The advent of variable frequency drives (VFDs) have allowed designs to increase the capacity of modern conveyors to levels unimaginable six or seven years ago. (Photo credit: Joy Global)
Conveyors serve as the arteries for coal mines. Once raw coal is cut from the face, it’s loaded onto a network of conveyors that funnel it to a mainline slope belt that eventually delivers it to the surface where another set of overland conveyors brings it to the prep plant. After it’s washed, another conveyor will likely transport the clean coal to a loadout facility. The coal business relies extensively on conveyors.
Similar to any piece of mining equipment, if conveyors are not properly designed and maintained, they can pose a hazard. For today’s safety-minded mine manager, hazards are not an option. Aside from injuries, they can be costly as far as downtime and possible citations from regulators.
Moreover, a number of new high-capacity longwall operations have been commissioned mostly in the Illinois Basin (IB) and northern Appalachian (NAPP) coalfields during the past five to six years. Historically, longwall systems were added to room-and-pillar mines and the high-production levels overwhelmed a system that was not prepared to handle the additional tonnage. Today’s modern longwall mines have been designed to accommodate that production capacity and the slope conveyors often have to handle production from two longwalls and multiple sections.
The engineers that design and build conveyors have been working to meet these challenges. Some of the recent slope installations are mind-boggling as far as installed horsepower and capacity. New technology has allowed them to make significant advances in horsepower and torque. Similarly, new pulley and idler arrangements can withstand the tensions placed on these systems. Much of the gear can also be used in the more common room-and-pillar mining operations.
Advancements in Slope Conveyor Capacities
The industry is heading toward higher horsepower, higher capacity slope conveyor systems, explained Vince Richardson, district sales manager-conveying products for Joy Global. He credits the implementation of medium-voltage, variable frequency drives (VFDs) for making many of these advancements possible. In the past seven years, the Joy Global conveyor team has designed and built seven major slope conveyors, which includes:
• a 4,000-hp, 5,000 ton per hour (tph), 72-in. slope conveyor in the ILB;
• a slope in the ILB with two 84-in. conveyors (one with an 8,000-hp rating and the other with 6,000 -p) with a capacity of 10,000 tph;
• an 84-in., 6,000 hp 8,100-tph slope conveyor in the ILB;
• a 72-in. 4,000-hp, 7,600-tph NAPP installation;
• an 84-in., 8,000-hp, 10,000 tph for another NAPP mine; and
• the latest installation, a 72-in. system rated for 10,000 hp at 7,000 tph in the IB, which is believed to be the largest applied horsepower for any operating slope conveyor.
Static Shaft Pulleys (SSPs), with bearings mounted internally, offer safety and operational advantages. (Photo credit: PPI)
As little as seven years ago, slope conveyors could not easily achieve these high capacities. “The first 5,000-tph system mentioned is the lowest capacity and eight years ago that would have been the maximum capacity,” Richardson said. “Now we have two systems that are running 10,000 tph. We have been fortunate. Using the right equipment design and technology, we have essentially doubled the capacities for slope conveyors.”
These slope belts have a lot in common. All of these installations use 4,160-volt (medium voltage) VFDs. “We work with integrators on the VFDs,” Richardson said. “They build the VFDs and configure the programming and components. We incorporate those units into our overall system design and scope of supply.”
All of these systems also employ constant-tension winches for take-up control. These systems have been around for about 20 years, but they have not been used on slope conveyors until recently. Most slope belts use gravity take-ups, which require a lot of maintenance. These new conveyor systems integrate constant-tension winches with VFDs. “Much of the time it sits there with 100% torque and zero speed until it needs to react,” Richardson said. “When it needs to take up slack or pay it out, it does so instantaneously.”
Most of these conveyors run steel-cord belting. The first 5,000-tph application uses a fabric belt and it also employs a booster drive midway down the slope.
Working with the mine operator, Joy Global’s conveyor group would engineer the installation to meet their needs. “Using our software modeling package, we would design a system to meet their needs and we’re probably the only ones with that capability and experience,” Richardson said. From concept to commissioning, most of these projects run three to four years.
Using three inputs — capacity, length and lift — within 30 minutes, the Joy Global conveyor team can model the conveyor telling the mine operator the horsepower requirements, the pounds per inch of width (PIW) rating to determine belting and the hardware options.
During the detailed engineering aspect of the project, a lot of technology is applied to pulley design, according to Richardson. “When 8,000 hp to 10,000 hp is introduced into a system through the pulley, they become very critical in the operation,” Richardson said. “We fabricate much of the equipment ourselves. Then we work with the customer and contractors to install the equipment.”
As an example, these larger installations are using 21-in. diameter bearings on the drive pulley. The drive pulleys can weigh as much as 32 tons. For the 8,000-hp systems, the drive pulleys are 72-in. diameter and the face width is 93 in. The drive pulleys on the 10,000-hp system are 87-in. diameter by 84-in. face width.
“With these high capacities, we needed to develop an idler that could withstand these loads,” Richardson said. “With all of these installations, we have moved into Conveyor Equipment Manufacturers Association [CEMA] F applications. Six years ago, CEMA F applications had not been defined.”
For the troughing idlers, Joy Global had to develop a product that could withstand the load. “We had to design
a shaft for the rollers that would provide minimal deflection,” Richardson said. “We developed special rolls and frames to accomodate the heavy loads generated by these large capacity systems.” One 84-in. troughing idler with rolls in
it weighs about 400 lb. The idlers are spaced 5 ft on top and 10 ft on the bottom.
