Conveyor belts that run off-center are a common and potentially costly problem in aggregate facilities. Running off-track can cause edge wear or other damage to the belts themselves; damage to other components of the system, such as idlers and pulleys; and possibly spillage of materials being moved through the plant.

Belt training is the process of adjusting idlers, pulleys and loading conditions to ensure that a conveyor belt does not run off center. It is a complex process with many variables. Whether training an existing conveyor belt that has gone off track or a new one that has just been installed, start with the following check list:

* Are head and tail pulley shafts parallel to each other?

* Are all belt idlers correctly bolted down and perpendicular to the frame to which they are bolted?

* Are training idlers installed properly and pointed in the correct direction?

* Are all drive components-speed reducer, V-belt drive, electric motor-in good working condition?

* Does the gravity take-up have the proper amount of weight?

* Have the belt skirting and wipers been properly adjusted?

* Are all conveyor pulleys and belt idlers free from material build up?

* Are lagging materials on conveyor pulleys worn or damaged?

* Has all foreign material been removed from around the conveyor?

If you can answer "yes" to all these questions and the conveyor belt still does not track properly, then check the conveyor structure to make sure it is straight. A laser alignment system is the best way to determine the structure's straightness.

Safety should be a primary concern when making adjustments to the conveyor system, just as it is with all work done around heavy equipment. To avoid potential hazards, train a conveyor belt when it is empty.

When working on the system, start at the head pulley and work toward the tail pulley on the return side of the belt (bottom side). Locate the maximum point of off-tracking and back up to the fourth, fifth or sixth idler before that point to begin making adjustments.

Keep in mind that a conveyor belt will always move toward the part of an idler or pulley it touches first. Therefore, any idler or pulley not running parallel with its counterparts serves as a de-training device.

Make small adjustments to one idler at a time and check for improvement before making another adjustment. Multiple idler shifts or dramatic adjustments can compound the tracking problem. Always follow the direction of belt travel when making corrective shifts.

After making adjustments, jog the belt for several revolutions while checking at head, drive, take-up, tail and along the carry and return runs for smooth passage.

Using training devices or making a series of adjustments as described above normally will correct off-track problems. However, if these measures do not correct problems, also look for off-center loading of the belt, bent or damaged structures or defective conveyor belting. If unable to identify and correct the problem, it might be necessary to consult a qualified engineer or specialist.

Correctly training a conveyor system prevents costly wear and tear and other more serious damage to the system components. The adjustments also should contribute to making the workplace safer by preventing materials spillage. The relatively short time it takes to train a conveyor may save serious downtime.

Even under normal running conditions, the use of training devices may be necessary to keep a belt running true. Aggregate production facilities frequently use four types of trainers:

1. Self-aligning idlers move a belt back to the center when it shifts to one side or the other. Make sure these idlers are mounted in the direction indicated on the side of the unit.

2. Vertical edge guides-free-rolling cylinders mounted just outside the edges of the belt-contain the belt when it shifts and help prevent edge damage. They are most often used on the return side of the belt where the belt responds more easily to lateral pressure.

3. Forward tilt idlers, with no more than a 2degrees tilt from vertical, help prevent lateral movement from the belt. A normal troughing idler can be used as a tilt idler by shimming under the bracket feet. The crown of the idler should not tilt more than about 11/48 in. per foot of belt width, to a maximum of about 31/48 in.

4. V-type returns are used primarily on high-tension conveyor belts, such as steel cord belts, where there is typically increased contact between the idler and the belt.