Reading Conveyor Belt Cracks Right: Ageing, Overload or Fitting Error?

Not long ago I was standing under a belt with a maintenance lead, torch in hand, and we were looking at two spots in the rubber. One was a fine web of hairline marks, barely deeper than a scratch. The other was a clean split that ran in from the belt edge towards the middle. To him both looked equally threatening. Yet they were telling two completely different stories, and only one of them was a job that could wait until knocking-off time.

Four cracks, four stories: the damage patterns

On the plant floor, “crack” is one of those words that say too much and too little at the same time. Too much, because everyone immediately pictures a write-off. Too little, because it covers wildly different kinds of damage. Before you reach for the phone and order a replacement belt, it is worth a moment with the torch. The crack pattern tells you half the diagnosis.

Roughly, the cases sort into four patterns. The fine surface cracks sit only in the topmost layer of rubber and spread as a net-like web across larger areas. The transverse crack runs at a right angle to the direction of travel, usually in from one edge. The longitudinal crack runs in the direction of travel, often set off by a foreign object that has lodged itself and works into the carcass. And then there are the small punctures and holes, point damage from sharp-edged material. Broken rollers or jagged lumps of material that snag on the structure can cause all of it, from a scratch to the kind of break that brings the plant to a stop.

The decisive question with any crack is not “how bad does it look” but “how deep does it go”. A crack that sits only in the cover is a different case from one that reaches the tension members. The cover is wear. The carcass is structure. The table below is the compass I carry in my head during a visual inspection.

You can see it already in the last column. The same umbrella term stands for the calm watch-and-wait case just as much as for the immediate emergency stop. That is why classification comes before any repair. Whoever does not read the crack but only patches it often fixes the symptom and lets the cause run on.

What the cause gives away: the mechanical signature

The crack that runs right through, the one where load-carrying capacity is really at stake, almost always has a mechanical signature. Something cut, pinched or overloaded the belt. The trick is to read the shape of the damage back to what caused it, instead of blindly doctoring the cover.

You know the usual suspects from the plant. A broken or jammed roller that no longer runs true can positively score an edge. A jagged piece of material wedged between belt and steelwork acts like a fixed chisel on every revolution. Sharp-edged, heavy material at the loading point punches punctures and notches. And a foreign object that lodges itself and works into the carcass opens the dreaded longitudinal rip, where even a strong, heavy belt can be destroyed over a great length in no time. How to spot this worst-case damage early and stop it is a topic of its own.

Two observations help with the diagnosis. If the damage always sits at the same point along the line while the belt runs past, the cause is in the installation: a roller, an edge, a pinch point. If it travels around with the belt, it sits in the belt itself. And recurring cracks at exactly one position are never chance. There is a cause still sitting there. More on that at the end.

When the plant is not at fault, but time is

The fine surface cracks are a different matter. No foreign object struck here, time did the work. Rubber is a living material, and it ages whether the belt runs or not.

The driver is ozone, with UV light lending a hand. Ozone raises the acidity at the rubber surface and reacts with its molecular structure. The result is exactly these fine cracks on the surface, together with a noticeable drop in tensile strength. UV light, whether from the sun or from certain lighting, speeds the whole thing up through photochemical reactions that push the oxidation of the rubber surface along. The rubber turns brittle and loses mechanical strength. It is like an old rubber boot left one summer too long on the terrace: the surface goes dull, then cracked, and one day it splits at the first firm bend.

A word on origin, because the question keeps coming up in workshop conversations. Some people in the trade report from experience that very cheap belts of unclear origin do not always come with the ozone and UV protection that a brand-name belt has. That is a field observation, not a tested statistic, and I would not put it as a blanket claim about a whole region of origin. What is solid is the other side: a belt leaves the factory highly protected against ozone and UV. Whether that protection survives to the point of fitting is decided less in the catalogue than in the storeroom.

The most expensive belt is the one that never ran

I once saw a replacement belt that spent a whole summer in the yard, stood upright in the blazing sun, because there was no room in the shed. When it was fitted a year later, the cover already showed fine cracks before the belt had carried a single gram of material. The operation had bought expensive tension-member fabric and then left its service life out in the yard. A belt belongs in cool, dry, covered storage. Otherwise you buy service life and store it right back outside.

The storage rules are not something someone made up, they live in a guideline of their own and lean on ISO 5285. Sensible storage is at around 15 degrees, at most 25 degrees, cool, dark and protected from the weather. If a belt has been stored in sub-zero temperatures, it should sit for at least 24 hours above 10 degrees before further work, so it gets its elastic properties back. That sounds like fussing over details. But it decides whether you fit a belt or an aged roll.

And then there is the ozone in the storeroom itself, which almost nobody thinks about. The sun outside is not the only source; equipment in the shed makes ozone too: certain fluorescent and mercury-vapour lamps, high-frequency machines, electrical sparks and discharges. A replacement belt parked for months right next to a source like that ages quietly away in the dark. The most expensive belt is not the one that tears. It is the one that ages without ever having run.

Repair, or out it comes?

Once the diagnosis is in, the money question follows: patch or replace? An everyday image helps here. A stone chip in the windscreen can be repaired as long as it is a small crater. Once the crack runs right across the whole screen, nobody fills it any more, then the screen comes out. With a belt the logic is the same, only with one clear line.

I know that line from a repair I had to talk an operation out of. They wanted to patch a transverse crack that ran from the edge halfway across the belt. Patching gets you nowhere there. Once a transverse crack reaches across that much of the width, the damaged section has to be cut out and a new splice made, or it tears open again at the same spot. The line is not drawn by feel but by extent. As an objective figure: once a transverse crack reaches from the belt edge across 25 percent of the belt width or more, the point for a new splice is reached. Below that, repair is possible; above it, it becomes a cut-out with a fresh splice.

For the smaller case, the clean puncture or the short crack, there are interim fixes that get the belt back into service quickly. One is cold vulcanisation. Another is mechanical crack-repair fasteners, set with bolts. According to manufacturer data they suit belts up to about 30 millimetres thick and score points for quick on-site fitting, low cost and next to no loss of material. For a fast, temporary repair you set such fasteners about 100 to 150 millimetres apart along the crack. One important caveat belongs with them, and it too comes from the manufacturer: these crack-repair fasteners are there to repair, not to join two belt ends. Use them for the wrong job and you build in the next failure.

A repair fastener is a bridge, not a new build. It carries the belt through until the planned repair, it does not replace it. Treat the bridge as the permanent answer and at the next stoppage you are standing at the same crack again.

Why the same crack keeps coming back

The most maddening crack is the one you have already repaired three times and that still comes back at the same spot. It is like a pothole you keep filling in while the lorry follows the same rut every day. As long as the cause is still there, every repair only buys time.

Recurring cracks at a fixed position almost always point to an unsolved trigger: a loading point that throws sharp-edged material at the same spot every time, a foreign object that slips through on a regular basis, or belt tracking that lets the belt chafe against an edge. Before you set the fourth patch, walk the loading point and the line. Fix the trigger first, then the repair is worth doing.

The second lever is the choice of material for the next belt. There is an international test standard for the tear strength of textile conveyor belts, one that measures the tear-propagation resistance of an incipient crack. But it only describes how the test is run and sets no minimum values. A test mark alone therefore tells you nothing about how tough the belt is against tearing further. What matters more is that the cover grade suits the conveyed material: sharp-edged, heavy material needs a different cover from fine sand. Which grades exist for that and what the marking means in detail is a chapter of its own.