Continuing from an earlier piece (available here) that was published as part of World Coal's Handling Week, Paul Harrison, Martin Engineering, USA, expands on remedies for conveyor belt mistracking by focusing on different belt training methods.
Part one of this
Part two continues by describing remedies to some of the issues described in part one. The text reviews different belt training methods, the effectiveness of each device and ways to ensure belts stay aligned by controlling other factors in the conveyor system.
This roller tracker uses a ribbed
A common procedure to correct a wandering belt is to slightly adjust the return and carrying idlers against the direction of the mistracking. Unfortunately, even slight over-compensation can cause friction or pinching and may reduce the life of both the idlers and the belt. Moreover, the approach does not work on reversing belts. An even more serious consequence is that, over time, a number of the idlers may be misaligned, ‘fighting’ each other to correct the alignment.
When idler training is not successful as a long-term solution, operators may be faced with a situation where the training procedure is repeated on a frequent (sometimes daily) basis. At that point, managers should consider installing some form of engineered belt training solution to mitigate the problem.
Engineered belt trackers are devices that sense the position of a belt and, through a mechanism or geometry change, actively adjust its path. Some of the most common types are discussed in the following:
Belt misalignment switches are sensors that are installed at intervals along the length of the conveyor on both sides of the belt near the outer limit of a safe belt path. The wandering belt pushes a lever arm and activates a switch, which either sets off an alarm or, in the case of severe mistracking, interrupts the conveyor’s power circuit, stopping the system. Costly downtime and lost production make these devices less of a solution to the problem of misalignment and more of an indicator of a severe problem. Vertical edge guides are meant to be positioned perpendicularly to the belt’s path to keep the edge away from the conveyor structure and should not be used to compensate for persistent misalignment problems. Performing more of a damage control function than true alignment, they allow the belt to strike a rolling surface instead of the structure. Most practical on short, low-tension systems and not particularly effective on thin belts, operators have experienced severe damage when the belt rides up over the guide into the structure or rolls over on itself.
The in-line sensing roll trainer has a carrying roll on a central pivot bearing with vertical guide rolls mounted on both sides.
Vee idlers and rollers, set on both the cargo side and return side of the belt, use a trough configuration and edge brackets that rely on a
Dynamic belt-tracking systems use the force of the mistracking belt on an arm that moves an idler, creating a steering action that directs the belt back into the
In-line sensing roll trainers have vertical guide rolls that are mounted on both sides of the belt, in line with the roller, with the
Torsion-spring trainers improve upon the leading sensing-roll trainer design by removing one sensing roll and incorporating a spring into the pivot, which keeps the one remaining sensing roll in constant contact with the belt. As the belt mistracks in either direction, the idler will compensate by pivoting and steering the belt. Since the sensing roll is in constant contact, there is no delay in reaction, no pinching due to continuous ‘re-tuning’ of the idler and less material accumulation at the pivot point. The drawbacks include the fact that it cannot use a troughed idler set and the sensing roller requires relatively frequent replacement due to constant use. Multi-pivot belt trainers use longer arms than other designs, positioning the guide rolls further from the pivot roller, as well as closer to the belt edge. The closer proximity allows guide rolls to sense very slight misalignments and make immediate corrections. Rather than waiting for a powerful mistracking force, the longer arms require considerably less force to move the pivot roller. The result is
Rubber covered rollers create higher steering friction with the belt than 'steel
Reversing belt trainers
“Once any belt training mechanism or procedure has been initiated, you can’t just throw the system in reverse,” Mueller warned. “That could cause immediate, expensive and hazardous consequences.”
If operators have manually trained the idlers in one direction opposite to the misalignment, suddenly they’re faced with an idler that, when the belt reverses, exacerbates the mistracking and causes it to drastically drift to one side almost immediately. This means that every time the belt is to be reversed, the system must be shut down and all idlers must be straightened or set in the opposite position. Most engineered training solutions react the same way and will pivot in the wrong direction. “Only mechanisms that are designed to detect both forward and reverse mistracking can
- HARRISON, P., ‘Centred: the art of conveyor belt tracking (Part one)’, World Coal Vol. 26 No. 7 (July 2016), pp. 29 – 33.
This is an excerpt from an article that was first published in World Coal August 2016. To register and receive your free trial of the magazine, click here.
Read the article online at: https://www.worldcoal.com/handling/29032017/centred-the-art-of-conveyor-belt-tracking-part-2/