Figure 1 – Contact with the support structure can cause serious belt damage.
By Andrew Timmerman
When a conveyor starts to deviate, both the safety and productivity of the system quickly decline, leading to increased operational costs. Even a minor belt misalignment can result in various problems, ranging from subtle annoyances to significant disasters involving the conveyor system.
A misaligned belt can also come into contact with the stringer, leading to fraying, shredding, or splice damage. If this issue is not noticed immediately, large lengths of valuable belting can be ruined, and the structural steel itself may sustain damage. In fact, a high-speed belt edge rubbing against the support structure can cut through steel mounts with remarkable speed, leaving a razor-sharp edge that creates a safety hazard.
Additionally, a compromised bracket or support can lead to a catastrophic idler failure, which could damage other system components. All these consequences of mistracking result in increased expenses, higher maintenance and reduced efficiency.
Senior product specialist for Martin Engineering, Dave Mueller, points out that 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,” Mueller said, “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 most cases, mistracking is a problem that can be corrected. Belt behavior is based on a set of principles that serve as guidelines for “belt training.” Training a belt involves adjusting the conveyor structure, rolling components and load conditions to properly center the belt. Wandering is prevented by first understanding the basic patterns of belt behavior and then following established procedures to carefully align the structure and components to correct any fluctuations in the belt’s path.
Figure 2 – When the belt is not center-loaded, the cargo weight pushes the belt toward the more lightly-loaded side.
Mistracking Indicators
Belt drift can start in any part of the conveyor system, and recognizing mistracking is the first step toward correction. All of the traits listed below could indicate mistracking.
Edge fraying – This is likely an indication that the belt is rubbing against the conveyor frame at some point, which degrades the edge, reduces the usable width and increases the risk of a fire.
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-center 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-center at the head or tail pulley – This type of drift can lead to a fast-moving belt coming into contact with the conveyor stringer structure. There is also a risk of splice failure, which puts the entire system
at risk of the fast-moving belt detaching and causing serious injury to workers nearby.
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 adequately clean the entire surface, leading to excessive carryback. Material accumulates on the pulleys and structure, fouling the return side of the belt, which results 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-center and cause excessive spillage. This may also be caused by inadequate transfer point design.
Figure 3 – Contact with the structure can break mechanical splices with expensive consequences.
Identification Begins at The Head Pulley
Starting at the head pulley, inspect the belt 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 center as the belt travels through the head pulley. If a splice is crooked, the belt’s path will quickly jump to one side as the splice crosses the pulley.
If it is discovered that these factors cause mistracking, adjusting the conveyor’s rolling components will not resolve the issue. The only options are to either replace the belt or, in the case of crooked splice(s), re-splice the belt, assuming there is enough extra belting in the take-up system to allow for the removal of the faulty splice section.
During the observation procedure, if the belt shifts to one side and remains there, the issue may stem from one of three factors: the head pulley lagging is inconsistent, the last few carrying idlers before the pulley are misaligned, or the head pulley itself is not properly adjusted.
Causes
Mistracking just after the head pulley on the conveyor’s return has two primary causes. The first could be that the lagging is missing on one end of the pulley, making the pulley’s diameter off-center, which places uneven pressure on the belt and causes it to wander. If this is not the issue, it’s possible that the belt cleaning system has been mounted slightly askew, applying greater pressure on one side of the belt and pulley. This uneven friction can also result in mistracking.
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 pointed out. “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.”
Belt and Splice – If the belt is poorly manufactured or stored incorrectly, it can bow or camber. Poor installation of a vulcanized or mechanical splice may result in a splice that causes belt tracking issues. Exposure to environmental elements or chemicals can degrade the belt’s carcass (plies or cords) and cover. Faults and damage resulting from mismatching the belt with the application and/or operating environment can lead to frequent replacements.
Conveyor Structure – Inaccurate alignment of the conveyor stringer structure can have subtle yet lasting effects on the belt’s performance. Structural misalignment can occur due to age, impact from machinery, seismic activity, or ground settling.
