Holding conveyor costs down and improving uptime adds to the bottom line
by steve fiscor, editor-in-chief
Conveyors serve as the arterial system for coal mines and prep plants. Longwalls and continuous miner sections are often limited by conveyor capacity and availability. Likewise, prep plants rely on conveyors to deliver raw coal and to transport clean coal and tailings to the refuse impoundment.
Most of these operations run 24/7 and, if these systems go down, the mine begins to hemorrhage money. Hundreds of workers are forced to do secondary jobs or sit idle until the systems are restored, or in the worst-case scenario, the miners are sent home. Raw coal is cash flow for the mine and a 3-million ton-per-year operation can lose as much $15,000/hour when its mainline conveyor goes down.
Programs that optimize costs and maintain availability are essential in today’s business climate. At $40 to $50 per foot (ft), most coal operators can’t afford to keep 2,000 ft of belting in inventory for emergency purposes, but other options are available. Regularly scheduled inspections prevent catastrophic failure and working with the right service providers could be instrumental.
Coal operators should also carefully consider the drive systems for their high-reliability conveyors. For the same reasoning, it is also expensive to keep a new spare high-tension engineered-class pulley in inventory. Yet, if that pulley fails, building and shipping a new one takes precious time. Rebuilding one from the yard and keeping it as a spare might be a more economical approach.
High-reliability conveyors, like the mainline and slope conveyors, for underground coal mining operations employ engineered class pulleys, which are attached to large shafts and supported by large diameter pillow-block bearings. They are designed for high tensions and high horsepower. These pulleys can be found in the drives, at the conveyor’s head and tail, and at the take-up. A traditional arrangement would employ at least five of them.
During the last few years, PPI has been developing a program for refurbishing engineered class pulleys. “We take an expensive item, perform an evaluation and replace worn parts, making it as close to new as possible before returning it to the customer as a viable spare for half the price of a new one,” said Jeff Poe, field engineering manager for PPI. “If a coal company does not want to purchase a new engineered-class pulley, they can ship a used one to us and we will disassemble it, inspect it and suggest repair services.”
While local machine shops might have the ability to turn a large diameter shaft and repair the rim shell, they most likely do not have the expertise required for the rework of a major high-reliability pulley. Tragically, they could severely damage the pulley arrangement if they try to remove the locking mechanism with torches. “Local machine shops do great work, but there’s quite a bit of expertise required as far as bending moments and fatigue cycles that factor into these large assemblies,” Poe said.
The before and after photos show a high-reliability pulley that was recently pulled from service at a West Virginia coal mining operation and refurbished by PPI. The lagging is worn and there appears to be some separation as well. The bearings have not been changed for years. The arrangement also has a backstop mounted on it, which is also a wear item.
The coal company asked for a full inspection. If it could be reused, they asked for a full rebuild including the backstop. PPI used non-destructive tests (NDT) to evaluate the shaft. Ultrasonic testing (UT) and magnetic particle testing can be used to generate a health report for the shaft. PPI looks for defects on the shaft’s surface and internally. If it can, it gives the shaft a clean bill of health.
PPI does the same thing for the rim shell. They strip the lagging from the pulley. They place it in a lathe and do a skim cut to remove any remaining lagging and any corrosion. They use the same types of NDT on the rim shell looking at the thickness and the integrity of the welds.
“In this case, both the shaft and the rim shell passed the test and we added new lagging, reassembled it with new bearings, sent the backstop out for repair and reattached it,” Poe said. “We ship a refurbished pulley [right photo] to the mine, which is almost as good as new.”
Most of the time, PPI finds surface defects on the shaft, but usually no internal defects. The seals are supposed to rotate with the shaft, but sometimes they will get hung up and the shaft will spin under the bearing seals, Poe explained. “If this is not caught quickly, it will wear grooves in the shaft,” Poe said. “Once that happens, you have a shaft that is undersized for the seals, which allows contaminants to enter. If the miners are washing coal accumulations off the bearings, they may inadvertently force water and grit into the bearing.”
Another form of shaft wear occurs where the pulley connects to the shaft. At that connection, there is a relative bending moment that occurs between the shaft and pulley with each rotation, which leads to shaft frettage. A slight amount of movement takes place between the shaft and the bearing locking mechanism and the shaft frets a small amount of metal. “The steel on the shaft oxidizes [rusts] and a small amount of polishing takes place with the shaft bending under the locking mechanism,” Poe said. “If it’s been in service for any length of time, we recommend replacing the locking mechanism.”
If the shaft is compromised, it will drive up the refurbishment costs considerably. “In that case, we contact the mine and tell them the shaft no longer passes CEMA criteria,” Poe said. “We never know what we will find until we do the tests, but most of the time we find some surface damage.”
Working with certain universities around the country, PPI has developed a 3D finite element analysis (FEA) program that is applicable to its pulleys. Not just any pulleys, but the way we make pulleys, Poe explained. “For a 4,000-hp, high-reliability conveyor, the program would determine the thickness of the tube according to PPI’s standards,” Poe said. “For example, the FEA might determine a 2-in. rim shell thickness. Once we remove the lagging, we can see that the rim shell is worn to 1.75 in. in some locations. We warn the customer that it has worn beyond the acceptable limits and look at ways to rebuild it.”
Also, knowing how the pulley will be used is incredibly important. When PPI disqualifies one for compliance, coal companies usually ask them if they can qualify it for use in another application that has the same face width. PPI can certify them for certain applications.
