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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-Temperature
‘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.
Selection Strategies
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.
Lubrication Intervals
The method of application combined with the application cycle dictates the rate of application. The rate of relubrication is the amount of lubricant fed into the component in a given time.
Greased components require a constant supply of lubricant at the load zone to sustain the hydrodynamic film much the same as oil lubricated components. The reserve grease contained in the cavity in the housing serves as an oil reservoir that components draw from for lubrication.
When grease is resupplied to the housing, the oil reservoir is replenished. The longer the duration between cycles, the greater the likelihood that the reservoir will deplete and the component will run to a semi-dry (mixed film) condition.
The oil in the load zone is squeezed and pushed away over time. If the relubrication volume is insufficient, or the cycle is sporadic (greater risk with manual lubrication), the likelihood that the oil film will dissipate leading to mixed film conditions increases. When these conditions are prevalent, the grease selection must be one that resists the squeezing action and tendency to dissipate. Greases formulated with heavier viscosity base oils and chemical and mechanical film forming additives can be helpful in these circumstances.
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