How to extend bearing life
Chris Rehmann of AES Seal discusses the preventative steps that can be taken to avoid bearing failure.
Bearings are precision components that require clean lubrication in adequate amounts to ensure a long, trouble free life. Even very small amounts of contamination or slightly elevated temperatures can lead to bearing failure.
A study into equipment reliability conducted at an industrial plant concluded that 40% of overall rotating equipment failures (pumps, mixers, etc) were due to bearing failure. They further estimated that 48% of these bearing failures were due to particle contamination and a further 4% were due to 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.
One of the most effective ways of preventing contamination from water, dust or other process fluid is with a modern labyrinth bearing protection seal. These kinds if upgrades are relatively expensive and have been proven to work, with return on investment of only a few months in some cases.
Conversely, bearings without adequate seals fail because of a range of issues, including dust contamination, humidity and moisture contamination, excessive heat, and lubrication issues.
1. Dust contamination
Dust within the production environment is a major problem for bearings. Even light dust, suspended in the air, will eventually find its way into a bearing. Although the bearing housing offers some protection against dust, ingress will still occur. For example, one of the significant 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 both heavy and light dust, through some bearing seals back into the housing. Over an extended duration this dust builds up inside the bearing and eventually leads to contamination of the oil, abrasion and bearing failure. If bearing seals are to work effectively they must facilitate this ‘breathing cycle’ in order to extend bearing life, whilst 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 (Figures 1
and 2). These use a positive air purge to enhance the performance of the labyrinth in combination with our mechanical seal pressure balancing technology to maximize the performance of the seal and minimize air consumption.
2. Humidity and moisture contamination
Moisture can enter bearing housings through old-style labyrinth seals or lip-seals as airborne water vapour, or via a stream of water from 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 a cause of many contamination problems, too. Even though the air in a production plant may appear to be ‘dry’, moisture is always present. Warm air is able to hold more water vapour and therefore hot air around machinery will have a higher relative humidity.
The pathway for water vapour entering the bearing starts when the bearing housing 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, it forms minute water droplets inside the bearing. Just as dust accumulates, 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.
Reducing the risk
To reduce the risk of humidity and moisture contamination, the bearing housing would have to be kept above dew point to prevent condensation forming. However, as this is not practical, the best way of dealing with it is to use modern labyrinth bearing protection which, when the shaft stops rotating creates a perfect vapour seal against both moisture and dust.
These labyrinth designs also protect against other sources of moisture contamination such as powerful waterjets. There are labyrinth seals available that are also able to operate in totally flooded or submerged environments, providing the bearing with complete protection.
3. Excessive heat
Bearings overheating is 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 discolouration 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 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 or insufficient cooling and lubricant failure.
Overheating is a major issue as 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 of extending the life of the lubricant and ensuring it remains in optimum condition is to use a modern labyrinth bearing protector because it has been proven to protect against contamination ingress and lubricant egress.
4. Lubrication issues
Another reason high up on the list why bearings fail is improper lubrication, which accounts for around a third of all failures. There are a number of reasons that lead to an issue with inadequate lubrication ranging from poor lubricant viscosity, prolonged service or infrequent 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, contamination from the outside is a major cause of compromised performance of the lubricant.
The challenge of creating optimum lubrication conditions is that it’s a balancing act between over-lubrication and under-lubrication. Both these create a problem as does contamination, or the use of a lubricant not suited to the equipment. And, consistency, viscosity, oxidation resistance and anti-wear characteristics all play a huge part in the selection of a lubricant. Usually, the application will dictate the amount, type and frequency of lubrication needed.
Extending bearing life
In recognition of the vulnerability of bearings, more advanced labyrinth bearing protection seals have been developed that can offer protection against all the types of contamination discussed above.
For instance, LabTecta66, which is non-contacting in operation to avoid shaft wear, incorporates patented dynamic lift technology to protect against the ‘breathing’ issues that contribute to 52% of all bearing failures centred around contamination. This dynamic lift technology uses the centrifugal force of rotating equipment to open a temporary micro-gap allowing expansion of the oil-air mixture in the bearing housing, which consequently allows the equipment to breathe.
When the equipment stops rotating the micro-gap immediately closes forming a perfect seal. This prevents dust and moisture being sucked back into the bearing housing and therefore prevents contamination (Figure 4). Rated to IP66 of the ingress protection code, LabTecta66 is capable of reducing water contamination of the bearing oil from as high as 83% to just 0.0003% compared to lip-seals, even when exposed to high pressure water jets. The range is ATEX certified for use in explosive environments and a number of special designs make it suitable for a wide and varied 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 as compared to lip-seals, which means that shafts already damaged can be retrofitted without costly replacement.
Bearings are precision elements within the production process and require an ongoing supply of a very clean film of lubricant in the appropriate amount in order to ensure long life and low maintenance. One of the most effective ways of achieving this is by using modern labyrinth bearing protectors, which are highly effective at preventing the entry of contaminants as well as the loss of lubricant. What’s more, they can be used to increase equipment reliability in pumps, electric motors, fans, pillow blocks, steam turbines and gearboxes.
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