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Streamlining the Scale House

Posted By admin On August 1, 2008 @ 4:17 pm In Applications,Articles,Features | No Comments

Optimizing a load of sand and gravel on a trial-and-error basis can be a time-consuming – and costly – endeavor.


At the Punta Gorda, Fla., mine of Coral Rock, Inc., drivers with overweight loads “would have to go back, open their tailgate, dump a little bit on the pile, then close the tailgate and come up to the scale house again to get reweighed,” says Bob Milhous, Coral Rock’s executive sales and logistics manager. “This took about 10 minutes per round trip, and sometimes they would have to go around four times before they got it right.”

Such inefficiencies ended at the Punta Gorda mine in 1989, when Coral Rock, Inc., installed a Lightning Loader manufactured by Petersen Industries, Inc., of Lake Wales, Fla. Now, optimizing a truckload at the mine’s scale house takes just a minute or two, Milhous says.

Load optimization is a goal that sand and gravel miners share with the truckers who haul these materials. Sand and gravel are sold by weight. If a truck is underloaded, the mine sells less, and the driver makes less money per trip. If a truck is overloaded or if the load is improperly distributed, the driver risks being cited and fined by local law enforcement authorities. And, if a truck exceeding state requirements is involved in an accident, the mine where the truck received its load will share liability with the driver and the truck’s owner.

Modern front-end loaders can measure their payload’s weight while filling a truck, but can’t calculate the distribution of that weight across the truck’s axles. Some dump trucks with air suspension have load cells that indicate weight, but even if a driver knows he is overloaded, he still must do something about it.

The Lightning Loader optimizes loads at the scale house by adding material to an underweight truck from a nearby reserve pile, or by transferring excess material from an overweight truck to the reserve pile. The machine is a knuckleboom grapple loader, a first cousin to the truck-mounted grapples used to pick up bulky trash and yard waste.

The grapple loader stands on a pedestal that supports a rotating head, from which extends a boom with a bucket mounted at its far end. The operator controls the loader from within the scale house. From there, he watches the scale, ensuring the precise weight. Flexible controls allow the operator to add or remove as little as 20 pounds of material at a time from a truck. The process takes just 65 seconds on average, expanding a mine’s potential sales volume by allowing many more trucks per hour to move across the scale.


Adaptation for aggregates

The Lightning Loader evolved from a device that Petersen Industries’ founder, John E. Petersen, developed in 1958 for bulk handling of oranges in Florida’s citrus groves.

“Trash officials in Lakeland saw these knuckleboom things picking up oranges, and asked Petersen if something similar was available to pick up bulky trash,” says Lee F. Rathbun, the firm’s vice president and co-owner. “They tried some things. The device kept getting refined and improved.”

Rathbun doesn’t know who initially used a truck-mounted Petersen Industries trash loader at a mine’s scale house, but company records show that E.R. Jahna Industries, Inc., a Lake Wales sand-mining company, purchased the first knuckleboom grapple loader mounted on a stationary base in 1960.

Another early customer, the EPK Clay Division of The Feldspar Corp. in Edgar, Fla., bought a grapple loader between 1965 and 1967. According to Petersen Industries’ service records, it was still in operation a quarter-century later, in 1991.

Coral Rock’s Milhous replaced his original machine in 2005 after 16 years of service. When installing the new loader, he kept the old control system because it was easier than replacing the controls.

Rinker Materials Corp.’s Florida Sand Division, based in Davenport, Fla., has a total of eight grapple loaders, two each at sand mines in Clermont, Davenport, Lake Wales, and Palmdale. “We bought our first loader in the mid-1970s,” says David Dorriety, regional purchasing manager. “Very seldom do we have to change one. The oldest unit we have is 23 years old, and we haven’t replaced one in seven or eight years. It’s a good, durable machine.”

Gerald Welch, plant manager at Rinker’s Lake Wales Sand Mine, says its first grapple loader was installed in 1984. A second machine was purchased in 2005 when the mine added a second scale house. “They are very reliable as long as you take care of them, grease them, change the hydraulic oil, change the filters,” he says.

Milhous also says the grapple loader is easy to maintain, and replacement parts are easy to obtain and install. “Hydraulic lines occasionally break,” he says. “Changing a hose takes just 20 minutes. About twice a year, we used to send someone out to get a hose made, which took a couple of hours. Now, we keep a whole set of hoses on hand.”


