase the stroke by 10 per cent and you’ll increase Gs by the same proportion. Increasing speed 10 per cent however will increase Gs by 21 per cent because acceleration is proportional to the square of the speed. Be cautious when you are changing screen speed.
As far as screening dynamics are concerned, speed takes a “back seat ride” with stroke and acceleration up the front.
There are businesses (such as ToThink Engineering) that have their own vibration meters. Some are simple and some are quite sophisticated. Within reason, all are useful. For some vibrational issues you need someone to test with more accurate equipment that also has a frequency analysis capability. Nevertheless, for gross checking of screen motion, scribing circles/ovals/lines on a piece of paper stuck to the side of a screen is useful too. With a bit of practice you can record the motion (magnitude and shape) of a screen. Just make sure you only hold the pen on the paper for very short periods so you can see individual circles/ovals/lines. Then measure the maximum dimension. That’s your stroke.
Use a tachometer or one of the vibration apps available on your smart phone and you will then have the speed. If the app gives you Hz then multiply it by 60 to get rpm. Use the formula above to calculate the Gs and check these numbers against the information provided in your equipment operation manual or contact the supplier of the screen.
Having these facts puts you in a better place to understand any issues and know where to turn to resolve them. I often hear about minor disputes between quarry sites and screen media or equipment suppliers when those involved don’t know how the screen is running. Some such issues have been known to linger for years. This is simply an indication of where our industry is at with vibrating equipment and the opportunities we have to do things better and smarter. Good information leads to good decisions.
Occasionally sites will change screen mesh on a deck to a more exotic and more costly product in an attempt to solve an issue (eg blinding) but they didn’t know if the screen was running at the appropriate Gs before they decided to make the purchase. Please don’t get me wrong. There are some fantastic screen media products available that can help with blinding, pegging, etc, but you should always make sure a screen is running correctly before you consider going that extra strep by changing the media type.
If you buy a new screen, make sure the supplier provides the intended stroke, speed and Gs for that machine and make sure that’s how it’s running before they leave it in your hands. That’s your benchmark. Either get someone in to test and record these parameters or carry out your own tests. Unless there is good reason to change any of those parameters then make sure they stay that way. That means you need to repeat these checks periodically.