Corrugated packaging is one of the most economical, rugged and environmentally sound ways to ship goods. More than two-thirds of the world’s retail commodities, from boxed chocolates to heavy industrial components, are transported in corrugated boxes. The efficient and cost-effective production of containerboard depends on good lubrication. Without it, the entire corrugating production line would come to a screeching halt.
This article focuses on maximizing the life of heated roll bearings in single facers, the machines at the heart of the corrugating line. Single facers press the flutes into paper, which is then sandwiched with glue between liners and eventually cut and shaped into cartons. Interchangeable rolls - typically six- to nine-feet long, steel rolling pins with ridges - actually press the flutes into the paper. These rolls have a large industrial roller bearing on each end and are constantly exposed to steam heat, making the paper more malleable.
The life expectancy of the rolls depends on many factors, such as the amount of recycled paper in the product, the type of coating on the roll and number of shifts at the plant per day. The rolls generally will last from one to five years. Maintenance managers strive to make the heated roll bearings last the life of the rolls. Because heated roll bearings are designed to survive 40,000 hours of continuous operation, they should outlast the roll. In reality, however, the premature failure of these heated roll bearings accounts for many hours of unplanned downtime and lost production. Often those failures can be traced directly to the lubricant.
Approaches to Wear Prevention
Bearing wear is caused by surface-to-surface contact. The primary function of a lubricant is to create a film of oil between the rolling element and the raceway. Even when using a grease, which is formed by combining an oil with a thickener, it is still the oil that provides the lubrication. The rolling element actually rides on a protective film of oil and is not subjected to wear-inducing, metal-to-metal contact. In the high-temperature operating environment of heated roll bearings, the oil must be stable. As temperatures increase, oil becomes thinner. The thinner the oil film, the less bearing protection. Because some oils are more volatile than others, they actually burn at high temperatures. This leaves carbon deposits in their wake, which accelerates bearing wear and damages lip seals, creating safety and environmental hazards. Proper lubricant selection has recently become more critical with the increased use of tungsten carbide as a coating on rolls. Compared to the more traditional chrome coating, tungsten carbide can double the life of the rolls, placing even greater demands on the bearing lubricant.
Traditionally, corrugating plants have relied on mineral oils to lubricate heated roll bearings, first with a drip system and then with a force-fed, closed-loop system introduced in the early 1980s. In fact, for single facers manufactured before 1988, there were few, if any, alternatives to petroleum oils or greases. Mineral oil, however, is not the optimal lubricant for high-temperature applications. It begins to degrade at or before 210°F (99°C), well below the operating conditions of a heated roll bearing, which can easily reach 350°F (177°C) and higher. To compensate for its high-temperature limitations, single facers lubricated with mineral oil require elaborate auxiliary systems to keep the oil circulating and to keep its temperature low.
Though they are maintenance-intensive, these auxiliary systems make mineral oil an acceptable lubricant for heated roll bearings, as long as all system components are operating properly. When they don’t function properly, petroleum-based oils can become messy and create unwanted consequences. For example, a clogged line can cause oil trapped in the bearing to heat up and degrade. Carbon debris, a byproduct of burning oil, accumulates and will eventually wear out the lip seals. The oil, in turn, runs down the side frames, onto the floor, posing in-plant safety hazards. Eventually, it finds its way into the sewer system, the equivalent of a hazardous waste spill. Simply said, a circulating oil system in this application is labor-intensive. Because the circulating system has several components, the chance of failure increases, jeopardizing the reliability of the bearings and corrugating line.
Current lubricant technology offers an alternative to mineral oils to help bearings last the life of the roll. Introduced to the corrugating industry in 1988, perfluoropolyether (PFPE) oils, thickened into a grease with nonorganic olytetrafluoroethylene (PTFE), provide superior wear protection for heated roll bearings. In fact, most OEMs now use PFPE/PTFE greases in new single facers, and many plants are replacing older mineral oil-based lubrication systems with PFPE/PTFE grease.
Several factors make PFPE superior to hydrocarbon oils for the lubrication of heated roll bearings. PFPE withstands high temperatures, hostile chemicals and volatile atmospheres. Unlike hydrocarbons that begin to degrade at 212°F (100°C), PFPE is chemically stable to 650°F (343°C). It will not burn and does not react with oxygen. When it does begin to break down, the byproducts vaporize, leaving no gummy tars or deposits. PFPE grease, which forms its own seal around the bearings, also resists common solvents and long-term contact with steam and boiling water. PFPE’s density also contributes to its superiority as a severe-service lubricant. About twice as dense as the typical hydrocarbon lubricant, PFPE provides superior load-carrying capability and film-forming characteristics, critical factors in extending the operating life of a bearing.
