Double Belt Sandwich Conveyors: Steep Inclines Between Two Belts

Most belt conveyors give up somewhere between 18 and 22 degrees. The material starts to slide, the load rolls back down the incline, and the standard fix is to reach for a chevron cover or a sidewall belt. There is a third option, and it has quietly moved coal, foundry sand, filter cake and cement raw meal on some of the tightest routes in heavy industry. Put the load between two belts running face to face, squeeze it, and the same conveyor that could not hold 20 degrees will climb 60, 75, even 90. That machine is the double belt sandwich conveyor.

If you have spent years around troughing idlers and single belts, the first thing that surprises you is how little of the conveying duty is done by gravity. On an ordinary belt the load sits on top and friction alone keeps it there, so the maximum angle is whatever the friction coefficient allows, usually 16 to 20 degrees for dry lump material and noticeably less once it is wet or rounded. In a sandwich machine the load is trapped. An upper belt presses down, a lower belt carries up, and the two edges are closed against escape, so the material rides inside a moving pocket rather than on an open surface. Angle stops being the governing variable, and clamping force takes its place.

That single change rewrites most of the design. Carrying capacity, belt selection, pulley layout and cleaning all follow from the fact that you are now running two belts instead of one, loaded from the side or the end, under a pressure you have to choose deliberately. This article walks through the clamping physics, the material types that suit a double conveyor belt and the ones that do not, the boundary against chevron and sidewall belts, the belt bodies and edges you need, and the drive, tensioning and tracking points that decide whether the machine runs for a decade or fails inside a year. We finish with the question too few suppliers answer honestly, which is when a double belt is simply the wrong answer.

Send us your incline, material and tonnage — we will tell you whether a double belt beats a chevron belt

01How a Double Belt Sandwich Conveyor Holds Its Load

The working principle is containment rather than friction. Two belts are arranged one above the other, with the carrying surfaces facing each other across a small gap. The lower belt behaves like the carrying run of any conventional conveyor. The upper belt is inverted, supported on its own idlers and pulleys, and its only job is to press the material down onto the lower belt so that the combined assembly forms a closed channel. Material enters at one end, usually through a chute that feeds the pocket between the belts, and discharges at the other when the upper belt lifts away and the load falls free.

What makes this work is that the material no longer has a free surface. On an open belt, a rounded particle on a 30-degree slope has nothing above it, so it rolls or slides the moment the driving friction is overcome. Inside a sandwich pocket the same particle is confined on all sides except the direction of travel, and the vertical pressure applied by the upper belt raises the effective friction between the material and both belt surfaces. The steeper the intended incline, the more pressure is needed, which is why sandwich machines at 90 degrees look almost like a press and machines at 40 degrees look like a slightly modified troughed conveyor.

Three mechanical details separate a machine that works from one that plugs. The first is a continuous, even seal along both edges, because any gap becomes a leak path and fine material finds it within days. The second is a controlled gap, because if the pocket is too tall the material tumbles inside it and the pressure never develops, while if it is too short the belts pinch and the edges wear through. The third is matched belt speed, because the two belts must run at essentially the same velocity or the load is sheared and, on a long unit, driven into a jam.

Sandwich conveyors come in three common arrangements, and the choice between them is set by the route rather than by preference. The S-shape wraps the pair of belts up and over in a rough S, which lets one continuous assembly take a horizontal feed, turn up a near-vertical lift and return to horizontal at the top without a transfer point. The C-shape makes a single broad curve, useful where headroom is limited and the lift is short. The linear or straight-incline version simply runs the two belts up a fixed slope with a feed and a discharge at the ends, and it is the most robust of the three because it has no curved section to control.

I have stood under all three types, and the one that causes the least trouble over a five-year horizon is almost always the plain straight incline. Curves look elegant on a drawing, but every curved section needs guide rollers, edge pressure control and a spare part nobody stocks. If the layout can accept a straight lift, take it.

