High-Temperature Grease Selection
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High-Temperature Grease Selection
Grease selection at elevated temperatures is one of the most consequential decisions an industrial maintenance team makes. When a kiln truck bearing seizes at 280 degrees Celsius, or a continuous caster roller table loses lubrication mid-campaign, the cost is measured in hours of unplanned downtime. Yet high-temperature grease selection is frequently reduced to scanning a datasheet for the highest dropping point number and picking the winner. This article addresses the questions that plant engineers, kiln and oven operators, and steel mill maintenance teams ask most frequently when specifying greases for high-temperature service. Each answer draws on established industry practice, published bearing life data, and the chemistry that governs how lubricants degrade under heat, providing a practical framework for evaluating greases against your specific operating conditions.
Frequently Asked Questions
Q1: What actually defines a "high-temperature" grease?
A high-temperature grease is defined by its ability to perform reliably at sustained operating temperatures above approximately 120 degrees Celsius (250 degrees Fahrenheit), the practical ceiling for conventional lithium soap greases with mineral base oils. Above this threshold, three degradation mechanisms accelerate: the thickener structure softens and may collapse, the base oil oxidizes exponentially (oil life roughly halves for every 10 degrees Celsius rise per the Arrhenius equation), and volatile oil fractions evaporate leaving hardened, carbonaceous residue that blocks lubricant pathways. A genuinely suitable high-temperature grease must address all three requiring a thickener with a dropping point well above the operating temperature, a synthetic base oil (PAO, ester, silicone, or PFPE) with inherent oxidation resistance, and antioxidant additives to scavenge free radicals before chain-reaction degradation begins. Industry convention divides the spectrum: above 120 degrees Celsius demands a high-temperature grease, above 180 degrees Celsius requires careful synthetic base oil selection, and above 230 degrees Celsius enters the domain of fluorinated greases or solid lubricants.
Q2: What is the difference between dropping point and maximum operating temperature?
Dropping point is the temperature at which grease transitions from semi-solid to liquid under laboratory conditions (ASTM D2265 or D566). It measures thickener thermal stability, not a useable operating limit. Maximum operating temperature is always substantially lower. The established derating guidelines are: for dropping points below 150 degrees Celsius, subtract approximately 40 degrees Celsius; for dropping points between 150 and 200 degrees Celsius, subtract approximately 55 degrees Celsius; for dropping points above 200 degrees Celsius, subtract approximately 85 degrees Celsius. A lithium complex grease with a 260 degrees Celsius dropping point therefore has a practical continuous service limit around 175 degrees Celsius. An ExxonMobil study found no reliable correlation between dropping point values and actual grease life in dynamic bearing tests one grease ranking highest by dropping point finished last in wheel-bearing life testing, while another with a lower dropping point delivered the best dynamic performance. For engineering decisions, rely on DIN 51821 (FE9) grease life test results, ASTM D3527 wheel bearing life tests, and manufacturer-published continuous service temperature ratings. The dropping point only tells you when the thickener melts; it reveals nothing about how fast the oil oxidizes, which is typically the real failure mechanism.
Q3: Which thickener types are suitable for high-temperature industrial applications?
Several thickener families serve elevated temperature service, each with distinct strengths. Lithium complex greases, with dropping points around 260 to 280 degrees Celsius, are the most widely used general-purpose high-temperature option and serve reliably to approximately 150 to 175 degrees Celsius continuous when paired with PAO or ester base oils. Calcium sulfonate complex greases offer the highest dropping points among common thickeners at approximately 290 degrees Celsius and above, with inherent extreme-pressure and anti-corrosion properties that make them a strong choice for steel mills and wet environments. Polyurea thickeners are ashless they leave no metallic residue when degraded and typically deliver three to five times longer service life than lithium complex in sealed bearings at elevated temperatures, which is why they are standard for factory-filled electric motor bearings. Organoclay (modified bentonite) thickeners have no defined melting point since clay does not melt, providing excellent structural stability where soap thickeners collapse; the practical limit is the base oil, not the thickener. PTFE-thickened greases paired with PFPE base oils represent the extreme end of the spectrum with continuous service above 260 degrees Celsius. Compatibility varies significantly: mixing lithium complex with calcium sulfonate greases can cause softening and oil separation, while lithium complex and polyurea blends show relatively high compatibility in ASTM D6185 testing.
Q4: When should I specify a PFPE/PTFE grease instead of a conventional high-temperature grease?