PFPE grease brings at least three other benefits to a corrugating plant’s maintenance program. Compared to mineral-based lubes, PFPE reduces maintenance. With a PFPE grease, the entire auxiliary system used to keep lubricating oil cool and flowing freely is no longer needed. Second, PFPE grease improves in-plant safety and housekeeping immediately by eliminating the potential for oil leaks and the environmental hazards that accompany them. Third, roll changes are quicker, because PFPE leaves no carbon deposits to clean from reusable single-facer components.
A Case Study from Brazil
Despite the advantages of PFPE, the cost of converting a single facer from oil to grease is sometimes too high for many companies to manage. Some single facer OEMs charge up to $20,000 for the housings and seals necessary to convert heated roll bearings from circulating oil to synthetic grease. Then there’s the cost of the grease. Some single facers require as much as 36 pounds of PFPE/PTFE grease per fill, about $2,700 per machine. This does not include top-ups during operation.
Though these costs are not small sums in the United States, they seem even larger in Brazil. Consider the case study of a corrugating plant: Ibéria Industria de Embalagens Ltda. The company’s 26,500 square-meter plant is located in Aguai, a city in the northeastern sector of São Paulo, Brazil. São Paulo’s per capita income is only $8,300, a statistic that served to magnify the conversion budget proposed by Ibéria’s maintenance supervisor, Olivio Mattiazzi.
Ironically, Mattiazzi’s track record didn’t help sell his conversion proposal to company management. He did an excellent job keeping his two 1995-vintage, 97-inch single facers operational. The oil-lubricated single facers were able to produce 6,500 tons/month at an average speed of 9,000 linear meters/hour, despite more than six years of service in a plant that operates around the clock, seven days a week.
“I knew we could improve productivity if we converted to grease,” Mattiazzi said. “The circulating oil system was very labor intensive. We always had to watch closely for electrical, pneumatic and mechanical failures - any of which could stop production. The system required hydraulic and electronic components that had to be imported, making them even more costly.”
But all these were the hidden costs of maintaining an oil system, and such costs are difficult to quantify.
Mattiazzi found a less expensive way to convert from oil to PFPE/PTFE at Capma Ltda. in Jundiai, São Paulo. Capma represents several international firms that supply products to the corrugating industry, including FARO Bearings of Sarasota, Florida and Nye Lubricants in Fairhaven, Massachusetts. By combining products from FARO Bearings and Nye Lubricants, Capma paved the way for the conversion at the Aguai plant.
In the early 1990s, a specialty-bearing manufacturer in Italy recognized that many heated roll bearings in single facers required an unusually large amount of expensive grease. Company engineers noted that most of the grease fills the housing, and that the bearing itself requires a much smaller volume of grease for proper lubrication. With this in mind, the manufacturer designed and introduced the first shielded versions of its standard corrugating roll bearings. They contain stamped metal shields around the open bearing. In addition to keeping wear-inducing contaminants out of the bearing, these shields confine the PFPE/PTFE grease to the bearing, eliminating the need to fill the entire housing.
In a typical single facer, for example, the shielded bearings reduce the amount of PFPE/PFTE grease needed by 40 to 70 percent. Importantly, these shielded bearings are designed to fit existing housings, so they eliminate the cost of re-machining the housing. Further, because the housings are not filled with grease, roll-changes are faster and cleaner.
The combined efforts of the suppliers provided the Brazilian plant with an economical conversion scheme. Instead of paying more than $10,000 for new seals, flanges, grease, bearings and other components needed for its oil-to-grease single facer conversion, the plant paid less than $6,000 per single facer - a budget approved by plant management. The shielded bearings also keep the cost of PFPE/PTFE grease to a minimum. Since the conversion, Mattiazzi uses only one tube of grease for each pressure roll bearing during assembly. Every two months, one more tube is used to relubricate all six heated roll bearings. “This new system cuts lubrication expenses about 33 percent and we no longer have to dispose of about 1,200 liters of oil every year,” Mattiazzi said.
“With the higher-temperature capabilities of the PFPE grease, we actually set new speed records for the plant just two days after the conversion. The bearings work more smoothly and with less vibration, thereby allowing higher velocity on the machines, and increasing production. Our single facers now average 9,650 linear meters/hour, nearly eight percent faster than with the oil lubrication. Further, after the conversion, the single facers average 8,154 tons/month, a 25 percent improvement over the old lubrication system.”
Since the change from oil to grease, Mattiazzi’s plant also eliminated the problem of oil contamination in the work place. It also realized savings in hydraulic equipment, electronic components, maintenance, and treatment and clean up of the oil, he said.
Bringing the Message Home
In the United States alone, hundreds of corrugating plants continue to use old oil-based single facers as primary equipment. Many more plants continue to maintain oil systems as their secondary lines. Despite the high maintenance and environmental dangers associated with oil systems, conversion costs have been considered prohibitive. The cost savings demonstrated by this case study offer a tested plan for similar plants to take a closer look at conversion. In an industry that operates on high volumes and low margins, small changes can make a big difference. Changing the oil in a single facer’s heated roll bearings - once and for all - is one change that’s worth investigating.