Temperature range – the SKF traffic light concept
The temperature range over which a grease can be used depends largely on the type of base oil and thickener as well as the additives. The relevant temperatures are schematically illustrated in diagram 1 in the form of a "double traffic light".
The extreme temperature limits, low and high, are well-defined.
- The low temperature limit (LTL), the lowest temperature at which a grease enables the bearing to be started up without difficulty, is largely determined by the base oil and its viscosity.
- The high temperature limit (HTL) is determined by the type of thickener and its dropping point. The dropping point is the temperature at which a grease loses its consistency and becomes a fluid.
As indicated in diagram 1 by the red zones, SKF does not recommend using a grease above or below its temperature limits to lubricate bearings. Although grease suppliers indicate the specific values for the low and high temperature limits in their product literature, the temperatures for reliable operation are indicated by the SKF values for the following limits:
- low temperature performance limit (LTPL)
- high temperature performance limit (HTPL)
Within these two limits, which is indicated by the green zone in diagram 1, the grease fulfils its function reliably and the relubrication interval or grease life can be determined accurately. Since the definition of the high temperature performance limit is not standardized internationally, care must be taken when interpreting supplier data.
At temperatures above the high temperature performance limit (HTPL), grease degrades with increasing rapidity and the byproducts of oxidation have a detrimental effect on the lubricant. Therefore, temperatures in the amber zone, between the high temperature performance limit (HTPL) and the high temperature limit (HTL), should only be allowed to occur for very short periods and not longer than a few hours.
An amber zone also exists for low temperatures. With decreasing temperature, the consistency (stiffness) of grease increases and the tendency of grease to bleed decreases. This ultimately leads to an insufficient supply of lubricant reaching the contact surfaces of the rolling elements and raceways. In diagram 1, this temperature limit is indicated by the low temperature performance limit (LTPL). Values for the low temperature performance limit are different for roller bearings than for ball bearings. Since ball bearings are easier to lubricate than roller bearings, the low temperature performance limit is less important for ball bearings. For roller bearings, however, serious damage can result when the bearings are operated continuously below this limit. Short periods in this zone, such as during a cold start, are not harmful because the heat caused by frictional heat brings the bearing temperature into the green zone.
Temperature zones differ from grease to grease and can only be determined by functional bearing testing.
Typical temperature zones for commonly available NLGI 2 greases without EP additives, which are normally used for rolling bearings, are shown in diagram 2. Since the data for each grease type is a summary of many greases of more or less similar composition, the transitions for each zone are not distinct, but fall within a certain range.
Temperature zones for SKF greases are shown in diagram 3. These temperature zones are based on extensive tests conducted in SKF laboratories.