Identifying and Correcting Common Causes of Conveyor Belt Mistracking
Belt wander can be caused by a number of problems. Factors contributing to belt wander include misalignment of conveyor components, off-center loading of cargo, accumulation of fugitive material on rolling components, poor belt splices, structural damage, ground subsidence, and many others. These problems may occur in any combination, greatly complicating the process of correction.
Despite the complexity of these problems, they are solvable. Misaligned components can be straightened, chutes can be redesigned to load the cargo in the center of the belt, material accumulations can be prevented or removed, belt splices can be improved, and operators can be trained. The challenge comes in identifying which of the long list of possibilities is the specific cause of a given belt’s problems. Once the cause of mistracking is identified, it can be corrected.
Faults with the Splice
Improper belt splicing is a significant cause of mistracking. If the belt is not spliced squarely, the belt will wander back and forth on the conveyor structure. This can usually be seen at the tail pulley. The belt will wander the same amount each time the splice reaches the tail pulley, only to return to its original position after the passage of the splice. If the splice is bad enough, it can negate all alignment efforts. The solution is to resplice the belt squarely.
Cupped Belt
A second significant cause of belt mistracking is a cupped belt. A cupped belt will track poorly because of differences in friction as it lies in the troughed idlers. Belt cupping is almost always a result of unequal shrinkage between the top and bottom covers of fabric. Heat, chemicals, trough angles, and over-tensioning can also cause belt cupping. This problem can usually be avoided by keeping the proper aspect ratio between top and bottom cover thicknesses: usually 3:1 or less. In some cases, the belt will cup as the top cover rubber properties change as a result of aging or exposure to chemicals. A cupped belt is hard to track consistently because tracking depends upon the friction between the belt and the rolling components. If the belt is so badly damaged that the contact area is reduced, the ability of the components to keep the belt in line is also reduced.
Using A Correctly Spec'd Belt
Incompatible belt properties, incorrect dimensions, improper tension settings, and mismatched load capacity can cause deformation, misalignment, and uneven weight distribution. Environmental factors like moisture, heat, or chemicals can further degrade an unsuitable belt, leading to irregular surfaces. Additionally, the belt's interactions with conveyor components and the structural design of the conveyor require compatibility; a mismatched belt can cause uneven wear and further tracking issues. Overall, using a belt not tailored to the specific application results in frequent and persistent mistracking problems.
Incorrect Belt Storage
Improper storage techniques can damage the belt, resulting in poor performance when installed. The importance of following correct procedures increases with the length of storage time and the size of the roll. The costs for handling, shipping, and storing the conveyor belt are minor compared to the purchase price, so proper procedures should be followed to protect this investment.
Structural Problems
To be able to keep the belt running straight, the structure must be properly established and corrected if damaged. Most structural damage occurs when the conveyor structure is struck by mobile equipment. Structural damage can also occur as a result of corrosion or settling of the foundations.
Component Problems
It is equally important that the components be properly installed and maintained in relation to the belt for reliable belt travel. One major source of belt wander is gravity take-up systems that are out of alignment or that have too much side-to-side movement, or “slop.” The take-up pulley, like all other main pulleys, must remain in alignment with the belt throughout the take-up’s travel, or the belt will mistrack.
Rotating components can have a significant mistracking effect on the belt. Rotating components that have become frozen or inoperative due to material buildup or those with material accumulations that alter their circumference can be major contributors to erratic belt tracking. Consequently, transfer points should be engineered, constructed, and maintained to prevent material spillage. An effective multiple-cleaner belt cleaning system should be installed to prevent material carryback. If necessary, cleaners can be installed to clean snub, take-up, and other pulleys.
Environmental Conditions
Strong winds on one side of the conveyor can provide enough force to move the belt off its centerline or even blow the belt off the idlers. The solution is to install retaining rings known as “wine hoops” over the conveyor to keep the belt in place, provide a windbreak on the windward side, or enclose the entire conveyor.
Should rain, ice, or snow be blown onto one side of the conveyor, the result would be a difference in friction on the idlers. This difference may be enough to push lightly-loaded belts off the proper path. Even the difference created when the sun warms one side of a belt in the morning is enough to cause a belt to wander. Here again, the solution would be a form of conveyor cover.
Material Loading Faults
Mistracking that arises from loading problems is generally easy to spot, because the belt will run in one position when loaded and another position when unloaded. This observation may be confused on older conveyors where years of adjustments performed to “fix” the belt’s path have altered the natural track of the belt.
The load’s center of gravity will seek the lowest point of the troughing idlers. When the belt is not center-loaded, the weight of the cargo pushes the belt off-center toward the conveyor’s more lightly-loaded side. This can be corrected by proper loading-chute arrangements or through the use of deflectors, grids, or chute bottoms that can be adjusted to correct the placement of the load on the belt.
Reversing Belts
Reversing conveyors can be a special source of frustration. When the belt direction is reversed, the tension areas in the belting change location in relation to the drive pulley and loading area(s). Imagine having a conveyor that has a head drive, and at the flip of a switch, it becomes the tail drive.
When the top side of the belt is running toward the drive pulley, the tight side of the belt is on top. However, when the belt is reversed and the top side is running away from the drive pulley, the tight side is now on the bottom. The carrying side of the conveyor changes from being pulled to being pushed. A belt being pushed is inherently more unstable than a belt being pulled; thus, it is more difficult to train.
This poses especially difficult problems because the components now contribute differently to the tracking problems. The belt may run fine in one direction and wander all over when reversed because different sets of rollers and pulleys control the steering of the belt. To overcome this type of problem, the system should be surveyed to determine which components are out of alignment. Corrections should be made as required to get all rotating components in alignment.