ONLINE EXCLUSIVE: Conveying Efficiency
Dedicated to producing light-density building insulation ranging from R11 to R38 in both kraft-faced and unfaced batts and rolls, K21 offers the flexibility to change products quickly or run different lengths and widths simultaneously. Combining K21's capability with two conventional 6-ft-wide production lines K11 and K12, the Kansas City plant annually supplies enough material to insulate about 1 million homes, plus a variety of specialized insulation products for metal buildings and other commercial applications such as HVAC duct wrap. With 1.7 million square feet of manufacturing space, it's the biggest of four CertainTeed insulation plants in the U.S.-all part of the diverse global enterprise of France's Saint-Gobain Group-and ranks among the largest and most modern insulation plants in the world.
In this business, belt conveyors are the assembly line and are expected to run 24/7. Production starts with molten glass, which is spun into fibers, sprayed with a thermosetting resin binder, and then deposited across a collection belt. The depth of fiber deposited, which is controlled largely by varying belt speed, determines the density of the insulation. This raw fiber matt transfers onto a steel "chain" conveyor for passage through a tunnel oven. There, a similar chain belt lowered from overhead compresses the matt to the desired thickness, ranging from 3.5 to 12 in., while heat up to 600ºF (316ºC) cures the binder to lock in that maximum dimension. The resulting combination of fiber density and matt thickness largely determines the insulation's R-value (resistance to heat flow). The product stream then returns to belt conveyors and stays there until packaged and unitized for shipment.
With so many places where problems can occur and spoil its volume-production advantage, K21 challenged maintenance management to find ways that would minimize the constant threat of belt failures, and would also provide the fastest, easiest way to recover from those that evade prevention. Today, a belt availability of around 97% shows that challenge has been well met.
Belts Vulcanized LengthwiseThroughout the plant, the belting is generally 1/4- or 3/16-in.-thick, two-ply construction with a smooth PVC top cover and a woven polyester bottom cover for low friction, riding on steel slider beds. K21 built upon CertainTeed's earlier experience pioneering 8-ft-wide production lines at its Chowchilla, Calif., plant, which demonstrated that increasing output with wider lines incurs a disproportionately small difference in front-end energy use-the industry's major expense-thereby yielding more product at lower unit cost. But while K21 pushed economy-of-scale to new heights, it also pushed belt widths well beyond commercially available standards. To form the initial fiber matt in a 12-ft-wide stream, belting had to be custom-fabricated by longitudinally vulcanizing a 118-in.-wide belt-the biggest standard width readily available-along one edge of a 30-in.-wide belt. The resulting 148-in.-wide stock then could be trimmed to satisfy the line's need for several different widths in the 10- to 12-ft range.
An even bigger challenge was that custom-fabricating the 24-ft-wide belt required longitudinally vulcanizing a 52-in.-wide belt between two 118-in.-wide belts, forming a single slab 106 ft long. This width is essential because after the 12-ft-wide fiber matt is cured, it passes through slitters that create multiple streams of specified batt widths, which must then be spread out by separations of 6 to 9 in. for the application of Kraft facing. For that, the streams transfer onto the 24-ft-wide "diverging belt," where overhead-mounted plows gradually wedge them apart as the belt moves them forward. The sideways-sliding movement of multiple batt streams requires the diverging conveyor to have a continuous, smooth crosswise surface.
After separation, the batt streams transfer onto a side-by-side pair of 12-ft-wide belts by bridging a gap through which streams of Kraft facing can feed up from below. One side of the Kraft is coated with molten asphalt, and with that side up, the facing streams arch over the receiving belt's tail pulley so the batt streams arrive centered on top of the sticky asphalted surfaces. Batt streams and facing then pass together through what's called the "tractor section," where an overhead pair of 12-ft-wide belts presses the batts onto their asphalted facing for proper bonding.
