Heavyweight Conveyor Systems are described as those generating tension on the conveyor belting equal to or greater than 160 pounds per inch of width during normal operation. Heavyweight conveyor systems are employed in the continuous haulage of bulk materials and are largely found in industrial applications. The principal components are the conveyor belt, the pulleys and idler rolls, and the motors and controls that provide and regulate the necessary power to drive the system.
The Conveyor Belt
The Conveyor Belt is an endless band that transfers the material conveyed between two points. A conveyor belt is comprised of a reinforced member, or carcass, and a protective covering. Conveyor belts are generally a composite of rubber or some elastomer such as PVC and fabric or some other reinforcement. The elastomer is the external covering which provides wearability and protection from the handling environment. The fabric carcass internally serves as the strength bearing member for supporting the load and to control stretching of the belt.
Pulleys and Rollers
These mechanisms drive and support the belt and material conveyed. The head pulley is usually the point where the material discharges. The tail pulley is generally near the area where material loads onto the belt. The driving roll is motorized and moves the belting. The drive could be located at the head, the tail, or placed at some other point along the conveyor system.
A snub roll can be placed near the drive to provide a controlled degree of wrap, preventing slippage. A soft rubber covering is often applied to the drive roll to assist in preventing belt slippage. Carrying idler rolls help support the belt and material conveyed. They are usually in a trough configuration to prevent spillage while increasing the capacity of the material able to be conveyed. A series of transition idlers gradually shape the belt in to and out of this troughed position. Return idler rolls support the belt on the bottom side after the material discharges. The take-up roll prevents slack in the belt as it stretches due to starting and stopping or loading and unloading. Bend pulleys are sometimes used in the take-up area for additional belt support.
Motors and Controls
These components generate power and regulate its use in driving the system. Proper selection of drive motors must be preceded by a determination of the system horsepower requirements. Torque characteristics must be analyzed to select the motor which will adequately start the conveyor into motion without exceeding the elastic properties of the conveyor belt. Reduced voltage is frequently used to achieve a smooth start by beginning slowly and then accelerating up to full speed.
Modern conveyor systems use computers to control many functions of operation. Sensing devices can electronically feed information to the computer for analysis and determination as to whether or not changes should be made for more efficient operation.
Many safety devices are incorporated into the average conveyor system. These include emergency stop switches, closed circuit television monitoring, and warning horns and alarms. Electrical interlocking techniques are used to stop a series of conveyors, should one of those require emergency shut down.
There are potentially many other elements of heavyweight conveyor systems. These are either a cement-like chemical bonding system or fastening devices made of metal. Loading chute designs provide a controlled material flow onto the belt to reduce belt surface abrasion. Rubber skirting is placed around the loading area to further prevent spillage. Scrapers/wipers placed against the belt surface help remove particles on the belt after material discharges.
Electronic rip detection systems can be utilized to provide automatic shut-off should the belt become severely damaged. Systems with high inclination angles such as grain elevators use buckets or other devices attached to the belt to carry the load.
In Application Engineering, the person calculating the tension per inch of width (PIW) of the particular conveyor is determining the power requirements and the belt tension by using engineering formulae.
The power required to drive a conveyor belt is composed of three (3) major parts:
- The power to drive the empty conveyor (horsepower).
- The power required to move the material horizontally.
- The power required to move material vertically. This would be a plus
value for an incline conveyor, and a minus for a declined conveyor.
Before beginning any selection process, emphasis should be given to the importance of obtaining accurate conveyor operating data. Data sheets are a valuable tool to use in getting all of the pertinent information required. Usually available to the belt engineer is the original prints found in the plant's files or by "walking" the system and obtaining the information from actual observation and measurements. With this data, the belt engineer can calculate tensions, required take-up, motor horsepower, and the PIW strength, along with many other pertinent considerations.
First, the Effective Belt Tension (TE) must be calculated. TE is the sum of the tension required to move the empty belt (TC), the tension required to move the load horizontally (TL), and the tension required to lift the load (TH).
Long, complicated, overland conveyor systems may require steel cable tension members and extensive belt engineering expertise; however, a percentage of conveyors do not involve tension (PIW). That is when the actual operating tensions are low; a much higher PIW belt may be required to satisfy the load support, troughability, and impact resistant requirements that are found in the belt manufacturer's manuals.
Heavy black belt "engineering" can often be very complicated. After considering all of the calculations and considerations, the following important question should always be asked before making a new recommendation: "How did the old belt fail?". This can spotlight on where the old belt was weak and where the new belt should be reinforced. On new systems, in general, the belt should be as narrow as possible (depending on lump size) and run as fast as possible, within acceptable limits, to convey the required tonnage.
The Installation of a Heavyweight Belt
The Installation of a Heavyweight Belt can range from small belt that is readily man handled, to a belt literally miles long hauling bulk materials over variable terrain. Regardless of size, four essential items must be achieved in proper belt installation:
- Protect the belt from injury during installation.
- Obtain intended orientation, i.e. right side-up, texture or cleats facing in correct direction, threaded as designed over all pulleys, idlers, slider beds, etc.
- Engage drive and take-up systems correctly (no excess tightness or slack)
- Apply secure splice, appropriate for intended operating conditions
Advancements in technology continue to provide a wide array of tools that aid with belt installations, such as steel cord stripping machines and automatic heated-rubber compound dispensers.
Few conveyor belts run perfectly centered at all points around their operating path, especially when newly installed. Most belts require a break-in period to obtain acceptable tracking. Acceptable tracking means following a path that neither loses the load being carried nor permits belt edge damage from rubbing against fixed elements of the conveyor system.
The tracking or training of a belt consists of the application or removal of belt steering forces to obtain an acceptable central path on the conveyor. Tracking/training procedures include consideration of the following among others:
- Recheck of conveyor for freedom from tilt, sag, bend, or other structural misalignments.
- Recheck of pulleys and idlers for misalignment or build-up.
- Recheck of belt splice accuracy and straightness.
- Observation of belt operation empty and loaded for any specific mistracking tendencies or locations along the conveyor
- Use of training idlers or idler adjustments (skew, tilt, etc.) to overcome any observed mistracking. This is often a" trial and error" procedure.
- Variation of belt tension within a controlled range to determine if tracking is affected.
- Elimination of factors detrimental to good tracking such as random spillage, off-center loading, build-up of materials on idlers and pulleys, excess wetness or lubricants on conveyor components, malfunctioning scrapers or wipers, wind load, frost or other environmental conditions.
Care and Maintenance
Conveyor and belting care and maintenance are important factors in any plant that has to move significant tonnage of bulk or unit materials with reliable consistency. Unfortunately, conveyors are sometimes taken for granted until a breakdown happens. To avoid such circumstances, various plans and programs are available to the conveyor operator.
The core of all of these programs is a periodic inspection plan. These are relatively inexpensive to perform and they can be tailored to the specific need of individual plants. The purpose of these inspections is twofold:
- Identify and apply repairs to minor problems before they become major.
- Provide a forecast and planning basis for larger, but inevitable maintenance operations or replacements.
A typical inspection checklist would include:
- Belt condition (overall & specifics)
- Belt tracking and training
- Drive unit condition and operation
- Take-up, type, function and setting
- All other pulleys and idlers
- Scrapers, wipers and other accessories
- Loading or spillage problems (if any)
- Housekeeping and lubrication, etc.
- Safety problems (if any)
After inspection and reporting, a management directed follow-through and repair protocol is necessary for optimum results. In-plant personnel or outside contractors can perform these services as they are required.