Centred: the art of conveyor belt tracking (Part 1)

The moment a conveyor belt begins to wander, the safety and productivity of the system quickly degrades and the cost of operation rises. Even a slight belt misalignment can lead to a variety of issues, from small annoyances to full-blown catastrophes involving a conveyor system.

Fugitive material can bury the load zone, resulting in idler failures, belt mistracking and even fires.

For example, spillage from non-centred cargo can get into idlers and pulleys, causing them to seize, leading to friction damage on the belt and, potentially, a fire. A misaligned belt can also come into contact with the stringer, causing fraying, shredding or splice damage. If this condition is not noticed right away, great lengths of valuable belting can be destroyed and the structural steel itself can be damaged. In fact, a high-speed belt edge rubbing on the support structure can cut through steel mounts with surprising speed, leaving a razor-sharp edge that poses a safety risk. Further, a compromised bracket or support can cause a catastrophic idler failure, which could damage other components in the system. All of these consequences of mistracking result in added expenses, higher maintenance and reduced efficiency.

According to a Senior Product Specialist at Martin Engineering, Dave Mueller, beyond the many causes for mistracking, the belt training system that came with the conveyor may in some cases actually worsen the problem. “We’re seeing increased belt speeds and greater cargo loads across most industries,” explained Mueller. “But some systems are equipped with belt tracking devices that aren’t able to handle those higher thresholds. We often see OEM trainers tied off with rope or chain in an attempt to drive a belt back into line.”

In the vast majority of cases, mistracking is a problem that can be corrected. Belt behaviour is based on a set of principles, which serve as the guidelines for ‘belt training’. Training a belt is the process of adjusting the conveyor structure, rolling components and load conditions to correctly centre the belt. Wandering is prevented by first understanding the basic patterns of belt behaviour and then following established procedures to carefully align the structure and components to correct any fluctuations in the belt’s path.

Mistracking indicators

Belt drift can begin in any part of the conveyor system; identifying mistracking is the first step toward correction. All of the traits listed below could be indicators of mistracking:

• Edge fraying: Probably an indication that the belt is rubbing on the conveyor frame at some point, degrading the edge, reducing the usable width and increasing the chance of a fire. Increasingly, the US Mine Safety and Health Administration (MSHA) is writing citations if belt edges are frayed.
• Excessive spillage: Could mean that one side of the belt has drifted higher on the trough angle, allowing cargo to discharge along the belt path.
• Idler fouling: An off-centre load and uneven belt plane can foul idlers. Bearing abrasion can then cause the idler to seize and the ensuing friction against the running belt can erode its coating and increase the risk of fire.
• Off-centre at head or tail pulley: This type of drift can lead to a fast-moving belt coming in contact with the conveyor stringer structure. There is also a chance of splice failure, which puts the entire system in jeopardy of the fast moving belt detaching and causing a serious injury to any workers that are in close proximity.
• Lack of tail pulley protection: On many systems, the belt collects lumps of spilled material on the non-carrying side. If these objects are not removed, they can become trapped between the tail pulley and the belt, causing mistracking and often doing significant damage to both.
• Uneven discharge: As the belt drifts to either side of the head pulley, the belt cleaners do not properly clean the entire surface, causing excessive carryback. Material collects on the pulleys and structure, fouling the return side of the belt, resulting in slippage, lost product and other negative effects.
• Uneven loading: If the belt path leading from the tail pulley into the loading zone is uneven, the cargo can be loaded off-centre and cause excessive spillage. This may also be caused by an inadequate transfer point design.

When the belt is not centre-loaded, the cargo weight pushes the belt toward the more lightly-loaded side.

Identification begins at the head pulley

The first step in solving tracking problems should be a complete walk of the belt, inspecting the entire length for issues such as frozen idlers. When an idler bearing seizes, the constant belt movement can wear through the shell with surprising speed, quickly developing a flat spot that produces friction and diverts the belt, sometimes leaving a razor-sharp edge that poses a potential threat to workers and to the belt itself.

Starting at the head pulley, the belt should also be inspected for cupping, bow/camber (a long curvature) or crooked splicing. When observing the empty belt running over the head pulley, a cambered belt will drift to one side in the middle of the camber and then slowly return to the centre as the belt travels through the head pulley. If a splice is crooked, the belt’s path will jump quickly to one side as the splice travels across the pulley.

If it is discovered that these factors are the cause of mistracking, adjusting the conveyor’s rolling components will not correct the issue. The only options are either replacing the belt or, in the case of crooked splice(s), re-splicing the belt, assuming there is enough extra belting in the take-up system to allow removal of the faulty splice section.

During the observation procedure, if the belt moves to one side and stays there, the problem may be one of three things: the head pulley lagging is not consistent; the last few carrying idlers before the pulley are out of alignment; or the head pulley itself is not properly adjusted.

Mistracking just after the head pulley on the conveyor’s return has two main causes. The first might be that the lagging is missing on one end of the pulley, so the pulley’s diameter is off-centre, placing uneven force on the belt and causing it to wander. If this is not the case, then the belt cleaning system may have been mounted slightly askew, putting greater force on one side of the belt and pulley. This uneven friction can also lead to mistracking.

Causes

Once operators and maintenance professionals properly identify the type of misalignment, they can then seek out the cause. “We’ve observed that there are three groups of common causes for mistracking,” Mueller said. “One is a fault with the belt or splice, another group is the conveyor’s structure, components or environment and the last is due to improper material loading.”

This is an excerpt from an article that was first published in World Coal July 2016. To register and receive your free trial of the magazine, click here.

Read the article online at: https://www.worldcoal.com/handling/27032017/centred-the-art-of-conveyor-belt-tracking-part-1/