Conveyor Belt Scrapers: Types, Selection and Typical Wear Signs

When I walk into a plant I have never seen before, the first place I look is under the belt. Most of the time the picture is the same: crusted return idlers, a cone of fines under the return run, and a shovel leaning against the nearest support, ready to hand. That shovel is the most honest measuring instrument in conveyor technology, because wherever it gets used daily, the scraper has stopped doing its job, or there never was one.

What a conveyor belt scraper really does

Carryback is the material that does not drop off at the discharge point but stays stuck to the belt and rides back with it. Along the way it settles exactly where nobody wants it: on the return idlers, on the pulleys, in the steelwork and on the floor under the conveyor. The belt picks it up the way a wet shoe sole picks up sand at the beach, quietly but reliably, on every single revolution.

The consequences are rarely spectacular, but they are relentless. The material cakes onto idlers and pulleys, builds up on one side, and before long the belt runs off track. Manufacturers of cleaning systems list carryback as one of the most common triggers for mistracking and the knock-on wear that follows from it. Then there is the cleaning work itself. A US mining authority, quoted via a manufacturer, puts roughly 100 grams of carryback per square metre as a level you can still live with. Sounds like next to nothing. On a belt 1,200 millimetres wide running at 2 metres per second around the clock, it still comes to around 7 tonnes of material that wants sweeping up from under the conveyor, day after day.

And the idlers? One manufacturer reports on a German opencast lignite mining company that analysed its idler replacements: around 30 percent traced back to wear from escaped material, not to normal ageing. Three idlers in ten. Every one of those replacements costs parts, fitter hours and usually belt downtime on top. Scale that up to a full year and you will see why I consider the scraper the most underrated component on the whole conveyor.

The main scraper types at a glance

Scrapers differ mainly in where they sit on the conveyor and how coarsely they are allowed to work. Catalogues call them belt cleaners, belt scrapers or cleaning systems; behind the names is the same principle: an edge or a brush that takes off the belt whatever it will not give up on its own.

By the way, when a quotation talks about a PU scraper, that is not a design of its own but the blade material, polyurethane. More on that in a moment, because in day-to-day practice the material question decides between success and aggravation more often than the design does.

Primary or secondary: why one scraper is often one too few

On paper, a primary scraper on its own looks sufficient. Two trade portals arrive independently at similar figures: a primary scraper removes 60 to 80 percent of the initial carryback, or 60 to 70 percent by the more cautious count. Read the other way round, that means 20 to 40 percent rides on. Only the combination with a secondary cleaner lifts the cleaning performance above 90 percent, according to both sources.

No system reaches one hundred percent; neither the belt surface nor the blade edge is anywhere near flawless enough for that. Which is why the same sources recommend a division of labour instead of a single oversized scraper: a rugged pre-cleaner up front, a precise second stage behind it. It is like sweeping up at the end of a shift. The big broom takes the pile, for what is left you need the hand brush, and neither of the two can replace the other.

Whether the second stage pays off is decided by your material. Dry, coarse gravel practically jumps clear at the discharge point on its own. Moist, fine or sticky material hangs on, and that is exactly when the secondary cleaner earns its keep. What carryback costs as a whole system and which cleaning concepts exist beyond blades is a topic that deserves an article of its own.

Blade material: PU, rubber, tungsten carbide, ceramic

One operation I looked after had fitted a tungsten carbide blade on a soft belt. Not out of conviction, but because it happened to be sitting on the shelf. It cleaned well enough. In the process, it dragged fine grooves into the cover and wore itself down unevenly, because it never sat cleanly on the soft surface. A scraper is meant to clean, not to engrave. The blade material has to suit the belt and the conveyed material, not whatever the storeroom happens to hold.

Polyurethane, PU for short, is the middle ground among blade materials. It is softer than tungsten carbide and wears faster, but it treats the belt gently: a PU blade never gets hard enough to damage the cover or repair patches, and over time it moulds itself to the shape of the belt. That is exactly why it is recommended for new belts as much as for older ones with an uneven surface. For fine, powdery material such as cement or flour, suppliers likewise point to smooth PU blades, because they hug the belt closely.

Tungsten carbide is the other end of the scale. It ranks among the hardest blade materials available, copes with heavy wear and with higher temperatures too, and is the obvious choice for coarse, abrasive material on new, undamaged belts. On a wavy or pre-damaged belt, though, a carbide blade can snag on the irregularities and ruin blade and belt in one go. There are in-between solutions as well: one manufacturer offers carbide-tipped blades on a carrier of polyurethane or rubber which, by its own account, even survive the belt running backwards without damage.

Rubber blades are the flexible and usually cheapest option. Suppliers recommend them for uneven or pre-damaged belt surfaces and for moist, sticky material; in return they wear faster than PU or carbide. Ceramic, finally, comes in where long service life is meant to justify the extra cost.

