Eliminating Conveyor Belt Misalignment

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, on 18. Sep. 2009 - 15:16

Eliminating Conveyor Belt Misalignment / Side-Travel Problems

by Aaron Gibbs, Vice President,

ASGCO Manufacturing, Inc., USA

One of the most common reasons for unscheduled down-time on conveyor material handling systems is conveyor belt side travel problems. One of the challenges in tracking conveyor belts is that each belt and system it rides on has its own characteristics, and therefore there is no one answer to track every type of conveyor belt.

“Tracking” or training is defined as the procedure required to make the conveyor belt run ‘true’ when empty or fully loaded.

Tracking or training of the conveyor belt is a process of adjusting idlers in a manner that will correct any tendencies of the belt to run other than true.

The structural conveyor components such as the supporting structure, pulleys, take-ups and idlers must be properly aligned. These should be the first things looked at when trying to identify belt tracking problems. Conveyor sections should be ‘square’ and ‘horizontal’ (side to side). All pulleys, snub rollers, troughing and return idlers must be square with the frame (perpendicular to the belt center line), parallel to each other and level.

Non-structural factors such as conveyor cleanliness, belt camber, belt tension and squared splices are also important factors to examine when determining belt tracking concerns. Cleanliness is essential to good belt tracking.

The “Handlebar Effect” (see Figs. 1 + 2)

Looking at the conveyor belt in the direction of belt travel and steer the idlers in the same way as bicycle handlebars until you move the conveyor belt to the desired position.

The training effort is caused by the friction force of the scuffing action of the conveyor belt as it passes over the cocked idler.

Carryback on idlers and pulleys can also cause the “Handlebar effect”. The belt will always move and make contact with the side it touches first, therefore always ensure that proper belt cleaners are installed and maintained to help eliminate this possibility. (Fig. 3).

The conveyor belt tracks off, more commonly, on the return side of the belt causing a wide array of problems; rubbing and tearing the conveyor belt edges on the conveyor structure, return idler brackets and causing spillage throughout the load-zone because the conveyor belt is off to one side or the other.

Mis-tracking causing the conveyor belt to cut into the frame of the conveyor. (Fig. 4).

Severe side travel off right side of tail pulley causing material spillage and conveyor belt damage. (Fig. 5).

Conveyor belt mistracking through the load causes material spillage on the walkways. (Fig. 6).

Mistracking in the load can cause off-center loading and spillage on the short side of the conveyor and the load not being centered on the trough or carry side of the conveyor belt. (Figs. 7 + 8).

Basic Rules of Conveyor Belt Tracking:

• The conveyor structure must be “True” (relative to the center-line) and “Level” (side-to-side).

• All pulleys, snub rollers, carrying and return idlers must be “Square” with the frame (perpendicular to the belt center-line) and parallel to each other.

– Is the pulley lagging worn?

– Belt Tension must be great enough to prevent slippage between the drive pulley and the belt.

Tension must also be enough to cause the belt to conform to the pulley crowns. Slippage will cause excessive wear to t both drive pulley lagging and the return side of the belt.

• Cleanliness is essential to good conveyor belt tracking. No buildup of carryback on the idlers or pulleys.

• Ensure the belt is stopped when making adjustments and remember that is better to make several

small adjustments and be careful not over compensate.

• Start tracking and adjusting idlers on the return run, working toward the tail pulley.

• This is followed by adjusting the idlers on the top run in the direction of belt travel.

– Tracking a conveyor belt should never be done by adjusting pulleys. This can cause uneven conveyor belt stretch and or pulley and shafting problems.

• START WITH THE BELT EMPTY... and begin to add partial and full load.

• Tracking adjustments should be spread over some length of the conveyor preceding the area of trouble

• Load the material in the center of the conveyor belt.

• Conveyor belt must be straight (less then 1⁄2% percent camber), in good working condition (not cupping or damaged) and ends must be squared and laced properly.

• Accurate squaring of the belt ends prior to splicing is essential.

• If the belt runs to one side at a particular point or points along the conveyor structure the cause will probably be due to the alignment, or leveling of the structure or to the idlers and pulleys immediately preceding that particular area or combination of these factors.

• “The belt moves toward the end of the roll / idler it contacts first”.

As stated, cleanliness is essential to proper conveyor belt tracking. The key to controlling carryback is the installation and maintenance of an effective belt cleaning system. It is a sound practice to use a multiple cleaner system to provide "more than one pass" at removing the material. These systems are typically composed of a precleaner on the face of the head pulley to remove the majority of material, and one or more secondary cleaners installed further along the belt return to remove residual fines. Tertiary or follow-up cleaners can be positioned even further back along the conveyor return to remove any last material. Criteria to consider in the selection and placement of a belt cleaning system include:

• Far forward-Installation should be on the conveyor return as close to the face of the head pulley as possible to remove material as close to the original material trajectory as possible.