Efficient design

Rathbun says the grapple loader was designed to operate in a harsh environment. “That’s hard on electronics, so we try to stay away from electronics,” he says. “Also, with electronic joysticks and solenoid valves, you have a binary open-and-closed situation. You lose proportionality. We use either direct hydraulic controls, or pilot-operated hydraulics with a low-pressure valve to control the loader remotely by operating a bigger valve spool somewhere else.”

Welch says proportional controls are a major asset. “With on and off controls, you have to bounce the bucket back and forth. With proportional controls, you can open it just enough to get the right amount of sand out of the bucket depending on how slow or fast you move your joystick.”

Another adaptation to the mining environment is the slewing mechanism (for swinging the rotating head to the left or right). Currently, most cranes employ a turntable with bearings and a small pinion gear that turns a larger bull gear. “The problem,” Rathbun says, “is that the gears aren’t enclosed in oil. They’re open. They get lubricated and collect dirt and dust. We have a spindle to support our rotating head and a totally enclosed hydraulic motor with a planetary gear to provide slewing force. Our mechanism is sort of old-fashioned and may be a little heavier, but it doesn’t have exposed gears.”

The spindle shaft turns in a large bushing made of Nylatron, a cast nylon product from Quadrant Engineering Plastic Products, Inc., a German company with U.S. headquarters in Reading, Pa. “You grease it from the inside, so each time the clean grease ejects dirty grease and doesn’t let debris in,” Rathbun says.


Going the extra mile

At the Dolese Bros. Company’s St. Helena Sand and Gravel Mine in Pine Grove, La., a dredge in a 20-acre pond pumps sand, gravel, and pea gravel three-eighths of an inch in diameter from a depth of 20 feet. These materials are sorted into piles, from which wheel loaders fill commercial dump trucks. Loading takes place a full mile from the scale house.     The mine installed a grapple loader in December 2006 and began using it in January 2007. “This is the first time I’ve ever been around one,” says Ricky Miley, plant manager. “I’m very impressed with it.”

He says the grapple loader paid for itself in its first six months of operation. Operating costs include electricity, filters, hydraulic oil, grease, and an occasional repair part. Such costs will vary based on a given machine’s intensity of use.

Rinker’s four Florida mines load 300 to 400 trucks a day, but the Davenport mine may load 600 in a day. At Lake Wales, Welch estimates his loader’s operating costs for electricity and maintenance at about $600 a month. Other operators report monthly operating costs ranging from a couple hundred dollars to $1,500, depending on type and frequency of use.


Like playing a video game

Cemex Florida Aggregates’ Inglis quarry consists of a 300-acre aggregate pit on a 900-acre site. Miners at the 75-feet deep pit produce manufactured sand. The quarry has two scale houses, one for aggregate and one for sand. Each has its own grapple loader. “We bought the first one in 2004 and the second in 2006,” says Billy Miller, assistant manager. “For the quarry as a whole, we’re gaining a good couple of hours a day in productivity.”

Training a new operator involves very little time and effort, he says. “It takes 15 to 30 minutes to show somebody how to run it, and three or four days for that person to get really good at it,” Miller adds.

Operating the machine involves no heavy lifting, and the hydraulic controls are easy to manipulate. Rebecca Rivers runs a grapple loader at the scale house of Rinker Materials’ Union Sand Mine in Ludowici, Ga. “It’s easy to learn,” she says. “Anyone who can play a video game can operate it. The controls are pretty simple.”

At Dolese Bro.’s St. Helena mine, the operator is Miley’s secretary, Melissa Collins. “She operated it the first day, just took her time,” he says. “You take your foot off the foot pedal or move your hands away from the controls, and it will stop.”

Collins says the loader is “probably easier to operate than a farm tractor. It took me about a week to be really comfortable with it. I started off slowly, but after six months I’ve gotten a lot quicker with it.”

Once the operator is trained, each truck load has the potential to yield boost profits. Milhous estimates that the average truck weight may be off by a ton. Depending on the number of trucks crossing your scales each day, that additional tonnage could have a big impact on the bottom line. “How many times do you want to weigh the same truck?” Rathbun notes. “If it increases the productivity of your operation, if you can move a lot more trucks in a day, a small addition to your fixed costs could make a big improvement in your revenues.”


Article and photos courtesy of Petersen Industries, Inc.



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