Market forces and increasing regulatory demands have exerted considerable pressures on pulp and paper operations. As a result, mills are striving to achieve maximized equipment uptime and optimized plant productivity by focusing on process efficiency and machine reliability.
Turning to critical components that will influence the efficiency and reliability of pumps and machinery, pulp and paper plants are realizing the vital part rolling element bearings play in the papermaking process.
Larger paper machines, for example, can incorporate as many as 1,500 bearings. Regardless of their number, location and particular function in an application, the reliability and service life of bearings are inextricably linked to maintenance and operating conditions.
Several conditions can result in premature bearing failure, including excessive sliding of surfaces (causing abrasive wear), improper loading combined with inadequate lubrication (leading to a severe type of wear called smearing) or the ingress of water (causing corrosion/tatic etching).
Abrasive wear is caused by the ingress of debris particles or inadequate lubrication. The nature of the foreign particles (hardness and size) and the severity of the contamination will influence the degree of material removal caused by abrasive wear. This wear results in removal of the original bearing surfaces and, at a macro level, a change to the internal bearing geometry. Increased friction and elevated operating temperatures usually accompany abrasive wear, resulting in bearing failure from surface initiated fatigue. Similarly, inadequate lubrication can result in asperity contact between rolling elements and raceways.
While the design of a bearing minimizes macro sliding contact points (typically limited to interaction between rolling elements and cage pockets or roller ends with flanges), some sliding in a bearing's rolling contact occurs because of geometry and elastic deformation. In most cases, this sliding is minimal and is often referred to as a micro slip.
Micro slip can occur in spherical roller bearings, which are the most common type of bearing in paper machines. The phenomenon exists in the raceways' contact with a spherical roller element, since pure rolling exists at only two points of contact: between the roller's inner and outer ring. Outside these points of pure rolling, micro slip occurs. The same phenomena are also present in ball bearings.
Avoidance Actions: The best ways to avoid abrasive wear in paper machine bearings are to use proper oil film or improve the cleanliness of the lubricant with filtration. Using a lubricant with anti-wear properties may also improve boundary lubrication present at startup and increase operating speeds.
Adhesive wear, or smearing, is the transfer of materials that occurs between two contacting surfaces that slide in relation to each other. Adhesive wear is often the result of light operating loads (below requisite minimum calculated values) or instantaneous acceleration of large rolling elements upon entrance into the bearing loaded zone.
Such damage is also associated with localized high temperatures. Adhesive wear may also occur between roller ends and flanges if lubrication is inadequate and metal-to-metal contact develops between these sliding surfaces. Smearing is an abnormal type of wear that develops between the rollers and raceway, typically found entering and exiting the load zone when the oil film is broken. This can occur on lightly loaded, high-speed applications, as well as on slow-speed, heavily loaded applications.
Smearing is undesirable because the affected surfaces become progressively rougher, increasing surface stress in the damaged regions. This added stress results in higher vibration levels and excessive heat that can alter the metallurgical structure of the steel in that region of damage. Unless addressed, the cycle continues toward premature bearing failure.
Avoidance Actions: The best way to avoid smearing is to confirm that the applied load exceeds the bearing's minimum load requirement. The use of oil lubrication instead of grease can also reduce the potential for smearing. When bearing size or lubrication type cannot be easily adjusted, using coatings or reducing the number of rolling elements are alternate avenues. While some polyglycol oils can prevent smearing, end users should consider the cost and environmental issues associated with these oils. In addition, some polyglycols dissolve in water, which will decrease lubricant viscosity and increase risk of corrosion.
Corrosion is perhaps the most common reason for short service lives in paper machine bearings. Bearing applications in paper machines are exposed to the ingress of water, especially in the forming and press section of the machine. Water, either from the papermaking process or from washing down the machine, is dangerous. The risk of corrosion is highest in non-rotating bearings. Free water in the lubricant of a non-rotating bearing will accumulate at the roller-to-ring interface, resulting in static etching.
In the dryer section of the papermaking process, bearings are subjected to another type of corrosion. High temperatures and aggressive chemicals may cause a type of corrosion referred to as etching. Some lubricant additives have proven to be especially aggressive at high temperatures.
Avoidance Actions: The best ways to avoid corrosion are to keep the lubricant free from water and to use a lubricant with good rust inhibitor additives. Proper sealing systems also impart heightened levels of protection against water and contaminant particles. New bearing technology with corrosion restraint steel also is available.