There are numerous criteria to consider when choosing a high-temperature grease for hot, grease-lubricated equipment.

The choice should embrace 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 number of greases that should accommodate excessive temperature conditions poses a number of the more difficult lubrication engineering decisions.

Given the variety of options the lubrication engineer must be selective and discriminating when sourcing grease to meet high-temperature requirements; it is extraordinarily vital to select a high-quality grease.

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 a number of hundreds of degrees, and will experience sustained temperatures of 250ºF to 300ºF (120ºC to ±150ºC).

Automotive assemblers dangle painted metal parts on long conveyors and weave them via massive drying ovens to dry painted metal surfaces. Working temperatures for these gas-fired ovens are maintained round 400ºF (205ºC).

In these two cases, the choice criteria differ appreciably. In addition to heat resistance, the grease for use in a hot steel mill application could require exceptional load-carrying capability, oxidation stability, mechanical stability, water wash resistance and good pumpability, and at a worth suitable for giant-quantity consumption. With all of the essential factors to consider, it is useful to have a grease choice strategy.

Selection Strategies

A reasonable starting level for selecting a high-temperature grease is to consider the nature of the temperatures and the causes of product degradation. Greases may very well be divided by temperatures along the lines in Table 1.

There is general correlation between a grease’s useful temperature range and the expected value per pound. As an illustration, a fluorinated hydrocarbon-based (type of synthetic oil) grease may work successfully as high as 570ºF (300ºC) in area applications but may additionally value hundreds of dollars per pound.

The grease’s long-term conduct is influenced by the causes of degradation, three of which are particularly important: mechanical (shear and stress) stability, oxidative stability and thermal stability. Oxidative and thermal stresses are interrelated. High-temperature applications will usually degrade the grease by thermal stress, in conjunction with oxidative failure occurring if the product is involved with air. This is analogous to what is to be anticipated with most industrial oil-lubricated applications.

When choosing lubricants for oil-lubricated applications, one typically begins with the consideration of base oil performance properties. This can be a good starting point for grease products. Grease consists of three parts: the base oil, the thickener and the additive package. There may be a variety of options from which the producer creates the final product. Table 2 consists of a few of these options. 1

Base oils could be subdivided into mineral and synthetic types. Mineral oils are essentially the most widely used base oil element, representing approximately 95 % of the greases manufactured. Artificial esters and PAO (synthetic hydrocarbons) are next, followed by silicones and a few different unique synthetic oils. 2

The American Petroleum Institute divides base oils into 5 categories which might be useful in initially choosing base oil by performance limits.

The Group I products are naphthenic and solvent-refined paraffinic petroleum stocks with a high proportion of unstable ‘unsaturated’ molecules that tend to promote oxidation. Additionally, there are polar products that stay within the Group I base oils called heterocycles (nitrogen, sulfur and oxygen- containing molecules). Though the polar products are reactive, they assist to dissolve or disperse additives to produce the ultimate product.

The Group II and Group III are mineral oils that have extensive processing to remove the reactive molecules and saturate (with hydrogen) the molecules to improve stability. In a sense, these base oils are more like the Group IV artificial hydrocarbons (PAOs) than the Group I mineral oils. The oxidative and thermal properties could be excellent as a consequence of the removal of the reactive heterocyclic molecules.

The Group IV synthetic hydrocarbons (SHC fluids) are produced by combining two or more smaller hydrocarbons to synthesize larger molecules. These fluids might have slightly better stability, but command a higher price. The Group V base oils have a defined however different degradation path (not primarily thermal or oxidative).

Mineral and synthetic base oils degrade thermally in conjunction with oxidative degradation if the product is in contact with air. The break point at which the individual oil molecules in a highly refined (Group II+, Group III) mineral oil and synthetic hydrocarbons will begin to unravel, releasing carbon atoms from the molecular chain, is about 536ºF to 608ºF (280ºC to 320ºC). three,four

The grease manufacturer will select supplies given their familiarity, and perhaps availability, of the raw materials. If the producer makes a particular type of synthetic base fluid and is intimately acquainted with the assorted destruction mechanisms of that fluid, then it is likely that this type of synthetic base will often be chosen for new product development.

Thickeners

The materials selected because the grease thickeners could also be natural, similar to polyurea; inorganic, resembling clay or fumed silica; or a cleaning soap/advanced soap, comparable to lithium, aluminum or calcium sulfonate complex. The usefulness of the grease over time depends upon the package, not just the thickening system or the type of base oil. As an illustration, silica has a dropping level of 2,732ºF (1,500ºC) as one extreme example. 5

Nevertheless, because grease performance will depend on a combination of supplies, this doesn’t characterize the helpful temperature range. Some clay-thickened (bentonite) greases might equally have very high melting factors, with dropping points noted on the product data sheets as 500ºC or greater. For these nonmelting products, the lubricating oil burns off at high temperatures, leaving behind hydrocarbon and thickener residues.

The natural polyurea thickener system provides temperature range limits just like the metal cleaning soap-thickened grease, however additionally it has antioxidation and antiwear properties that come from the thickener itself. Polyurea thickeners might become more well-liked but they are tough to fabricate, requiring the dealing with of several poisonous materials.

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