PFPE (perfluoropolyether) base oil greases thickened with PTFE should be specified when conditions push past the limits that synthetic hydrocarbon greases can tolerate. The primary triggers are: continuous operating temperatures above 230 degrees Celsius where PAO and ester base oils oxidize rapidly regardless of antioxidants; environments combining high heat with chemical aggression such as kilns with reactive process gases, furnace fans in solvent-laden atmospheres, or bearings exposed to steam and corrosive condensates; applications where relubrication is physically difficult or impossible; and situations where conventional greases carbonize and form hard deposits blocking lubrication paths. PFPE greases are chemically inert, containing only carbon, fluorine, and oxygen with no hydrogen bonds for oxidation to attack. They do not carbonize, form no hard residues, and have no measurable dropping point or values exceeding 300 degrees Celsius. SKF LGET 2 is a widely recognized example with an operating range from minus 40 to plus 260 degrees Celsius. KLUBER's BARRIERTA L 55/2 serves similar applications in kiln cars, calenders, and furnace bearings. The tradeoffs: PFPE/PTFE greases cost roughly two to four times more than premium PAO-based greases, are approximately twice as dense at around 1.9 grams per cubic centimetre, and must never be mixed with hydrocarbon or silicone greases. Conversion requires thorough flushing with fluorinated solvents. For steel continuous casting roller tables, hot-rolling backup roll bearings, and tunnel kiln cars, PFPE/PTFE greases can extend relubrication intervals from months to years, justifying the upfront cost through reduced maintenance labour and downtime.
Q5: How do silicone greases compare to synthetic hydrocarbon (PAO) greases at high temperatures?
The comparison between silicone and PAO greases hinges on a fundamental tradeoff between thermal stability and load-carrying capacity. Silicone base oils offer outstanding oxidation resistance and thermal stability, remaining functional to approximately 200 to 250 degrees Celsius with minimal viscosity change, very low volatility, and fluidity down to minus 80 degrees Celsius. Their weakness is lubricity: silicone oils form a weak film and cannot support heavy loads in rolling-element bearings. In four-ball wear testing, silicone greases consistently show higher wear scar diameters than PAO counterparts, and they tend to creep and migrate, contaminating adjacent surfaces. PAO greases provide the opposite profile: excellent lubricity and film strength for boundary and hydrodynamic lubrication, good load-carrying capacity for ball and roller bearings and gears, and adequate high-temperature performance up to approximately 160 to 190 degrees Celsius. Above 190 degrees Celsius, PAO oxidation accelerates beyond what even premium antioxidant packages can manage. The practical selection rule: for rolling-element bearings, gears, or significant mechanical loads at temperatures up to 190 degrees Celsius, PAO-based grease is appropriate. For low-load sliding contacts, static joints, or slow-moving mechanisms above 200 degrees Celsius, silicone grease may offer longer service life. For applications demanding both high load capacity and temperatures above 230 degrees Celsius, move past both to PFPE/PTFE greases.
Q6: How does temperature affect relubrication frequency, and how do I calculate the adjustment?
Temperature is the most aggressive accelerator of grease degradation, and its effect on relubrication intervals is dramatic and non-linear. The foundational rule: grease oxidation rate doubles for every 10 degrees Celsius (18 degrees Fahrenheit) increase. Mobil's guidance states that for every 2.8 degrees Celsius above 71 degrees Celsius, the relubrication interval should be halved. NTN-SNR specifies shortening to two-thirds for every 10 degrees Celsius above 80 degrees Celsius. FUCHS advises halving for every 15 degrees Celsius rise above 70 degrees Celsius. To illustrate: a bearing with a standard interval of 3,000 hours at 70 degrees Celsius drops to approximately 750 to 1,500 hours at 85 degrees Celsius, 370 to 750 hours at 100 degrees Celsius, and potentially below 100 hours at 120 degrees Celsius. Elevated temperature accelerates base oil oxidation, increases evaporation, softens the grease structure causing leakage, and can trigger thickener degradation simultaneously. A practical field technique: monitor bearing housing temperature after relubrication and use the temperature recovery profile to fine-tune the interval. If bearing temperature has risen measurably before the next scheduled greasing, the interval is too long. Fresh grease acts as both coolant and purge mechanism, so at high temperatures slightly larger quantities delivered more frequently help stabilize bearing operating temperature.
Q7: How do I select the right base oil viscosity for a high-temperature grease?