As far as maintenance, the mines need to have a plan in place to monitor bearing vibration and temperature. The bearings are drilled and tapped with sensors installed. All of that data is brought back to an interface with set alarm limits, which reports to the mine’s central control room. Richardson said that Joy Global is looking at other technologies that can make these systems run more efficiently. There are a lot of R&D efforts under way to make these conveyors smarter than they are now, while at the same time looking at historical trends for maintenance purposes.
Static Shaft Pulleys
Jeff Poe, a field applications engineer for PPI, who specializes in underground mining, couldn’t agree more. “Today there is a great need for more reliable equipment that lasts longer and yet remains cost competitive,” Poe said. As an example, Poe described how PPI has developed a pulley where the bearings are mounted on the inside of the drum. Essentially the pulley rotates around a stationary shaft much like an idler. There are some obvious advantages from a contamination and load point of view and it has been readily embraced by underground coal miners.
Training idlers for underground operations, such as the return idler shown above, keep the belt from drifting. (Photo credit: PPI)
Sealed inside the pulley, the bearings are protected from contamination. “Tail pulleys will typically see a lot of muck and spillage that piles up around the bearings on these pulleys,” Poe said. “With static shaft pulleys [SSPs], the contamination falls onto a stationary shaft with mounting blocks and it doesn’t affect the performance of the system.”
Because it is no longer rotating, dynamic shaft fatigue is no longer an issue, Poe explained. “The shaft will deflect once initially and it never changes,” Poe said. “The bearings adjust to the deflection and that’s how it runs for the life of the product.”
The bearings are better protected and they last longer. They are designed to be greased while in operation. Because the shaft is no longer rotating, guarding the area is no longer a concern as long as it is not a tripping hazard.
Another difference between mounting blocks and pillow blocks is that pillow block bearing housings have to be aligned and shimmed to prevent the seals from binding. If the seals are bound, the bearings will overheat and fail prematurely. With the SSPs, there is no shimming. More importantly, in the underground environment, miners tend to lose the shims when they change the bearings. With the SSPs, they square the pulley to the conveyor structure and bolt it down.
They have a few limitations. A small diameter pulley that requires a large shaft and bearing could be a problem. If the bearing housing diameter is greater than the drum diameter. SSPs are also currently running on 84-in. belt conveyor applications.
The Importance of Training Idlers
There is a growing need for better training idlers underground, but the standard that the underground coal operators have accepted over the years has created an impediment for R&D.
An underground mine may have miles of conveyor. Therefore, it is advantageous to the mine to have standardization amongst their conveyor components. However, it can be very difficult to introduce better, more efficient conveyor components, when a manufacturer is also tasked with compatibility with the old.
A prime example is the troughing return idlers bolted to C channel, Poe explained. “Above ground the bolt centers across the width of that idler are normally belt width plus 9 in.; that’s the standard width,” Poe said. “Underground, coal mines use a low-profile offset box idler and the centers are belt width plus 13.5 in. That was standard set by Continental many years ago and the mines have accepted it.”
With wider belts underground, most mines did not take full advantage of training idlers. Training idlers swivel in the center and have guide arms with vertical rollers that track the belt into place. As the belt wanders over, it hits the guide arms, swivels the idlers and helps train the belt.
The belts were allowed to wander naturally. Any idler that is no longer rotating or functioning properly presents a hazard in that situation and the Mine Safety and Health Administration is writing citations for it more recently. If the belt rubs any of the steel structure, the C channel, drop brackets, etc., inspectors will also write citations for that. Coal operators can no longer allow the belt to rub the steel so there is a need for training idlers.
PPI has developed a standard line of training idlers for underground applications. “They are not much different in appearance from training idlers used on the surface, but that they have been designed for that special underground width,” Poe said.
Poe realizes that a lot of miners are tracking conveyors manually by knocking an idler one direction or another with a hammer. “When loading conditions change and no one is present, who knows if the belt is properly trained?” Poe asked. “If maintenance crews take that approach routinely, after about a year, all of the idlers have been knocked one direction or the other. No one knows where to start because everything is out of wack.”
Lightening the Load
As Richardson mentioned earlier, some of these idlers can weigh as much as 400 lb and nobody is getting any younger. The weight of conveyor structure has become a hot topic in the underground mining industry. There has been a huge push for lighter weight rolls, Poe explained, but there are trade-offs and the industry is trying to decide if the trade-offs are worth it.
Poe has worked with several belt coordinators who have tried the lightweight rolls above and below ground. Lightweight rolls for underground use must be static conductive. That materials exists, but it can’t withstand the loads that are placed on it.
If miners require a 72-in. belt running at 5,000 tph, he needs a CEMA E idler, which should be able to support 1,800 lb. In general, plastic rolls for belt widths above 48 in. cannot carry the CEMA class loads. “We have a line of plastic idlers and we know the limitations,” Poe said. “It doesn’t take long, minutes, for a conveyor to wear a flat spot on a plastic roll. Plastic rolls have their place, but not in high capacity underground conveyors.”
PPI is working with lightweight metal rolls that meet the CEMA load ratings. “It’s difficult to beat steel,” Poe said. “We are developing a steel idler that weighs less than the traditional idler and still meets the CEMA rating. If we can remove 20% to 30% of the weight, then we will have cleared some huge hurdles, especially when it comes to wide belt width applications.”
Joy Global and PPI have developed a lot of different technologies and hardware that meet the needs of modern coal operators as far as safety, longevity and availability.