November 30, 2010
Cost, convenience, and cosmetics. These were the major concerns facing NRG Systems, Hinesburg, Vt., when the company decided to do something about the dies in its flaring machine.
The company manufactures wind assessment equipment for the wind energy industry, including the TallTower™, which is a tilt-up meteorological system designed specifically for wind resource measurements (see Figure 1). Up to 200 feet tall, the tower consists of a series of large-diameter, thin-wall galvanized steel tubes, mostly 87 in. long, which are joined on-site. The individual tube sections are unpacked, and each tube slides into the next until the tower is the specified height. When completed, the crew tilts the tower up from the ground and into position using a gin pole and a winch.
NRG Systems stresses that the towers are easy to transport to remote sites and easy to set up by the customer’s crews without the need for climbing, cranes, drills, or concrete foundations. This means that the tube sections must go together flawlessly.
Ease of assembly is based on a single machine back at the plant. The company built the machine for one purpose: flaring the tower and gin pole tubes.
The flare is accomplished by a single-piece, conical die. The narrow part of the die enters the tube end, and as the tube is forced forward, the wide part of the die creates the flare.
This, said Richard Kelley, NRG Systems’ manufacturing engineer, is where the problem began to surface. Over time the compressing action—caused by galvanized tube working against the steel die—caused the die to wear. For years the company manufactured the die from hardened tool steel (A2 or D2 steel hardened to Rockwell 60).
“People [at NRG Systems] didn’t think about it much when a die lasted a year or more, but when we got really busy, these things started wearing out more quickly,” Kelley said. NRG Systems uses several dies to flare several tube diameters (6, 8, and 10 in.). On average, Kelley said, each die had to be replaced every three months. Because a replacement die costs $1,000 to $1,100, NRG Systems could have ended up spending close to $10,000 per year for dies alone, had the company continued using the uncoated hardened steel dies.
But the die replacement cost was only one consideration. The labor involved in changing the dies was an inconvenience, Kelley noted. Furthermore, another factor added to the labor cost (and inconvenience): maintenance. As the die began to wear, small surface bumps began to appear on the tool. In turn, the bumps were creating small grooves or scrapes inside the flared tube, and these didn’t help the tubes fit together, Kelley said.
To remedy this problem, Kelley had to dress the tool every couple thousand flares. “I’d take a die grinder and manually grind off the high points so it would flare the tubes smooth again.” With a lifespan of 7,000 cycles per die, each die had to be dressed about three times during the life of the tool.
The combination of cost and inconvenience prompted Kelley to seek some kind of solution. Before joining NRG Systems, Kelley worked for a company that also had experienced a die wear problem, and he acquired some firsthand knowledge of protective coatings.
“We did a lot of ceramic coating and tungsten carbide coating as well,” Kelley said. “Eventually we bought our own flame coating gun and applied the coatings in-house. It wasn’t as good as a commercial coating, but it helped.”
An Internet search turned up several coating companies, one of which was Longevity Coatings, a thermal spray coating firm located in Allentown, Pa0. Mark Purington, owner of Longevity, studied the NRG Systems application and submitted a couple of options to Kelley. Both involved a tungsten carbide coating (see Figure 2).
“This was not so much a wear problem as a friction problem created by incompatible metals trying to slide against each other to perform a task,” Purington said. “I recommended a thin coating of tungsten carbide because of the friction and compression issues.”
Specifically, Purington recommended LONG 944 tungsten carbide. One reason for this recommendation is the coating’s extreme hardness. He pointed out that some other materials, such as ceramic, are just as hard.
“But, typically, ceramic has a high coefficient of friction against steel,” Purington said. The LONG 944 tungsten carbide, on the other hand, is “the most lubricious of all the carbide coatings we make. Its high crystalline content makes it a very slippery coating.”
Hard Versus Slippery. One way to stop wear, Purington explained, is to apply a very hard coating. Another way is to make a very slippery coating— an important consideration because NRG’s application involved compression of two metals.
But the selection and application of the right coating are not the only considerations, Purington pointed out. The way the coating is finished is also crucial.
Finish Versus Microfinish. “A lot of times a finish to the coating is what makes it work in the application,” he said. By way of an example, he cited another customer who had a wear problem. In that case, Longevity applied a very fine finish to the customer’s substrate to prevent particulates from working their way into the substrate and, thereby, affecting wear.
“We worked the finish down so fine that the invading particulates trying to displace the surface had no place to take hold. The particulates were substantively larger than the biggest surface microdepression and had no place to bite in.”
NRG Systems did not need a fine finish; the applied coating is simply polished, Purington explained.
“We diamond-finish it to about a 60 Ra. If required, we could go much finer, down to 1 or 2 Ra. But in this case, it’s not desirable. We try to fashion the finish to the job.” In fact, he added, “a little roughness in this case is not a bad thing. The surface is diamond-finished smooth but with incomplete cleanup. This roughness gives the die less mating surface, leading to a lower level of friction, and that too adds to the slipperiness of the surface.”
On Purington’s recommendation, NRG Systems also switched from A2 hardened tool steel to a less expensive steel. It selected a 4140 steel, a chromium-molybdenum alloy, which cut the die cost from about $1,000 to $700.
Applying and polishing the tungsten carbide coating runs about $300-$350. This brings the cost for a new, coated die to the same price as an uncoated, hardened steel die. However, the coated die lasts nearly three times as long (20,000 cycles versus 7,000 for the uncoated die), effectively cutting the die replacement cost by one-third (see Figure 3). Another benefit is that Kelley no longer has to dress the bumps as he did with the former dies. This, in turn, makes for a nicer-looking product when it comes time to ship the tubes to customers.
“Typically, manufacturing decisions are made based on cost versus performance,” Kelley said, but in this case the company benefited from lower cost and better performance. Longevity was helpful with its recommendations and quick to respond to our issues. The coated tools allow us to produce a consistent-quality product, without constant monitoring and maintenance hassles.”
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