Monday 10th Aug, 2020

The common causes of ceramic lagging failure

David Molesworth, Managing Director of pulley lagging specialist Elastotec, explains some of the most common reasons that ceramic pulley lagging fails.

David Molesworth, Managing Director of pulley lagging specialist Elastotec, explains some of the most common reasons that ceramic pulley lagging fails.

Ceramic pulley lagging is often used for pulleys that require a long, trouble-free life to avoid unplanned conveyor shutdowns.

For mining companies, these shutdowns can cost thousands of dollars per minute. While ceramic lagging can be more expensive when compared with other options, its longevity helps make financial sense.

A key contributor to the longer service life of ceramic lagging are the tiles bonded to the rubber backing. These are made from aluminium oxide, an extremely hard ceramic material that provides exceptional wear and abrasion resistance.

It is critical to the performance of the lagging that these tiles remain in place during service. Because pulley lagging is used in demanding, dynamic applications where the tiles are loaded and unloaded every time the pulley rotates, constant flexing can quickly wear out areas of weakness in the adhesion between the tiles and rubber.

This is one of the biggest causes of pulley lagging failure and can occur for two reasons: debonding of the tiles from the rubber backing and physical damage, which tears the tiles from the rubber backing.

Tile debonding

Aluminium oxide tiles must be treated with chemical adhesion promoters or adhesives in order for them to attach to the rubber backing. There is a vast range of adhesives available from different manufacturers and most will provide a strong initial bond between the tile and rubber if applied correctly.

This can be tested by holding the lagging in a fixed position and applying a load to the tile. A good bond has been achieved when the load applied to the tile is eventually able to tear it out of the backing with a layer of rubber still attached to the tile. If the rubber remains in contact across the entire tile surface, this is called a 100 per cent rubber tear bond.

Adhesion of ceramic tiles relies on a chemical reaction between the tile, the adhesive and the rubber backing. Ceramic tile loss due to de-bonding occurs when these chemical reactions break down and stop working effectively.

Break downs can be caused by poor lagging manufacturing practices, or lagging subjected to outdoor exposure, ultraviolet light, sub-zero temperatures, prolonged service above 50°C, and contact with acids, bases or oils.

David Molesworth, Managing Director of pulley lagging specialist Elastotec, says it is critical that extensive adhesion testing is carried out under the full range of conditions that the ceramic lagging will be subjected to in service to ensure that the tile/rubber bonds remain effective.

“Unfortunately, very few ceramic lagging manufacturers do this testing,” he says.

To identify if tiles have de-bonded, Molesworth advises to inspect the recesses in the rubber backing where tiles are missing. If they are smooth and have a detailed imprint of the back of the tile, often seen as ribs or marks, it is likely the adhesion has failed.

“Ceramic tile loss due to de-bonding is always a production fault and is the lagging manufacturer’s responsibility,” he says.

Physical damage

This form of tile loss occurs when the load applied to the tiles in service exceeds the strength of the rubber backing layer, resulting in the tiles being torn out of the rubber backing. The tile is often removed with a layer of rubber bonded to the tile on all surfaces that were in contact with the rubber.

Molesworth says this is often seen on non-drive pulleys and rarely on drive pulleys.

“In particular, we see physical damage causing tile loss on high-tension non-drive pulleys subject to high localised shear stresses, such as head pulleys with short transitions or bend pullets in contact with the dirty side of the belt and subject to carry-back,” he says.

“On the latter types of pulleys, when the system is new the lagging performance can be okay, but as the centre of the belt wears, the localised shear stress in the centre of the belt increases dramatically and eventually will result in the tiles being torn from the lagging.

“The presence of carry-back on the uneven belt surface can accelerate this.”

Elastotec has spent the past seven years studying the various causes of tile loss and failure and has developed a range of measures to eliminate tile de-bonding and tile loss due to physical damage.

The company’s Hot Vulcanised Ceramic Lagging can handle the harsh environments often found in the mining industry, such as a non-drive pulley with continuous exposure to carry-back. The featured image shows that although the tiles have been cracked in this environment, none have de-bonded nor are missing. The pulley remained in service until a bearing failure necessitated a pulley change out.

Eight years of outdoor ageing adhesion data, testing in temperatures from -40°C to 60°C, and data from more than 800 pulleys installed internationally, has allowed Elastotec to guarantee no tile loss due to de-bonding from its ceramic lagging.

Molesworth says consultation with conveyor maintenance personnel and evaluation of the operating requirements for each conveyor is required before the best lagging can be selected for each application.

“Ceramic lagging is not the cure-all solution for all forms of lagging failure,” he says.

“Failures that occur due to high localised shear forces, and/or the presence of carry back causing tiles loss due to physical damage, require a different type of lagging.”