None of this is exotic technology. The belts themselves are ordinary rubber belts, often a standard EP carcass on the lower run and a lighter body on the upper run. What is uncommon is the discipline around pressure, gap and edge sealing, and that is where buyers get caught. A conveyor belt manufacturer who understands sandwich duty will ask about angle, material bulk density and throughput before quoting a body, and will push back if the numbers do not support the application.

02Clamping Force and Line Pressure: The Numbers That Decide Everything

The clamping force in a sandwich conveyor is expressed as line pressure, the force perpendicular to the belt surface per unit width of belt. It is measured in newtons per millimetre of belt width, or in the older imperial habit, pounds per inch. A typical steep-incline sandwich belt runs somewhere between 3 and 12 N/mm of line pressure, and the correct figure for a given job is found by looking at the load's resistance to sliding rather than by copying a competitor's machine.

Sound clamping design starts with the classic condition that the friction available on both faces must exceed the gravity component driving the material down the slope. Written out simply, the belt-to-material friction coefficient times the clamping force, taken over both the upper and lower contact, has to be greater than the load's weight times the sine of the incline angle, with a safety factor on top that most engineers set between 1.3 and 1.6. On a horizontal or gently inclined machine the sine term is small and pressure is trivial. At 70 degrees the sine term dominates and the pressure requirement climbs steeply, which is why the pressure system, not the belts, becomes the expensive part of a vertical lift.

The friction coefficient is the number people underestimate. Dry sand on a smooth rubber cover might sit at 0.5, but the same sand on a wet, dust-contaminated cover can drop below 0.3, and a rounded, free-flowing plastic pellet behaves far worse than a rough crushed ore. Coarse, angular material with a broad particle size distribution interlocks well and holds a high effective friction. Smooth, spherical, monodisperse particles slide almost like ball bearings and force you to raise pressure, slow the belt or refuse the job. This is the single most important material property for a double conveyor belt, and it is not something a simple abrasion test will tell you.

Pressure is delivered in one of two ways. The simpler scheme relies on the weight of the upper belt, its idlers and a held-down return strand, which is adequate for shallow angles in the 30 to 40 degree range and requires almost no active control. The active scheme uses a series of pneumatic cylinders, or on some designs a mechanical linkage, to push the upper belt down against stops, letting the operator dial the pressure up and down as the material changes. Active pressure costs money and demands air supply, but it is the only way to reach the 60 to 90 degree class, and it lets you de-tension the upper belt when running empty to cut wear.

One field measurement stuck with me. On a 900 t/h coal line feeding a silo, a sandwich unit at 55 degrees had been losing pressure gradually and the operators blamed the belts. We found the air regulator had been set two bar below the design value during a maintenance weekend, and the loss of roughly 2 N/mm of line pressure was enough to let a wet weekend batch of coal slip backward through the pocket. Restoring pressure fixed a problem that two belt changes had not.

Notice how the table pairs every pressure band with the failure that band tends to produce. That pairing is deliberate, because pressure choice is really a bet about which failure you would rather prevent. Run light and you save belt wear but risk slip. Run heavy and you stop slip but drive edge fatigue and material degradation. Almost every real sandwich conveyor lives on the compromise between those two, and the operator's pressure log is the single best indicator of whether the design was understood by the people running it.

03Material Fit: Fine, Powdery and Free-Flowing Cargo

The double conveyor belt was invented around a specific problem, which is the vertical or near-vertical lifting of material that cannot be elevated any other way in a single pass. The materials that drove its adoption in the 1970s and the ones that keep it busy today are not lumpy mine rock. They are fine, often dry, and frequently difficult. Cement raw meal, pulverised fuel ash, foundry sand, phosphate, soda ash, filter cake, limestone fines and fine coal are the classic cargo. They share a small median particle size, a high degree of flowability when disturbed, and a tendency to fluidise or aerate, which is exactly why an open belt cannot hold them on a slope.

