1. Where is a Metcar^® standard dryer bearing used?

The standard dryer bearings that are manufactured by Metallized Carbon Corporation are used mainly in veneer, gypsum board, ceiling tile, and insulation board dryers. After forming, these products must be heated to remove the water added during the forming process. Gypsum board, ceiling tile, and insulation board dryers are long, continuous dryers, with chain driven rollers that carry the boards through the dryer. The rollers are usually about four inches in diameter by at least nine feet long. These rollers weigh about 100 pounds each and have a bearing journal on each end. This is where Metcar ^® standard dryer bearings and assemblies are installed. There may be thousands of rollers in one dryer. The principal manufacturers of these dryers are Coe Manufacturing, AKI/BMH, Flakt, and Babcock Gmbh.

Oil and grease lubricated bearings fail prematurely in these dryers because the heat in the dryer volatilizes oil-grease lubricants. Without the oil-grease lubricant, metal to metal contact causes the balls in ball bearings or the metal sleeve in bronze sleeve bearings to wear quickly. Metcar solid self-lubricating, carbon-graphite bearings have been preferred in this application for more than fifty years.

2. What is Metcar^® carbon-graphite material?

Metcar^® is the trade name for a family of self-lubricating materials that rely on graphite for their self-lubricating properties. These materials are made by bonding graphite powder and other inorganic materials with a molten binder. The material is formed by compression molding and then is fired to very high temperatures to carbonize the binder. The material at this stage is called carbon-graphite and contains a percentage of porosity by volume. This porosity can be filled, by vacuum - pressure impregnation, to improve the lubricating qualities and physical characteristics of the material. The most common impregnates are metals (such as copper), inorganic lubricants, and oxidation inhibitors. When carbon-graphite material is further heated to about 4,000° F, the carbon binder is graphitized, and the material is then called electro-graphite. Electro-graphite materials are also impregnated with inorganic lubricants, and oxidation inhibitors to improve lubricating qualities and physical characteristics. These electro-graphite based and carbon-graphite based materials, both impregnated and unimpregnated, help make up the Metcar^® family of materials.

3. What are the characteristics of Metcar carbon materials that make it preferable to other self-lubricating materials for dryer bearings?

Metcar materials are:

• Low in friction
  
• Wear resistant
  
• Mechanically strong
  
• Maintenance Free
  
• Oil and grease free
  
• Dimensionally stable
  
• Heat resistant (will not melt , soften, or catch on fire)
  
• Easy to assemble into metal casings
  
• Solid and Self-lubricating throughout (not a thin coating)
  
• Economical to use

4. Which Metcar Grade is best for my dryer?

The Metcar Grade recommended for each dryer application depends on the maximum temperature and the load on the bearings. For lightly loaded bearings (under 20-psi load) with the maximum bearing temperature under 700° F, Metcar Grade M-11 is recommended. For dryer bearings with heavier loads (this is most board dryers) and with maximum bearing temperatures of 700° F, Metcar Grade 1515 Copper is recommended for longest wear life. For all dryer bearings where the bearing temperature is between 750° and 1000° F, maximum, Metcar Grade 2500 is the high temperature standard of the industry.

As part of the Metcar^® family of high performance materials, the company's RESISTOX^® grades are specifically designed for high temperature applications such as conveyor type dryers. RESISTOX^® Grade 9800 is recommended where bearing temperatures are 1000° F. RESISTOX^® Grade 9200, with its oxidation resistant coating, is designed to operate at temperatures to 1100° F.

5. What are some causes for premature bearing wear in a board dryer?

A. Misalignment of the bearing assembly

Often times the rollers in a board dryer will become misaligned due to several circumstances. Warping of structural members due to heat or physical stress can cause the surfaces where the bearing assemblies are mounted to move out of alignment. The ground and flooring beneath the dryer may settle or shift causing the dryer to move. Similarly, if the shaft is bent or warped, the bearing and journal are never in alignment.

