WET News

18 WET NEWS MAY 2016 Bearings are precision components but even iotas of particle or moisture contamination or slightly elevated temperatures can lead to bearing failure. AESSEAL's Chris Rehmann explains. The challenge: Caring for bearings INSIGHT Insight Pumps & mixing technology Archimedean screw ƒ tted with AESSEAL bearing seals in a UK sewage plant A study into equipment reliability conducted at an industrial plant concluded that 40% of overall rotating equipment failures (pumps, mixers, for intstance) were due to bearing failure. It was estimated that 48% of these bearing failures were due to particle contamination and a further 4% were because of corrosion (caused by liquid in the oil). That means bearing oil contamination accounts for just over half (52%) of all bearing problems and 21% of all rotating equipment failures (Bloch 2011*). The most common types of bearing oil contamination are: Particle contamination Particle contamination is a major problem for bearings. Even microscopic particles suspended in the air, can eventually † nd their way into a bearing. Although the bearing housing o‡ ers some protection, ingress will still occur. For example, one of the signi† cant contributors to bearing oil contamination is the breathing process that occurs with all rotating equipment. When equipment rotates the bearing housing heats up and the oil / air mixture inside expands and is forced through the seal. The problem arises when the equipment cools because the oil / air mixture also cools and contracts sucking air from the external atmosphere, laden with contaminants, through inadequate bearing seals back into the housing. Over an extended duration these contaminants build up inside the bearing and eventually lead to contamination of the oil, abrasion and bearing failure. If bearing seals are to work e‡ ectively they must facilitate this 'breathing cycle' in order to extend bearing life, whilst preventing particle contamination. Humidity and moisture contamination Moisture can enter bearing housings through old-style labyrinth seals or lip-seals as airborne water vapour, or via hose-down operations. It can also enter through a variety of other means such as the breather vent, or from the widely used non-pressure balanced constant level lubricators, or abraded oil ring material. Water vapour present in the atmosphere is also a cause of many contamination problems and also enters the bearing through the breathing process described above. Excessive heat Bearings that overheat are another common cause of failure. The trick is to get the bearing running at optimum temperature and to do that requires adequate lubrication (not too much, not too little). A sure sign that you have a bearing overheating is discoloration of the rings, balls and cages, ranging from shades of blue to brown. Unless the bearing is made of special alloys, temperatures in excess of 200°C (292F) can anneal the ring and ball materials resulting in loss of hardness and, in extreme cases, deformation of the bearing elements. The most common cause of overheating is excessive speed, inadequate heat dissipation / insu˜ cient cooling and lubricant failure. Overheating is a major issue as even slightly elevated temperatures can cause oil or grease to degrade or bleed, reducing e˜ ciency of the lubricant. Under even higher temperatures, oxidation causes loss of lubricating elements and the formation of carbon, which may clog the bearing. Lubrication issues Another challenge for engineers is ensuring bearings do not fail because of improper lubrication, which accounts for around a third of all failures. Reasons for inadequate lubrication range from poor lubricant viscosity, prolonged service or infrequent When the equipment rotates the bearing housing heats up and the oil / air mixture inside heats up forcing air through the seal. changes, excessive temperature, wrong type of lubrication or over lubrication (the rolling elements of a bearing operate at their optimum temperature when the minimum amount of lubrication is used). And, of course, as we have seen above, contamination from the outside is a major cause of compromised performance of the lubricant.

• Cover