Fine material actually suits containment well. The smaller the particle, the more contact points it makes with both belt faces and the more effectively a modest line pressure converts into holding friction. A 3 mm minus material saturated in the pocket behaves almost like a soft solid and will ride a 70 degree incline with far less pressure than a coarse, poorly graded gravel of the same bulk density. This is the opposite of the intuition most people bring from open-belt conveying, where fine material is the nuisance.

The nuisances in sandwich duty are moisture, stickiness and temperature. A wet, cohesive clay will adhere to the upper belt, build up on the return side and progressively close the feed gap, and no amount of pressure will help because the material is no longer free to be clamped. Oily or greasy residues from machining swarf and food waste behave similarly, and there a conventional oil-resistant conveyor belt cover is only part of the answer, because the real problem is adhesion rather than chemical attack. Hot material above roughly 120 degrees C forces you away from standard covers entirely and toward a heat-resistant conveyor belt body on both runs, and it also changes the pressure frame, since thermal growth in a long upper belt is not negligible.

Abrasion is easier to manage than adhesion. Almost any mineral route will benefit from a wear-grade cover on the lower belt, which does the carrying work, and you can specify an abrasion-resistant conveyor belt cover there while keeping a smoother, lower-friction cover on the upper belt where the priority is releasing material cleanly. That asymmetry is one of the small design freedoms a sandwich machine offers and single-belt conveyors do not, and a good conveyor belt supplier will build the two runs to different specifications on request rather than quoting the same body twice.

Where does the material come from? In practice, most sandwich units sit downstream of a fine-grinding or sizing step, which is why the cement plant is the single largest home for the technology, followed by minerals processing and the fine end of mining and quarrying. You also see them in food and chemical plants lifting powders and granules in tight footprints, though there hygiene and cross-contamination rules add a cleaning burden that the machine type does not naturally love.

Read that table as a filter rather than a menu. If your material falls in the first three rows and stays dry, a sandwich lift is a reasonable candidate and the design conversation can move on to angle and pressure. If it falls in the last two rows, the right move is usually to stop considering a double belt and look at an open belt with pockets instead, and we come back to that boundary in detail later.

One more material property deserves its own sentence, because it quietly ruins more sandwich machines than any other. Angle of repose and the related angle of surcharge tell you how the material behaves when poured, but inside a clamped pocket the relevant figure is the internal friction angle, which for most free-flowing powders sits between 25 and 40 degrees. If the intended incline is many times that figure, you are relying entirely on applied pressure, and you should be honest about whether the pressure system can be maintained reliably in your plant. On a dusty site with a single small compressor, it often cannot.

04The Steep Incline Envelope: Angle, Speed and Capacity

Angle and capacity trade against each other in a sandwich conveyor just as they do on an open belt, but the mechanism is different. Going steeper costs pressure, and pressure costs belt wear, material degradation and energy, so the practical envelope is set as much by operating cost as by geometry. In the field, most installed double belt units run between 30 and 75 degrees, with a strong cluster in the 45 to 65 degree band where the pressure requirement is manageable with a standard active-clamp package.

Belt speed is the second axis. Sandwich belts generally run slower than open belts doing comparable tonnage, because the pocket has to fill evenly and the clamping contact needs time to develop. Speeds of 1.0 to 2.0 m/s are typical, and pushing beyond about 2.5 m/s on a steep unit tends to aerate fine material and reduce the effective friction you worked so hard to create. A wide, slow belt beats a narrow, fast one on almost every sandwich job I have seen.

Capacity itself follows from belt width, pocket depth and speed, with a fill factor that rarely exceeds 0.75 of the geometric pocket because the material needs room to be pressed without being extruded. On a 1,200 mm wide belt with a 100 mm pocket running at 1.5 m/s, you can move the sort of tonnage that a much wider, faster open belt would carry on a gentle slope, which is the price of the steep angle. That trade is often worth it where the alternative is a long inclined gallery or a bucket elevator.

