Mobile Screens

A mobile screen is a self-contained screening plant, usually mounted on tracks and sometimes on wheels, that separates material into different sizes. Material fed into the hopper passes over one or more vibrating screen decks or through a rotating drum, and the fractions that pass through or stay on each deck are carried away by separate conveyors to their own stockpiles. The whole plant, with its power unit and conveyors, can move around a site and be transported on a low loader.

Mobile screens work alongside mobile crushers in quarries and recycling, but they are also used on their own to separate soil from stones, clean topsoil, process compost and wood chips, and grade sand and gravel. Screening turns a mixed material into saleable or reusable products of defined sizes, which is why a screen is often the first machine a small recycling or aggregates business buys.

Brief History of Mobile Screens

Screening is as old as the use of aggregates, and simple sieves and inclined bar screens were used for centuries. Mechanically vibrated screens and rotating trommels came into use in quarries and mines in the nineteenth and early twentieth centuries, mostly as fixed installations.

The mobile screen as a single self-contained unit developed in the second half of the twentieth century. Powerscreen, founded in Northern Ireland in the 1960s, was one of the pioneers of mobile screening plants, and Northern Ireland became an important centre of the industry, with other makers such as Finlay. Wheeled units came first, followed by tracked screens that could move easily around the site and be set up quickly. Since then the range has grown to cover heavy scalpers, precise multi-deck screens, trommels and compact units, with hydraulic folding conveyors, diesel-electric drives and control systems for linking with crushers.

How a Mobile Screen Works

Hopper and feeder. Material is loaded into a hopper, usually by a wheel loader or excavator, and a belt or apron feeder carries it to the screen box at a steady rate. A tipping grid can remove very large pieces first.
Screen box. One or more decks of mesh, punch plate, bars or fingers are shaken by a vibrating mechanism. Smaller particles fall through the openings, while larger ones move along the deck.
Decks. A two-deck screen produces three fractions, a three-deck screen four. Deck angle and stroke can often be adjusted.
Trommel drum. In trommel screens, a rotating perforated drum tumbles the material, which falls through the holes if small enough.
Conveyors. Each fraction is carried by its own conveyor to a stockpile. Many units have hydraulically folding conveyors and can swing them for more stockpile space.
Power. A diesel engine drives the hydraulics or a generator for electric drives, and many units can run from mains power.

Types of Mobile Screens

Scalpers

Heavy-duty screens with robust decks of bars, fingers or punch plate. They handle blasted rock, demolition waste, topsoil and other difficult feed, and are often used before a crusher to remove fines and soil.

Inclined Screens

The most common type for aggregates, with two or three decks set at an angle to produce accurately sized fractions of sand, gravel and crushed stone.

Horizontal Screens

Screens with near-horizontal decks that give longer retention time and high accuracy. They are also easier to integrate with crushers in closed circuit.

Trommel Screens

Rotating drum screens used for sticky, light or bulky material such as topsoil, compost, wood chips, municipal waste and some recycling streams.

Compact Screens

Smaller units, sometimes towed, for landscaping, small recycling yards and contractors that screen modest volumes.

Rinsing and Washing Units

Mobile screens fitted with spray bars and combined with sand washing equipment to produce clean aggregates.

Size Classes

Class	Typical transport weight	Typical capacity	Typical use
Compact	under about 20 t	up to about 150 t/h	Landscaping, small recycling jobs
Medium	about 20 to 35 t	about 150 to 400 t/h	Contract screening, quarries, recycling
Large	over about 35 t	400 t/h and more	Large quarries, heavy scalping

Figures are indicative. Screens are also described by the size of their screen box, such as the length and width of the top deck.

