High-Temperature Grease Guide

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

The choice must 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 details to resolve, the collection of greases that should accommodate excessive temperature conditions poses some of the more difficult lubrication engineering decisions.

Given the number of options the lubrication engineer have to be selective and discriminating when sourcing grease to fulfill high-temperature requirements; it is extremely important 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 expertise sustained temperatures of 250ºF to 300ºF (120ºC to ±150ºC).

Automotive assemblers hang painted metal parts on long conveyors and weave them via massive 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 sizzling metal mill application may require exceptional load-carrying capability, oxidation stability, mechanical stability, water wash resistance and good pumpability, and at a value suitable for big-volume consumption. With the entire essential factors to consider, it is useful to have a grease choice strategy.

Selection Strategies

A reasonable starting level 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 along the lines in Table 1.

There may be general correlation between a grease’s useful temperature range and the anticipated worth per pound. As an example, a fluorinated hydrocarbon-based (type of synthetic oil) grease could work effectively as high as 570ºF (300ºC) in space applications but may additionally value hundreds of dollars per pound.

The grease’s lengthy-time period habits 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 typically degrade the grease by way of thermal stress, in conjunction with oxidative failure occurring if the product is involved 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 can also be a very good starting point for grease products. Grease consists of three elements: the bottom oil, the thickener and the additive package. There’s a variety of options from which the producer creates the final product. Table 2 includes a few of these options. 1

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

The American Petroleum Institute divides base oils into five categories which can be 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 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 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 artificial hydrocarbons (PAOs) than the Group I mineral oils. The oxidative and thermal properties could be excellent as a consequence of the removal of the reactive heterocyclic molecules.

The Group IV synthetic hydrocarbons (SHC fluids) are produced by combining two or more smaller hydrocarbons to synthesize bigger molecules. These fluids could have slightly higher stability, however 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 involved 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). 3,4

The grease manufacturer will select supplies given their acquaintedity, and maybe availability, of the raw materials. If the producer 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 synthetic base will often be selected for new product development.

Thickeners

The materials chosen as the grease thickeners could also be natural, similar to polyurea; inorganic, resembling clay or fumed silica; or a cleaning soap/advanced cleaning soap, reminiscent of lithium, aluminum or calcium sulfonate complex. The usefulness of the grease over time will depend on the package, not just the thickening system or the type of base oil. For example, silica has a dropping point of two,732ºF (1,500ºC) as one excessive example. 5

However, because grease performance depends upon a mix of materials, this doesn’t characterize the helpful temperature range. Some clay-thickened (bentonite) greases could similarly have very high melting factors, 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 gives 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 widespread but they are troublesome to fabricate, requiring the dealing with of a number of poisonous materials.

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