The two 12-ft-wide belts then angle away from each other and later sub-divide their cargo onto smaller belts. These ultimately guide the streams to various machines that chop and form measured portions into individual batts or rolls, then pass them to packaging stations on a network of narrower belts ranging down to 9 in. wide.
Depending on the belt speed needed to meet the density specification at the front of the line-which can vary from 50 to 350 feet per minute-the entire process can take from nearly half an hour to less than four minutes.
"That's if everything runs smoothly," quips Maintenance Manager Don Roberts. "K21 is a handful-you can easily get into trouble. There are a lot of ways to lose a belt, mostly unpredictable."
Tracking always headed the list of problems, especially on the wider belts, he recalls. However, tracking has been greatly improved by adding V-guide "profiles"-strips of rubber with a trapezoid-shaped cross-section 7/16-in. high, bonded onto the underside of the belting along both edges. These profiles overlap both edges of the conveyor's steel slider beds and pulleys to restrain the belt from migrating toward either side. Edge profiles were favored over center guides to avoid the expense of retrofitting the line with specially grooved pulleys and slider beds. Before the profiles were added, off-tracking damage held belt availability down around 87%.
"Even with profiles, belts still can track off and get torn up as a result of fiber wads building up on end pulleys or a piece of profile breaking away," Roberts notes. "Belts also can be damaged when workers cut jammed materials out with utility knives. The fiber spinners also sometimes drop globs of molten glass that melt big holes through the belts. We had one incident where a plow in the diverging section slipped too low and ripped open our 24-ft belt."
On the K21 line, it's still not uncommon to replace more than a dozen belts in a year, sometimes considerably more. "Months might pass with no problems, or we might replace five or six belts in one week. Whatever happens, we just have to be ready to deal with it fast."
Fasteners Faster, CheaperRoberts explains that when the K21 line was built, most belts started with vulcanized splices. "That makes sense for first-time installation because at that point we usually have time to vulcanize, and if done right a vulcanized splice should last as long as the belt itself. In reality, though, the belts often don't last very long," he says. "Once production is under way, almost any of those belts can get damaged suddenly. Then, we can't sit and wait for a vulcanizing contractor to drive out here and get set up. We never know how long that might take, but we know that once the crew is here and everything is ready, the cutting and cooking will still take at least six hours after that. So there's a lot of downtime involved in vulcanizing, in addition to fairly high cost."
According to Roberts, a much better solution is a hinged-plate splicing that attaches to the belt with staples. "This is much faster and less costly than vulcanizing, its wide-plate surface stands up to slider-bed wear much longer than other styles of mechanical fasteners, and our own people can install it quickly. It gives us a lot more flexibility in figuring out the fastest way to get a downed line running again," Roberts explains.
The company uses Alligator Ready Set™ Staple fasteners supplied by Flexco, which feature top and bottom plates approximately 1-in. wide, with two high-tensile staples pre-staked in each top plate. The top and bottom plates are joined at one end by two hinge loops to form single-piece fastener segments that sandwich the belt ends. A continuous strip of these segments is fitted across both belt ends and installed by hammering the staples through both plates and the belt. Oriented crosswise to the belt, the staples penetrate between rather than through load-bearing carcass fibers, and clinch into protective pockets on the bottom plate's underside, compressing both plates into the belt. Designed for pulley diameters as small as 2-in., the splices pass easily around CertainTeed's smallest 6-in. pulleys.
Belt tensions are minimal here, with even full belts carrying relatively light loads as insulation averages around 150 grams (5.3 oz.) per square foot. "With such low tensions, splice hinge pins rarely need replacement," Roberts notes.
Unlike vulcanized splices, in which internal deterioration can remain hidden until it turns into a surprise failure, mechanical fasteners allow wear or damage to be monitored, so pre-emptive replacement scheduled for a convenient time can avoid suffering surprise downtime at the wrong time.