There is a real comparison to be made with the vertical screw and the bucket elevator, and it comes up in almost every project. A bucket elevator can reach 90 degrees and handles coarse, abrasive, hot material that a clamped pocket would destroy. What it cannot do is turn corners, take a horizontal feed without a transfer, or match the gentle handling of fragile or friable fines. Where the route needs to change direction and the material is fine, the sandwich belt usually wins on layout and on degradation.

Two structural choices appear once you cross about 60 degrees. The first is that the lower belt increasingly looks like a heavy-duty body carrying a tension load, and a steel cord conveyor belt becomes attractive on long, high-tension lifts even though the material is fine, because the carcass has to survive the drive tension rather than the abrasion. The second is that the upper belt stays deliberately light and flexible, and a standard fabric body such as an rubber conveyor belt with an EP carcass is usually the right call, as long as it is built to deform around the load rather than fight it. Anyone who tells you the two belts should be identical has probably not maintained a steep unit, and a manufacturer such as the team at our own conveyor belt factory will normally specify them separately.

05Double Belt versus Chevron, Cleated and Sidewall Belts

This is not a chevron selection guide, and it is not meant to be. The reason to put the comparison here is that most buyers arrive at the double belt question after already being offered a chevron conveyor belt or a sidewall conveyor belt, and the honest advice is that for the large majority of inclines, one of those two is the correct answer and a sandwich machine is overkill. The double belt earns its place in a narrow but genuinely difficult slice of applications, and buying it for anything else wastes capital.

Start with what each family is actually good at. A chevron or cleated belt keeps an open surface and adds raised profiles that stop material from rolling back, which works beautifully for coarse, angular, fairly dry material up to about 30 to 35 degrees and becomes unreliable past that as the profiles fill and stop doing their job. A sidewall belt does the same trick with rigid cross-cleats and continuous sidewalls, and it comfortably reaches 60 degrees or more with lump material, which is why you see it on crushed rock, ore and construction aggregate. Both of those keep the material open to the air, which is fine when the material is lumpy and free-draining and a liability when it is fine and fluid.

The double belt is the answer when the material behaves like a liquid, when the angle is beyond what an open belt can hold, and when a sealed pocket is the only way to keep fine particles from rolling back or leaking. It is also the answer when the route must turn a corner without a transfer point, because the two belts can be curved together in a way that no troughed open belt can manage. If your material is crushed stone at 25 degrees, none of that applies and you should buy a chevron belt and spend the savings on a heavier cover.

Cost tends to settle the argument. A sandwich machine costs roughly two to three times an equivalent open-belt conveyor once you count the second belt, its drive, its support structure, the pressure system and the more complex chute work. It also consumes more power per tonne, because you are running two belts and overcoming the clamping contact. You accept that premium only when the alternative is not a cheaper belt but a different, more expensive machine altogether, such as a bucket elevator or a vertical screw. If your fallback option is a chevron belt, the chevron belt usually wins.

One pattern I keep seeing is a plant that specifies a sandwich unit because the drawings show a very steep lift, then relaxes the geometry during value engineering. The angle drops from 70 degrees to 40, the double belt stays in the specification, and the plant ends up paying twice the price for a machine that a sidewall belt would have handled. Keep the comparison honest right up to the purchase order, and re-run it whenever the route changes, because the boundary sits at the angle the material and the pressure system can genuinely hold, not at the angle printed on the original layout drawing.

There is a related family of belts that shows up beside sandwich machines on the same sites, and it is worth linking the two in your mind because they often share a spare-parts store. An industrial conveyor belt of the general-purpose kind is what most of the surrounding plant runs on, and a sandwich machine is usually one specialised conveyor inside an otherwise conventional plant. If you are sourcing for a whole site rather than one machine, it is cheaper to buy from one wholesale conveyor belts programme than to source each conveyor separately, and a transmission belt manufacturer or V-belt manufacturer covering the drive belts alongside the conveyor bodies can simplify the spares list considerably. That is also where a reliable conveyor belt distributor relationship pays off, because a sandwich unit that sits idle waiting for a specific upper-belt width is an expensive way to save a few percent on the original order.

