Maintenance to Corrosion in Bearings - Report Example

CORROSION IN BEARINGS Student Number Email Address Institution Abstract Industrial equipment is installed with important components referred to as bearings. Lubrication of the bearing will definitely minimize friction and increase its lifespan. Maintenance of bearing can lead it to live to its full lifetime. This is because problems in bearing are very costly and most often result in the breakdown of the entire machine. In fact, an investment in bearing requires some significant capital. For efficient performance, reliability, and high life expectancy, bearings have to be handled with utmost care. The handling includes the installation, storage, removal, inspection and cleaning. When good care and proper maintenance is observed in a bearing, its life expectancy will definitely increase. Especially in an environment when the conditions are harsh involves heavy loads and is susceptible to contamination, good maintenance will increase the useful life of the equipment. Some bearings have coatings that protect them from corrosion. However, there are many types of bearings that have no coatings hence susceptible to corrosion. The risk is so much that even the slightest moisture can lead to the bearing becoming corroded. This explains why it is imperative to ensure that roper care is taken during storage. Bearings should be free from not only moisture but also contamination from any chemical. To protect them from corrosion, bearing is therefore coated with anti-corrosion substance and then covered with a piece protective paper. Only bearing that is ready for installation should be removed from the wrappers. Oiling of the bearing will enhance their lifespan especially when the temperature changes or when the humidity is extremely high. Introduction Bearings are elements of machines used to reduce friction on marts that are in motion by constraining their relative motion into one desirable motion (Michael, Blazel, Kimball & Rivera-Lopez, 2008). This desired movement could be specified as linear or free rotation. In addition to these motions, bearings can serve to reduce friction by control of the normal forces vector in control of parts in motion. One of the bearing parts serves to support the other. There are various types of bearings. Plain bearings are bearings whose surfaces are coated with lubricants. They are slack but attain goods stiffness when there wear and the speeds are relatively low. When well lubricated, they have high shelf life and are used in many applications. Rolling element bearings on the other hand use roller or balls to reduce friction. They are moderately stiff and may sometimes require extra maintenance other than lubrication. They are used for low friction cases and high mechanical loads appliances. Jewel bearings are normally small and are applicable for work that needs to be very precise carrying low loads as is the case with clocks. Fluid bearings have two edge seal held faces separated by a fluid. They have no friction and require no maintenance. They are used where low friction is needed for heavy loading. In magnetic bearings, magnets are used to act as separation of the faces. They don’t suffer friction and require no maintenance. However, eddy currents may lead to inconsequential movements ( Lu, Berzins, Goodyer & Jimack, 2005). Figure 1 is a representation of a ball bearing. Figure: Ball Bearing In the course of operation, bearings are subjected to heavy mechanical loads, high speeds and play during load application and friction among others (Khonsari & Booser, 2001). All these cause the bearing mechanical properties to depreciate and the surface to wear leading to exposure to the environment. When the surface of the bearing is exposed, it may be corroded. Corrosion in bearings could be as a result of condensation of moisture as the temperature changes or wear of the seals that could allow water leakage inside the bearing (Ahmad & Institution of Chemical Engineers, 2006). Corrosion of bearing surfaces can also occur when the bearing is not fully dried after washing. According to Harnoy (2003), corrosion refers to degradation of material properties when it chemically reacts to its environment. In bearings, this chemical attack may occur over a wide area or it can be linear. For instance, it can attack the line at which a race and roller of stationary bearings come into contact. There is various classification of corrosion which could cause damage to bearings depending on environment, corrosion damage morphology or the material itself. These include general, pitting, crevice, Inter-granular, de-alloying and erosion corrosion modes. Corrosion can also occur as a result of fracture induced by the environment. In general corrosion, electrochemical or chemical reactions eat away the exposed surface of a material proceed continuously and at a constant rate. Consequently, the exposed metal loses its thickness and surface appearance is changed (Idaho National Laboratory, 2005). By reducing the thickness of the material and the decrease in mechanical strength, the bearing may fail and in severe cases, leaking is experienced. Pitting corrosion on the other hand involves high corrosion rates concentrated on a spot within the material surface. Mostly, it is prevalent where a coating which would otherwise hinder the inner material from corroding has flawed or in metallic materials that combat corrosion by forming a hydroxide, native oxide or a film