Bearing the harsh environment
03 July 2017
With its freezing climate, the Arctic is one of the most extreme and barren environments on Earth. Despite this, four million humans have learned to live there, and these inhabitants expect crucial machinery to operate effectively.
Here Chris Johnson, Managing Director of SMB Bearings, explains the lubrication requirements for bearings operating in low temperatures and other harsh environments.
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. In fact, according to an RKB study, improper lubrication is the cause of 80 percent of bearing failures.
The two basic types of lubricants available are oils and greases. Generally speaking, oils are preferred where free rotation is required but, due to the difficulty in keeping oil inside the bearing, this may be at the expense of bearing life unless complex oil re-circulation systems are used. Lubricating oils were traditionally refined from petroleum but today, there are a wide range of synthetic oils, silicon oils and fluorinated compounds available to choose from. Grease, on the other hand, is a semi-solid or solid lubricant and is simply an oil combined with a thickener and may include anti-wear, extreme pressure or anti-oxidant additives.
Regardless of whether you use an oil or grease, low temperatures can dramatically affect the effectiveness of a lubricant. Operating in reduced temperatures will result in a higher viscosity lubricant, which can restrict the lubrication flow within bearings. Naturally, too high a viscosity can cause excessive friction and, in turn, damage the bearing and the machinery it is being used in.
Lubricants have different optimum temperature ranges. For example, while some low temperature lubricants may be able to operate at sub-zero temperatures, the lubricant may not fare as well when the machinery is up and running and the ambient temperature of the bearing increases. When choosing a lubricant, the entire range of temperatures in which the machinery will function should be taken into consideration. Naturally, it is helpful if the consistency of a lubricant does not change too dramatically over its temperature range.
The pour point of oil describes the lowest possible temperature at which the oil will flow. When the temperature drops below the pour point of an oil, it will become partially or completely solid. In this instance, the oil will either dramatically increase the bearing’s rolling resistance or stop it from rotating altogether. When establishing low temperature limits for greases, the pour point of the base oil is an important consideration.
The thickener in a grease acts as a kind of sponge to ensure that lubricating oil stays securely inside the bearing. Thickeners will release and re-absorb oil allowing a lubricating film to form between at the points of friction. At temperatures near to or below the base oil’s pour point, they can no longer do this effectively and accelerated bearing wear can occur. In addition, a larger amount of thickener in a grease, while useful at high temperature, can cause the grease to stiffen too much at low temperature.
More than lubrication
Lubrication is not the only consideration when purchasing bearings to operate in low temperature environments. It is also important to select the most suitable bearing material for your application. For example, SMB Bearings supplies 316 stainless steel bearings with polyethylene, PEEK or PTFE cages. Bearings from this range can be used in cryogenic applications down to almost -200°C and are suitable for high temperature use up to 250°, depending on the cage material.
The 316 range is ideal for use in harsh environments, but bearings manufactured from this material are semi-precision and only suitable for relatively light loads due to the difficulty in hardening 316 grade steel. The more commonly used 440 grade bearings are ideal for high load and high precision applications. However, 440 grade bearings will suffer from corrosion in many harsh environments where 316 grade stainless steel excels.
In very corrosive environments or in extreme temperatures where higher loads or speeds must be tolerated, full ceramic bearings are recommended. Full ceramic bearings have excellent corrosion resistance and those with PTFE cages or 316 stainless steel cages can perform at temperatures close to -200°C while full complement (cageless) silicon nitride bearings can operate at high temperatures of up to 800°C.
In such extreme temperatures, lubrication can be a problem. One option could be to run the bearing “dry” without any lubrication at all. Full ceramic bearings will operate quite happily without lubrication but 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.
When designing applications to operate in harsh environments, it is easy to forget the importance of lubrication choice. Incorrect lubrication can not only slow down the rotation speed of a bearing, but it could potentially restrict its movement completely. This obviously has disastrous consequences for machinery operating in extreme – and often remote – environments. That’s why it’s vital that machine original equipment manufacturers approach bearing suppliers that have the skills and industry knowledge to advise on the best lubrication choice, otherwise they could be left out in the cold.
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