There are numerous criteria to consider when choosing a high-temperature grease for warm, 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, working temperature, atmospheric contamination, loading, speed, relubrication intervals, etc.
With the number of particulars to resolve, the selection of greases that must accommodate excessive temperature conditions poses a few of 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 extremely necessary to select 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 should experience sustained temperatures of 250ºF to 300ºF (120ºC to ±150ºC).
Automotive assemblers cling painted metal parts on long conveyors and weave them by means of giant drying ovens to dry painted metal surfaces. Operating temperatures for these gas-fired ovens are maintained round 400ºF (205ºC).
In these two cases, the selection criteria differ appreciably. In addition to heat resistance, the grease for use in a hot metal mill application might require exceptional load-carrying capability, oxidation stability, mechanical stability, water wash resistance and good pumpability, and at a price suitable for large-quantity consumption. With the entire essential factors to consider, it is helpful to have a grease choice strategy.
Choice 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 may very well be divided by temperatures alongside the lines in Table 1.
There may be basic correlation between a grease’s useful temperature range and the anticipated price per pound. For example, a fluorinated hydrocarbon-based mostly (type of synthetic oil) grease might work effectively as high as 570ºF (300ºC) in space applications however may additionally value hundreds of dollars per pound.
The grease’s long-time period habits is influenced by the causes of degradation, three of which are particularly necessary: 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 way of thermal stress, in conjunction with oxidative failure occurring if the product is in contact with air. This is similar to what’s to be expected 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 is also an excellent starting point for grease products. Grease is composed of three elements: the bottom oil, the thickener and the additive package. There’s a variety of options from which the producer creates the ultimate product. Table 2 consists of some of these options. 1
Base oils can be subdivided into mineral and artificial types. Mineral oils are essentially the most widely used base oil part, representing approximately ninety five % of the greases manufactured. Artificial esters and PAO (synthetic hydrocarbons) are next, adopted by silicones and some other exotic synthetic oils. 2
The American Petroleum Institute divides base oils into 5 categories which might be useful 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 within 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 experience intensive 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 can 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 larger molecules. These fluids may have slightly better stability, however command a higher price. The Group V base oils have a defined but totally 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 level at which the person 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 materials 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 varied destruction mechanisms of that fluid, then it is likely that this type of artificial base will often be selected for new product development.
Thickeners
The materials selected as the grease thickeners may be natural, corresponding to polyurea; inorganic, resembling clay or fumed silica; or a soap/complicated soap, akin to 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. For instance, silica has a dropping point of 2,732ºF (1,500ºC) as one extreme example. 5
Nevertheless, because grease performance will depend on a mix of materials, this does not signify the useful temperature range. Some clay-thickened (bentonite) greases may equally 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 organic polyurea thickener system affords 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 may turn into more common however they’re difficult to fabricate, requiring the handling of a number of poisonous materials.
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