Outdoor conveyors – High winds require “wind loops” to keep the belt in line. Exposure to extreme temperatures on one side of the conveyor can cause components to expand, leading to changes in friction.
Improper Loading – The center of gravity of the load will seek the lowest point of the troughing idlers. Therefore, if the belt is not centered loaded, the weight of the cargo will push the belt toward the conveyor’s less-loaded side. This issue can be resolved by installing a central loading transfer chute or using deflectors, grids, or adjustable chute bottoms to ensure proper load placement on the belt.
Figure 4 – This roller tracker uses a unique 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, this approach does not work on reversing belts. An even more serious consequence is that over time, several of the idlers may become misaligned, “fighting” each other to correct the alignment.
When idler training fails as a long-term solution, operators may encounter a situation where the training procedure must be repeated frequently (sometimes daily). At that point, managers should consider implementing an engineered training solution to address the issue.
Engineered training solutions are devices that sense the position of a belt and, through a mechanism or geometric change, actively adjust its path. Some of the most common types include:
Belt Misalignment Switches – The wandering belt pushes a lever arm and activates a switch, which either sets off an alarm or stops the system. Costly downtime can result from these systems.
Vertical Edge Guides – Positioned perpendicular to the belt’s path to keep the edge away from the conveyor structure and should not be used to compensate for ongoing misalignment problems. Most effective on short, low-tension systems, they are not particularly suitable for thin belts, as operators have experienced the belt rolling over on itself.
Vee Idlers and Rollers – Set on both the cargo side and return side of the belt, these use a trough configuration and edge brackets that rely on a centering 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 – The raised section of the pulley (the crown) first contacts the belt, while the outer sections of the belt on both sides creates a force that drives it toward the center.
Dynamic Belt-Tracking Systems – They utilize the force of the misaligned belt on an arm that moves an idler, creating a steering action that guides the belt back to the center.
In-line sensing roll trainers feature vertical guide rolls mounted on both sides of the belt, aligned with the roller, with the centerline running through the idler’s pivot point. The movement of the belt against either guide roll causes the roll to shift in the direction of misalignment, pivoting the entire idler.
Leading Sensing-Roll Trainers – Using either a pivoting carrying roll or a troughing set, short arms on both sides of the frame are placed in front of the pivoting roller and end in guide rolls situated 25 to 75 millimeters
(1 to 3 in.) from the belt edge.
Torsion-Spring Trainers – These enhance the leading sensing-roll trainer design by eliminating one sensing roll and adding a spring to the pivot, which maintains the one remaining sensing roll in constant contact with the belt edge.
Multi-Pivot Belt Trainers feature longer arms than other designs, positioning the guide rolls farther from the pivot roller and closer to the belt edge. This proximity allows the guide rolls to detect very slight misalignments and make immediate corrections. Instead of relying on a powerful mistracking force, the longer arms require significantly less force to move the pivot roller. As a result, there is improved correction with no pinch points and reduced wear on conveyor and tracking equipment, leading to a longer and more efficient service life.
Figure 5 – The in-line sensing roll trainer has a carrying roll on a central pivot bearing with vertical guide rolls mounted on both sides.
Total Conveyor Analysis
“Installing trackers is the economical solution, but operators should conduct a thorough analysis and consider addressing other causal issues,” Mueller said. “By focusing solely on belt alignment, plant personnel may miss other opportunities to increase production and relieve some of the burden on their system.”
Off-centered loading is a significant cause of mistracking. To address these issues, some conveyor accessory manufacturers provide well-engineered modern equipment, such as load-centering transfer chutes, high-impact cradles, adjustable slider cradles, a variety of cleaner blades designed for specific applications and redesigned chute box technology.
Keeping the belt centered and moving quickly is the key to high production, a low cost of operation and a safer workplace. “Misalignment causes downtime and costs money,” Mueller concluded, “but nothing causes more downtime and expense than a destructive belt fire or other catastrophe as a result of inattention to mistracking problems.”
Timmerman is the global engineering manager for Martin Engineering.
Source: www.coalage.com
