Heat, either directly applied or generated in service, plays a significant role in the life of a bearing, being both beneficial and potentially detrimental at different stages of its lifecycle. SKF's Phil Burge, Marketing and Communications Manager explains how heat can be used beneficially as an aid to bearing installation, and describes the negative effects that excessive heat will have on lubricants when the bearing is in service.
Heat is the inevitable result of friction in rotating machinery and bearings are in the front line where friction is concerned. Their role is to reduce it, thereby minimising internal temperature rises and their unwanted consequences such as lubricant deterioration and premature bearing failure. But heat has a very positive role to play at the earliest stage of the bearing’s lifecycle when it is prepared for installation on a shaft or within a housing.
An estimated 16% of all bearing failures are believed to be caused by forcing bearings into place when mounting, and this can have serious knock-on effects. Quite apart from the fact that the bearing is very likely to fail prematurely due to resultant raceway damage, the system in which it is installed will run less efficiently or may even undergo catastrophic failure, leading to lost production and costly downtime.
Some like it hot
There are various industry-approved ways to mount a bearing. SKF, for example, has developed a variety of methods for cold mounting, including its proprietary Drive-up, Oil Injection and SensorMount techniques; but heat, too, can be particularly useful when mounting bearings. Hot-mounting a bearing – by creating a temperature difference between the bearing ring and the shaft or housing - can be achieved using several techniques, though some of the more traditional approaches, such as oil baths, are no longer recommended due to health and safety considerations and environmental problems associated with disposal.
More modern methods include aluminium heating rings, electric hot plates (for small bearings), heating cabinets and infrared radiators. SKF recommends using induction heaters, which quickly apply heat evenly to the bearings. Induction heaters are safe to use because the heater and yoke never get hot; however, they can cause bearings to be magnetised, and are nowadays equipped with a device that automatically demagnetises the bearing prior to mounting.
Before a bearing is placed in service, the operating temperature range of the application should be established to ensure that the correct size/type of bearing and associated lubricant is specified. Where high operating temperatures are to be expected, either as a result of ambient conditions or generated within the bearing itself, special bearing designs and materials of construction that avoid the heat-induced structural changes that can occur throughout the bearing geometry should be used. SKF has developed a range of both standard and customised high temperature bearings that are capable of operating at temperatures as high as 350 °C.
Bearing temperature should also be monitored regularly (or even continuously) in service and as close as possible to the bearing’s outside diameter, as this will indicate any excursions from the norm, suggesting lubricant problems or damage to the bearing’s raceways. In addition to vibration, temperature is also a prime condition monitoring parameter, and many instruments are available that measure and analyse both vibration signatures and temperature trends simultaneously to provide reliable, real time machine health monitoring.
Heat and its effect on lubricants
Critically, elevated temperatures may also degrade the bearing lubrication, particularly if the wrong type and quantity of lubricant is used, leading to premature bearing failure. Choice and quantity of lubricant, its application and regular inspection are therefore among the most important maintenance functions as far as bearings are concerned.
The ‘high temperature limit’ (HTL) of a grease lubricant is established by its ‘dropping point’ - the temperature at which it becomes a fluid. Before the onset of the HTL is a grease’s so-called ‘high temperature performance limit’ (HTPL); at this stage a grease starts to degrade chemically, the by-products of oxidation having an irreversible, detrimental effect on the lubricant and consequently bearing performance. Oxidation leads to a chemical reaction producing volatile elements and lacquer formation, which severely degrades the grease’s lubricating properties.
SKF offers a traffic light system to display a grease's operating temperature rating: green representing the temperature range over which the grease will perform reliably; amber indicating the stage between HTPL and HTL where the performance of the grease is likely to be unreliable, and finally red - the point at which the grease has deteriorated to such an extent that it has become unusable. A similar traffic light system for grease is applicable to low operating temperatures.
Lubricants, both oil and grease based, are available in many grades and viscosities, and in formulations suitable for use at both high and low extremes of temperature. The temperature rating of the recommended lubricant must be determined at the earliest stage of machine design - dependent upon predicted temperature rises within that machine - and used consistently throughout its operational life.
Temperature monitoring devices are tools that no industrial manufacturing plant should be without. A rapid rise in bearing temperature may be indicative of lubricant deterioration or a sign that the bearing is developing a mechanical fault. So, whether temperature is measured continuously using thermocouples or during regular maintenance checks using a thermal camera, this simple maintenance task is likely to have a significant impact on a bearing’s life expectancy.