Whatever the reason, this misalignment can cause premature bearing wear. The shaft journal does not sit parallel to the bore of the bearing, thus causing "point" contact rather than "line" contact. This 'point' contact means that all the weight of the shaft journal is resting in one or two places and may even cause additional force to be exerted due to torquing. When the shaft journal makes full line contact on a bearing surface, the load is distributed evenly and the bearing life will be greatly improved. Metallized Carbon Assemblies are designed with self-aligning features to ensure proper shaft journal and bearing alignment.

B. Excessive wear due to rough shaft surface finish

Carbon materials are soft compared to a steel shaft. If the shaft surface is too rough (greater than 32 micro inch rms) the shaft will scratch, score and grind away at the carbon bearing and cause a high wear rate. Because the carbon bearing can not generate a thick enough film on the shaft surface to cover the roughness peaks on the shaft, the scoring, scratching and grinding will continue until the bearing is worn out. If the shaft has a smooth surface finish (32 micro inch rms or better) the carbon material can burnish the shaft surface with a graphite film and the wear rate will be low. The graphite film on the shaft can cover the roughness peaks on the shaft and prevent excessive wear. The wear rate will be low and the bearing inside diameter will look polished. It is important to select a carbon graphite material that will provide the best wear resistance for the application.

C. Oxidation

At temperatures over 700° F in air, carbon-graphite based materials start to oxidize by the chemical reaction O₂ + C = CO₂. This is the same reaction that occurs when charcoal burns in a barbecue grill. Therefore, to avoid oxidation, Metcar carbon-graphite based grades such as M-11 and 1515 must be kept below 700 degrees Fahrenheit. Metcar electro-graphite based grades, such as 2500, 9800, and 9200 that are treated with oxidation inhibitors can withstand temperatures up to 1050 degrees Fahrenheit for several months without significant oxidation. It is very important to make sure the grade chosen is capable of performing at the maximum operating temperature of the equipment.

6. Why are there so many different housing designs?

The dryer manufactures over the years have not standardized on how the dryer bearings should attach to the dryer structural frame. Housings with vertical mounting, horizontal mounting, "saddle mounting", offset vertical mounting, flush mounting, angle mounting, flange mounting, and "wing mounting" are all used. Also, our four different housing types have been designed for easy cartridge replacement. Our 100 series housings allow the replacement bearing cartridges to be attached with "clip locks" that slide in from the axial direction. Our 200, 300, and 700 series housings allow replacement bearing cartridges to be attached by "clip springs" that attach from above the bearing.

7. Why are there so many different replacement cartridge designs?

Cartridges are designed either solid or split. The solid cartridges are normally used on the idler side of the roller where the bearing can be easily slipped off the end of the shaft. The split cartridges are used on the driven side of the roller where the drive sprocket is "outside" the bearing. The split cartridge allows the bearing cartridge to be changed without removing the drive sprocket. Cartridges are also designed with and without thrust faces. To carry axial load, the carbon material is extended past the end of the steel casing. A collar on the shaft rubs against this thrust face to prevent the roller from moving in the axial direction. Cartridges also come in different lengths depending on the length of the shaft journal and the load the bearing must carry.

8. How long do dryer bearings last?

The wear life of dryer bearings depends mostly on the grade of carbon bearing material used, the bearing load, the shaft speed, the dryer temperature, and the shaft surface finish. Dryers with slow speed, light load, at 500° F, a 32 micro inch steel shaft surface finish, and with Metcar grade 1515 bearings have reported wear life as high as 20 years. Dryers with normal operating conditions usually experience a bearing wear life of about four years. The wear life on most Metcar bearing cartridges can be doubled by simply rotating the cartridge 180° and reinserting it into the housing.

9. Why is selecting the optimum dryer bearing assembly important to the management of a board plant?

There are three board plant-operating costs associated with dryer bearings. First, there is the cost to purchase bearing assemblies and replacement cartridges. Second, there is the cost of maintenance labor to replace worn out bearing cartridges. Third, there is the cost of lost production due to scheduled and unscheduled dryer shut downs to replace worn out bearing cartridges. When these three costs are added together, they can have a significant impact on the operating efficiency of a board plant.

All Metallized Carbon bearing assemblies, replacement cartridges and hardware are designed to increase the Mean Time Between Maintenance and reduce the down time required to maintain the bearings.