On service life, one supplier gives a rough orientation for normal conditions: PU 6 to 12 months, tungsten carbide 12 to 24 months, ceramic 18 to 36 months. That is the figure of a single supplier, not a standard, and your plant may sit well above or below it. The same supplier reports a cement works that claims to have stretched the service life of its belt by 40 percent with ceramic secondary cleaners. A single case, but one that points the way. Ask the plant that runs it, not the brochure.

Contact pressure and adjustment: the most common mistake

The most expensive mistake with scrapers does not cost a cent at purchase: too much pressure. At one plant, the colleague from maintenance gave the head scraper another turn tighter on every walk-round, on the logic that more pressure means a cleaner belt. After a few weeks the cover looked as if someone had gone over it with coarse sandpaper: long grooves, drawn cleanly in the running direction. We backed the pressure well off. The cleaning got better, not worse, and the belt finally got some peace.

Behind this sits a simple relationship: there is an optimum range for contact pressure, and cleaning performance falls away in both directions from it. Too much pressure raises the friction between blade and belt. The blade wears faster, the cover suffers, the drive draws more energy, and the belt does not get any cleaner for it. With tyre pressure, everyone accepts that there is a correct range and that any deviation costs money. The same holds for the scraper, except the target value is not printed on a sticker in the door frame.

Too little pressure is just as treacherous, and that surprises many people: the blade then cleans worse yet wears faster at the same time. Material gets pulled through the gap between blade and belt at belt speed and mills fine grooves into the edge. Those grooves widen until the edge is downright frayed. You can spot an edge like that at a glance during a visual inspection, once you know what you are looking for.

On top of that comes a creeping effect: as a PU blade wears, its contact area with the belt grows. The pressure per unit of area drops, even though nobody has touched the tensioner. Mechanically tensioned systems therefore need regular re-tensioning, otherwise even a well-chosen system cleans only half-heartedly after a few months. Manufacturers regard missed re-tensioning as the main reason why otherwise suitable scrapers disappoint in service; good tensioners can be adjusted without tools for exactly that reason. You will find no concrete pressure figures here, and that is deliberate: generally valid, documented values do not exist, and the right range depends on the system and its manufacturer.

Typical wear signs: when the blade is due

A scraper announces its end long before it gives up entirely. You just have to know where to look. The three most reliable signs: more carryback than usual under the belt, fresh spillage around the head pulley, and a blade edge that is visibly or unevenly worn.

During the visual inspection, the first look belongs to the blade edge, because that is where wear shows first. The second look goes to the running conveyor: does the blade still sit against the belt along its whole length? Do stripes of material stay behind on the belt after the scraper? A worn blade works like an old windscreen wiper. It has stopped wiping, it just smears, and the stripe on the belt is its confession.

Suppliers recommend making blades, mountings and the tensioning mechanism a fixed part of the regular visual inspection, and watching the conveyor in a test run after every adjustment. Fixed replacement dates by calendar, on the other hand, have never worked for me: a blade wears by operating hours and material, not by date. How much reserve can sit in a blade is shown by a single case study with condition monitoring: there, a vibration sensor reported the state of the secondary cleaners, and the operator was able to stretch the blade change from 25 to 30 days out to 40 to 45 days without the cleaning getting any worse. One single plant, not a standard. But a hint at what watching the condition is worth compared with guessing.

Anything that makes the swap quick is useful too. Some suppliers deliver systems with tool-free blade change (pull the pin, swap the blade, set the pin) and special blades for hot, chemically aggressive or heavily abrasive environments. If the change takes ten minutes instead of two hours, it actually gets done.

The scraper as a spare part: what your supplier needs to know

At some point, despite all the care, a new purchase is due, and then the quality of your enquiry decides the quality of the quotation. What follows is field experience from many spare-part enquiries, not a standard. A supplier who knows the following points can offer you a system that fits instead of the nearest catalogue match:

• belt width and, if known, belt speed
• conveyed material with its properties: dry, moist, sticky, abrasive, sharp-edged
• installation point: head pulley, return side or the underside of the belt
• the existing scraper type and, if available, the model
• photos of the scraper and of the installation area
• operating environment: wet, dusty, hot, oily or with food contact
• the result you want: coarse cleaning, fine cleaning, protection of the return side or less spillage

The photos matter more than most people think. A picture of the installation space answers questions nobody thinks to ask on the phone: how much room is there under the pulley? How does the fitter get at the tensioner? And the cleaning goal belongs in the enquiry because it steers the choice of system. Someone whose main aim is to protect the tail pulley needs something different from someone who has to hand the belt over broom-clean to the next transfer point. None of this is wizardry. But it is craft, and it starts with a complete enquiry.