• Out of the flow-The cleaners should be installed below the material trajectory, so they are not barraged by lumps.

• Safe for the belt-Use systems that minimize or eliminate any risk of damage to the belt, splices, or to themselves. They should clean at low pressure and incorporate a method of relief for the passage of splices and other obstructions.

• With multiple-cleaner installations, it may be necessary to add systems-an expanded dribble chute or a scavenger conveyor-to return the material removed by follow-up cleaners to the main material flow.

Applying corrective equipment and tracking techniques can optimize conveyor belt and other critical equipment life. Making the fuel handling systems of coal fired power plants and mining companies safer and more productive. However, training idlers should not be the permanent fix. The belt should run true and training idlers are to be installed for adverse and transient conditions.

Laser Alignment of the conveyor system is one method to find out the exact problems of an existing conveyor system or one that is being put in new to help accurately align all components. (Fig. 9). It can provide for a highly accurate method (.003” at 500’) aligning the conveyor system or even non-conveyor applications such as:

Tripper Rails and Structural foundation elevations. Laser Alignment is a highly accurate system to align framework and conveyor components, which is key to minimizing the costs of the conveyor operation.

The most common piece of conveyor-tracking technology equipment, that is 25 – 50 years old, is a training or tracking idler. These trackers do work in some applications but have their problems, such as damaging the edge of the belt, damaging the belt carcass, not working on wet applications and center pivot seizures. Figs. 10-12 below are common examples of failed conveyor belt tracking idlers. Problems occur when the external pivot builds up with carryback and seizes the center pivot enableling it to react when the conveyor belt makes contact with the guide idler. These trackers eventually get “tied-off” in one direction and start to cause more problems like edge damage and ply separation to the conveyor belt.

After identifying the need in the market for a better tracker, ASGCO (Complete Conveyor Solutions) of Allentown, Pennsylvania examined the problems with existing trackers and combining that with company experience, engineers set out to design a tracker. It was agreed that a good tracking roller / idler:

• Must operate in both dry and wet conditions

• Requires a minimum force to activate the tracking mechanism

• Does not rely on the edge of the belt to activate the tracking mechanism

• Maintains good traction with belt

• Durable design

The ASGCO Tru-Trainer belt tracking idler (Fig. 13) utilizes a unique and highly effective tracking action, which is non- damaging to the belt and kicks-in immediately if the belt begins to track off-line. Because it does not rely on contact with the belt edge in order to guide the belt, belt edge damage, which occurs frequently with other

tracking systems, is avoided. The Tru-Trainer has special tapers (similar to a crown pulley) on opposite sides of the roller, which cause the roller to pivot about an internal, vertical pivot axis. The internal central pivot is perpendicular to the belt and this results in the tracking action always being on the same horizontal plane as the belt. For this reason the Tru-Trainer works equally well with reversing / shuttle conveyors.

As the belt starts to move off center it will contact the taper section of the roller on that side of the belt. The difference between the peripheral rotational speeds of the tapered portion of the drum and the center causes the drum to pivot about its internal pivot. The extent, to which the roller will pivot, will depend on the extent to which the belt has moved off its central position. Since the roller is no longer perpendicular to the direction of the belt travel, the skew roller immediately steers the belt back to its central position.

Once the Roller has steered the belt back to the center, the opposite side of the belt will contact the taper on the opposite side of the roller and this will cause the roller to realign itself perpendicularly to the belt.

Since the surface of the belt remains in contact with the roller over the entire width of the belt, the tracking action is far more effective and immediate than those types of trackers that pivot away from contact with the underside of the belt.

The bearings are mounted in the bearing housings in both ends of the drum, to provide support for the roller drum over its full length. This ensures concentric vibration-free rotation. Each Tru-Trainer uses a pair of 37/16” ball bearings to facilitate the design (internal movement of the shaft within the inner sleeve) and therefore are well within their rpm and load specifications. This allows conveyor belts with speeds exceeding 1100 FPM to be tracked effectively.

The center pivot is unique and simple, consisting of a stainless steel pin within a solid mild steel center shaft. This combination of the dis-similar materials overcomes any possibility of seizure. As an added precaution graphite anti-seize compound is added and the whole unit is seal welded for life.

The drum of the roller is a 1⁄2” of hot vulcanized 65 durometer abrasion resistant natural rubber. For extreme wear applications as seen in underground and above ground coal and mineral mining applications, we recommend our 3⁄4” Skalper urethane cover. The bearing housing, front and back seal have been specifically developed for entire Tru-Trainer product range, to ensure effective sealing in adverse conditions.

