On-Line Cover Thickness Measurement for Steel Cord Belts

Field technicians have been measuring remaining cover thickness in conveyor belting, pretty much since it was first being fabricated.

These are static measurements done across the belt, with the conveyor stopped and isolated. In fabric reinforced belting, ultrasonic probes are used and in steel cord belting, both ultrasonic and magnetic based sensors apply.

These measurements provide data which can be used to plot cover wear rates against time/tonnage to predict end-of-life, if this is the belt’s mode of failure. This procedure is critical in high-wear applications. Also the shape of a wear profile is determined, and this is important in assessing the effectiveness of transfer chute design, and other structural elements.

When I first got involved in conveyor belt monitoring in 1979, I was in Dampier, Western Australia talking to the operator of a steel cord conveyor which transported primary crushed iron ore. I asked what condition monitoring he did and he told me, just remaining cover thickness measurements. But he also told me he had no confidence in the results. When asked why, he said....

“...because we are in the rubber growing business. We measure the covers every three months and sometimes we have more rubber than last time, sometimes even more than last year...”

He showed me the cover wear trending graph, which looked more like the entire Himalayas than the expected downward gentle slope. He suspected the equipment and/or the technicians.

It was some years before I realised what was actually going on here.

In performing remaining cover thickness measurements, some knowledge of the manufacturing process is essential, and I refer here specifically to steel cord belting. Most measuring technicians have never been in a belt fabrication plant, or seen a steel cord belt line operating.

Even though steel cord belt is delivered in a roll (or cassette) of continuous belting, the actual manufacturing process consists of a number of fixed-length events. The steel cords themselves can be considered continuous, as the rope is available in very long lengths, however, other components/elements cannot.

The belt is produced in a number of ‘cooks’ or ‘cures’. This is done in a vulcanising press, which has a finite width and obviously length, typically in the range 9.0 – 30m. For the cover thickness in any one cure to be absolutely consistent over the entire area of belting, the press platen faces must be precisely parallel – 'hot' end and 'cold' end, and the rubber volumes uniform, top and bottom. In my experience, this is not always so. I have seen a carry cover thickness variation of as much as 2.0mm, point-to-point over the surface of one cure length of a new belt. This variation creates a unique press “signature” or palm print, and is present in every press length of belting fabricated on this production line.

Then there is the rubber cover stock. This comes in rolls of un-cured (raw) rubber, which has been rolled or calendared in lengths of typically 50m. The thickness of the cover stock is subject to its own manufacturing tolerances, so there is no absolute guarantee that each roll of stock has exactly the same thickness. This means that at the end of every cover stock roll length, maybe 50m or so, there may be a change in cover thickness in the finished product.

Also, since the finished belt roll might be in the 100 – 500m range, there obviously will be a number of cover stock ‘factory cover joins’. This is the location, in the fabrication process, where one roll ends and is joined to the beginning of a new roll. The join is a manual process and requires a high degree of skill to ensure that, after cure, the belt maintains uniform cover thickness across the zone of the join.

To sum up, even though a roll of belting may have been ordered with say 12 x 7mm covers, there is every chance that there will be variations in these dimensions, along the roll. This means that any lateral cover thickness measurements MUST be made at the EXACT same location along the belting, over time, if cover wear trending is to be meaningful.

Beltscan specified this requirement in the early eighties, and this is the technique which generally applies worldwide, today.

Whilst the static measurement technique is OK for determining wear profiles and rates of wear in the covers, it has one serious drawback. In predicting end-of-life in a section of belting, it is necessary to establish where in the length of the section is the location of MINIMUM cover thickness. This is one location where cover thickness measurements MUST be taken. In the normal course of events in the life of any section of belting, this location is typically never determined. This gives rise to ‘surprises’, where steel cords are seen in a cover of a belt section, which, based on measurement data, still has cover remaining.

To overcome this drawback, Beltscan has included in its Belt Guard range of instruments, an on-line LONGITUDINAL cover thickness device. This device is typically fitted over the tail pulley, when the carry cover is being measured, or somewhere in the return run for the pulley cover.

It is important to understand that this device gives a true remaining cover thickness measurement, NOT just cover PROFILE. There are devices offered which measure belt profile only, this data is meaningless for end-of-life trending.

There are two versions of the BeltGuard device available, the Belt Guard-RCT-1 and the Belt Guard-RCT-2.

The Belt Guard-RCT-1 is a contact device, which is very effective and is relatively simple and inexpensive. It measures two values,

1.Remaining cover thickness, and
2.Lateral position of the belt on the structure.

The data is stored in the device in simple .csv format and can be downloaded for viewing or integration into trending regimes.

The Belt Guard-RCT-2 (shown in the image) is a non-contact device, where contact with the belt surface is not acceptable. The device is much more complex and more expensive than the RCT-1. Normally the cover thickness sensor is ‘parked’ safely away from the belt surface, and then deployed remotely when measurements are needed to be made.

In basic form, the devices are set at a fixed location across the belting, this location having been identified as the highest lateral wear point. The ‘strip’ of cover being measured along the belt is +/-100mm. wide. The transverse location of the belting on the structure needs to be known so that belt wander does not give rise to false data. Data is able to be captured only when the sensor is above the desired strip of belting.

In advanced form, the devices can be made to travel laterally, measuring strips as they go, so that a complete map of the cover, laterally and longitudinally can be obtained and 3D graphics of the COVER THICKNESS, not profile, created.

Author: Barry Brown