Malcolm Watson and Tony Synnott discuss the grease versus oil debate for bearings, and how to achieve best practice in lubricant replenishment and replacement.
Lubrication of industrial bearings is an essential tool for minimising friction and reducing wear that could cause premature failure, by preventing direct metallic contact between the bearing's rings, rolling elements and cage. An appropriate lubrication regime will also prevent foreign material from entering the bearing while guarding against corrosion or rusting.
Any discussion of lubrication inevitably leads to a comparison between oil and grease lubricants, and a discussion of the pros and cons of each type. The right method of lubrication must be selected to match the application conditions and purpose to gain the best performance from the bearing, so selection of either will depend on the temperature of the operating conditions, and the speed of the process.
There are several different types of oil lubrication methods - oil bath, drip feed, splash, circulating, jet, oil mist and oil air.
Oil lubrication is generally suited to higher speed and higher temperature applications and has a number of benefits. Its fluidity is good, lubricant replacement is easy, and removal of foreign material is also an easier process than with grease lubrication. It has a high limiting speed, and heat transfer is easily possible using the process of forced oil circulation.
However, oil can be prone to leaking unless measures are taken to effectively prevent it. Careful storage and sealing are also required.
It is essential to use a lubricating oil which has suitable viscosity at the bearing operating 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 13mm3/s at the actual operating temperature, while tapered roller bearings and spherical roller bearings should be lubricated with an oil of 20mm3/s viscosity or more. This figure rises to a minimum of 32mm3/s at operating temperature for spherical thrust bearings.
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% to 80% 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/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. Tony Synnott is with Brammer, Newark, Nottinghamshire, UK