06Top Belt, Bottom Belt and the Edges Between Them

The two belts in a sandwich machine do different jobs, and specifying them as a matched pair is one of the most common and most expensive mistakes. The lower belt carries the material, transmits the drive tension and takes the abrasion, so it is built like a heavy-duty carrying belt with a robust carcass and a wear-grade cover. The upper belt only contains, so it is built light and flexible, with a cover chosen for release rather than for abrasion, and a carcass just strong enough to carry its own weight, the clamping loads and the small tension needed to run true.

Get the lower belt right first. It sees the full drive tension of a steep, often long lift, which on a tall unit can push you toward a high-tensile fabric carcass or, as we saw earlier, a steel cord body. It also sees the working face of the load and the sharpest particles, so a thick abrasion-resistant cover on the load side is standard practice, and on a mineral duty an impact and cut resistant conveyor belt cover is worth the premium wherever the feed lands. The return side can be a lighter, smoother cover, because all it meets is idlers.

The upper belt is where sandwich design gets subtle. It has to conform to the load surface rather than bridge over it, or the pressure concentrates in a few spots and both the material and the belt suffer. That means a thinner body, often a two-ply or three-ply fabric carcass, and a cover with genuinely good release. A cover that grips tenaciously will carry fine material back on its return run, dropping it along the whole length of the machine and turning a clean installation into a mess within a month. Specifying a smooth, low-friction upper cover is not laziness, it is maintenance prevention.

Both belts need to run at matched speed, and they need pulley diameters that respect the flexibility of the upper body. A pulley sized for a stiff lower belt will over-flex a thin upper belt and crack its cover at the edges, so the two pulley sets are often deliberately different in diameter. Where the machine has a curved section, the upper belt also carries guide rollers on its edges, and those rollers are a wear point that has to be accessible for replacement without dismantling the whole unit.

The edges are the part buyers forget, and they are the part that decides whether the machine stays clean. A sandwich conveyor needs a continuous, flexible seal along both sides of the pocket, and the seal is usually formed by flanged or rounded belt edges running against each other, sometimes with an additional edge rubber profile. If the two belts are the same width and the same edge profile, the seal is easy to control. If the upper belt is narrower, or its edges are square while the lower belt's are rounded, you get a gap, and fine material will find it. On a coal or cement duty a millimetre of edge misalignment is enough to start a slow leak that coats the structure below within weeks.

Supporting hardware matters here too. The idlers under the lower belt are the familiar troughed type, whereas the idlers over the upper belt are usually flat and spaced to let the belt sag slightly onto the load rather than bridge it. Because the upper run needs to follow the profile of the material, flat return idlers often do double duty on the top strand, and the frame that holds them is what applies the clamp. The whole assembly is a small system of conveyor components working together, and on a sandwich machine the components matter as much as the belts, which is worth remembering when you compare quotes that differ mainly on the belt price.

07Drive, Tensioning, Tracking and Cleaning

A sandwich conveyor has two belts, and the question of how to drive them is the first thing an experienced millwright asks. In principle you can drive both belts from one motor through a mechanical linkage, and on short, lightly loaded units that keeps the layout compact and guarantees the speeds stay locked. In practice, most steep units use two independent drives with a speed-matching control, because the two belts rarely need the same torque and a single drive forces one of them to be oversized. The trade is that independent drives can drift apart, and a few centimetres per minute of relative slip will slowly shear the load and wear the pocket.

Tensioning is where a lot of sandwich machines are lost. The lower belt is tensioned like any conveyor, with a screw or gravity take-up sized for its full drive tension. The upper belt needs its own take-up, and it needs enough stroke to cope with thermal growth and with the belt relaxing over its service life, because an upper belt that goes slack stops sealing the edges. On a hot duty the take-up stroke can be surprisingly long, and a unit that was tensioned correctly at commissioning can lose its seal within a season if the take-up runs out of travel. Design the upper take-up with spare stroke and you avoid a whole class of leaks.

