From the "Lubricating Grease Guide":
Where grease is handled in a dispensing system, the grade chosen may be related more to the capabilities of the system than to the requirements of the application. In service, consistency differences are important. Consider, for example, the use of grease in a gear case. The grease should be carried into the meshing gear teeth, which are thus kept covered with lubricant. If the grease is firm, the gear teeth may cut a channel through the grease and run dry, causing damage to the teeth. Unless operating temperature is high, therefore, soft-to- semifluid greases are ordinarily recommended.
In ball or roller bearings, carrying grease to the rolling elements is considered undesirable, because that grease will churn, soften, heat up, and work its way out through the seals. Thus a common recommendation for a ball bearing is to have the bearing space only about one-third full of grease of firm consistency - often NLGI No. 2. If the bearings support a vertical shaft, the grease had best be still firmer - probably NLGI No. 3.
Read more about the "Lubricating Grease Guide"
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