Viscosity: A measure of a lubricating oil’s resistance to flow. Viscosity index: Defined as the rate of change of viscosity with respect to temperature. Importance: It is the most important property determining the performance of lubricating oils under the influence of temperature. A lubricating oil must have sufficient viscosity to maintain a lubricating film on the surface. For machine parts moving at low speeds under high pressures, a high viscosity oil should be used as it is more resistant to squeezing out between rubbing parts. For lower pressures and high speeds, lighter oils can be used. If the viscosity is too low, it is not possible to form a liquid oil film between two moving or sliding surfaces, resulting in excessive wear.

Unlike oils, greases cannot distribute heat by convection, resulting in faster oxidation or carbonization. At high temperatures, normal grease can soften and flow, form crusts or harden, causing improper lubrication. In practice, high temperature greases may be recommended for processes where the hourly temperature is above 1200C. Selection of a high temperature grease for an application includes the following: 1. Definition of the temperature interval 2. Lubrication intervals 3. Cooling cycles during operation 4. Oil spillage to another component during operation 5. Atmospheric pollution 6. Dropping point of the grease 7. Type, viscosity and viscosity index of the base oil 8. Type of thickener 9. Composition and properties of additives 10. Loading speed 11. Possibility of atmospheric contamination Energy, corrugated cardboard and paper, mines, steel production, pellet mills, tires, hot rolling, automotive paint shops, furnaces in the food and pharmaceutical processing industries are some industries that typically require high temperature grease.

Motor bearings are subjected to high radial and low axial loads, but the loads can fluctuate due to the frequent start-up and shutdown depending its operation. Due to its intermittent operation, the friction zones in the motor bearing will constantly shift. With each on/off cycle, the motor starts from zero and reaches its rated speed. As a result, the friction partners experience maximum friction during boundary lubrication and move into the low-friction hydrodynamic zone. High temperatures and extreme vibration are other operational variables that grease must deal with. The grease releases some oil to create the required lubricating film between the friction surfaces. The intense heat generated during the boundary and mixed friction zone can cause the base oil to evaporate and the base oil viscosity to drop. The process continues for the designed life of the grease and the specified relubrication interval. However, in some cases, if the sealing is not effective, the bearings are exposed to external anomalies such as excessive dust, heat, water or corrosive process environment, etc. Also, when the motor is stopped unplanned for long periods of time, the oxidation process accelerates. Bearing metal act as catalysts for the external dust influence, operating temperature, wear particles and the oxidation process. The end result of this oxidation process during prolonged stopping can lead to hardening of the grease or ‘hard cake-like formation’, which can damage bearing life.

Sudden hardening, softening or severe discoloration of the grease can be a good indicator to check the grease. Observations can be confirmed with multiple tests to analyze the quality of the grease. A combination of the following tests can help to reach a conclusion. a) Penetration test b) Dropping point test c) IR test in comparison with unused sample d) Four ball anti-wear test e) Wear analysis.