STEEL CORD BELT RIPS – A TALE IN TWO PARTS
THE FIRST PART
Prior to 1980, the most common catastrophic event to befall the owner/operator of a conveyor employing steel cord belting, was a lateral fracture, or a splice pulling apart. This was an event of varying impact.
At the bottom end of the scale, for a short plant conveyor, there was some cleaning up, re-stringing and splicing, loss of conveyor availability (and associated costs) and a degree of soul searching.
In the mid range, in the event the conveyor was a long overland structure traversing undulating terrain, the impact was more severe. The bitter ends of the belting could end up some kilometres apart, presenting the belt crew with a significant challenge in re-stringing. The splice(s) had to be constructed well away from any splicing station or any controlled environment. Halfway up a mountain at minus 20C??
Also there was the usual clean up and consequential losses.
At the top end of the scale was the underground-to-surface conveyor. The fracture almost invariably occurred at a high tension location, typically around the portal. The high tension implies that the conveyor was heavily loaded, so many hundreds of tonnes of product cascaded back down the slope, carrying away the structure and blocking the tunnel/drift. This of course denied man/materials and dolly car access, making restoration a nightmare.
There have even been occasions where an unloaded incline conveyor has parted while being used for man-riding purposes. The results are too horrible to contemplate.
THE SECOND PART
In late 1979, the introduction of steel cord belt carcass/splice condition monitoring began. This saw the incidence of lateral fractures and splice failures reduce drastically, to the point that they are almost unheard of today.
Now the biggest catastrophic event facing a belt conveyor owner/operator is a longitudinal rip.
This can be an event of enormous magnitude. There have been many instances of belts ripping end-to-end, with staggering negative consequences.
As a means of negating these events, a number of regimes is available to prevent longitudinal rips from occurring. Most of these involve magnetic devices removing tramp metal, a common cause of a rip, from the product stream. However, petrified tree roots and hostile hard rocks etc., which are immune to magnetic removal, can be very effective ripping agents.
Despite all good efforts, longitudinal rips do occur, somewhere in the world, probably every week or month.
The next line of defence is to detect the rip and stop the conveyor running, to minimise belt damage length.
BELT RIP DETECTION - VARIOUS DEVICES
There have been many methods introduced to attempt to minimise the length of a belt rip. These range from simple "home-made" mechanical trip switches to embedded antennae/elements in the belting. Some belting manufacturers offer lateral members in the belt, ex manufacture, designed to help dislodge any foreign objects or, at least, shut down the conveyor system.
Several of the most common methods are discussed below:
i) MECHANICAL SYSTEMS
There are many variations of mechanical systems in existence. Nearly all of these depend on the physical movement of a plate, wire or rod caused by either the conveyed material falling between the ripped belt strips, a ragged belt surface or directly by the object responsible for the rip. There are a number of problems with these types of systems:
It cannot be assumed that the ripped belt strips will actually separate enough to allow sufficient material to fall through, to activate the system within an acceptable time. Indeed it cannot even be assumed that there is any product present at the time of a rip. It also cannot be assumed that the object causing the rip will protrude through the belt in such a fashion as to activate the trip detection device.
None of the mechanical systems is able to detect rips that do not extend fully through the belt.
An improvement over these methods involves the continuous measurement of the belt width. Significant changes in this width can be used to initiate a belt stop. This method relies on there being a change in belt width, co-incident with a longitudinal rip. It is generally thought that in thick steel cord type belting such changes may not occur. However, a body of evidence exists to indicate that this secondary effect does often occur, either as an overlapping or parting of the edges, predominantly in the thinner fabric re-enforced type belting.
ii) EMBEDDED ANTENNA SYSTEMS
These systems involve embedding a number of transverse conductive antennae within the belt, at regular intervals along the belt length. An electronic signal of some form is induced into one end of the antenna, and received at the other end. The system is required to "see" an antenna within a specific time/distance interval. This interval is based on the distance between antennae and the belt speed.
A rip in a belt would theoretically damage the antenna and prevent lateral transmission of the electronic signal along the antenna. Since the system electronics would not then recognise the damaged antenna, within the given period, the conveyor system would be shut down. Inspection of the antenna site is then required.
Although more effective than any mechanical device, there have been some continuing problems with the embedded antenna systems. Most of these problems relate to antenna failure, commonly the result of impact damage to the antenna, or flexural failure of the antenna material. In order to return the system to full operation, the antenna must be replaced. This is expensive and also requires exposing the belt internals to a hostile environment.
Other disadvantages with the embedded antenna systems are:
A minimum amount of cover rubber is required to receive the antenna, typically 5.0mm. Further, to reduce possible impact damage, the antennae are commonly installed in the pulley cover of the belt. Thus, a partial-depth belt cut that does not extend through to the pulley cover, will not damage the antenna, and will not stop the conveyor. Contrary to popular belief, many of the most troublesome rips have not involved full penetration of the belt.
Because the antennae are installed at regular intervals eg. 30 - 100m, along the belt, the cost of owning and maintaining such a system rises in long conveyors.
Retro-fitting the system to installed belts or replacing damaged antennae are both expensive and potentially detrimental to the belt condition, as this operation has been known to damage the protective zinc coating on the cables. Also, any improper bonding of the incoming antenna rubber and the parent belt will eventually cause the implant to lift at the edges, promoting attack of the steel cords by the elements. This has commonly led to catastrophic failure of the belt carcass, through corrosion/rusting.
A variation of this approach uses lateral magnetically permeable implants, which, when magnetised and then damaged, create detectable fringing magnetic fields.
More recently, RFID tags have been vulcanised into the belt during manufacture. Reading of these tags in a running conveyor aids in identification of places of interest – splices, antennae etc. and an understanding of where the endless belt is longitudinally on its structure. These tags are subject to damage as for the antennae themselves and effective reading of the them on the fly has proved difficult and sometimes unreliable in high-speed conveyors.
iii) ULTRASONIC RIP DETECTION
The embedded antenna system is the most common rip detection system found today.
A new device that is based on the use of ultrasonics, has been developed. This system, known as the "Belt Guard BG10k", is similar to an antenna system in that a signal is transmitted across the belt and any interruption of this signal indicates a longitudinal belt issue.
An important advantage of the ultrasonic system lies in the fact that nothing is required to be embedded in the belt. The belting itself is the transmission medium. Transducers are mounted under the belt transversely and in contact with the pulley cover. A signal, or pulse of ultrasound can be transmitted from one transducer to another, as long as the belt is homogeneous, between the transducers. If there is significant interruption to the pulse of ultrasound, an alarm condition is generated, stopping the conveyor. Even if the edges of the rip are in intimate contact and/or wet, the attenuation of the ultrasound is substantial because of the unique nature of propagation being used.
Whether a signal reduction is `significant' can be controlled by a number of operator adjustments.