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High-Temperature Grease Guide
There are many criteria to consider when selecting a high temperature grease for hot, grease-lubricated equipment.
The selection must include consideration of oil type and viscosity, oil viscosity index, thickener type, stability of the composition formed by the oil and the thickener), additive composition and properties, ambient temperature, operating temperature, atmospheric contamination, loading, speed, relubrication intervals, etc.
With the variety of details to resolve, the selection of greases that must accommodate extreme temperature conditions poses some of the more challenging lubrication engineering decisions.
Given the variety of options, the potential for incompatibility problems and high prices for a given high-temperature product, the lubrication engineer must be selective and discriminating when sourcing products to meet high-temperature requirements.
‘High’ is relative when characterizing temperature conditions. Bearings running in a steel mill roll-out table application may be exposed to process temperatures of several hundreds of degrees, and may experience sustained temperatures of 250oF to 300oF (120oC to ±150oC).
Automotive assemblers hang painted metal parts on long conveyors and weave them through large drying ovens to dry painted metal surfaces. Operating temperatures for these gas-fired ovens are maintained around 400oF (205oC).
In these two cases, the selection criteria differ appreciably. In addition to heat resistance, the grease to be used in a hot steel mill application may require exceptional load-carrying capability, oxidation stability, mechanical stability, water wash resistance and good pumpability, and at a price suitable for large-volume consumption. With all of the important factors to consider, it is useful to have a grease selection strategy.
A reasonable starting point for selecting a high temperature grease is to consider the nature of the temperatures and the causes of product degradation. Greases could be divided by temperatures along the lines in Table 1.
There is general correlation between a grease’s useful temperature range and the expected price per pound. For instance, a fluorinated hydrocarbon-based (type of synthetic oil) grease may work effectively as high as 570oF (300oC) in space applications but may also cost hundreds of dollars per pound.