High-temperature grease: how to match it to the actual operating temperature

First establish what is actually hot

The air temperature around a machine is not necessarily the grease temperature. At the lubrication point, heat near the contact also depends on friction, speed, load, bearing design and heat dissipation. A housing measurement can help, but it does not always describe conditions in the raceway or on a sliding surface.

1. the stabilised temperature after warm-up and possible peaks during start-up, stops and load changes;
2. the type, designation, size and mounting arrangement of the bearing or other lubricated component;
3. speed, load direction, load variation and the time spent in each part of the cycle;
4. sealing, ventilation and the available heat path away from the contact;
5. water, steam, dust, chemicals, process material and other contamination sources;
6. the current grease, dispensing method and the history of temperature, vibration or leakage.

Separate the continuous temperature, a short peak and the temperature after shutdown. A product that tolerates a brief peak in a documented condition does not necessarily provide adequate life during continuous operation at the same temperature.

Dropping point is not operating temperature

Dropping point is the temperature at which, in a laboratory test, a drop separates from a heated grease sample. It does not answer how long a grease will protect a particular bearing under a given load and speed. NLGI states that this test has very limited relevance to service performance and should not be used to determine the upper operating temperature of a grease. NLGI defines operating temperature as the range in which the lubricated device can be started up acceptably and lubrication remains adequate; it depends on the base oil, thickener and additives, and the upper limit is typically determined with high-temperature bearing tests.

Bentonite greases need the same care. In the Molyguard catalogue, products with this thickener, such as ANP 40 and COMPOUND RA 40, have no numerical dropping point, and COMPOUND RA 40 is described as a non-melting compound. The absence of a dropping point still does not make a product suitable for any temperature or application: the catalogue gives each of them a defined operating-temperature range and a specific intended use.

Look for the safe performance limit, not one large number

In the product documentation, look for the operating-temperature range, the high-temperature performance limit, bearing-test conditions and information about life. SKF derives the high-temperature limit (HTL) of a grease from its dropping point, the temperature at which the grease loses its structure irreversibly; this limit must not be exceeded at any time. The safe upper operating temperature, the high-temperature performance limit (HTPL), is lower, and that is the value to compare with the measured temperature.

If a data sheet gives only dropping point, do not treat it as a complete answer. Ask for data for the application or check the bearing and machine instructions. Missing test conditions should reduce confidence in the selection, not be filled with an assumption.

Which grease-data fields should you compare?

1. base oil and thickener — they affect behaviour, ageing and compatibility;
2. operating-temperature range and how it was established;
3. speed and load for which the application or test results are stated;
4. NLGI consistency grade, remembering that it is not the whole specification;
5. water, corrosion, shear and contamination resistance where the process requires it;
6. compatibility with seals, materials and the grease already in the point.

The NLGI number describes consistency, not operating temperature. Two products with the same grade may have different base oils, thickeners, temperature ranges, water resistance and high-speed behaviour. “NLGI 2” alone therefore does not settle the choice.

Higher temperature shortens grease life

Higher temperature accelerates ageing and can shorten the relubrication interval. SKF materials give an indicative rule of halving grease life for each 15°C increase within the safe operating range. This is a planning guide, not a universal conversion for every product and bearing.

Temperature is not the only cause of overheating. Too much grease can increase drag and temperature, while too little can damage the lubricating film. After changing a product or dose, monitor temperature, vibration, leakage and grease condition instead of judging the change from one reading.

What the TC Kolor catalogue shows — and what it does not

The TC Kolor high-temperature grease category contains ANP 40 and COMPOUND RA 40. The catalogue describes ANP 40 as a compound with metal-oxide micropowders for foundry applications, and COMPOUND RA 40 as a non-melting copper compound for bolts operating at very high temperatures. Those descriptions are not a basis for recommending either product for a bearing.

In the bearing grease range, AERO 300 is described as a grease for heavily loaded roller bearings at high temperature, with a lithium-complex thickener. GS ROLLER DR is described as an HT grease for bearings in dryers, steamers, stenters and rolling mills, with calcium sulphate. In the catalogue table, both products have an operating temperature of up to +200°C (220°C peak) and dropping points of 275°C and 300°C respectively. That gap shows why dropping point should not be read as an operating temperature. These are examples of reading application, composition and temperature range in the catalogue, not automatic recommendations for every point.

The catalogue does not replace the instructions for a particular bearing or an application review. Before ordering, compare actual temperature, speed, load, sealing, environment and the changeover method with the product documentation. If one of these facts is missing, the enquiry needs more information.

A practical selection process

1. Identify the lubrication point: component, designation, machine manufacturer and mounting arrangement.
2. Measure the temperature at normal operating conditions and record peaks in the cycle.
3. Record speed, load, direction of movement, duty time and environmental conditions.
4. Compare the data with the grease data sheet, machine instructions and bearing manufacturer’s recommendation.
5. Check compatibility with the existing grease, seals and materials; do not mix products just because they share an NLGI grade.
6. Set the dose, application method and interval, then plan a check of temperature, vibration and leakage after the change.

A decision example without guessing

Suppose the lubrication point is a roller bearing next to a heating zone. Measuring the temperature in the room or searching for a product marked “HT” is not enough. Record the housing temperature at steady state, possible peaks, roller speed, load, sealing and the grease already in use.

If the measurement exceeds the current range or the housing is much hotter than the surroundings, first investigate the heat source. Then compare products using the stated test conditions and application. When the data sheet does not contain the information needed for a decision, ask the supplier or bearing manufacturer rather than choosing the largest number in an advertisement.

How to prepare an enquiry

Include the machine name and model, bearing designation, a photograph of the lubrication point, continuous temperature and peaks, speed, load, sealing, environment, current product, quantity, frequency and dispensing method. This allows the specific point to be assessed instead of treating “high-temperature grease” as a sufficient specification.

With this information, send your enquiry to TC Kolor using the contact form. For comparing greases for bearings that run hot, start with the bearing grease category, which includes AERO 300 and GS ROLLER DR. The final selection requires application data and compatibility confirmation.

• SKF, Bearing grease selection chart
• SKF, Rolling bearings — Selecting grease or oil
• SKF Evolution, Grease lubrication mechanisms in rolling bearing systems
• NLGI, Grease glossary
• TC Kolor, Molyguard 2022 catalogue