The concept of measurement is subjective when characterizing temperature conditions. Bearings operating in a steel rolling mill rolling table application may be exposed to process temperatures of several hundred degrees and may have to operate at continuous temperatures of 250ºF to 300ºF (120ºC to ±150ºC). Automotive assemblers hang painted metal parts on long conveyors and use large drying furnaces to dry the painted metal surfaces. The operating temperatures of these gas-powered furnaces are kept at around 205ºC (400ºF). In these two cases, the selection criteria differ significantly. In addition to heat resistance, the grease to be used in a hot steel mill application may require exceptional load carrying capacity, oxidation stability, mechanical stability, water washout resistance and good pumping capability, and an affordable price for large volume consumption. It is useful to have a grease selection strategy, taking into account all the important factors that need to be considered.

A reasonable starting point for selecting high temperature grease is to consider the nature of temperature and the causes of product degradation. There is a general relationship between the useful temperature range of a grease and its expected price. For example, a fluorinated hydrocarbon-based (synthetic oil type) grease can work effectively up to 300°C (570ºF) in aerospace applications but can be a bit expensive. The long-term behavior of the grease, especially mechanical (shear and tension) stability, oxidative stability and thermal stability are important causes of degradation. Oxidative and thermal stresses are interrelated. High temperature applications will often degrade grease through thermal stress in conjunction with oxidative failure that occurs when the product is in contact with air. This is an expected effect in most industrial lubricated applications. Selection should always be made with these behaviors of the grease in mind.

The materials chosen as oil thickeners can be organic, such as polyurea; inorganic, such as clay or sooty silica; or a soap/complex soap structure, such as lithium, aluminum or calcium sulfonate complex. The benefits of the grease over time depend on the package, not just the thickening system or the type of base oil. However, since grease performance depends on a combination of materials, it does not represent the useful temperature range. Some clay-thickened (bentonite) greases can similarly have very high melting points, with dropping points indicated in product data sheets as 500ºC or higher. For these non-melting products, the lubricating oil burns at high temperatures, leaving behind hydrocarbon and thickener residues. The organic polyurea thickener system offers similar temperature range limits to metal soap thickened grease. But in addition, it has anti-oxidation and anti-wear properties that come from the thickener itself. Polyurea thickeners may become more popular, but they are difficult to produce and require the handling of various toxic materials.

When selecting high temperature grease, follow these steps: Determine the actual temperature range. The operating temperature may be lower than it appears. Use a contact or non-contact sensor to measure the operating temperature of the grease. Are the machines intermittent or continuous? If continuous, look for a high-end product that meets operational requirements. Are heating and cooling cycles accompanied by machine operating intervals? Consider whether moisture is caused by the atmosphere or by contact. What is a reasonable relubrication interval or opportunity? If relubrication will be difficult, consider a premium product to achieve a lower cost of use, even though it is more expensive. Consider potential cosmetic issues. Can the product drip onto a component during operation? The frequency and volume of relubrication must be balanced against product contamination issues.