October 4, 2010
When mining equipment manufacturer Bucyrus, Houston, Pa., redesigned its line pans for coal removal, it increased the size and thickness of the equipment. The redesign provided the perfect opportunity to move to an automated welding cell to keep up with the additional 150 lbs. of filler metal that would have to be added to each line pan.
Bucyrus International Inc. knows something about earthmoving. Its products have been used in huge excavation projects all over the world, even in the creation of the Panama Canal.
When it came time to redesign its armored face conveyor (AFC) product line almost five years ago, the company’s engineers knew it had to present beefier and more robust conveyor components. The longwall conveyor equipment routinely is exposed to the battering involved in removing coal from the face and up and out of the mine. The goal was to have the conveyors move as much as 5,000 tons of coal per hour out of a mine—and survive to do the job again the next day.
Upgrading the conveyor components meant designs using heavier plate sections and tougher grades of wear-resistant steel. Those designs meant much more welding time would be required. The additional welding time meant higher production costs—if Bucyrus maintained a completely manual approach to welding.
Bucyrus kept the cost down by keeping its eye on automation. In 2006 the company invested in automated welding cells to produce its conveyor components in a cost-effective manner.
The most effective way to illustrate the impact of the robotic welders in the fabrication shop of Bucyrus‘ 120,000-sq.-ft. plant in Houston, Pa., is to focus on the line pans, the backbone of the AFC. As many as 170 pans run on a conveyor face. When the plant’s fabrication shop was presented with the new, more durable AFC design, manual welding the pans would have required twice as much time in the weld shop compared to the previous design. The reason: The new design requires two to three times more weld metal.
However, according to Bucyrus welding engineer Derek Hall, thanks to automated robotic welding on the new pan designs, labor time actually has dropped by 25 to 30 percent.
“The new design, which allowed us to double the AFC’s warranty life,” Hall said, “is significantly beefier, using, for example, a 40-mm-thick bottom plate for the pans compared to a 30-mm plate before.”
The redesign also called for new abrasion-resistant steel, moving to AR450 from AR400, and the inclusion of a replaceable inserted wear tub that allows the miner to replace the tub in the existing pan frame—a feature that didn’t exist in the previous design.
“All of this adds up to 150 lbs. of weld metal needed to fabricate each pan,” Hall said. “Investing in robotic welding seemed a logical choice.”
Two C profiles connected by a base plate comprises the frame for each pan. The profile sections—roughly 6.5 feet by 1 ft.—are welded to the bottom plate of the top trough—about 6 ft. by 4 ft. by 1.5 in.—and then welded to the pan base. This design allows for the separation of wear parts and structural parts and the easy removal and replacement of the worn top troughs.
Wear parts are made from very hard and wear-resistant materials, such as Hardox® 450 from SSAB, which has HB 425-475 hardness and 205-KSI tensile strength (see “Welding Hardox Wear Plate”). The structural parts are fabricated from high-strength quenched-and-tempered steels, primarily A514 T1 or its equivalent.
Pan-assembly components come from the plant’s burn shop, which houses a pair of large oxyfuel cutting machines, as well as a plasma arc cutting machine for plate to 0.75 in. thick. The larger of the two OFC machines runs as many as five cutting torches simultaneously; the second, smaller machine powers a pair of cutting torches.
After the blanks are cut, electrically driven carriages with oxyfuel torches mounted on them are used to burn bevels as part of weld prep. Additionally, an oxyfuel cutting robot is used to prep nonlinear weld joints.
To automate the pan welding process, Hall and his team specified two identical twin-robot welding cells from Motoman Robotics, West Carrollton, Ohio. Each cell houses a pair of six-axis, extended-reach robots, capable of approximately 10 ft. of horizontal reach, mounted on a 9-ft.-long servo-driven base track. The setup optimizes access to the 6-ft. by 5-ft. by 14-in. weldments (see Figure 1).
Before robotic welding begins, each conveyor pan, which weighs 3.5 tons once completed, is fixtured and tack welded manually. Four employees on each of three shifts are involved in the fitting and welding of the huge weldments.
