What makes a good grease? (page 2/2)
Extreme pressure / antiwear properties
These are determined by the 4-ball test, a wear test that measures the load carrying capability of greases. It gauges the force level when the top ball welds to the 3 others.
The lowest operating temperature is determined by testing the greases torque and the base fluid’s pour point at low temperatures. The highest operating temperature is determined by evaluating the evaporation loss and thermal stability of the base fluid. The operating temperature indicated in the PI sheet is only meant as a guideline for actual use. Other factors like operating conditions, atmospheric conditions and type of application should be taken into account.
This indicates a grease’s heat-resistance. As grease temperature rises, penetration increases until the grease liquifies and loses its consistency. Dropping point is the temperature at which a grease becomes fluid enough to drip. It indicates the upper temperature limit at which a grease retains its structure, not the maximum temperature at which a grease may be used. Some greases are able to regain their original structure after cooling down from the dropping point.
Corrosion and rust resistance
This is a grease’s ability to protect metal parts from chemical corrosion. A grease’s natural resistance depends on the type of thickener. Corrosion resistance can be enhanced by corrosion and rust inhibitors.
This is a grease’s ability to withstand the effects of water with no change in lubrication. A soap and water lather may suspend the oil in the grease, forming an emulsion that can wash away or, to a lesser extent, reduce lubricity by diluting and changing grease consistency and texture. Rusting becomes a concern if water comes in contact with iron or steel components.
High temperatures harm greases more than oils. Grease cannot dissipate heat through convection like a circulating oil. As result, high temperatures cause accelerated oxidation or even carbonization where grease hardens or forms a crust. Effective grease lubrication depends on the grease’s consistency. High temperatures induce softening and bleeding, causing grease to flow away from the areas where it’s needed. The mineral oil in grease can flash, burn, or evaporate at temperatures above 177 °C (350 °F). High temperatures above 73-79 °C (165-175 °F) can dehydrate certain greases such as calcium soap grease and cause structural breakdown. The higher evaporation and dehydration rates at elevated temperatures require more frequent grease replacement.
If a grease’s temperature is low enough, it will become so viscous that it can be classified as a hard grease. Pumpability suffers and machinery may break down due to torque limitations and power requirements. The temperature at which this happens depends on the shape of the lubricated part and the power supplied to it. As a guideline, the base oil’s pour point is considered the grease’s low-temperature.
Learn more about grease
What make a good grease? : page 1 / page 2
Grease components : page 1