Tracking the upper belt is harder than tracking a normal belt, because the upper belt has no load to settle it and it is often thin enough to be twitchy. Training idlers on the return side help, and the same self-aligning hardware that keeps an open belt centred will keep an upper belt centred too, though it needs lighter contact force or it will fight the belt instead of guiding it. A misaligned upper belt closes the seal on one side and opens it on the other, and the side that opens leaks immediately.

Cleaning is where sandwich conveyors differ most from conventional practice, and it is worth slowing down here. Both belts must return clean, because any carryback on either face closes the gap, and a closing gap pinches the edges and accelerates wear. The lower belt takes standard primary and secondary scrapers at the discharge. The upper belt is harder, since its working face is underneath, and it usually needs a light scraper plus, on sticky material, a wash station or an air knife. Where the material is genuinely adhesive, cleaning is the limiting factor on how steep and how fast you can run.

The supporting hardware deserves a mention because it carries the clamping reaction. The lower run rides on conventional troughed idlers, and a job that moves a lot of fines benefits from sealed bearing housings that keep dust out, of the kind described in this guide to a dust-proof idler with a labyrinth seal. The upper run usually runs on flat idlers, and because that strand applies the clamp, its spacing controls how evenly the pressure is distributed. Where the machine uses conventional troughing on the lower strand, standard troughing idlers and training idlers do most of the work, with impact idlers under the feed point if the material arrives with any fall. It is worth reading up on how self-aligning idlers can reduce the tracking problem on both strands, since alignment faults on a sandwich machine are harder to fix once the unit is built than on an open conveyor.

The pulleys and drive components are standard, though the geometry is tighter. Head and tail pulleys on the lower belt follow normal practice, and the upper belt has its own smaller head and tail pulleys where the belt leaves and rejoins the pocket. Belt-to-pulley friction has to be checked at both ends, because the upper belt typically has less wrap and must not be the first to slip under load. A complete set of conveyor pulleys sized for both strands, and conveyor rollers selected for the dust and moisture of the job, will outlast a cheaper package by a wide margin on this duty.

08Failure Modes and When a Double Belt Is the Wrong Answer

Sandwich conveyors fail in a fairly predictable set of ways, and almost all of them trace back to pressure, edge condition or cleaning rather than to the belt carcass itself. That is useful, because it means a maintenance programme built around those three things will keep a double conveyor belt running for many years, while a programme built around belt changes will not.

Now to the part the brochure never includes. There are applications where a double belt sandwich conveyor should not be used at all, and recognising them early saves a very expensive project. If the material is coarse and lumpy, above roughly 30 mm, the lumps bridge the feed gap and crush the pocket, so the belt faces take impact damage that no cover grade will survive for long. Choose a chevron or sidewall belt instead. If the material is wet, cohesive or adhesive, such as damp clay, sticky filter cake or oily swarf, it will stick to the upper belt and carry back, and the seal will slowly close with its own product. In that case an open belt with adequate cleaning is more forgiving.

If the route is only moderately inclined, say under 30 to 35 degrees, a double belt is simply the wrong tool and a chevron or sidewall belt will do the job for a fraction of the money. If the material is very hot, above the working limit of standard rubber covers, the two clamping surfaces both face the heat and the problem doubles, so a bucket elevator or a heat-rated alternative deserves the comparison. If the plant cannot reliably maintain compressed air or the mechanical pressure system, an actively clamped vertical lift will drift out of specification, and a passive design that cannot reach the required pressure is no better. And if the material is fragile and must not be crushed, remember that clamping is compression, so a sandwich machine is a poor choice for friable product even when the angle seems to demand it.

