Lubrication is a vital aspect of bearing care and will have a considerable impact on how well and how long a bearing will perform in service. Poor lubrication is estimated to be responsible for more than a third of all premature bearing failures; and as bearings are at the heart of most machines, this can lead to significant downtime. Quite apart from important considerations such as the frequency of lubrication and the quantities of lubricant used, it is vital to match the grade or type of lubricant to the operating conditions of the bearing. Get this right and its service life will be optimised, ultimately reducing incidences of unplanned machine downtime.
Grease or oil?
In the first instance, specifying a lubricant for a bearing comes down to the relatively simple choice between oil and grease. Grease offers a number of advantages over oil, and by far the largest majority of bearings (between 80 and 90%, in fact) are grease lubricated. Having said that, oil does have some important niche uses in bearing lubrication, particularly in high speed applications that require maximised heat transfer from the bearing’s contacting surfaces.
In some applications, so called 'Solid Oil' delivers benefits that grease or oil alone cannot provide. This is a polymer matrix saturated with lubricating oil, which completely fills the free space in a bearing; Solid Oil is particularly useful in applications that are difficult to access for routine maintenance purposes. For extremely high temperature applications, where any grade of oil or grease would otherwise be burnt off, a solid, graphite-based lubricant is the more appropriate choice. But returning to that most popular of lubricants for bearings – grease – let’s consider its characteristics and how it behaves in service.
Characteristics of grease
Grease is composed of a thickener and a base oil. The thickener functions as a container for the base oil and behaves like a water-filled sponge. When a load is applied to grease, the thickener releases the base oil in a process known as oil bleeding or oil separation. When the load is released, the thickener normally reabsorbs the base oil.
The thickeners used in most common greases include lithium, calcium or sodium soaps. Lithium and sodium soaps have a wide operating temperature range, typically up to 120°C, while calcium soaps only have an operating temperature range of up to 80°C. However, calcium soaps offer superior protection against water, including saltwater. Meanwhile, a variety of favourable grease properties can be obtained by adding more complex soap formulations to the base oil.
Greases are divided into various consistency grades according to a scale developed by the US National Lubricating Grease Institute (NLGI). Greases with a high consistency (so-called 'stiff' greases) are assigned high NLGI grades, while those with a low consistency ('soft' greases) are given low NLGI grades. There are nine NLGI grades in total, but for rolling bearing applications the relevant grades are NLGI 1, 2 and 3. While every application must be considered in terms of speed, temperature, load, Etc. when selecting an appropriate grade, grease based on a mineral oil and lithium thickener, with an NLGI 2 grade, is sufficient for most applications.
Other considerations
The most important considerations when choosing an appropriate grade of grease are its base oil viscosity, operating temperature range and load-bearing capacity. Viscosity is temperature and pressure dependent. The viscosity of the base oil, for example, decreases with rising temperature and increases with falling temperature. Conversely, the viscosity of the base oil increases with increasing pressure. It is also worth noting that for every 10 to 15°C increase in temperature, the viscosity of the mineral base oil drops by a factor of two.
The operating temperature of a bearing will affect grease selection and this is measured as close to the bearing’s outside diameter as possible. The choice of grease will also depend on the 'speed factor' of the bearing, which is a function of its rotational speed and mean diameter. Where load is concerned, the choice depends on the ratio between the dynamic load rating (C-value) of the bearing and the load to which it is subjected. Grease datasheets will list these variables and suggest corresponding grade choices.
Oil selection
Although less than 20% of rolling bearings are lubricated with oil, there are important factors to consider when selecting which type of oil to use.
Lubricating oil consists of 95% base oil mixed with additives that achieve or enhance certain performance properties such as anti-rust. Of the three types of oils – mineral, synthetic and natural, the latter is rarely used for rolling bearings due to a risk of quality impairment and acid formation.
Mineral base oils are generally preferred for rolling bearing lubrication, whereas synthetic base oils are generally considered for bearing lubrication under special operation conditions such as at very high or low temperatures. However, in selection, all contributing factors must be considered, including application, bearing type and load, temperature, speed and shaft orientation, as well as external influences such as vibration.
Whether selecting mineral or synthetic – as with grease - the most important property is viscosity, as this will determine the consistency and thickness of the oil film created between the bearing’s rolling and sliding elements.
It is also essential to check anti-wear and extreme pressure additive requirements for slow rotating bearings under heavy loads. Some extreme pressure additives can have a detrimental effect on bearing materials, shortening bearing service life dramatically, particularly above 80°C. If specific operating conditions exist, the properties of the oil should complement the conditions. When bearings have to operate over a wide temperature range, for example, oil with the least changes in temperature variation should be selected.
Lubrication management
Under the influence of time, temperature, mechanical working, ageing and the ingress of contaminants, lubricants in a bearing arrangement deteriorate and gradually lose their lubricating properties. Re-lubrication after a certain period of operation becomes necessary and, once more, it will be vital to ensure that the correct type and quantity of lubricant is used. Effective lubrication and good relubrication practices are critical to reducing unplanned downtime and increasing the overall life cycle of equipment, helping improve profitability and reduce costs.
For further information please visit: www.skf.co.uk