Once complete, the weldments are then rolled over to one of two nearby heating stations where metal is preheated to 125 degrees F in a closely controlled process. The preheating is done to prevent hydrogen cracking during the finishing welds.
After preheating, the automated welding takes place in two stages. In the first of the two robotic welding cells, the intermediate deck plate, bottom plate, C profiles, and forged connectors come together to form the core pan. The process takes 3.5 hours of nearly continuous welding and deposition of 90 lbs. of weld metal.
For the most part, the robots lay down groove welds, depositing as many as 10 passes per joint. The robots move in tandem to weld simultaneously on each joint, one starting at the end of a joint and the other in the middle, and both robots traversing in the same direction (see Figure 2).
“After each pass the robots are programmed to head to a nozzle-cleaning setup,” Hall said. “And after three or four passes, we program the robots to pause while an operator deslags each weld surface to ensure we get good, repeatable arc starts.”
The completed core pan then moves to the second cell, where another set of welding robots spend 90 minutes joining several accessories to the core pan, depositing a maximum of four weld passes per joint. On the other side of the cell, the robots work in tandem for 40 minutes to weld up the replaceable tubs. All told, the welding robots in the second cell add 60 lbs. of weld metal to the pan assembly.
Finally, tub and core pan come together after seven hours of manual welding, which occurs outside of the robotic welding cells.
To keep up with orders, the robotic cells run three shifts per day. The goal is to complete six pans per day.
Both robotic welding cells burn 0.045-in.-dia. weld wire, ER100-S1 from ESAB, with a 90 percent argon/10 percent CO2 shielding gas mix. The gases are kept in bulk tanks outside the plant and mixed on-site.
Weld power comes from big transformer-rectifier power supplies. Water-cooled GMAW guns—custom swan-neck models from Tregaskiss—deliver wire from 550-lb. barrels. The typical weld procedure calls for power sources set at 350 amps, 31.5 V, a wire travel speed of 14-18 IPM, and weaving on cover passes.
Even with the robotics, good quality isn’t assumed. Once manual assembly welding is complete, the plant’s certified welding inspectors take over (see Figure 3).
“The core pan is critical,” Hall said. “It gets loaded hard and sees a lot of bending and twisting in service. A few years ago, we’d see weld rejects from ultrasonic testing on almost every pan, adding several hours of excavating and manual rewelding.”
The main cause of rejects in the pan was a lack of fusion in the welds, according to Hall. Two new quality procedures have helped to bring reject levels down to the point where Bucyrus needs to repair welds in only one out of 20 pans.
“The first of these procedures is a new touch routine that Motoman Robotics’ engineers helped us develop. It allows the robots to accurately locate the start point of each weld. Our fixtured assemblies, due to the tolerance stackups and to the distortion imparted by preheating and by depositing the multipass welds, can cause weld-joint location to vary by as much as 0.5 in.,” Hall said.
“We consider the touch routine developed proprietary,” he added, noting that developing the procedure to accurately locate the weld joints in space “is the only way we could have gotten the process to work.”
Also key to such a dramatic reduction in weld rework has been the weld shop’s use of what Hall called a “shakedown.”
Once per week, during production welding of the core pan, the cell controller temporarily halts welding after each pass to allow an operator to inspect the welds. The programmer looks at each weld pass, checks for torch alignment, and adjusts the taught points as needed. His record of each adjustment then is used to calibrate the equipment.
“While this shakedown procedure adds as much as 90 minutes to the weld time for that one assembly, the payoff in improved weld quality has been significant,” Hall said.
SSAB offers plenty of advice about welding Hardox wear plate at its Web site, www.hardox.com. Here are some rules of thumb to avoid hydrogen cracking:
Minimum recommended preheat and interpass temperatures for Hardox 450 (used by Bucyrus) is 125 degrees C for plate to 40 mm thick and 200 degrees C for plate 40 to 80 mm. Maximum recommended interpass temperature is 225 degrees C.
In high-humidity conditions, increase the minimum recommended preheat temperature by 25 degrees C.
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