In hazardous environments, such as those found in the oil and gas industry, bearings are often subjected to demanding application conditions. Components used in these hostile environments are therefore designed to fulfil rigorous industry standards such as NEMA ratings. Here Chris Johnson, managing director at steel bearings supplier SMB Bearings, explains five considerations when choosing steel bearings for harsh operating conditions.
The specific environmental conditions in which a product is used affects its specifications and importantly, these specifications must be determined at the start of the machine building process. From extreme temperatures and thin-film lubrication to contaminated and corrosive environments, bearings used in hazardous area environments might be expected to carry extreme loads and deal with frequent machinery stops and starts.
Not all bearing steels are equal
Various materials have been used by the bearing industry, undergoing different heat treatment and processes to attain the desired properties to maximise bearing life and performance. Chrome steel is the most common material for ball bearings due to its high load capacity and low-noise properties.
440 grade stainless steel bearings are sometimes used in marine environments and are a common choice for corrosion resistance. However, they do have a limited resistance to seawater or salt spray. 316 grade stainless steel bearings are often specified for more aggressive environments provided loads and speeds are low. Whereas full ceramic bearings are a popular choice for higher load applications.
A more recent alternative is high nitrogen stainless steel. This ultra clean steel with a fine and uniform microstructure has very high corrosion resistance, high wear resistance, high rolling contact fatigue strength and high impact toughness. High nitrogen steel bearings are supplied with ceramic balls to provide marine grade stainless steel bearings for higher loads and speeds than even 440 grade stainless steel can achieve.
When the steel is heat treated, the process produces large, brittle chromium and chromium-molybdenum carbides that deplete the surrounding steel matrix. When high nitrogen stainless steel is hardened and tempered, however, fine chromium nitrides are formed. The resulting, smaller chromium-depleted zones around the nitrides make the high nitrogen stainless steel much more corrosion resistant and durable.
For example, in sour gas compressors, SKF has reported that its high nitride stainless steel hybrid bearings can provide six to ten times the service life of conventional bearings. This results in reductions in compressor maintenance and operating costs. Similarly, a European Commission report explored how improving the properties of steel bearings through the development of a high nitrogen austenitic steel functional grade, increased the reliability of the studied wind turbines.
Another level of armour against contamination is a protective coating. Chromium and nickel plating offer good corrosion resistance in highly corrosive environments. However, coatings will eventually separate from the bearing and need continual maintenance. This isn’t the most practical option for hard to access locations.
Bearing lubrication choice will affect the rolling resistance, speed, noise and most critically, the lifespan of a bearing. However, when a bearing is required to operate under demanding conditions, the selection of lubricant becomes even more important. SMB Bearings can supply sealed bearings with waterproof greases that contain corrosion inhibitors. These lubricants protect the internal surfaces of the bearing and can be matched to the specific application environment.
In such extreme temperatures, lubrication can be a problem. One option could be to run the bearing ‘dry’ without any lubrication. Full ceramic bearings will operate without lubrication but can be lubricated with waterproof grease for extended life. However, for stainless steel bearings, the rotational speeds must be extremely low. Chromium steel bearings are not an option as they will corrode without a protective coating of oil.
In harsh environments, contamination protection is of utmost importance, so opting for a contact seal is favourable to ensure contaminants do not enter the steel bearing. For equipment that may be exposed to moisture, a contact seal will also offer increased water resistance. This will stop grease washing out of the bearing, allowing it to do its job in lubricating and protecting the internal surfaces of the bearing. An alternative option is a metal shield but this offers greatly reduced protection against moisture.
Conventional approaches to maintenance in hazardous industries have relied on experts. These traditional methods are costly and inflexible as many of the problems that result in unplanned downtime simply cannot be picked up in a visual inspection.
Total cost of ownership also suffers due to companies wasting time and money by monitoring assets manually, then applying reactive corrective maintenance, instead of predictive maintenances through digital monitoring.
However, remote condition monitoring adoption has been slow in hazardous areas, partly due to the lack of cost-effective and easy to install solutions. Another obstacle is the often-challenging environments in which the equipment operates. Fortunately, solutions like sensors that can be integrated into the bearing housing themselves to monitor wear, are becoming more accessible to businesses thanks to falling costs and, along with software and firmware, technological advancements that make their use more flexible.
In addition to potentially causing downtime and lost productivity, the need for component replacement can present safety concerns in harsh operating environments. Therefore, bearings that offer little to no maintenance improve employee safety and reduce potential workplace accidents.
With correct product selection in the first instance, design engineers can overcome most environmental challenges, allowing their bearings to enjoy a longer service life thus reducing total cost of ownership.
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