A vibrating screen is a deceptively simple machine. A motor shakes a box carrying one or more mesh decks, small particles fall through the holes and larger ones travel over the end. That picture is true as far as it goes, but it hides everything that decides whether a screen grades material accurately or spends its life blinded, overloaded and wearing out early. So how does a vibrating screen work underneath that simple picture? This guide explains it in plain terms: where the vibration comes from, what it does to the material on the deck, and why stroke, speed and angle matter more than horsepower.
The short answer
A vibrating screen works by throwing material off its deck in a rapid, repeating cycle. Each throw lifts the bed of material, shuffles it so that fine particles sink and coarse particles rise, and lands it slightly further along the deck. Particles smaller than the apertures fall through as they meet the mesh; everything else keeps travelling until it discharges over the end. The whole trick is controlling that throw so every particle gets a fair chance to be sorted.
Where the vibration comes from
Unbalanced motors
The most common drive is an unbalance motor: an ordinary electric motor with adjustable out-of-balance weights fitted to each end of its shaft. As the shaft spins, the offset weights pull the whole machine round in a small orbit many hundreds of times a minute. Fit two of these motors side by side, counter-rotating, and their sideways forces cancel while their straight-line forces add, producing a clean linear stroke.
Exciter drives
Larger and heavier screens often use a gearbox-style exciter instead: a sealed unit with geared, counter-rotating eccentric shafts, driven by an external motor. The principle is identical. The exciter simply packages bigger unbalanced masses in a housing designed for the forces involved.
Springs and isolation
The screen body sits on springs. This matters for two reasons. It lets the body vibrate freely so the energy goes into the material rather than the building, and it protects the supporting structure from fatigue. The brief shudder you feel when a screen starts up or shuts down is the machine passing through its resonant speed, which is normal, provided it does not linger there.
What happens to the material on the deck
Stratification
Vibration makes a bed of mixed particles behave almost like a liquid. Larger particles rise to the top and finer particles sink between them towards the mesh. This is stratification, and it is the reason a screen can process a deep bed of material at all. Without it, the fines in the top of the bed would never reach an aperture.
The probability of passing
A particle passes the deck when it happens to meet an aperture squarely, with room to fall through. A particle half the aperture size does this easily and passes almost immediately. A particle at 90 or 95 per cent of the aperture size needs many attempts, which is why near-size material is the hardest to screen and why deck length exists: it buys those particles more attempts before they run out of deck.
How material travels
Each vibration cycle throws the bed forward as well as upward, so material walks along the deck at a rate set by the stroke, the speed and the deck angle. Feed the screen too fast and the bed becomes too deep for fines to reach the mesh in time; too slow and the deck runs half empty, wasting its area. Screening performance is largely the art of managing bed depth.
A worked example
Picture a double deck screen fed with 0 to 40mm crushed stone, cutting at 20mm on the top deck and 4mm on the bottom. Material lands at the feed end and the bed immediately starts to stratify: the plus 20mm stone rides across the top deck and discharges over the end as the coarse product. Everything smaller drops to the bottom deck part way along.
On the bottom deck the process repeats at the finer cut. The 4 to 20mm stone travels over the end as the mid product, while the 0 to 4mm falls through into the fines hopper. Three products from one machine, and every design question, from stroke to deck angle to media choice, comes back to giving each particle enough chances to find the right hole before it runs out of deck.
Linear, circular and elliptical motion
A single rotating shaft gives circular motion, the traditional choice for inclined screens where gravity helps material travel down the deck. Twin counter-rotating shafts or motors give linear motion, which can convey material along a horizontal deck and suits low headroom installations. Elliptical motion sits between the two, combining steady travel with a strong throw. None of the three is universally better. The right motion depends on the duty, which is why manufacturers ask so many questions before quoting.
Stroke, frequency and g-force in plain terms
Stroke is how far the deck moves in each cycle. Frequency is how many cycles it makes per minute. Together they set the acceleration of the deck, usually quoted as a multiple of gravity. Most screens run somewhere between 3g and 5g.
Coarse material wants a long stroke at low speed: big, unhurried throws that shift heavy particles and clear the deck. Fine material wants a short stroke at high speed: quick agitation that keeps fines moving through the bed. Turning the g-force up beyond what the duty needs does not improve the cut, but it does shorten the life of every weld, spring and bearing in the machine.
The role of the screen media
The mesh or panel the material actually touches is called the screen media, and it is as much a part of how the machine works as the drive. Open area, the share of the surface that is hole rather than wire, sets how quickly material can pass. Woven wire offers the most open area and the sharpest cut. Polyurethane panels trade open area for wear life. As media wears, apertures grow, and the screen quietly starts making a coarser cut than the one it was set up for, which is why media condition is part of how a vibrating screen works, not just a maintenance line item.
Why screens blind and peg
Blinding is a coating problem: damp fines and clay build up on the mesh until the apertures close. Pegging is a wedging problem: particles very close to aperture size jam in the holes and stay there. Both reduce open area, which quietly ruins the cut long before anyone notices a blocked deck.
Screen designers counter them in several ways: ball decks that tap the mesh from underneath, electrically heated decks and mesh cleaning systems that stop damp fines bonding to the wire, flexible polyurethane media that flexes pegged particles free, and finger screen decks for genuinely sticky feeds. Aperture shape helps too. Slotted apertures pass damp, elongated material that would peg a square mesh.
Frequently asked questions
What is stratification and why does it matter?
Stratification is the sorting of a vibrated bed, with coarse particles rising and fines sinking towards the mesh. It matters because it delivers fine particles to the deck surface where they can pass. A screen fed too deeply for stratification to finish is inaccurate no matter how well it was built.
Why do many screens run at an angle?
The incline uses gravity to move material along the deck, which allows circular motion and simpler drives. It also presents apertures to the material at an angle, which changes the effective cut size. Horizontal screens rely on linear motion to convey material instead, and are chosen where headroom is tight or a very accurate cut is needed.
Does turning up the vibration improve screening?
Rarely. If the stroke and speed already suit the material, more g-force just increases wear and fatigue. Poor screening is more often a bed depth, media or feed distribution problem than a vibration problem, and those are the places to look first.
What limits how much a screen can handle?
Deck area, in the end. Every deck has a rate beyond which the bed is too deep to stratify and the cut falls apart. Feeding it evenly across its width, keeping the media clean and choosing the right open area all push the limit higher, but past a point the duty simply needs more square metres.
How long does a vibrating screen last?
The body and drive of a well specified screen are usually measured in decades, with springs, media and liners treated as consumables along the way. What shortens life is running duties the machine was not designed for: heavier feed, more g-force or more hours than the fatigue calculations assumed. A screen matched to its duty and maintained on condition outlives most of the plant around it.
The same principles at every scale
Everything above applies whether the machine is a small laboratory screen or a triple deck unit grading hundreds of tonnes an hour. Mogensen applies these principles across its range of vibrating screens and sizers, designed and built in Grantham, Lincolnshire. If a screen on your site is not behaving the way this guide says it should, our technical support team can usually tell you why, and what to change.
