When you're working in a machinery situation where the temperatures get a little heated, choosing the right grease is a critical decision to get right. How do you make the right decision?
The internet has some excellent resources out there that can help guide you to making the right decision.
One such resource is the Machinery Lubrication web site, where we found an excellent primer on all the factors that you need to consider in selecting the right kind of high temperature grease.
We've referenced much of that information here. For the full article, check out Machinery Lubrication's web site here.
What to consider?
There are many criteria to consider when selecting a high-temperature grease for hot, grease-lubricated equipment - 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 details to resolve, the selection of greases that must accommodate extreme temperature conditions is a challenging decision, and given potential for incompatibility problems and the high prices for many high-temperature grease, you really have to be selective and discriminating when making the decision of which grease to use.
‘High temperature’ is relative when characterizing temperature conditions. Bearings running in a steel mill roll-out table application may be exposed to process temperatures of several hundreds of degrees, and may experience sustained temperatures of 250ºF to 300ºF (120ºC to ±150ºC). Automotive assemblers hang painted metal parts on long conveyors and weave them through large drying ovens to dry painted metal surfaces. Operating temperatures for these gas-fired ovens are maintained around 400ºF (205ºC). You can see that there's a wide temperature range to consider here.
In these two cases, the selection factors differ appreciably. In addition to heat resistance, the grease to be used in a hot steel mill application may require exceptional load-carrying capability, oxidation stability, mechanical stability, water wash resistance and good pumpability, and at a price suitable for large-volume consumption. With all of the important factors to consider, it is useful to have a grease selection strategy.
Where to start?
A reasonable starting point for selecting a high-temperature grease is to consider the nature of the temperatures and the causes of product degradation that are inherent to whatever place you're going to be using it. There is general correlation between a grease’s useful temperature range and the expected price per pound. For instance, a fluorinated hydrocarbon-based (type of synthetic oil) grease may work effectively as high as 570ºF (300ºC) in space applications but may also cost hundreds of dollars per pound. How well the grease holds up long-term 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 generally degrade the grease through thermal stress, in conjunction with oxidative failure occurring if the product is in contact with air. This is similar to what is to be expected with most industrial oil-lubricated applications. Bell Performance's X-tra Lube Grease provides some advantages in these types of situations because of the residual lubricating power of its additive package, that continues providing lubricating protection after other parts of the grease may have broken down due to these factors in normal use.
What's in a grease?
For the uninitiated, grease consists of a base oil with a thickener added, plus extra additives thrown in to bulk up certain essential properties of the grease. Think of the whole thing like a sponge - the thickener is the spongy materials that holds the base oil together like a sponger holds liquid.
When selecting lubricants for oil-lubricated applications, one often begins with the consideration of base oil performance properties. This is also a good starting point for grease products. Base oils can be subdivided into mineral and synthetic types. Mineral oils are the most widely used base oil component, representing approximately 95 percent of the greases manufactured. Synthetic esters and PAO (synthetic hydrocarbons) are next, followed by silicones and a few other exotic synthetic oils.
The American Petroleum Institute divides base oils into five categories based on their performance limits (Groups I-V). They tend to differ by how well they resist thermal or oxidative break down, and how well they disperse and hold the additives that are added to the grease product. Generaally speaking, the higher Groups are more resistant but cost more.
Mineral and synthetic 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 synthetic 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 materials given their familiarity, and perhaps availability, of the raw materials. If the manufacturer makes a particular type of synthetic base fluid and is intimately familiar 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.
The materials selected as the grease thickeners may be organic, such as polyurea; inorganic, such as clay or fumed silica; or a soap/complex soap, such as lithium, aluminum or calcium sulfonate complex. The usefulness of the grease over time depends on the whole package together, not just one thing like the thickening system or the type of base oil. For instance, a silica grease thickener has a dropping point of 2,732ºF (1,500ºC) as one extreme example. However, because grease performance depends on a combination of materials, this does not represent the useful temperature range. Some examples of the kinds of thickeners you may have to choose from could be:
Organic polyurea - offers temperature range limits similar to the metal soap-thickened grease, but additionally it has good antioxidation and antiwear properties that come from the thickener itself. Polyurea thickeners might become more popular but they are difficult to manufacture, requiring the handling of several toxic materials. While the thickener has a high dropping point, the composition begins to thermally degrade at temperatures which limit its usefulness over time at high temperatures.
Calcium sulfonate - similar to the polyurea, it possesses inherent antioxidant, rust-inhibiting properties, but in addition has inherent high dropping points and EP/antiwear properties.
Metal soap or complex soap thickener system - lithium complex-thickened grease (which is what X-tra Lube Grease contains) has maximum temperature limits superior to that of simple lithium grease, because the thickener offers higher thermal degradation limits. Collectively, metal soap thickeners have thermal degradation limits that range between 250ºF to 430ºF (120ºC and 220ºC). However, unless the grease composition is properly fortified against oxidation and thermal degradation, the end product showing a dropping point of 500ºF (260ºC) or greater would not be any more useful for long-term service than a grease with a low dropping point.
Grease additives tend to bulk up properties for greases in fashion similar to lubricating oils: oxidation stability, corrosion resistance, wear resistance, low temperature flow characteristics, water resistance, etc. The additive must be capable of working synergistically with the thickener and the oil to lead to a balanced, stable mixture of the three distinct components.
Make sure about compatibility
Because greases are a complex mixture of balanced chemicals, there's potential for issues if you try to mix different greases in the same place, and this is especially true when high temperatures are involved. You know this is happening to you if the grease you just added thins out too quickly. This problem is typically solved by adding more grease until the old residual grease is flushed out. But if that isn't an option, there's not much choice other than dismantling the equipment and cleaning it out. While the only exact way to avoid this problem is to test every grease you plan to use, the general rule is that there's less potential for problems if you stay within the same faily of thickener (lithium to lithium, lithium complex to lithium complex, aluminum complex to aluminum complex, etc.).
It's also wise to pay attention to the issue of moisture sensitivity. If a particular grease component is sensitive to moisture, then regardless of the grease’s ability to withstand the heat alone, the use of the product must be weighed against the risk of process moisture degradation of the grease. It could be unwise to use a water-soluble glycol oil type of grease in an application that is subject to high moisture, such as a conveyor wash system. Even though the fluid may be capable of resisting thermal breakdown from the heat of the drying system, the moisture poses a performance risk that may not be entirely eliminated.
Selecting a High-Temperature Grease: The Guidelines
Determine the real temperature range. The operating temperature may be less than what it seems. Use a contact or noncontact sensor to measure the operating temperature of the grease. Does it exceed 392ºF (200ºC)?
Is your grease need intermittent or continuous? If it is continuous, then look for a top-tier product that meets the operational requirements, so that it will hold up better in continuous use.
Do heating and cooling cycles accompany machinery operating and nonoperating intervals? If so, you need to consider whether moisture development is going to affect the grease you use.
What is the reasonable relubrication interval or opportunity? If relubrication is going to be difficult, then consider a top-tier product to achieve a lower use cost even though it’s more expensive.
Consider any cosmetic issues. Can the product drip onto a component in process? Relubrication frequency and volume must be balanced against product contamination issues.
This post was published on January 24, 2013 and was updated on September 4, 2014.