Fig. 14 shows the coal handling conveyor 48” belt width 600 FPM Return Tru-Training Idler

For the load-carrying side the Tru-Trainer Trough tracking idler / roller was developed. Due to the fact that the load carrying side is in a trough formation a different tracking configuration was needed. Applying Tru-Trainer Flat Return principles of tracking in the horizontal plane, wing rollers were incorporated into the design to activate our standard central pivot system. Appreciating the potential damage caused by wing rolls intercepting the belt at 90°. The activating wing rolls are set to have a maximum effective contact between the belt edge and the wing roller of 25° -55° and are compatible with all troughed belts from 15° - 45°. These wing rolls are set to miss the edge of the existing troughed conveyor belt by between 1/4”on either side.

As the belt moves off-center the belt slides up the wing roll and thus causes the Tru-Trainer to pivot on its internal pivot and steer the belt back to center with minimum force. The unique aspect of the Tru-Trainer Trough tracker is that the center roll does all the steering and not the wing rolls, as this is where the most

effective steering forces are exerted. (Fig. 15).

For extremely wide or heavy belts the Tru-Trainer Dual Return tracking roller was developed to accommodate the excessive forces encountered in these applications. An external central pivot mechanism was designed instead of using a single cumbersome large drum, enabling a concentric and balanced rotation to be achieved. This results in an extended bearing life. Two separate tapered rolls are mounted onto the pivot mechanism, which can be individually replaced as wear occurs. The operating principle is identical to the Tru-Trainer Flat Return tracking roller, the advantage of the split configuration has resulted in a highly sensitive tracking action.

Fig. 16 shows the coal handling conveyor 60” belt width 700 FPM Dual-Return Tru-Training Idler.

Tru-Trainer series of conveyor belt tracking idlers are a patented design that offers reliable and effective conveyor belt training. The training idler employs a unique and effective internal tracking mechanism, which is non-damaging to the belt and reacts immediately if the belt begins to drift off center.

Because it does not rely on contact with the belt edge in order to guide the belt, belt edge damage, which occurs frequently with other tracking systems, is avoided. The center pivot is unique and simple, consisting of a stainless steel pin within a solid steel center shaft. Advantages include:

• Increases Conveyor Efficiency and Belt Life

• Reduces Spillage and Increase Tonnages

• Improves Safety and Reliability

Fig. 17 shows the PRB coal ship loading conveyor 96” belt width; 750 FPM Dual-Return Urethane Tru-Training Idler

The most critical areas to install training idlers are 30’ before the tail pulley to ensure proper tracking over the tail pulley and through the load zone. The other areas are just after the belt leaves the head pulley and before and after the gravity take-up if present. Self training idlers should be installed on 100’ centers on the return side, unless the conveyor structure is out of square and then it may be required to install them on 50’ centers.

In the end, belt tracking is essential to a well running conveyor system. It is far less expensive to fix the problem then let it drag on year after year causing additional problems to the conveyor material handling system.

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Fig. 1: The “Handlebar Effect” - Looking at the conveyor belt in the direction of belt travel and steer the idlers in the same way as bicycle handlebars until you move the conveyor belt to the desired position.

Fig. 2: The “Handlebar Effect” - The training effort is caused by the friction force of the scuffing action of the conveyor belt as it passes over the cocked idler.

Fig. 3: Carryback on idlers and pulleys can also cause the “Handlebar effect”. The belt will always move and

make contact with the side it touches first, therefore always ensure that proper belt cleaners are installed and maintained to help eliminate this possibility.

Fig. 4: Mis-tracking causing the conveyor belt to cut into the frame of the conveyor.

Fig. 5: Severe side travel off right side of tail pulley causing material spillage and conveyor belt damage.

Fig. 6: Conveyor belt mistracking through the load causes material spillage on the walkways.

Fig. 7 + Fig. 8: Mistracking in the load can cause off-center loading and spillage on the short side of the conveyor and the load not

being centered on the trough or carry side of the conveyor belt.

Fig. 9: Laser alignment

Fig. 10: Training or tracking idler

Fig. 11: Training or tracking idler

Fig. 12: Training or tracking idler

Fig. 13: Internal view of the Tru-Trainer Flat Return Idler

Fig. 14: Coal handling conveyor 48” belt width 600 FPM Return Tru-Training Idler

Fig. 15: Tru-Trainer Dual Return

Fig. 16: Coal handling conveyor 60” belt width 700 FPM Dual-Return Tru-Training Idler

Fig. 17: PRB coal ship loading conveyor 96” belt width; 750 FPM Dual-Return Urethane Tru-Training Idler