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

The selection should embody 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 particulars to resolve, the choice of greases that should accommodate excessive temperature conditions poses a number of the more difficult lubrication engineering decisions.

Given the number of options the lubrication engineer must be selective and discriminating when sourcing grease to fulfill high-temperature requirements; it is extraordinarily important 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 could also be uncovered to process temperatures of several hundreds of degrees, and will experience sustained temperatures of 250ºF to 300ºF (120ºC to ±150ºC).

Automotive assemblers grasp painted metal parts on long conveyors and weave them by giant drying ovens to dry painted metal surfaces. Working 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 sizzling metal mill application could require distinctive 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 helpful to have a grease selection strategy.

Selection Strategies

A reasonable starting level for choosing a high-temperature grease is to consider the character of the temperatures and the causes of product degradation. Greases could be divided by temperatures along the lines in Table 1.

There is basic correlation between a grease’s helpful temperature range and the expected value per pound. As an example, a fluorinated hydrocarbon-primarily based (type of artificial oil) grease could work effectively as high as 570ºF (300ºC) in area applications but can also price hundreds of dollars per pound.

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

When selecting lubricants for oil-lubricated applications, one usually begins with the consideration of base oil performance properties. This can also be a good starting level for grease products. Grease is composed of three parts: the bottom oil, the thickener and the additive package. There’s a wide range of options from which the producer creates the final product. Table 2 contains some of these options. 1

Base oils could be subdivided into mineral and artificial types. Mineral oils are the most widely used base oil component, representing approximately 95 percent of the greases manufactured. Artificial esters and PAO (synthetic hydrocarbons) are next, adopted by silicones and a few different exotic artificial oils. 2

The American Petroleum Institute divides base oils into five classes that are helpful in initially selecting base oil by performance limits.

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

The Group II and Group III are mineral oils that have in depth 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 synthetic hydrocarbons (PAOs) than the Group I mineral oils. The oxidative and thermal properties could be superb as a consequence of the removal of the reactive heterocyclic molecules.

The Group IV artificial hydrocarbons (SHC fluids) are produced by combining or more smaller hydrocarbons to synthesize bigger molecules. These fluids could have slightly higher stability, however 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 touch with air. The break level 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). 3,4

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

Thickeners

The supplies chosen as the grease thickeners may be organic, comparable to polyurea; inorganic, comparable to clay or fumed silica; or a cleaning soap/complicated soap, akin to lithium, aluminum or calcium sulfonate complex. The usefulness of the grease over time depends on the package, not just the thickening system or the type of base oil. As an example, silica has a dropping point of two,732ºF (1,500ºC) as one excessive example. 5

Nevertheless, because grease performance depends on a combination of materials, this doesn’t characterize the helpful temperature range. Some clay-thickened (bentonite) greases may similarly have very high melting points, with dropping factors 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 turn out to be more widespread but they are difficult to manufacture, requiring the handling of a number of toxic materials.

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