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

The choice should 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 collection of greases that should accommodate extreme temperature conditions poses among the more difficult lubrication engineering decisions.

Given the variety of options the lubrication engineer should be selective and discriminating when sourcing grease to satisfy high-temperature requirements; it is extraordinarily vital to pick 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 exposed to process temperatures of a number of hundreds of degrees, and should experience sustained temperatures of 250ºF to 300ºF (120ºC to ±150ºC).

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

In these cases, the selection criteria differ appreciably. In addition to heat resistance, the grease to be used in a scorching steel mill application may require distinctive load-carrying capability, oxidation stability, mechanical stability, water wash resistance and good pumpability, and at a worth suitable for large-quantity consumption. With all the vital factors to consider, it is helpful to have a grease selection strategy.

Choice Strategies

A reasonable starting point for choosing 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 may be normal correlation between a grease’s helpful temperature range and the expected price per pound. As an example, a fluorinated hydrocarbon-primarily based (type of artificial oil) grease might work effectively as high as 570ºF (300ºC) in space applications but may additionally cost hundreds of dollars per pound.

The grease’s lengthy-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 through thermal stress, in conjunction with oxidative failure occurring if the product is in contact with air. This is similar to what’s 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 is also a very good starting point for grease products. Grease consists of three elements: the bottom oil, the thickener and the additive package. There may be quite a lot of options from which the producer creates the final product. Table 2 contains a few of these options. 1

Base oils might be subdivided into mineral and artificial types. Mineral oils are essentially the most widely used base oil component, representing approximately 95 p.c of the greases manufactured. Artificial esters and PAO (synthetic hydrocarbons) are next, followed by silicones and some different unique artificial oils. 2

The American Petroleum Institute divides base oils into 5 classes that are useful in initially choosing base oil by performance limits.

The Group I products are naphthenic and solvent-refined paraffinic petroleum stocks with a high share of unstable ‘unsaturated’ molecules that are likely 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 help to dissolve or disperse additives to produce the final product.

The Group II and Group III are mineral oils that experience intensive processing to remove the reactive molecules and saturate (with hydrogen) the molecules to improve stability. In a way, these base oils are more like the Group IV synthetic hydrocarbons (PAOs) than the Group I mineral oils. The oxidative and thermal properties might be very good as a consequence of the removal of the reactive heterocyclic molecules.

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

Mineral and synthetic base oils degrade thermally in conjunction with oxidative degradation if the product is involved with air. The break level at which the person 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,4

The grease manufacturer will choose materials given their acquaintedity, and perhaps availability, of the raw materials. If the manufacturer makes a particular type of artificial base fluid and is intimately familiar 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 supplies selected as the grease thickeners could also be organic, similar to polyurea; inorganic, reminiscent of clay or fumed silica; or a soap/complex cleaning soap, such as lithium, aluminum or calcium sulfonate complex. The usefulness of the grease over time relies on the package, not just the thickening system or the type of base oil. As an example, silica has a dropping level of two,732ºF (1,500ºC) as one excessive example. 5

Nevertheless, because grease performance depends upon a combination of materials, this does not characterize the useful temperature range. Some clay-thickened (bentonite) greases might similarly have very high melting points, 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 organic polyurea thickener system affords temperature range limits similar to the metal soap-thickened grease, however additionally it has antioxidation and antiwear properties that come from the thickener itself. Polyurea thickeners may change into more common however they’re troublesome to manufacture, requiring the dealing with of several toxic materials.

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