For insurance against serious downtime, Roberts keeps a spare belt for each conveyor on all three insulation production lines, each one cut-to-length with the splicing pre-installed on both ends. "Very often the fastest way to get the line running again is to simply pull in the spare," he says. "With the fasteners already in place, we just hinge-pin the spare onto the end of the old belt as we run it out of the conveyor. When the new belt meets up with its trailing end, we mesh and pin the hinge loops, and we're back in business. Then we can repair the old belt off-line more conveniently or make a new spare." For that, the plant also stocks a supply of 12-ft-wide belting plus rolls of smaller widths.
A pre-vulcanized spare belt wouldn't help much, Roberts points out, because installing it would require taking the conveyor apart, reassembling it, then most likely retraining the belt.
"If only the splice is damaged, the fastest fix might be to cut the splice out and replace it on-line, especially on the longer belts," he continues. "Our maintenance crew can cut out and replace a Ready Set in one to two hours, depending on belt width."
The splicing system used on the K21 line is estimated to trim installation time by more than 20% compared to conventional staple-attached designs, because the continuous-strip format automatically sets proper spacing and alignment, while pre-staked staples eliminate the time-consuming steps of loading a staple dispenser and injecting staples into installation tooling. Pre-staked staples also ease inventory and purchasing efforts, and help minimize installation errors by eliminating the need to match staple sizes with fastener sizes.
"Sometimes only a short portion of the splice gets torn up, smashed or bent when a belt tracks off and starts curling over the edge of its rollers," Roberts observes. "The Ready-Set system lets us go in and replace just the damaged parts quickly. Hammer installation with small, portable tooling makes on-line repairs easy-sometimes even without fully separating the rest of the splice."
Although produced in strips, the fastener plates are defined by breakaway bridges so the needed number of segments can be broken off easily by hand.
Distributor Stocks Back-UpsFor the 24-ft-wide belt, custom fabrication requires weeks of lead time, so a second spare is stocked at the Kansas City headquarters of belting distributor IBT, which coordinates the lengthwise fabrication for CertainTeed. "If a belt's going to track badly or fail for some other reason, it usually does so in the first few hours of running, so it's wise to have a back-up for the back-up," says IBT Rubber Products Manager Dan Paustian.
For the 12-ft-wide belting, IBT also stocks a supply adequate to cover CertainTeed's longest single belt as back-up for the 10- to 12-ft spares and extra belting kept at the plant. As one of only a few belt shops in the Midwest equipped to handle and vulcanize belts up to 9.8-ft (3 m) wide, IBT slits and supplies the material for both 12-ft and 24-ft fabrications.
According to Paustian, vulcanizing a finger-splice into belts wider than IBT's 3-m press must be done in more than one "cook," or with a traveling-platen press designed to move across the belt. "The process is difficult and slow, and there aren't many traveling-platen presses around," he explains, "so bringing one in along with people who know how to use it would cost two or three times the price of these already expensive belts."
Wide belt vulcanizing also is potentially wasteful, he adds. "If the finger splice comes out just a little crooked, it won't track and has to be redone. Each vulcanized splice can consume a foot of belting material, which limits the number of times each belt can be vulcanized." That's rarely a problem with mechanical splicing, which consumes only a few inches of belt, and allows an economical way to restore belt length or replace damaged sections with inserts if necessary.
"From a maintenance standpoint, a large-scale continuous process like our K21 line would still be possible without a long-wearing mechanical splice like Ready Set, but it wouldn't be very practical," Roberts agrees.
For more information about CertainTeed's K21 line, contact Donald D. Roberts, CertainTeed Corp., (913) 342-6624, ext. 380, e-mail firstname.lastname@example.org , or visit http://www.certainteed.com .
For more information about mechanical splicing, contact Flexible Steel Lacing Co. (Flexco), 2525 Wisconsin Ave. Downers Grove, IL 60515-4200; (630) 971-0150; fax (630)971-1180; e-mail email@example.com .; or visit http://www.flexco.com .