Screening Efficiency

The performance of a mobile screen depends on more than its size. Screening efficiency describes how much of the material that should pass through a deck actually does, and it drops when the deck is overloaded, when the material is wet or clayey, or when the openings blind. Feed rate is therefore one of the most important settings: a screen fed evenly at a moderate rate usually produces cleaner products than one flooded with material. Deck angle, stroke and speed change how fast material travels and how often particles meet the openings, and many modern units allow these settings to be changed from the control panel.

Material shape also matters. Flat and elongated particles pass through openings less predictably than cubical ones, and near-size particles, those close to the opening size, are the most likely to cause blinding and pegging. Operators often use different media on each deck, for example finger decks on top for sticky feed and woven mesh or polyurethane below for accuracy. Anti-blinding devices such as ball decks or tensioned wire media help with difficult materials.

Main Manufacturers

Powerscreen and Finlay, both part of Terex.
McCloskey, a major maker of mobile screens and trommels.
Metso, Sandvik and Kleemann in the Wirtgen Group.
Keestrack, Rubble Master and Rockster.
Doppstadt and Komptech for trommels in waste and compost.

Main Uses of Mobile Screens

Producing sized aggregates in quarries and gravel pits.
Scalping before crushing and screening after crushing.
Recycling of concrete, asphalt and mixed construction waste.
Screening topsoil and excavated soil for reuse on site.
Compost, wood chip, biomass and waste processing.
Coal, ore and industrial minerals in mining.

Choosing a Mobile Screen

Know the feed. Material size, moisture, clay content and contamination decide between a scalper, an inclined screen or a trommel.
Define the products. The number of fractions and their accuracy set the number of decks and the choice of screen media.
Pick the right media. Wire mesh, polyurethane, rubber, punch plate, bars and fingers each suit different materials and wear levels.
Match the crusher. When the screen works with a crusher, the capacities should be balanced to avoid overloading either machine.
Plan the stockpiles. Conveyor length and height decide how much material can be stored before a loader must clear it.

Safety

Mobile screens have many moving parts, including vibrating boxes, conveyors, drums and rollers, so guarding and lockout are essential. Media changes and cleaning of blocked decks must only be done with the machine isolated, and many accidents involve workers reaching into running conveyors. Safe access platforms and walkways reduce falls. Dust, noise and flying material require protective equipment and exclusion zones, and a stable, level base is needed to keep the machine stable, especially with folded out conveyors. Moving and setting up the screen requires a clear area and careful coordination with loading machines.

Trends and Technology

Modern mobile screens use electric drives for the screen box and conveyors, which can be powered by an onboard generator or by the grid. Control systems link screens with crushers so that the line adjusts to material flow. Screen media are becoming more durable and easier to change, and quick change systems reduce downtime. Telematics report production, fuel consumption and service needs. Growth in recycling and in the reuse of excavated soil on site is increasing demand for screens that handle wet, sticky and mixed materials.

Lower-emission engines, dust suppression systems and quieter drives are helping screens work in urban recycling yards and on construction sites close to homes.

Key Terms

Deck: one layer of screening surface in a screen box.
Screen media: mesh, plates, bars or fingers that form the screening surface.
Scalping: removing oversize or fines before further processing.
Oversize: material that does not pass through a deck.
Trommel: a rotating drum screen.
Blinding: clogging of screen openings by sticky or near-size material.

Frequently Asked Questions

What is the difference between a scalper and a screen? A scalper is built for heavy, rough feed and coarse separation, while a standard screen is designed for accurate separation of cleaner material into defined sizes.

How many products can a mobile screen produce? Typically two to four, depending on the number of decks. Three-deck screens producing four products are common.

When is a trommel better than a vibrating screen? For sticky, light or bulky materials such as topsoil, compost and waste, where the tumbling action helps separate the material.

Can a mobile screen work with any crusher? Most can be linked to crushers of any make, as long as capacities and conveyor heights match.

Can a mobile screen handle wet material? Yes, within limits. Finger decks, scalpers and trommels cope better with wet and sticky material than fine mesh decks, and rinsing systems can be added where clean products are required.