Performance Roof Systems in Kansas City, Mo., experienced continual problems with bearings in a heated auger machine used in the production of industrial roll roofing. Temperatures of approximately 500°F caused lubricants for the ball bearings to quickly evaporate.
From that point, failure was only a matter of time. Tired of shutting down the machine to change bearings, maintenance supervisor Clayton Stiverson searched for alternatives and finally found a bearing material based on a self-lubricating graphite/metal alloy that provides a low coefficient of friction without any lubricant.
Figure 1. High temperatures and a dirty and dusty environment caused bearings on an auger machine to fail rapidly
"The new bearings have lasted 18 months and show no signs of wear," Stiverson said. "By eliminating the need to lubricate and replace the bearings, we're saving a considerable amount of money in maintenance downtime, labor and replacement part costs."
The heated auger machine plays an important part in the production of industrial roll roofing. To maintain the high temperatures required at that stage of production, hot tracing oil is continually circulated through a tube that runs through the center of the bearings. (Tracing oil is oil used to transfer heat.)
The oil that is removed is heated back to a high temperature before being pumped back into the machine. The bearings originally supplied with the machine continue to be maintenance problems. The machine has four pillow block bearings and each bearing cost $435 to replace. The cost of the labor and machine downtime added to the total cost of replacing the bearings.
"I replaced six bearings over a four-year period," Stiverson said. "In most cases the bearings failed almost as soon as they were installed. It was not hard to tell when that happened because soon after the lubricant evaporated, the bearings began to shriek. If we had the time and money to spend on bearings, we could have replaced them nearly every week.
With the machine running at a relatively low rpm, we were able to operate for quite a while even after the bearings failed. But eventually we had to replace them before they locked up completely. We tried to head off this problem by lubricating the bearings more frequently. But that didn't seem to make any difference. Whatever lubricant we applied quickly dried out and caked up."
Figure 2. Each time maintenance personnel had to replace a bearing, they had to work around obstructions. This
Stiverson researched various lubricants in an effort to find one that would stand up to the demands of the application. "I tried a number of different oils and greases, including some rated to 1,000°F," Stiverson said. "But they rarely lasted longer than 24 hours. The oils would typically evaporate and the greases would cake up and get hard as a brick. Interestingly, the lubricant that worked the best was 30-weight automobile engine oil.
But even this lubricant failed fairly quickly. It just couldn't withstand the exposure to high temperatures plus the dirt and sediment produced in the roofing process. After a while, the oil would assume the consistency of molasses and from that point the bearings would fail pretty quickly."
Next, Stiverson considered other bearing materials. Traditional nonlubricated bearing materials, made of polymers, have no chance of standing up to the high temperatures involved in the application. But he continued to search directories and trade journals for a material that could tolerate the combination of heat and dirt encountered in this application. Then he discovered Graphalloy® bearings from Graphite Metallizing.
"The advertisement claimed the bearings didn't require lubrication and could handle temperatures much higher than what we were running," Stiverson. "I was skeptical but decided to get more information."
Figure 3. Side view of auger shows from another angle how difficult it is to reach the bearings.
Stiverson talked to an application engineer from Graphite Metallizing about the problem. The engineer recommended Graphalloy®, a bearing material that consists of graphite filled with a metal to enhance the chemical, mechanical and tribological properties of the material.
The graphite structure can be compared to a deck of cards with individual layers which easily slide off the deck. This characteristic gives the material a self-lubricating ability, eliminating the need for external lubricants.
Graphalloy® requires no grease or oil, survives run-dry conditions, and eliminates galling and seizing in hot and dry conditions. The bushing material is ideally suited to applications where temperatures are too high to permit the use of oil or other lubricants because there are no lubricants to congeal or solidify.
Graphalloy® will not soften at high temperatures or extrude under load. Many grades are suitable for temperatures to 750°F in air, where oil-based lubricants burn off or oxidize and plastics fail. Special grades give good service up to 1,000°F and higher in nonoxidizing atmospheres.
The material provides additional properties beneficial in other applications. It maintains its integrity when submerged in hostile liquids such as acids, alkalies, hydrocarbons, black liquor, and liquid gases. The material provides a constant, low coefficient of friction rather than just a surface layer, helping to protect against catastrophic failure.
Lubrication is maintained during linear motion; lubricant is not drawn out and dust is not pulled in. Graphalloy® wear components also improve reliability under conditions such as low-speed operation, frequent starts and stops, and switch-overs from standby to continuous running.
Graphalloy® bushings are available in more than 100 grades of material in any desired size or geometry, including cylindrical with or without grooves, flange or double flange, split and metal-backed.
Figure 4. Graphalloy® Bushings
Stiverson spoke to the manufacturer and ordered four bearings in order to evaluate the performance of the material. The ball bearings originally used in the application had an 80 mm inside diameter, 37/16 inch outside diameter, and were 5½ inches long to fit the split pillow block in the auger machine.
"The decision to order all the bearings was easy because everything leading up to this didn't work," Stiverson said. "This wasn't guesswork. Our department has 90-plus years of experience in the maintenance field. We were confident we chose the right bearings.
The company did not offer this exact size as a standard product but provided a special item that met our requirements. These new bearings installed easier than the old ones. We put them in and hoped for the best. The bearings continued running with no problems. After a year, we inspected the bearings and could not see any visible signs of wear. Since installation, we've never removed the new bearings."
The new material proved successful in this application. "By eliminating the need for regular maintenance, the new bearing material has simplified the job of maintaining the auger," Stiverson said. The Graphalloy® bearings resist dirt and sediment because they are not lubricated. The grease used in the previous bearings collected dirt and sediment.
"This allows the maintenance staff to focus on other issues. The new bearings run much more quietly than the ones used in the past. We are satisfied with their performance and will consider them for other high-temperature applications as the need arises."