Bearings are precision components that require clean lubrication in adequate amounts to ensure a long, trouble-free life. Even small amounts of contamination or slightly elevated temperatures can lead to bearing failure.
A study of equipment reliability conducted at a major refinery concluded that 40 percent of rotating equipment failures (pumps, mixers, etc.) were attributable to bearing failure. It further estimated that 48 percent of bearing failures were due to particle contamination and 4 percent were due to corrosion (caused by liquid in the oil). In fact, bearing oil contamination accounts for 52 percent of bearing problems and 21 percent of rotating equipment failures.1 If water, dust or other process fluids enter a bearing, it is headed for trouble. Modern labyrinth bearing protection seals can help prevent these issues.
Figure 1. While the shaft is rotating, a micro-gap opens, allowing the thermal expansion in the bearing housing. While the shaft is not rotating, the micro-gap is closed, forming a perfect vapor seal (Images and graphics courtesy of AESSEAL)Dust Contamination
Dust in the production environment is a major problem for bearings. Heavy dust is made of particles as small as 50 microns that can become airborne.
Because they fall at about 200 millimeters per second, these particles are unlikely to move beyond the production area. Heavy dust is readily seen as a cloud with the naked eye.
Light dust, which is smaller than 50 microns in size, may stay in the air for more than 30 minutes. This type of dust can travel well beyond the manufacturing site, although it is commonly seen as a fine coating when it settles on machinery, bearing housings and other surfaces.
Both types of dust are a concern because even light dust will find its way into a bearing. Although the housing offers some protection, ingress still happens. One significant factor in bearing oil contamination is the breathing process that occurs with all rotating equipment.
When equipment rotates, the bearing housing heats up, and the oil and air mixture inside expands and is forced through the seal. The problem arises when equipment cools, because the oil and air mixture also cools and contracts, sucking air laden with dust from the external atmosphere through seals back into the housing. Over time, dust builds up inside the bearing and eventually leads to oil contamination, abrasion to components and bearing failure. Bearing seals must facilitate this breathing cycle to extend bearing life, while preventing dust contamination.
Some modern labyrinth seals with an air purge design are suitable for use in extreme environments and applications where contamination may completely cover the seal or equipment (see Image 1). These use a positive air purge to enhance the performance of the labyrinth in combination with mechanical seal pressure balancing technology to maximize the performance of the seal and minimize air consumption.
Image 1. Three months after running, the air purge still keeps dust away from the stator to rotor interface. Humidity & Moisture Contamination
Moisture can enter bearing housings through old-style labyrinth seals or lip seals as airborne water vapor or as a stream of water from hose-down operations. It can also enter through other ways, such as the breather vent or from the widely used non-pressure balanced constant level lubricators or abraded oil ring material.
Figure 2. When equipment rotates, the bearing housing heats up, and the oil and air mixture inside heats up forcing air through the seal. As equipment cools the oil and air mixture contracts, it sucks air from the atmosphere.Water vapor present in the atmosphere is also a cause of many contamination problems. Even though the air in a production plant may appear to be dry, moisture is always present. Warm air can hold more water vapor, so the hot air around machinery will have a higher relative humidity.
The pathway for water vapor entering the bearing starts when the bearing house begins to breathe. As the machine cools, this warm, moisture-laden air (along with airborne dust) is sucked back into the housing. As the equipment continues to cool and reaches dew point, minute water droplets form inside the bearing. This moisture builds up, causing corrosion and eventually failure.
Moisture and humidity alone contribute to damage within mechanical components, however when coupled with noxious elements from the air around the production process, it can create an even more corrosive combination for bearings.
To reduce the risk of humidity and moisture contamination, the bearing housing would need to be kept above dew point to prevent condensation from forming. However, since this is not practical, the best way to reduce the risk is use of modern labyrinth bearing protection. When the shaft stops rotating, the bearing protection creates a perfect vapor seal against both moisture and dust. These labyrinth designs also protect against other sources of moisture contamination such as powerful waterjets. Some labyrinth seals can operate in completely flooded or submerged environments, providing the bearing with complete protection.
Overheating is another common cause of bearing failure. To prevent overheating, users should get the bearing running at optimum temperature, which requires adequate, but not excessive, lubrication. Discoloration of the rings, balls and cages, ranging from shades of blue to brown, is a sure sign of bearing overheating. Unless the bearing is made of special alloys, temperatures in excess of 200 C (292 F) 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/insufficient cooling and lubricant failure.
Overheating is a major problem, because even slightly elevated temperatures can cause oil or grease to degrade or bleed, reducing efficiency of the lubricant. Under even higher temperatures, oxidation causes loss of lubricating elements and the formation of carbon, which may clog the bearing. The most effective way to extend the life of the lubricant and ensure that it remains in optimum condition is to use a modern labyrinth bearing protector. These devices have been proven to protect against contamination ingress and lubricant egress.
Lubrication Issues
Improper lubrication accounts for about one-third of all bearing failures. Poor lubricant viscosity, prolonged service or infrequent changes, excessive temperature, using the wrong type of lubrication or over-lubrication are common problems. External contamination is another major cause of compromised performance of the lubricant.
Creating optimum lubrication conditions is a balancing act between over-lubrication and under-lubrication. Both create a problem as do contamination or the use of a lubricant not suited to the equipment. Consistency, viscosity, oxidation resistance and anti-wear characteristics all play a role in the selection of a lubricant. Usually, the application will dictate the amount, type and frequency of lubrication needed.
Extending Bearing Life
Manufacturers have developed more advanced labyrinth bearing protection seals that can offer protection against all types of contamination. For example, one seal that is non-contacting in operation to avoid shaft wear incorporates patented dynamic lift technology to protect against the breathing issues that contribute to 52 percent of all bearing failures centered around contamination. This dynamic lift technology uses the centrifugal force of rotating equipment to open a temporary micro-gap, allowing expansion of the oil and air mixture in the bearing housing, which allows the equipment to breathe.
When the equipment stops rotating, the micro-gap immediately closes, forming a perfect seal. This prevents dust and moisture from being sucked back into the housing and therefore prevents contamination (see Figure 3).
Figure 3. While the shaft is rotating, a micro-gap opens, allowing the thermal expansion in the bearing housing. While the shaft is not rotating, the micro-gap is closed, forming a perfect vapor seal.Rated to IP66 of the ingress protection code, this seal can reduce water contamination of the bearing oil from as high as 83 percent to just 0.0003 percent compared to lip-seals, even when exposed to high-pressure water jets. The range is Atmosphères Explosives (ATEX) certified for use in explosive environments. Special designs make it suitable for a wide range of applications.
It is also designed with a thinner cross-section and seal length than competing devices, which means that it can be retrofitted on more equipment without having to carry out modifications. Furthermore, the design enables it to be positioned differently on the shaft than lip seals, which means that damaged shafts can be retrofitted without costly replacement.
Conclusion
When all of the issues that cause bearing failure are addressed, bearings should have a long, trouble-free life. Taking steps to address these problems before they happen can result in significant cost savings. Bearings are precision elements and require an ongoing supply of clean lubricant in the appropriate amount to ensure long equipment life and low maintenance. Modern labyrinth bearing protectors have been shown to prevent the entry of contaminants, as well as the loss of lubricant.
References:
Bloch, Heinz; "Pump Users Handbook: Life Extension" 2011