High-Temperature Grease Guide

There are a lot of criteria to consider when selecting a high-temperature grease for hot, 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 particulars to resolve, the number of greases that should accommodate extreme temperature conditions poses a few of the more challenging lubrication engineering decisions.

Given the number of options the lubrication engineer have to be selective and discriminating when sourcing grease to satisfy high-temperature requirements; it is extraordinarily 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 uncovered to process temperatures of a number of 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 means of giant drying ovens to dry painted metal surfaces. Operating temperatures for these gas-fired ovens are maintained around 400ºF (205ºC).

In these cases, the selection criteria differ appreciably. In addition to heat resistance, the grease to be used in a hot metal mill application could require exceptional load-carrying capability, oxidation stability, mechanical stability, water wash resistance and good pumpability, and at a worth suitable for large-volume consumption. With the entire essential factors to consider, it is beneficial to have a grease selection 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’s normal correlation between a grease’s useful temperature range and the anticipated worth per pound. As an illustration, a fluorinated hydrocarbon-based (type of artificial oil) grease could work effectively as high as 570ºF (300ºC) in house applications however might also cost hundreds of dollars per pound.

The grease’s lengthy-time period behavior 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 via thermal stress, in conjunction with oxidative failure occurring if the product is in touch with air. This is comparable to what is to be anticipated with most industrial oil-lubricated applications.

When deciding on lubricants for oil-lubricated applications, one typically begins with the consideration of base oil performance properties. This is also a superb starting level for grease products. Grease is composed of three components: the bottom oil, the thickener and the additive package. There is a wide range of options from which the manufacturer creates the ultimate product. Table 2 consists of a few of these options. 1

Base oils can be subdivided into mineral and synthetic types. Mineral oils are essentially the most widely used base oil component, representing approximately ninety five percent of the greases manufactured. Synthetic esters and PAO (synthetic hydrocarbons) are subsequent, adopted by silicones and some different unique artificial oils. 2

The American Petroleum Institute divides base oils into five categories which are helpful in initially choosing 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 are inclined to promote oxidation. Additionally, there are polar products that stay in the Group I base oils called heterocycles (nitrogen, sulfur and oxygen- containing molecules). Although 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 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 will 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 could have slightly higher stability, but command a higher price. The Group V base oils have a defined but 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 point 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,four

The grease producer will select materials given their acquaintedity, and maybe availability, of the raw materials. If the producer 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 chosen for new product development.

Thickeners

The materials chosen because the grease thickeners could also be natural, such as polyurea; inorganic, comparable to clay or fumed silica; or a soap/advanced soap, similar 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. For instance, silica has a dropping point of two,732ºF (1,500ºC) as one excessive example. 5

However, because grease performance is dependent upon a mixture of supplies, this doesn’t signify the helpful temperature range. Some clay-thickened (bentonite) greases may equally 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 much 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 into more in style but they are difficult to fabricate, requiring the dealing with of a number of poisonous materials.

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