Viscosity selection for high-temperature greases must account for the viscosity-temperature relationship. A base oil measuring 100 centistokes at 40 degrees Celsius may thin to approximately 14 centistokes at 100 degrees Celsius. If the bearing manufacturer specifies a minimum of 10 centistokes at operating temperature to maintain elastohydrodynamic film thickness, the margin erodes quickly as temperature climbs. The recommendation: select a higher base oil viscosity for high-temperature service than you would for the same bearing at ambient temperature. For continuous operation above 120 degrees Celsius, ISO VG 100 to 220 is commonly specified. Above 180 degrees Celsius, ISO VG 220 to 460 is typical, and some PFPE greases use 400 to 550 centistokes precisely to maintain adequate film thickness at extreme temperatures. The bearing speed factor (bore diameter in millimetres multiplied by RPM) provides a cross-check: below 100,000, ISO VG 220 to 1000 is appropriate; between 100,000 and 400,000, ISO VG 68 to 220; above 400,000, ISO VG 22 to 100. When both high speed and high temperature are present, the conflict between lower viscosity for speed and higher viscosity for temperature is resolved by selecting a synthetic base oil with a high viscosity index, maintaining viscosity more consistently across the temperature range than a mineral oil of the same nominal grade.
Q8: What are the warning signs that a high-temperature grease is failing in service?
Recognizing grease degradation before it causes bearing damage enables proactive intervention. The primary failure signs: hardening or carbonization where grease turns dark brown to black and becomes crust-like or powdery, caused by base oil evaporation this is particularly dangerous because hardened grease blocks relubrication pathways. Oil separation with a dry soap cake left behind indicates thickener collapse under thermal stress. Severe softening and leakage, where grease liquefies and drips from the housing, signals the operating temperature is approaching or exceeding the dropping point. An acrid or burnt odour indicates thermal decomposition of base oil or additives. Rising bearing temperature measured by infrared thermography or installed sensors, especially a gradual upward trend over weeks, indicates the grease is generating friction rather than reducing it. Increased motor current draw on electric motor bearings signals the same problem. Vibration analysis showing increasing high-frequency energy in bearing defect frequency ranges indicates the lubricating film is failing. A practical field test: extract a small grease sample, place it between two clean glass slides, and compare against retained fresh grease for colour, consistency, and hard particles. This provides a useful qualitative assessment requiring no laboratory equipment.
Q9: Can different high-temperature greases be mixed during a changeover?
Mixing incompatible greases is a common cause of high-temperature bearing failures after a lubricant changeover. Greases with different thickener chemistries should be assumed incompatible unless the manufacturer confirms otherwise. Lithium complex and calcium sulfonate complex greases exhibit the lowest compatibility in ASTM D6185 testing, with rapid softening, excessive oil separation, and loss of structural stability. Lithium complex and polyurea greases show higher compatibility, though polyurea chemistry varies between manufacturers. PFPE/PTFE greases are incompatible with all hydrocarbon and silicone greases. Even greases from the same thickener family but different manufacturers can be incompatible due to additive chemistry differences. The conservative changeover procedure: drain or purge as much old grease as possible while the bearing is warm; flush the housing with a compatible flushing oil or solvent recommended by the new grease manufacturer; fill to the correct level (25 to 35 percent of free cavity volume for high-temperature bearings); run under light load while monitoring temperature, then regrease after a short break-in period to purge residual old grease. Simply pumping in new grease until old grease purges out risks creating an incompatible mixture that can fail within hours.
Q10: When should solid lubricants such as molybdenum disulfide or graphite be used instead of, or in addition to, grease?
Solid lubricants become the primary strategy when operating temperatures exceed the practical limits of even advanced grease formulations. Molybdenum disulfide (MoS2) oxidizes above approximately 400 degrees Celsius in air forming abrasive molybdenum trioxide, but in reducing atmospheres or vacuum remains effective to much higher temperatures. Graphite requires adsorbed moisture to maintain its lamellar structure, effective to approximately 450 to 550 degrees Celsius in air but poorly suited to vacuum or dry inert gas. Boron nitride offers a ceiling of approximately 900 degrees Celsius in air and is chemically inert, though significantly more expensive. In the 260 to 450 degrees Celsius range, solid lubricants are applied as dry films, dispersed in volatile carriers, or incorporated into high-temperature grease as a backup lubricant. In steel mills, greases containing MoS2 or graphite are commonly specified for ladle turret bearings, caster strand guide rolls, and furnace charging equipment where radiant heat loads push bearing temperatures well above ambient. The critical consideration is particle size and dispersion: agglomerated particles can block filters and lubrication channels, while properly dispersed sub-micron particles fill asperities for boundary lubrication without interfering with oil film formation. A practical note: the dark grey-black appearance of MoS2 and graphite greases makes visual condition assessment unreliable. Rely on vibration analysis, temperature trending, and scheduled sampling instead.
