Centred: the art of conveyor belt tracking (Part 2)

Part one of this two part series covered the hazards of mistracking belts, indicators of misalignment, how to detect where belt wander is occurring, and how to identify the cause.¹ It was observed that conveyor belt mistracking is a major workplace hazard as one of the main causes of dust and spillage. A Senior Product Specialist at Martin Engineering, Dave Mueller, an expert in belt tracking, talked about how higher production demands can render older models of belt trackers obsolete. The author also discussed how observing the head pulley while it is running tells operators a lot about the positioning and health of the belt.

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 lagging made of durable polyurethane to increase performance.

Belt training

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 centring force to correct the belt path, which can add stress on the belt and lead to damage. These systems are more expensive and require more maintenance than a conventional return idler. Crowned pulleys, with larger diameters at the centre than at the edges, operate from the basic tracking principle that if the raised portion of the pulley (the crown) touches the belt first, it centres the belt, because the outer sections of the belt on both sides produce a force driving it toward the centre. These forces cancel each other out on a centred belt and, if it wanders to one side, the dynamic forces will be greater on that side, acting to push the belt back toward the centre. Crowned pulleys are most effective on conveyors with short, low-tension belts, but do not work with some blade cleaners because the pitched belt plane does not allow the flat surface needed for effective cleaning.

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 centre. The design can be vulnerable to the accumulations of fugitive material, which can block the range of motion or seize the pivot bearing. This can lock the belt-training idler into a position where it functions as a ‘misalignment’ idler and pushes the belt out of the proper track, actually worsening the problem.

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 centreline running through the idler’s pivot point. Movement of the belt against either guide roll causes the roll to move in the direction of the misalignment, pivoting the entire idler. Since belts always move toward the side they contact first, the pivoted idler then steers the out-of-track belt back to the proper path. On some systems, the corrective action of the central pivot idler can be abrupt, causing the belt to ‘kick over’, slamming it consistently from one side to the other and resulting in edge damage and overuse of the pivot bearing. Leading sensing-roll trainers are supplied as original equipment on many new conveyors. Employing either a pivoting carrying roll or troughing set, short arms on both sides of the frame are positioned in advance of the pivoting roller and end in guide rolls located 25 to 75 mm (1 to 3 in.) from the belt edge. Designed for use on the carrying side or the return side, movement of the belt against the guide roll directs the steering idler and corrects the belt path back toward the centre. These designs can be prone to material clogging in pivot bearings, causing them to freeze in an extreme position and resulting in constant mistraining. To remedy this, operators have commonly ‘tied off’ these devices with rope or wire, reducing or eliminating the functionality of the device.

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 better correction with no pinch points and less wear on conveyor and tracking equipment, for a longer and more efficient service life.

Rubber covered rollers create higher steering friction with the belt than 'steel can' idlers, providing a better grip.

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 centre these belts,” Mueller said. “Examples are torsion spring trainers with an opposite-end sensing roll and reversing multi-pivot trainers with four long-arm guide rolls. The forward guide rolls recognise the opposite direction of the belt and automatically switch to the back guide rolls, adjusting the pivot roller in the proper direction.”

References

1. 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/