There is one more case worth stating plainly. A sandwich conveyor is not a good answer when the whole point is to move a lot of tonnage over a long, gentle distance, because the cost and complexity only pay off on steep lifts. If the incline is gentle, buy a conventional heavy-duty belt and spend the difference on a better carcass, better idlers and a proper maintenance regime. The double belt earns its premium in a narrow band of steep, fine, free-flowing duty, and outside that band it is the most expensive way to solve a problem that a simpler belt already solves.

If you take one thing from this article, let it be that a double conveyor belt is a containment machine, not a friction machine, and that every design decision follows from the pressure you are willing to apply and maintain. Get the pressure right, seal the edges, keep both belts clean and matched in speed, and a sandwich unit will do things no open belt can. Get any of those wrong and no belt body in the catalogue will save you. When you are ready to look at a specific route, a good first step is a belt comparison against what an open belt would achieve, and our notes on self-aligning rollers and belt tracking and on how impact rollers protect conveyor belts are useful companions to this one. For a broader view of the family, see our conveyor belt manufacturer guide, and for long, high-tension lifts the maintenance notes in our steel cord belts inspection guide apply directly to the lower belt of a tall sandwich machine.

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09Frequently Asked Questions

Can a sandwich conveyor really lift material straight up, at ninety degrees?

Yes, and vertical lifts are a routine configuration, but they are the hardest version to keep working. At 90 degrees the gravity component is at its maximum, so the line pressure needed to hold the load is also at its maximum, typically above 12 N/mm of belt width and often designed case by case. The feed has to be uniform, because a sudden slug of material will either stall the column or push it out at an edge. Vertical lifts also concentrate wear in the pressure frame and the belt edges, so they only make sense where the alternative, usually a bucket elevator, is worse for the material.

How steep can these belts go before I should just use a chevron belt instead?

Below about 30 to 35 degrees, use the chevron or sidewall belt and keep your money. A double belt starts to make sense when the incline is beyond what an open surface can hold and the material is fine enough to be clamped, which in practice is the 40 to 70 degree band for most powders and fines. The deciding factor is not the angle alone but the material. A coarse, dry aggregate at 50 degrees is still better served by a sidewall belt, while a fine, free-flowing powder at 50 degrees is exactly what a clamped pocket is for.

What kind of material works best inside a clamped pocket?

Fine, dry, well-graded material with a bit of angularity is ideal, which is why cement raw meal, fly ash, foundry sand and fine coal dominate the installed base. The small particles make many contact points, so a moderate pressure develops high effective friction and the load rides the incline without slipping. What you want to avoid is material that is wet and cohesive, because it sticks to the containing belt and carries back, or material that is rounded and free-flowing like plastic pellets, because the particles roll against each other and the clamping force has little to grip.

Do both belts need their own drive and take-up?

In most steep units, yes. Two independent drives let each belt run at the torque it actually needs and keep the speeds matched through a control, whereas a single drive forces one belt to be oversized and risks relative slip. Each belt also needs its own take-up, and the upper one needs spare stroke, because a containing belt that loses tension stops sealing the edges. If the upper take-up runs out of travel, the machine leaks, so designers build in extra stroke to cover thermal growth and the belt relaxing over its life.

Why does fine material keep leaking out of the sides?

Edge leakage almost always comes down to the seal between the two belts rather than to the belts themselves. If the upper belt tracks off centre, the gap closes on one side and opens on the other, and the open side leaks immediately. A mismatched edge profile, or carryback that gradually closes the pocket gap, produces the same result. The cure is to correct tracking, renew the edge seal and fix cleaning so both faces return dry, because a sealed pocket only stays sealed while all three conditions hold.

How often does the upper belt need replacing compared with the lower one?

The containing belt is usually the shorter-lived of the two, but not because it wears from the load. It fails at the edges from flexing over small pulleys, from tracking forces and from any sustained over-pressure, and it suffers most where cleaning is poor and material carries back. On a well-set-up machine running below 60 degrees with good cleaning, the upper belt may last several years. On a hot, dusty vertical unit with the pressure frame running hard, expect to replace it far more often than the lower carrying belt, and budget for that in the spares plan.