When PPI designs large high-reliability pulleys, they design them for infinite life and infinite cycles, Poe explained. “That doesn’t mean we are guaranteeing them for 1,000 years,” Poe said. “It means that the shaft and pulley are guaranteed for 20-plus years. If the lagging wears out in five years and the bearings wear out in six years, don’t throw out the shaft and pulley, refurbish it.
The time to complete the rebuild depends on the availability of the new components. PPI can complete the NDT testing in 1 to 2 months. New 22-in. bearings, however, have an 18-week lead time right now.
A Belt Change Out in a Shift
The 2,200-ft refuse conveyor at a West Virginia prep plant had reached the end of its life and needed to be replaced. The job needed to be completed as quickly as possible and they turned to Raocat Rubber Co., who restrung the conveyor with new 42-in. belting, vulcanized the splices and returned the conveyor to service in six hours.
“The timeframes we must work within for coal operators are much different than other industries,” said Steve Tate, operations manager for Raocat Rubber. “The window is significantly smaller. If we did this same job at an aggregate operation, for example, we may have had two days, but because it was a coal operation, we had to complete it as quickly as possible.”
Raocat is a full-service conveyor belt distributor that has been servicing the mines since 1987. Tate has been there since 1989. “It’s a family-owned business with 22 employees and we take a lot of pride in the work we do,” Tate said.
To prepare for this project, Raocat did everything that could possibly be done before the existing conveyor was taken out of service. “We will combine rolls and prep the ends,” Tate said. “We will have everything staged and ready to go to the point where as soon as the conveyor is shut down, we are pulling belt within 15 minutes.”
This conveyor was relatively flat belt with a 600 PIW rating. The project required two rolls of belting. Raocat staged one roll in a set of stands and lapped one roll out on the ground, which allowed them to vulcanize the two pieces the day before the installation. All of the belt was pulled into place as one solid piece using a front-end loader. If everything went according to plan, and it did, the Raocat vulcanizing crew would have three hours to vulcanize the final splice.
“We call it time machine planning,” Tate said. “During the preplanning process, we look at all of the possible ways to save every possible minute.”
For a project like this, belt winders are too slow. A winder coils belt at 10 to 12 feet per minute (fpm). “It’s a slow process with a high-torque value and the winders only hold a small amount of belt,” Tate said. “In the time it takes to get a belt winder set up, we would already have the belt changed.”
Once the belt was de-energized and locked out, the crews cut the belt, spliced an end to the new belt with a set of fasteners, pulled the new belt into place with the front-end loader, removed the fasteners and vulcanized the final splice. “While one group is vulcanizing that final splice, another group is rolling up the old belting,” Tate said.
Raocat operates safely. Throughout the process, the company makes sure that nobody is in line with the belt coming on and going off. “We position lookouts at the head, tail and take-up, making sure everyone is in safe positions and we also watch to make sure we do not damage the new belt as its pulled into place,” Tate said. “We have been blessed. We haven’t had a lost time accident in eight or nine years. Our men are very conscientious. They argue like family and they look out for each other like family.”
The belting for this project was provided by ICL America. All belt manufacturers make quality belting these days, Tate explained. “The difference with ICL is that they keep inventory on the ground,” Raocat said. “Depending on the type of belt, it could cost anywhere from $20/ft to $100/ft. It’s one thing if you know the belt will fail and can plan for the purchase, but it’s hard to justify having $2 million in inventory for an emergency. These guys [ICL] have been rock stars at keeping critical inventory on hand. It makes my life easier and it doesn’t kill my cash flow.”
Tate believes strongly in preventive maintenance (PM). “My customers do not have emergencies,” Tate said. “Every Raocat customer is on a PM schedule. This belt was scheduled for replacement and we were able to plan for the installation. Two days before installation, we started seeing exposed fabric. We almost nailed that one to the day. Usually you are not that lucky, but we make a tremendous effort to keep our customers from having preventable catastrophes.”
ICL America Ltd., (ICLAM) may seem like a new name to some in the conveyor belt market, however, that is not necessarily the case. Starting in 2003, the company has been working behind the scenes with major players in the conveyor belt industry. Its grassroots extend back to the early 1990s. ICLAM started off in the mining industry, evolved into other markets and then returned to mining with expansions in several areas. Meanwhile, the market for the big players consolidated. There was turmoil and confusion, but ICLAM claims it stood steady through all of it. In 2015, the company said it started expanding again, competing head-to-head with the industry’s major players.
“ICL America Ltd. is emerging as a global leader for conveyor belting and the company has positioned itself for customer service, especially when it comes to tailor-made needs,” said John Blue, director of new business development for ICLAM. “We took advantage of the sound technical base we had built over the years, which allowed ICLAM to continue developing its technology and know-how through its own resource network. With more than three decades of experience in sales, product knowledge, research, development and marketing of conveyor belting for mining operations.”
ICLAM can provide all types of conveyor belt, including fabric, steel mesh, heavyweight, PVC, hybrid, MSHA-approved, and steel cord at widths up to 118 in. The company has fabric belt rated for 2,000 PIW and steel cord belting rated for ST-5400. It uses both direct and indirect distribution.
When a coal mine rips a conveyor belt, typically they contact a local service provider such as a local distributor to quickly repair and restore operations. This is where ICLAM said it shines by being able to provide a quick solution so that distributors can service the customers’ needs.