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Greases are divided into various consistency classes in accordance with the National Lubricating Grease Institute (NLGI). The consistency of lubricating grease should not change drastically when operated within its specified temperature range. Greases that soften at elevated temperatures may leak from the bearing arrangement. Those that stiffen at low temperatures may restrict bearing rotation or limit oil bleeding.
Greases with a metallic soap thickener and a consistency of 1, 2 or 3 on the NLGI scale are typically used for rolling bearings. The most common greases have a consistency of 2. Lower consistency greases are preferred for low temperature applications or to improve pumpability. Greases with a consistency of 3 are recommended for bearing arrangements with a vertical shaft where a baffle plate is arranged beneath the bearing to retain the grease in the bearing.
In applications subjected to vibration, the grease is heavily worked as it is continuously thrown back into the bearing by vibration. For these applications, higher consistency greases that do not change their consistency during operation may help.
Greases thickened with polyurea can soften or harden depending on the shear rate in the application. Therefore, in applications with vertical shafts, there is a danger that under certain conditions, polyurea grease leaks, irrespective of the operating temperature.
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