April 1, 2015
Conveyor systems vary widely in overall quality and efficiency. Consider that operations may obtain conveyors from a variety of sources — from in-house or local fabrication operations, to used equipment outlets, to opting for the latest technologies from major manufacturers. No matter the source, producers should ultimately evaluate each material handling system and component — not just on price or immediate availability — but based upon performance, safety, and ease of maintenance. Examining the latter criteria is the basis of a long-term and forward-thinking approach to choosing any piece of equipment.
Case in point: A recent industry water-cooler discussion focused on evaluating the differentiating factors between mast-type stackers and telescoping radial stackers. Is one type more advantageous than the other? How does each rate based upon the key criteria? Arguably, there are a number of varying opinions as to using either stacker type. Lafe Grimm, chief engineer for portable projects at Superior Industries, shares his take on how the two systems stack up.
Designed with an outer truss section that’s hinged to an inner section, the mast-type stacker assembly is supported by a massive mast or tower structure that extends well beyond the hinged interconnection. This mass and height is required to support a wire rope suspension system that raises and lowers the outer section during conveying and stockpiling. The wire ropes are the sole support of the outer section.
“Wire ropes can deteriorate over time. If left in service long enough, they will break – causing the potential of serious injuries, and costly equipment damage and downtime,” Grimm says. He explains that the fiber strands in the wire ropes are susceptible to deterioration due to overloading, extreme heat, and damage due to bending, flexing, and kinking. These effects are accelerated in a system that involves sheaves and a moving cable. “Also, with the higher profile of the mast-type stacker’s tower, the unit is more susceptible to wind load pressure,” he adds.
Alternatively, the telescoping radial stacking conveyor consists of a stinger conveyor mounted inside an outer conveyor of similar length. The stinger conveyor has the ability to move linearly along the length of the outer conveyor, thereby varying the location of the discharge pulley. The height of the discharge pulley is variable, as well as the radial position of the conveyor. The undercarriage design also delivers support and rigid lateral stability.
“We adhere to some very rigorous steel structure codes; and the load sharing hydraulic cylinders add even greater structural support and stability,” Grimm says. He adds that the cylinders used to raise and lower the stacker are specially designed for safety, as a counterbalance valve keeps the “raise” cylinders from lowering should there be a hydraulic failure. Also, he explains that, in extreme weather situations (such as hurricane winds), safety and loss prevention are aided by the ability to quickly and easily adjust the stacker by retracting the stinger conveyor and bringing the unit into a low profile.
According to Grimm, some mast-type stacker manufacturers describe the wire rope suspension system as allowing the discharge point of the stacker to remain slightly above the top of the pile, thereby decreasing material segregation and degradation. “In truth, mast-type stackers are not known for precise control and increased range of movement. As such, they are prone to deliver higher material segregation within the finished stockpile,” he says. To mitigate this situation, some operations have added spray systems or specialized chutes to reduce segregation issues and minimize particulate matter in the air.
“An automated telescoping radial stacker is the only solution to creating a fully desegregated stockpile,” Grimm says. “It stockpiles in very thin lifts or layers, with each layer consisting of a series of windrows of material. To accomplish this, the conveyor is in motion continuously, and the three-axis variation of the discharge pulley is essential in making the layered pile that overcomes segregation.”
Telescoping conveyors can also be programmed to create higher-volume stockpiles of many different shapes, sizes, and configurations. Grimm points to the fact that some site footprints are more suited to a rectangular or pie-shaped pile; or a producer may want to stockpile inline over a reclaim tunnel for more live storage. This stockpiling flexibility can result in higher production capacity at a lower cost, he says, adding that these advantages are particularly important to the portable operator who wants to build the largest stockpile possible before relocating or to the producer who wishes to maximize stockpile volume on a site with limited space.
Portability is another consideration, he says. The telescoping radial stacker has three styles of road or radial travel axles from mobile, to in-pit portable, to fixed radial stacking. “Mast-type stackers require a lot of clearance and are very expensive to move,” Grimm says.
Ease of maintenance
The wire rope used on mast-type stackers requires frequent and meticulous inspections by a designated competent person. Guidance on these inspections can be obtained from the American National Standard for Wire Rope for Mines; the Occupational Safety and Health Administration; and from the wire rope manufacturer’s specifications, which indicate the number of allowable wire strand breaks per equipment type. The number of broken wires on the outside of the wire rope is an indication of its condition and whether it must be considered for replacement.
Grimm stresses that these types of inspections can be painstaking and time-consuming, while often placing the inspector in perilous positions on lifts, or under the machine itself. Wire rope can also be very costly to replace. As a result, inspections may not be conducted as often as they should be — leading to potential hazards.
“The hydraulic systems on the telescoping radial stackers are far simpler to maintain; however they do require routine maintenance that includes monitoring fluid levels and cleanliness, changing oil filters, and checking for potential leaks,” Grimm says.
Grimm says that conveyor technology is always changing, and always being fine-tuned. “The mast-type stacker was far more common decades ago. That was when wire rope was used in all kinds of equipment — such as the early bulldozers — a time before the advent of hydraulic technology, which is safer and more reliable,” he says.
Unlike the fabrication of yesteryear, Grimm says, “Today’s telescoping radial stacking conveyors meet the criteria of safety, performance, and ease of maintenance. They are PLC-controlled, automated systems that significantly streamline material handling functions and are designed with key standard safety features at the forefront.”
Article courtesy of Superior Industries.