of salt. In environment rich with halide ions, pitting is a common phenomenon because these ions either prevent reformation of film coatings or stimulates their breakdown. Pitting initiation time also relies on the temperature magnitudes with its rate increasing at heightened temperatures. For instance, there are cases where a bearing is placed in a region with high operating temperatures or where friction is high. Once pitting has began, it proceeds at an exponential rate that is higher on the underlying material that on the exposed one (Idaho National Laboratory, 2005). Pitting is responsible for cracks that enhance material failure as well as perforations found in storage bottles among others. In Jones, In Haggard & In Greenwald (2014) describes the other type of corrosion as the crevice corrosion. An exposed material on the bearing surface such as occluded regions which do not freely mix with the environment, some of these regions could be the crevices. This leads to a difference in the interaction of these regions with the chemical environmental bulk that may result into corrosion within these occluded regions. When alloys are exposed to environments containing halide ions such as chloride ions or when the material is a passive metal, there are high chances that it will undergo crevice corrosion. Initiating the corrosive attack in crevices takes a long time but proceeds very fast once it has started with the rates being higher on the inside than on the outside of the material. Crevice corrosion is responsible for initiation of cracks which could advance to cause the bearing mechanism to fail. Inter-granular corrosion is a morphological corrosion prevalent in alloys formed by solidification process of the elements mixture (Kambič & Kalb, 2013). Due to nucleation of the crystals that form the bearing solid, the region separating individual crystals may have a chemical composition entirely different from the grains themselves. However much homogenization serves to reduce these differences, their elimination is impossible. Therefore, some combinations of the alloy environments may result into corrosion of these regions commonly known as inter-granular corrosion. Under this attack, the bearing surface remains unaltered but there is a rapid or slow reduction in mechanical strength that may result into ultimate breakage when a load is applied. Corrosion can also occur in bearings as a result or erosion. It this mode of corrosion, the top film that offers protection to the material is carried away. This is a result of the fluid environment moving relatively to the bearing. When the bearing material reacts with the surrounding environment, mechanical stress is applied to the metal. This stress leads to fracture of the material which if exposed to some environments such as organic solvents the cracked region can be eaten away (Kambič & Kalb, 2013). As a result of one or a combination of these corrosion modes, bearings fail mechanically and hence cannot perform their specified tasks or carry mechanical load. Failure is more prevalent when corrosion has attacked the outer bearing ring as compared to the bore. This is because once the bore of the bearing is eaten away; the ring to shaft fit is interfered with. When the rolling elements or the bearing raceways are corrodes, the bearing ceases to be useful (Neale, 2001). To combat the issue of corrosion in bearings, special coatings with anti-corrosion components may be applied on the bearing surface. However, this material is not used for most on the bearings in the market which gives an extra task of rust prevention when handling them to ensure no rust forms on them. Bearing failure can be as a result of several factors. Bearings may fail when contact fatigue, presence of foreign materials in the bearing material composition, corrosion or when bearings are not handled appropriately (Neale, 2001). Lubrication refers to the application of appropriate sub stances to reduce friction between mating surfaces, act as a medium of heat transfer or shield the component from dirt and rust. Inadequate lubrication as well as running conditions that are not favorable can also cause bearing failure. This is because enough lubricant must be applied for t he sliding and rolling surfaces to be separated when servicing the bearing (Lugt, 2012). Therefore, it is best to determine the right amount, grade, type, system by which the lubricant will be supplied. The specified viscosity of the lubricant and the additives must be specified In reference to the system bearing. If lubrication is not done in the right manner, the performance as well as appearance of the bearing will be damaged with the damage levels ranging from low to catastrophic. Figure 2 represents the effects of elevated temperatures when the heated lubricant stains the color of the bearing material. Figure 2: how high operation temperatures and lubricant stains cause discoloration of the bearing material surface. According to Idaho National Laboratory (2005), when the lubricant is lacking completely, the type of lubricant used is wrong or when the temperature changes with drastic variations on the conditions of running, the bearing peels or gets scored. This is the second level of bearing failure in relation to lubrication and can be displayed by figures (3) and (4) below. Figure 3: peeling of bearing surface due to application of high loads and thin film lubricant for high operating temperatures. Figure (4): scoring of the rig