Conveying problems explained and answered
Despite fitting thicker, heavier belts, we continue to have to replace belts too frequently because of rip and impact damage. What is the best solution? Rob van Oijen investigates.
The short answer to the question concerned is that there are no simple solutions, but there are several actions that will dramatically reduce damage, improve output and minimise costs. These fall into two categories: conveyor design and maintenance, and the conveyor belt itself.
Conveyor design and maintenance
A significant proportion of belt damage is caused by incorrect installation of auxiliary equipment, damaged, protruding steelwork and components vibrating loose and ultimately becoming detached. The best way to avoid such problems is regular inspection (at least daily) backed up by a preventive maintenance regime. This should include rapid replacement of worn or damaged components because they can cause so much collateral damage.
The loading point of any conveyor should always be a target for scrutiny because it is so often the source of damage problems as well as general wear and tear. Several key considerations include: ensuring central loading with material feeding at the same speed and direction as belt travel, repositioning or redesigning chutes to minimise the velocity of free-falling material, integrating screen bars or a ‘V’ slot in the chute to handle lumpy materials mixed with fines, utilising impact idlers or cradles to absorb impact energy, maintaining clearance between the skirting and belt to prevent objects from becoming trapped, and ensuring smooth flow at the delivery point to prevent blockages.
It is not what you expect...
The biggest single cause of catastrophic rip damage is trapped material such as large, sharp rocks or other foreign objects. If these become lodged in a chute or some other part of the conveyor mechanism, they can puncture the belt cover, penetrate the belt carcass and cut the belt lengthwise as it is pulled against the object. Even the thickest, heaviest of belts can quite easily be ripped apart over its entire length in a matter of minutes.
Total avoidance is not possible but reducing the chances of it happening and minimising the extent of the damage if it does occur is very achievable. The first step is identifying where objects are most likely to become trapped and taking appropriate action to minimise the risk by installing skirts or screening for example.
At the same time, areas where spilled materials build up need to be regularly inspected and quickly dealt with. Apart from increasing the possibility of objects becoming trapped, waste build-up is a common cause of failure of components such as idlers and drums and also a major contributor to premature belt wear.
The importance of daily inspections cannot be over-emphasised. The old management adage, ‘It is not what you expect, it is what you inspect’ certainly applies to conveyors. Regular, preventive maintenance, good quality components such as idlers and rollers and a clean working environment all helps to prevent damage and extend conveyor belt life. Other factors include making sure that any scrapers are correctly adjusted, and drum linings (where applicable) are in good condition. Belt tracking is also important because a mis-tracked belt can catch on the conveyor framework. Again, a clean environment is important because mis-tracking is often caused by material build-up on the bottom side of the conveyor belt or drums and pulleys.
Early warning systems
On some applications, the fitting of an early warning facility in the form of a rip detection system can certainly help minimise the extent of the damage by allowing the belt to be stopped as soon as rip damage starts to occur. Rip detectors work by using detection loops that are embedded at predetermined intervals in the belt during the manufacturing process. A transmitter and receiver are positioned on the conveyor structure opposite each other on either side of the belt (Diagram 1). This is usually behind the loading position to provide protection in the area where there is the greatest exposure to risk. Multiple detection locations are also possible.
The transmitter sends electro-magnetic signals to the receiver at regular intervals. When a signal is not received by the receiver, caused by a damaged detection loop for example, then an alarm is triggered or, depending on the type of system, the conveyor is automatically stopped. Even if this facility is part of the system and the conveyor drive automatically stops the belt for optimum safety they should still be physically monitored. A second method of condition monitoring is to continuously scan the integrity of the steel cords for breaks or damage. This also permits monitoring the behaviour of splices, checking for anomalies, and identifying damage or even impending failures. This method of scanning is typically combined with the rip detection.
Having the detection loops fitted during the manufacturing process is, of course, an additional expense and the detection systems themselves can also be quite costly. These costs need to be weighed against the cost of a major repairs, replacement belt(s) and the loss of output while the conveyor is out of service, which can be several weeks if you do not have a spare belt on site.
Not worth the sacrifice
Despite the undisputed importance of conveyor design, inspection, maintenance and having a clean working environment, when it comes to rip, tear and impact damage, the most significant factor is the conveyor belt itself. Even if the conveyor set up is perfect and continuously monitored and inspected, accidents will still happen. This is why the conveyor belt itself also needs to be a central focus of attention. There are three methods of approach – sacrificial belts, thicker, heavier belts, or specialist belts engineered for purpose.
It is estimated that in mining and quarrying especially, up to 80% of belts are replaced prematurely as a result of accidental catastrophic damage. Such damage is regarded by some as being inevitable, in which case the use of ‘sacrificial belts’ is the chosen approach. The most popular source of ‘cheap’ belts is Southeast Asia. The line of thinking seems to be that as the belt is going to be destroyed anyway, why fit more expensive, better-quality ones? The arguments against this view are, in my opinion, overwhelming. Firstly, there is the cost in time and money spent regularly buying replacement belts. Add to this the very significant cost of loss of output due to stoppages to make repeated repairs, the cost of the repairs themselves and, ultimately, the conveyor downtime and cost of fitting the replacement belts.
Another increasingly important argument against sacrificial belts is its environmental impact. Manufacturing industrial conveyor belts uses a lot of energy and materials, much of which are oil derivatives, so it produces a big carbon footprint. Add to this the considerable impact on the environment caused by discarded industrial belting. Rubber constitutes at least 70% of the material mass of a conveyor belt, most of which is synthetic. The inner plies are mostly polyester and nylon and are therefore not biodegradable. Recycling conveyor belts remains a complicated and expensive process so nearly 90% of conveyor belts end up on scrapheaps and in landfill.
Thicker is not the solution
Although fitting belts with thicker covers and heavier carcasses may seem entirely logical, it is rarely the best solution. Firstly, if the cover rubber is not resilient enough, then adding more of the same will make little or no difference. Neither will increasing the number of plies or the tensile strength because thicker, heavier belts are much less flexible.
Reduced longitudinal flexibility normally requires an increase in the diameter of the drive pulley. Increasing tensile strength by just one step usually means an increase in diameter of 25% or more. Failure to increase the pulley (drum) diameter can lead to dynamic stress failure, especially in splice joint areas. Reduced horizontal flexibility causes a decrease in troughability. There is also a price to pay in terms of increased power consumption, which can be quite significant.
Engineered for the task
The only practical solution is to fit a conveyor belt that has been specifically engineered to withstand the kind of punishment that would destroy a conventional belt. The genuinely specialist belts that are available use uniquely designed fabric plies that allow the transversal (weft) nylon strands to stretch. As the trapped object is being pulled through the belt, the strands gather into a bundle that eventually becomes strong enough to stop the belt in its tracks rather than propagate over a much long distance. The design of the fabric also allows the energy created by heavy impact to be dissipated over a much wider area rather than rupture the inner carcass.
Specialist belts such as Fenner Dunlop’s UsFlex and Ultra X range, are proven to provide more than three times the rip and impact resistance and last up to four or five times longer than conventional multi-ply conveyor belts. There are a number of imitators, but tests show that they typically have some 60% less rip and tear resistance. Although the initial purchase price may appear high, they are, without doubt, the most cost-effective solution. Over the lifetime of the belt, the cost will be substantially less than even the cheapest, thickest sacrificial belt with the added benefit of significantly fewer repairs, replacements and stoppages. As they say, price is what you pay but cost is what you actually spend.
Rob van Oijen is manager for Application Engineering at Dunlop Conveyor Belting.
