August 19, 2019 by L. (Tex) Leugner

Grease can be a complex subject, but a good understanding of the type of grease and its purpose can ensure that informed maintenance decisions are made. The National Lubricating Grease Association (NLGI) has a publication called "Grease Guide", which provides grease lubrication guidelines. If you follow this guide, you will provide good maintenance practices.​​​ The type of grease, the selected grade, and the amount of grease are directly dependent on factors such as the application, bearing type and speed, operating temperature, seal or retainer design, the possibility of contamination, relubrication intervals, and the amount of grease required.

Although there are multiple grease types based on thickener (soap), type (calcium, lithium, aluminum, or sodium) or NLGI grade (000 semi-fluid to six greases), this test only covers one of the most common grades. Two and three, as well as the more common types of thickeners, such as lithium, lithium complexes, and non-soap polyureas.

NLGI Class 1 grease is easy to stir in the bearing; therefore, a smaller amount must be used in high-speed applications of precision bearings, and only 20% of the bearing voids may be filled with grease. Bearings used in low-speed applications may have up to 80% of the voids filled with tertiary grease. Therefore, the amount used in the bearing will be less important. Therefore, NLGI grade two grease is very common in all industrial applications, while NLGI grade one is widely used in centralized lubrication systems.

1. How to choose the type and grade of grease for rolling bearings?

Logic: When operating conditions exclude the advice of the equipment manufacturer or lubricant supplier: Calculate the bearing surface speed or DN speed. The surface speed is determined by multiplying the shaft diameter (mm) by the bearing speed, and obey the following guidelines. If the DN speed is less than 200,000, the first or second level is used. At 200,000, using the third stage, it will remain in place and form a channel that allows the oil to separate from the thickener to lubricate the bearing.

When the speed is higher than 200,000, it is recommended to switch to oil. Ball, roller or needle bearing types will experience load changes within these speed ranges, and the required grease should have mechanical stability and excellent oxidation resistance. When the operating temperature does not exceed 121°C, lithium-based grease is widely used. In the case of continuous high temperature, lithium complex grease or polyurea grease is excellent.

2. How to choose the type and grade of grease for sliding bearings and open gears?

Logic: The selection and application of grease for journal bearings and open gears are opposite to those for rolling bearings. These applications require the grease to stay in place and may require a higher NLGI grade, such as 3, 4, or 5. Extreme pressure or solid lubricant additives (such as molybdenum or graphite) may be required to control wear.

3. Has your maintenance staff received practical training in grease lubrication?

Logic: Knowledgeable lubrication experts will understand the following precautions. If there is too much grease, internal friction will cause the temperature to rise above its dropping point, which will cause the grease to oxidize. Mixing incompatible greases can cause extreme softening, causing the lubricant to separate from the thickener (conventional polyurea greases are not compatible with most greases, while lithium-based greases are compatible with many types of greases).

The use of ordinary grease guns can generate pressures as high as 15,000 psi, and when lubricating the bearings in the bearing housings without grease discharge ports, the seals are often damaged. When lubricating the exhaust bearing box, remove the exhaust hole and apply grease with the bearing stopped until new grease flows out of the exhaust hole. With the vent hole still removed, slowly rotate the bearing so that excess grease can escape from the vent hole. After running for one to two hours, check the temperature of the bearing box to make sure it is normal, and replace the vent. The temperature at the enclosure should not exceed approximately 60°C.

When relubricating the motor, it may not be necessary to stop the motor, but relubricating at a standstill can minimize the possibility of grease leaking through the seal along the shaft. If relubrication is performed while the bearing is rotating, the bearing temperature will increase by 10 to 25°. As the newly added grease will generate internal friction, this is normal. After an hour or two of operation, the temperature should drop, depending on the application. If the temperature does not return to normal and the correct amount of grease is added according to the calculation below, do not add more grease, but determine the cause of the temperature increase.

4. Does your maintenance staff consider applying the correct relubrication interval and grease amount based on operating temperature and contamination?

Logic: Mineral oil starts to oxidize at 70°C. It is important to remember that “for every 15°C increase in temperature above 70°C, the service life of mineral oil-based grease in rolling bearings will be reduced by half.”

For example, the calculated bearing relubrication interval is 1,000 hours at 70°C; if the operating temperature reaches 85°C, this interval should be halved to 500 hours. At 100°C, the interval is reduced to 250 hours. At 115°C, the interval is further shortened to 125 hours. Therefore, when choosing a high-temperature grease, one should consider its oxidation resistance, base oil viscosity (for traditional grease) and the type of thickener used. Estimate the maximum temperature the grease can withstand.

For example, the typical dropping point temperature of lithium grease is 175°C. 175 minus the 40 degree standard; the resulting number 135 is approximately the maximum intermittent operating temperature that the grease can withstand before oxidation begins. The chart on page 22 provides guidelines for relubrication intervals and grease amounts for common rolling bearings, as well as their corresponding speeds at room temperature and environmental conditions.

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