as a result of inadequate lubrication film. In extreme cases of lubrication failure, the bearing totally locks up. When this happens, the bearing loses its original material and geometrical structure (Shreir, 2014). The rollers skew, the cage is destroyed, mass transfer of the metal occurs and the bearing completely seizes as in figure (5) below. Figure (5): Total bearing lock up In this report, we seek to explain how corrosion in bearings leads to their mechanical failure or poor performance. We will also look deeply into lubrication practices that can combat corrosion in bearings and how lubrication enhances bearing performance (Wiederhorn & Bolz, 1998). Critical Reflection and Discussion For efficient performance of a bearing, there is need for proper lubrication. This can only be achieved when the right type of lubricant is used, when the correct amount is applied and when the bearing is properly applied with the lubricant (Idaho National Laboratory, 2005). Lubricants serve the following functions in bearings: to minimize friction along mating surfaces, transfer heat, prevent corrosion, and protect it from dirt contamination. An Elastohydrodynamic effect is considered during the lubrication process. The film that is formed between the mating bearings is called the electohydrodynamic mechanism (EHD) ( Lugt, 2012). Two factors considered in EHD include; elasticity of the bodies in contact and the effect caused by the hydrodynamic mechanism. The thicknesses of the lubricant fill impacts directly on the performance of the bearing. The conditions that dictate the thickness include the loads, the velocity, pressure, and the lubricant viscosity (Mickalonis & Savannah River Site, 2000). Bearing Damage The various working conditions can cause damage in bearing. Some of these conditions are discussed below. Wear-Abrasive Presence of foreign particles can cause wear and tear of any bearing through bruising, abrasive or grooving. Some of these foreign materials may include; sand particles, metal debris from grinding, carbides resulting from gear wears among others. The cone rib and the roller ends wears out more compared to the races in bearing that are tapered. Continuous wear can eventually lead to reduction in fatigue life of any bearing and may create misalignment. It also causes clearance from within and increases the endplay. Sadly, abrasive wear greatly affects the entire working condition of a machine. The wear effect is transmitted to other areas of machine that uses bearing. Seals that are defective may be the entry point for these foreign particles (Mickalonis & Savannah River Site, 2000). When these bearings are improperly cleaned or when the filtration is ineffective, the foreign particles may accumulate hence completely causing a breakdown of the machine as in figure (6) below. Figure (6): Abrasive Wear Wear Pitting As stated by Lugt (2012) articles of foreign substances that roll through the bearing result into bruising or pitting of the races. Housings that are improperly cleaned may contain metal chips that eventually lead to early damage of the bearing. Some of these dirt contaminants are particles from the environment and sand. The internal wear is caused by wear from, brakes, clutches, joints, and gears. These particles contaminate the lubricant and find access to the bearing. The continuous contact between the particles and the bearing results to friction that eventually wears. Figure (7) below shows the impact of bruising Figure 7: Impact of Bruising Etching-Corrosion Races are coated with fine finish that makes them vulnerable to corrosion because of friction. When they are exposed to the foreign particles that roll continuously, the fine finish will be corroded. Lugt (2012) states that when the temperature changes, moisture condenses in the housing hence cause bearing damage. Seals that are worn out can become the passage for water and moisture. Bearings should be left to dry properly since that can also cause serious damage to bearings. Before storing the bearing, it is advisable to oil them or any other preservative before wrapping them in a paper. All bearings must be stored in dry place to protect them from corrosion. Figure below shows a corroded race. Figure (8): etching corrosion Inadequate Lubrication When the lubricant used is inadequate, the bearings become susceptible to damage. Lubrication should help to minimize friction. The correct lubricant grade, amount, type, supply system, right additives, and viscosity of the fluid should be properly applied to the bearing (Mickalonis & Savannah River Site, 2000). It is important to consider service conditions, history, speed, loading, sealing system, and obviously the expected lifespan. Failure to consider the above factors may result into an inefficient bearing performance. The figure below shows a bearing that has undergone complete lockup. Overload Overloading normally generate significant amount of heat which eventually cause serious damage to the bearing. The damage caused is similar to that cause by lack of lubrication. Sometimes the lubricant may lack the requisite film strength hence pre-exposing the bearing to damage. This happens especially when the loading is in heavy machines. The heavy loads normally cause fatigue on the bearing (Rees et.al, 2010). Figure (9) below shows the effects of heavy loads on cylindrical bearings. Figure (9): Impact of Overload Endplay When the endplay is in excess, the rollers become loose. This minimizes