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

The choice 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 details to resolve, the number of greases that must accommodate extreme temperature conditions poses among the more challenging lubrication engineering decisions.

Given the number of options the lubrication engineer have to be selective and discriminating when sourcing grease to meet high-temperature requirements; it is extremely necessary to pick a high-quality grease.

High-Temperature

‘High’ is relative when characterizing temperature conditions. Bearings running in a metal 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 cling painted metal parts on lengthy conveyors and weave them by giant 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 to be used in a sizzling steel mill application might require distinctive load-carrying capability, oxidation stability, mechanical stability, water wash resistance and good pumpability, and at a worth suitable for big-quantity consumption. With all the vital factors to consider, it is helpful 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 may very well be divided by temperatures alongside the lines in Table 1.

There may be basic correlation between a grease’s helpful temperature range and the anticipated value per pound. For instance, a fluorinated hydrocarbon-based mostly (type of artificial oil) grease could work effectively as high as 570ºF (300ºC) in area applications however may value hundreds of dollars per pound.

The grease’s long-term conduct is influenced by the causes of degradation, three of which are particularly vital: mechanical (shear and stress) stability, oxidative stability and thermal stability. Oxidative and thermal stresses are interrelated. High-temperature applications will usually degrade the grease through thermal stress, in conjunction with oxidative failure occurring if the product is in contact with air. This is analogous to what’s to be expected 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 good starting point for grease products. Grease consists of three elements: the bottom oil, the thickener and the additive package. There is quite a lot of options from which the producer creates the ultimate product. Table 2 includes some of these options. 1

Base oils will be subdivided into mineral and synthetic types. Mineral oils are probably the most widely used base oil part, representing approximately 95 % of the greases manufactured. Artificial esters and PAO (synthetic hydrocarbons) are subsequent, adopted by silicones and a few other unique 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 proportion 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 assist to dissolve or disperse additives to produce the final 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 way, these base oils are more like the Group IV artificial hydrocarbons (PAOs) than the Group I mineral oils. The oxidative and thermal properties might be excellent 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 might have slightly better stability, but command a higher price. The Group V base oils have a defined but totally different degradation path (not primarily thermal or oxidative).

Mineral and artificial 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 artificial hydrocarbons will start to unravel, releasing carbon atoms from the molecular chain, is about 536ºF to 608ºF (280ºC to 320ºC). three,4

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

Thickeners

The materials selected because the grease thickeners could also be natural, reminiscent of polyurea; inorganic, corresponding to clay or fumed silica; or a cleaning soap/advanced soap, resembling 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 2,732ºF (1,500ºC) as one excessive example. 5

Nevertheless, because grease performance relies on a combination of materials, this doesn’t represent the helpful temperature range. Some clay-thickened (bentonite) greases could similarly 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 organic polyurea thickener system affords temperature range limits similar to the metal soap-thickened grease, but additionally it has antioxidation and antiwear properties that come from the thickener itself. Polyurea thickeners may turn into more in style but they are tough to manufacture, requiring the handling of a number of toxic materials.

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