High-Temperature Grease Guide

There are many criteria to consider when selecting 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 number of particulars to resolve, the number of greases that must accommodate excessive temperature conditions poses among the more challenging lubrication engineering decisions.

Given the variety of options the lubrication engineer have to 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 may experience sustained temperatures of 250ºF to 300ºF (120ºC to ±150ºC).

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

In these cases, the choice 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 value suitable for big-volume consumption. With all the vital factors to consider, it is helpful to have a grease selection strategy.

Choice Strategies

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

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

The grease’s long-term conduct 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 typically degrade the grease by thermal stress, in conjunction with oxidative failure occurring if the product is in touch with air. This is comparable to what is 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 can also be a superb starting level for grease products. Grease is composed of three components: the bottom oil, the thickener and the additive package. There’s a wide range of options from which the manufacturer creates the ultimate product. Table 2 consists of some of these options. 1

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

The American Petroleum Institute divides base oils into five classes that are useful in initially deciding on 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 remain 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 ultimate product.

The Group II and Group III are mineral oils that experience extensive 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 will 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 could have slightly higher stability, but command a higher price. The Group V base oils have a defined however 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 person oil molecules in a highly refined (Group II+, Group III) mineral oil and artificial hydrocarbons will begin to unravel, releasing carbon atoms from the molecular chain, is about 536ºF to 608ºF (280ºC to 320ºC). 3,four

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

Thickeners

The materials selected because the grease thickeners may be organic, such as polyurea; inorganic, corresponding to clay or fumed silica; or a soap/complex cleaning soap, reminiscent of 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. For instance, silica has a dropping level of two,732ºF (1,500ºC) as one extreme example. 5

Nonetheless, because grease performance is determined by a combination of supplies, this doesn’t symbolize 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 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 would possibly turn out to be more fashionable but they’re tough to fabricate, requiring the handling of several poisonous materials.

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