When a bearing is required to operate under demanding conditions, the selection of lubricant becomes even more important. Chris Johnson explains the lubrication requirements for bearings operating in low temperatures
Mech

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 – even in the depths of winter.

Bearings are found in most engineered systems exploring space, from satellites to the Mars Space Rover. 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 a study by Swiss bearings manufacturer RKB Bearing Industries, improper lubrication is the cause of 80 per cent of bearing failures.

As the space environment is a vacuum, great care must be taken when choosing the bearing lubricant as most greases will vaporise to an unacceptable level. Ceramic bearings, however, are usually unlubricated so they perform well in a vacuum. This makes them the ideal candidate for satellites or other projects such as the Mars Exploration Rover, especially as Mars can experience temperatures as low as 125 degrees Celsius.

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 from which to choose. 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.

Temperature ranges


Lubricants have different optimum temperature ranges. For example, while some low-temperature lubricants may be able to operate at sub-zero temperatures found on some planets, 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, for low or high temperatures, 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 degrees Centigrade and are suitable for high temperature use up to 250 degrees, 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 degrees Centigrade while full complement (cageless) silicon nitride bearings can operate at high temperatures of up to 800 degrees Centigrade.

In such extreme conditions, whether on Mars or in the in the arctic, 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 outer space environments.

That is why it is 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.

Author:
Chris Johnson is the managing director of specialist bearing supplier, SMB Bearings
www: http://www.smbbearings.com/
e-mail: chris@smbbearings.com

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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 – even in the depths of winter. Bearings are found in most engineered systems...