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Lubrication selection is important for low temperature operations
By Chris Johnson, Managing Director, SMB Bearings
Bearing lubrication choice affects the rolling resistance, speed, noise, and most critically, the lifespan of a bearing. However, when a bearing is required to operate under demanding conditions, lubricant selection becomes even more important. In fact, according to an RKB study, improper lubrication is the cause of 80% of bearing failures.
The two basic types of lubricants available are oils and greases. Oils are preferred where free rotation is required but this may be at the expense of bearing life unless complex 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 to choose from. Grease, on the other hand, is a semi-solid or solid lubricant and is an oil combined with a thickener and may include anti-wear, extreme pressure, or antioxidant additives.
Regardless of whether you use an oil or grease, low temperatures can dramatically affect the effectiveness of a lubricant. Operating in reduced temperatures results in higher viscosity, which can restrict lubrication flow within bearings. Too high a viscosity can cause excessive friction and, in turn, damage both the bearing and the machinery.
Lubricants have different optimum temperature ranges. While some low temperature lubricants may operate at sub-zero temperatures, they may not fare as well when the machinery is running and the ambient temperature of the bearing increases. When choosing a lubricant, the entire range of temperatures at which the machinery will function should be considered. Naturally, it is helpful if the lubricant consistency does not change too dramatically over its temperature range.
The pour point of oil describes the lowest temperature at which the oil will flow. When the temperature drops below the pour point, the oil will partially or completely solidify. This 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 sponge to ensure that oil stays securely inside the bearing by releasing and re-absorbing oil, allowing a lubricating film to form at friction points. At temperatures near to or below the base oil’s pour point, they can no longer do this effectively and accelerated wear can occur. A larger amount of thickener in a grease can also cause it to stiffen too much at low temperature.
More than lubrication
It is also important to select the most suitable bearing material. SMB Bearings supplies 316 stainless steel bearings which can be used down to almost -200°C and up to 250°C, depending on the cage material. However, they are semi-precision and only suitable for relatively light loads due to the difficulty in hardening 316 grade steel. 440 grade bearings are ideal for high load and high precision applications but may suffer from corrosion.
Full ceramic bearings are recommended in corrosive environments or extreme temperatures where higher loads and speeds must be tolerated — they have excellent corrosion resistance and those with cages can perform at temperatures close to -200°C. Full complement silicon nitride bearings can operate at temperatures of up to 800°C.
In extreme conditions, lubrication can be a problem. One option is to run the bearing “dry” without any lubrication at all. Full ceramic bearings will operate without lubrication but for stainless steel bearings, rotational speeds must be extremely low.
When operating in harsh environments, incorrect lubrication can not only slow down the rotation speed of a bearing, but potentially restrict its movement completely. This obviously has disastrous consequences for machinery operating in extreme environments. That’s why it’s vital that manufacturers approach bearing suppliers with the skills and industry knowledge to advise on the best lubrication choice, otherwise they could be left out in the cold.