the contact with laces. The rollers then slide out into the zone with the load. The speed and the excessive friction of the rollers ultimately damage the bearing. The figure below shows the impact of excessive play. Misalignment Machining Bearings that are wrongly aligned will have a shorter lifespan. Seats and shoulders must be placed correctly so that they support the bearing. This will give the bearing more life. In case of misalignment, the load will not be evenly spread but will concentrate at one point hence causing fatigue. Damage Caused During Handling and Installation According to Shreir (2014) utmost care must be exercised during the handling and installation of the bearing. When carelessly handled, rollers may suffer serious damages. The rollers may be wrongly placed resulting into localized spalling. They may spread over the surfaces and create premature spalling. The bearing then will experience friction and begin to vibrate and make noise (Shreir, 2014). The figure below shows nicks and dents caused by mishandling of bearing. Mishandling of the rollers may also lead to damage of cages. Since cages are made from bronze, steel, or brass, they are prone to damage. This can lead to bearing damage (Shreir, 2014). Factors That Determine Lubrication Efficiency When the bearing is properly lubricated, corrosion and friction are greatly reduced. Similarly, when the lubrication is inadequate, bearing is susceptible to damage. Inadequate lubrication is divided into several groups namely; overfilling, mixing greases, worn-out grease, under filling, incorrect grease, incorrect lubrication, water contamination, systems and intervals among others. For instance, overfilling can lead to high temperatures when the machine is working (Shreir, 2014). Excess heat can lead to leakage of grease. Increase in temperatures also increases the rate of oxidation. Under filling can also cause high temperatures but due to friction. The bearings suffer from grease starvation when the grease in little. This causes the rollers and the races to experience friction. Because of this, the bearings are damaged. Executive Summary Due to wear resulting from operation at extreme temperatures and corrosion, most bearings fail before they exhaust fatigue life. For a bearing to attain optimum performance over its ideal fatigue life therefore, it must be well and regularly lubricated. This is because lubricants maintain a separation of the bearing mating surfaces and takes the heat produced from rubbing away. Lubrication also forms a coating on the surface of the material shielding it from the external environment hence solving the issue of corrosion (Shreir, 2014).. There are different types of lubricants used for different purposes. The most common ones are oil and grease lubricants with selection based on the conditions of operation and design of the bearing. Oils may be synthetic or petroleum products of crude oil and is suitable for use in high temperatures and speed. Grease, on the other hand, can be a solid or semi-liquid (Shreir, 2014). A thickening agent is added which serves to release oil or as a sponge when the bearing is subjected to a load or to change the grease characteristics and performance. However, special lubricants are used when the bearing is subjected to environments that are quiet running, handle food products, vacuum areas, conduct electricity as well as where oxidation and hence corrosion n due to friction may occur. Some bearings are pre lubricated. These are bearings which use chemically and mechanically stable greases and have a long fatigue life. They are used where grease may hinder proper performance of some part of or the whole mechanism and where it is difficult to keep the housing clean and non-contaminated. References Top of Form Bottom of Form Ahmad, Z., & Institution of Chemical Engineers (Great Britain). (2006). Principles of corrosion engineering and corrosion control. Boston, MA: Elsevier/BH. Harnoy, A. (2003). Bearing design in machinery: Engineering tribology and lubrication. New York: Marcel Dekker. Idaho National Laboratory. (2005). Corrosion. Idaho Falls, Idaho: Idaho National Laboratory. In Jones, L. C., In Haggard, W. O., & In Greenwald, A. S. (2014). Metal-on-metal bearings: A clinical practicum. Kambič, M., & Kalb, R. (January 01, 2013). Corrosion and lubrication properties of some ionic liquids. Ventil, 19, 364 Khonsari, M. M., & Booser, E. R. (2001). Applied tribology: Bearing design and lubrication. New York: John Wiley. Lu, H., Berzins, M., Goodyer, C. E., & Jimack, P. K. (November 01, 2005). High-order discontinuous Galerkin method for elastohydrodynamic lubrication line contact problems. Communications in Numerical Methods in Engineering, 21, 11, 643-650. Lugt, P. M. (2012). Grease Lubrication in Rolling Bearings. New York: Wiley. Michael, P., Blazel, B., Kimball, A., & Rivera-Lopez, R. (January 01, 2008). An Investigation of the Filterability of Synthetic Biodegradable Hydraulic Fluids. Proceedings of the National Conference on Fluid Power, 205-210. Mickalonis, J., & Savannah River Site (US). (2000). Corrosion Evaluation of Stellite Alloys 12 and 712. United States. Neale, M. J. (2001). Lubrication and reliability handbook. Boston: Butterworth-Heinemann. Shreir, L. L. (2014). Corrosion: Volume 1. Burlington, VT: Elsevier Wiederhorn, S. M., & Bolz, L. H. (October 01, 1998). Stress Corrosion and Static Fatigue of Glass. Journal of the American Ceramic Society, 53, 10, 543-548. Read More

