High-Temperature Grease Guide

There are numerous criteria to consider when selecting a high-temperature grease for warm, 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, working temperature, atmospheric contamination, loading, speed, relubrication intervals, etc.

With the variety of details to resolve, the collection of greases that should accommodate excessive temperature conditions poses some of the more challenging lubrication engineering decisions.

Given the number of options the lubrication engineer must be selective and discriminating when sourcing grease to fulfill high-temperature requirements; it is extraordinarily essential to pick out 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 several hundreds of degrees, and will expertise sustained temperatures of 250ºF to 300ºF (120ºC to ±150ºC).

Automotive assemblers hang painted metal parts on lengthy conveyors and weave them by massive drying ovens to dry painted metal surfaces. Working 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 to be used in a scorching metal mill application may require exceptional load-carrying capability, oxidation stability, mechanical stability, water wash resistance and good pumpability, and at a price suitable for big-quantity consumption. With all of the essential factors to consider, it is beneficial to have a grease choice strategy.

Choice Strategies

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

There’s general correlation between a grease’s helpful temperature range and the expected price per pound. As an example, a fluorinated hydrocarbon-based (type of synthetic oil) grease may work successfully as high as 570ºF (300ºC) in area applications but can also price hundreds of dollars per pound.

The grease’s long-time period conduct is influenced by the causes of degradation, three of which are particularly important: 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 touch with air. This is comparable to what’s to be expected with most industrial oil-lubricated applications.

When selecting lubricants for oil-lubricated applications, one typically begins with the consideration of base oil performance properties. This can be a very good starting point for grease products. Grease is composed of three elements: the base oil, the thickener and the additive package. There’s a wide range of options from which the manufacturer creates the ultimate product. Table 2 contains a few of these options. 1

Base oils could be subdivided into mineral and artificial types. Mineral oils are essentially the most widely used base oil element, representing approximately ninety five p.c of the greases manufactured. Synthetic esters and PAO (synthetic hydrocarbons) are next, followed by silicones and a few different unique artificial oils. 2

The American Petroleum Institute divides base oils into 5 categories which are helpful 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 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). 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 in depth 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 might 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 bigger molecules. These fluids might have slightly higher stability, but command a higher price. The Group V base oils have a defined however 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 contact 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 manufacturer will choose supplies given their acquaintedity, and maybe availability, of the raw materials. If the manufacturer makes a particular type of synthetic base fluid and is intimately familiar with the assorted destruction mechanisms of that fluid, then it is likely that this type of artificial base will typically be selected for new product development.

Thickeners

The supplies selected as the grease thickeners could also be natural, resembling polyurea; inorganic, equivalent to clay or fumed silica; or a cleaning soap/advanced soap, reminiscent of lithium, aluminum or calcium sulfonate complex. The usefulness of the grease over time is determined by the package, not just the thickening system or the type of base oil. For instance, silica has a dropping level of 2,732ºF (1,500ºC) as one excessive example. 5

Nevertheless, because grease performance is determined by a mixture of materials, this doesn’t signify the helpful temperature range. Some clay-thickened (bentonite) greases could 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 affords temperature range limits similar to the metal soap-thickened grease, however additionally it has antioxidation and antiwear properties that come from the thickener itself. Polyurea thickeners might turn out to be more fashionable but they’re difficult to manufacture, requiring the handling of a number of toxic materials.

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