FEATURE SEALS & BEARINGS
CHOOSING LUBRICATION for industrial applications
Where rolling element bearings are concerned, a variety of factors will dictate what type of lubricant is most appropriate for a particular application, as Phil Burge of SKF explains
L
ubrication is a fundamental maintenance activity yet poor,
inappropriate or non-existent lubrication is the root cause of up to three quarters of breakdowns of industrial equipment. 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 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 you will optimise its service life, ultimately reducing incidences of unplanned machine downtime.
OIL OR GREASE? 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, as will be explained later in this article and by far the largest majority of bearings (80-90%) are grease lubricated. However, oil has some important niche uses in bearing lubrication, particularly in high speed, high temperature applications that require maximised heat transfer from the bearing’s contacting surfaces. Appropriate choices come down to selecting either mineral or synthetic oil, but by far 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. In some applications, '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 would be the more appropriate choice. Returning to the most popular lubricant for bearings - grease - it is composed of a thickener and a base oil. The thickener functions as a container for the base oil
20 FEBRUARY/MARCH 2018 |IRISH MANUFACTURING
considerations when choosing an appropriate grade of grease are its base oil viscosity, rust-inhibiting capabilities, 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 worth noting that for every 10-15°C increase in temperature, the viscosity of the mineral base oil drops by a factor of two. The operating temperature of a bearing
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.
CONSISTENCY GRADES Greases are divided into consistency grades according to a scale developed by the US National Lubricating Grease Institute (NLGI). Greases with a high consistency ('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 speeds, temperatures, loads, 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. The most important
Specifying a lubricant for a bearing comes down to the relatively simple choice between oil and grease
will affect grease selection and this is measured as close to the bearing outside diameter as possible. The choice of grease will 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.
TRAFFIC LIGHT SYSTEM As an aid to selection based on temperature alone, SKF has introduced a traffic light system in which the range for greases is divided into low temperature limit (LTL); low temperature performance limit (LTPL); high temperature performance limit (HTPL); and high temperature limit (HTL) organised into five ‘zones’: red indicating LTL and lower, HTL and higher; amber indicating regions between LTL and LTPL and HTPL and HTL; and green representing the ideal operating range between LTPL and HTPL. Over time, temperature, mechanical
working, ageing and the ingress of contaminants, grease in a bearing arrangement deteriorates and gradually loses its lubricating properties. Re-lubrication after a certain period of
operation becomes necessary and it will be vital to ensure that the correct type and quantity of grease is used.
SKF (UK) T: 01582 496433
www.skf.com
CONNECTINGINDUSTRY / IRISHMANUFACTURING
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