They are used for low friction cases and high mechanical loads appliances. Jewel bearings are normally small and are applicable for work that needs to be very precise carrying low loads as is the case with clocks. Fluid bearings have two edge seal held faces separated by a fluid. They have no friction and require no maintenance. They are used where low friction is needed for heavy loading. In magnetic bearings, magnets are used to act as separation of the faces. They don’t suffer friction and require no maintenance.

However, eddy currents may lead to inconsequential movements ( Lu, Berzins, Goodyer & Jimack, 2005). Figure 1 is a representation of a ball bearing. Figure: Ball Bearing In the course of operation, bearings are subjected to heavy mechanical loads, high speeds and play during load application and friction among others (Khonsari & Booser, 2001). All these cause the bearing mechanical properties to depreciate and the surface to wear leading to exposure to the environment. When the surface of the bearing is exposed, it may be corroded.

Corrosion in bearings could be as a result of condensation of moisture as the temperature changes or wear of the seals that could allow water leakage inside the bearing (Ahmad & Institution of Chemical Engineers, 2006). Corrosion of bearing surfaces can also occur when the bearing is not fully dried after washing. According to Harnoy (2003), corrosion refers to degradation of material properties when it chemically reacts to its environment. In bearings, this chemical attack may occur over a wide area or it can be linear.

For instance, it can attack the line at which a race and roller of stationary bearings come into contact. There is various classification of corrosion which could cause damage to bearings depending on environment, corrosion damage morphology or the material itself. These include general, pitting, crevice, Inter-granular, de-alloying and erosion corrosion modes. Corrosion can also occur as a result of fracture induced by the environment. In general corrosion, electrochemical or chemical reactions eat away the exposed surface of a material proceed continuously and at a constant rate.

Consequently, the exposed metal loses its thickness and surface appearance is changed (Idaho National Laboratory, 2005). By reducing the thickness of the material and the decrease in mechanical strength, the bearing may fail and in severe cases, leaking is experienced. Pitting corrosion on the other hand involves high corrosion rates concentrated on a spot within the material surface. Mostly, it is prevalent where a coating which would otherwise hinder the inner material from corroding has flawed or in metallic materials that combat corrosion by forming a hydroxide, native oxide or a film of salt.

In environment rich with halide ions, pitting is a common phenomenon because these ions either prevent reformation of film coatings or stimulates their breakdown. Pitting initiation time also relies on the temperature magnitudes with its rate increasing at heightened temperatures. For instance, there are cases where a bearing is placed in a region with high operating temperatures or where friction is high. Once pitting has began, it proceeds at an exponential rate that is higher on the underlying material that on the exposed one (Idaho National Laboratory, 2005).

Pitting is responsible for cracks that enhance material failure as well as perforations found in storage bottles among others. In Jones, In Haggard & In Greenwald (2014) describes the other type of corrosion as the crevice corrosion. An exposed material on the bearing surface such as occluded regions which do not freely mix with the environment, some of these regions could be the crevices. This leads to a difference in the interaction of these regions with the chemical environmental bulk that may result into corrosion within these occluded regions.

When alloys are exposed to environments containing halide ions such as chloride ions or when the material is a passive metal, there are high chances that it will undergo crevice corrosion.

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