As their markets mature, producers often need to increase the capacity of existing conveying equipment while minimizing additional cost and downtime. In many cases, producers can modify their existing equipment by increasing belt speed, changing idler style or using a combination of these two options to "find" additional capacity on existing conveyor structures.
But prior to any conveyor modification, producers should step back and look at their feed system's operation. Before selecting a capacity increasing option, they should be aware of any problems that the option might cause.
Take a fresh look
Begin any proposed conveyor modification by studying the gradation of the aggregate conveyed. Many older concrete plants were designed to batch mixes using a larger-sized, rounded natural gravel. But today's commercial mixes use a coarse aggregate, whether stone or gravel, that has a certain amount of fractured faces and often has a finer gradation. The finer gradation allows a greater surcharge, and the fractured faces decrease possible material rolling, often facilitating capacity increases.
Producers often overlook a key production constraint on their conveyors--the lagging and wrap on drive pulleys. Worn or poor lagging can cause belt slippage that restricts the system's ability to reach full speed without damaging the belt. Improper wrap limits efficient power transmission from gearbox to conveyor belt.
Producers should study their annual aggregate use. If they're handling more specialty and lightweight aggregates they should consider installing a special conveyor. Lower bulk densities of lightweight aggregate wreak havoc on conveyors designed for normal aggregate weight.
Since production increases will directly affect belt wear, producers should study their existing belting. In any designed production increase, producers should remember that the most expensive--and most perishable--single item on the conveyor is belting. Amortization of conveyor belting is a major factor in the cost per ton of material handled.
Start with belt speed
For many producers, increasing belt speed is the best way to increase a conveyor's capacity. On conveyors with shaft-mounted drives, producers select belt speed by installing matched pairs of input and output sheaves. Matched sheave pairings are found in tables provided by the speed reducer's manufacturer.
How fast is too fast? With today's improved idlers and conveyor belting, a belt speed of 600 feet per minute (FPM) should handle most concrete aggregates. But faster operating speeds can increase wear on a belting's top cover. "Too fast" is a speed at which most of the material being conveyed is in contact with the belting's top cover. An optimum belt speed limits load-to-belting surface contact to about 20% to 25%, placing more "rock on rock." This decreases the belt-cover wear cost per ton of material conveyed.
Designers use the conveyor's length and speed to calculate a "frequency factor" to predict belt wear. For instance, on a relatively short conveyor (less than 100 feet long), a belt speed of 600 FPM would result in a low frequency factor, meaning the conveyor requires a belting with a very heavy and costly belt top-cover thickness. But if the conveyor is 1,200 feet long, the frequency factor increases, allowing minimal top-cover thickness. But in many long-conveyor situations, loading conditions, rather than speed, are likely to be the governing factor in belt selection.
Producers can vary the surcharge by changing chute design at points where material is transferred to a conveyor. The chute design must not allow a straight drop on the conveyor that does not impart velocity in the direction of belt travel.
Higher belt speed can also increase conveyor idler maintenance costs. The existing idlers may not have sufficient bearing-load capacity for the faster belt speed. Producers should consider upgrading idlers for proper idler life. CEMA "B"-rated idlers may have to be changed to CEMA "C," or CEMA "C" changed to CEMA "D. To limit idler surface wear while increasing idler load rating, increase the roll diameter, thus reducing idler shaft speed.
Increased belt speed may cause more coarse material rollback. After any speed increase, producers should monitor conditions at all loading levels to ensure that rollback isn't excessive.
Measure the idler angle
Sometimes increasing belt speed alone doesn't produce the desired capacity. Producers should consider replacing existing troughing idlers with ones having deeper troughs. Deep trough idlers reduce spillage as they "cup" the conveyor belt. They also eliminate sag by providing additional load support between the idlers because of the increased beam effect. Standard idler angles are 20 degrees, 35 degrees and 45 degrees.
Without any change in belt speed, a deeper idler creates a significant capacity increase. For instance, assuming a 20 degree load surcharge, changing from 20-degree idlers to 35-degree idlers of equal length roll idlers increases the belt's carrying capacity by 26.7%. A change from 35-degree to 45-degree idlers of equal length roll idlers increases capacity by 8%.
By combining a belt-speed increase with deeper trough idlers, producers can increase a conveyor's capacity by as much as 50%. (Note: With 45-degree idlers, the belt speed should be at least 400 FPM so that material moves through the transition distance without spilling.)
Deeper idlers also help prevent spillage. Many producers prefer a narrower belt width that handles plant capacity but turn to a wider belt to avoid spillage. If a conveyor with 20-degree idlers is spilling material, the amount will be reduced or may even be eliminated by substituting 35-degree idlers, when no greater load is imposed on them.
Another advantage of deeper idlers is less wear on the belting's top cover. With a larger cross-sectional load, the deeper trough idler reduces material contact with the belting surface, resulting in longer belting life.
Note that, when increasing troughing angles, producers should check the conveyor belting's working tensions and load support to be certain that they can handle the increased capacity.
Avoid the sag
When increasing existing conveyor capacity, producers should constantly check for sag between idlers while the belt is fully loaded. Excessive sag shortens idler life, increases belting wear, reduces actual capacity and increases operating costs. By keeping the belt travel path as level as possible, producers minimize material bounce and movement. In severe situations, excessive sag may induce material rollbacks that can damage equipment and injure personnel.
Sag increases wear on the belting's top by causing material to slide against the conveyor's forward motion at each idler. Belt sag may also promote material entrapment under otherwise correctly designed skirtboards, resulting in costly damage to belting. Resulting flow restrictions limit the volume of material that can be placed on the belt. To counteract sag, the conveyor's drive may require additional horsepower, increasing the plant's energy cost.
When all else fails
When speed and idler changes on the existing conveyor still don't provide adequate capacity, using a wider belt may be the producer's last resort. Usually this means replacing the entire conveyor, but there is another alternative. Some producers have installed modular, "bolted construction-type conveyor trusses" rather than the welded trusses. They can then widen the conveyor belt by purchasing truss side-frames only instead of a complete truss.
H. D. Patten is C.E.O.(retired) at E.F. Marsh Engineering Co. (Marco), St. Louis, Mo., with more than 50 years experience in conveyor systems design.