Q11: What fill volume is recommended for bearings operating at high temperatures?
Standard bearing fill guidelines recommend 30 to 50 percent of free cavity volume for normal conditions, but for high-temperature service the recommended fill drops to 25 to 35 percent. The reason is churning. Excess grease worked by rolling elements generates heat through internal friction. At elevated temperatures, the additional churning heat can push grease past its thermal limit, triggering a destructive cascade: churning heats the grease, it softens and oxidizes faster, degradation products increase friction, and the cycle accelerates to failure. Reducing fill volume minimizes churning losses while maintaining adequate lubricant availability. For very high-speed bearings at elevated temperatures, fill volumes as low as 20 percent may be specified. The complement to lower fill volume is more frequent relubrication: with less grease in the housing, replenishment must occur on shorter intervals. For sealed-for-life bearings with PFPE/PTFE grease in kiln car or furnace fan applications, the fill calculation must also account for thermal expansion: grease volume increases at operating temperature, and overfilling at ambient can force grease past seals when the bearing reaches service temperature. SKF and other manufacturers recommend approximately 30 percent fill of bearing free internal volume for high-temperature service, with the housing cavity at approximately 50 percent. These values are lower than ambient-temperature practice because the margin for churning-induced overheating is much narrower.
Q12: How do I evaluate whether a high-temperature grease from a specialty manufacturer such as KLUBER is appropriate for my application?
Evaluating a specialty high-temperature grease requires focusing on engineering data that correlates with your operating conditions. Start by measuring actual bearing operating temperature using a contact thermocouple or infrared measurement, not the process temperature of the furnace or kiln. The bearing outer ring is often 20 to 50 degrees Celsius cooler than the surrounding process due to heat dissipation through shaft and housing. Compare this measured temperature against the manufacturer's published continuous service temperature rating, not the peak rating. Review the base oil viscosity at your operating temperature. For a KLUBER product such as Klübertemp HM 83-402, designed for long-life service at extreme temperatures, the key questions are: what is the base oil viscosity at 40 and 100 degrees Celsius to calculate viscosity at operating temperature? What is the FE9 (DIN 51821) grease life at the temperature closest to your operating condition? What relubrication interval does the manufacturer recommend for your bearing type, size, speed, and temperature? Specialty manufacturers provide application-specific engineering support. When contacting a manufacturer, provide bearing type and size, rotational speed, actual measured bearing temperature, environmental contaminants, current relubrication method and interval, and a description of the failure mode you are addressing. Requesting FE9 test data at your operating temperature is reasonable for a premium product. Finally, calculate total cost of ownership: a specialty grease costing three times as much per kilogram but extending relubrication intervals from monthly to annually and eliminating an unplanned outage per year typically shows a compelling economic case when labour, downtime, and parts are accounted for.
Key Takeaways
High-temperature grease selection is an engineering decision, not a purchasing decision. The dropping point only tells you when the thickener melts and has no reliable correlation with bearing life: always derate by 55 to 85 degrees Celsius to estimate usable continuous service temperature. Select base oil viscosity at your actual bearing operating temperature, using synthetic base oils with high viscosity indices for applications above 120 degrees Celsius. Choose the thickener type based on the specific combination of temperature, load, speed, water exposure, and relubrication accessibility. Temperature shortens relubrication intervals dramatically: halve the interval for every 10 to 15 degrees Celsius above 70 degrees Celsius and verify with bearing temperature monitoring. Never mix incompatible grease families without thorough purge and flush. For extreme conditions above 230 degrees Celsius, PFPE/PTFE greases are the proven solution. When evaluating a specialty manufacturer's product, request FE9 life test data at your operating temperature and calculate total cost of ownership, not the per-unit lubricant price.
Need Help? Contact KOEED
KOEED's technical team can help you select the right industrial lubricant for your application. Contact Moritta@KOEED.COM with your equipment details and operating conditions for a personalized recommendation. KLUBER datasheets and MSDS available on request.