Power Transmission
temperature. An oil with low viscosity is used for high speed applications, while an oil with high viscosity is used for applications with heavy loads. In operating conditions where temperatures are substantial, using a high viscosity lubrication oil is recommended. Viscosity of lubricating oil inevitably decreases at higher temperatures so the exact operating temperature should be established when selecting a lubricant. Lubricants for radial ball bearings and cylindrical roller bearings should have a viscosity of at least 13mm²/s at the actual operating temperature, while tapered roller bearings and spherical roller bearings should be lubricated with an oil of 20mm²/s viscosity or more. This figure rises to a minimum of 32mm²/s at operating temperature for spherical thrust bearings.
Grease lubrication
Greases are made from a base oil, a thickener and a selection of additives. As a lubricant, grease has a number of advantages. Simple to store and seal, it is the best choice when the bearing is operating under normal speed and temperatures. Grease also does not tend to leak, so operating areas are seldom contaminated. The cooling effects of grease and fluidity are poor
compared with oil. Replacing a grease completely is sometimes difficult and the removal of debris particles from grease is impossible. It also deteriorates over time, so regular replenishment is essential. Due to its semi- solid nature, grease has a lower limiting speed, typically 65 per cent to 80 per cent that of oil. The consistency of the grease is also an important
factor to take into consideration; consistency is a measure of the grease thickness and is rated on a standard NLGI scale (National Lubricating Grease Laboratory) in numbers from 0 to 4. Generally grease with consistency numbers from 2 to 3 are used for most bearing applications, the number 3 grease being preferred for larger bearings (above 75mm bore) and in application of higher temperature. Number 0 or 1 grease will be soft enough to be used in centralised lubrication systems (number 0 being preferred for very long piping systems). It is vital to get the replenishment and replacement
of either grease or oil lubricant right to ensure long service life and high performance levels. Even if high- quality grease is used, there is a deterioration in its properties over time, so periodic replenishment is essential. Differing criteria apply depending on the type of
bearing used. In the case of ball bearings, for example, replenishing time can be extended depending on the type of grease used. For example, using high-quality lithium soap/synthetic oil may double the time interval between replenishment compared to a Lithium soap/
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mineral oil grease. Overfilling is something to be aware of, as this may cause a rise in temperature due to over churning of the grease, especially at high speeds. More grease should not be added at this stage, to avoid the risk of blowing out a seal. Overall, the length of time that a grease-lubricated
bearing will operate well without replenishment will depend on the size of the bearing, the type, the speed, the operating temperature and the type of grease used. The replenishing
time interval will also depend to a degree on the external forces on the bearing. For situations where very large forces are used it may be necessary to select a grease type containing an EP additive (Extreme Pressure). Replacing oil
lubrication is dependent on operating conditions, and the oil quality. In most cases, in operating temperatures of 50°C if the environment is clean, the replacement interval could be up to is a year. If the oil temperature is above 100°C, the oil should be changed at least once every three months. However, whatever
the application, following manufacturer’s guidelines alongside regular checks of the amount and condition of lubricant present in a bearing will help ensure that lubrication takes place when needed and that optimum performance is extracted from components. ●
Malcolm Watson is with Brammer, Newton Aycliffe,
UK.
www.brammeruk.com. Tony Synnott is with NSK Bearings Europe Ltd, Newark, Nottinghamshire, UK
“Viscosity of lubricating oil inevitably decreases at higher temperatures so the exact operating temperature should be established when selecting a lubricant.”
Malcolm Watson, Brammer, and Tony Synnott, NSK Bearings Europe Ltd.
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