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Calculation of the bearing free space & grease volume

Bearing free space can be determined by means of the formula below. The formula uses volume units [cm3, dm3] instead of weight units [g, kg] in order to avoid calculation errors (up to 100%) due to the different densities of greases.

 V=[ π/4 · B·(D2 - d2)·10-9 -G/ 7800] m3 d = bearing bore diameter [mm] D = bearing outer diameter [mm] B = bearing width [mm] G = bearing weight [kg]

Notes: Because of different bearing types, cages and designs, the a.m. formula does only give a rough estimation.

Once the bearing free space has been determined, the required grease quantity is calculated as a percentage of
the available free space. The correct quantity is important to ensure proper lubrication of all contact surfaces.Over lubrication can be just as detrimental as under lubrication: for example, too much grease in high-speed bearings may lead to overheating or the development of higher starting and running torque. Generally, grease quantity in a bearing should occupy about 25% to 35% of the free internal space. In some extremely high speed applications, the lubricant can be as low as 10% to 15%.

Consistency
The consistency is the degree of the grease plasticity. Grease with low consistency should be selected for application in very low-temperature ranges in order to guarantee the continuous release of oil. At higher temperatures, a higher consistency is recommendable, so that the grease does not soften too much. Greases with a stiffer consistency are preferred and beneficial for applications with outer ring rotation where centrifugal force tends to sling grease out of the bearing, and those vertical axis applications where gravity pulls grease away from its intended position.

Notes: Penetration is a measure which indicates consistency - the solidity of grease. A measurement device has a cone with a specified time. Penetration is the depth to which the cone penetrates (in units of 1/10 mm).

 NLGL NO. ASTM WORKED (penetration) Grease is numbered differently by the grease manufacturers. Numbers 250 and 300 of cup and fiber grease generally use penetration (at 25℃), while most versatile grease employ NLGI penetration numbers such as 0, 1 and 2. 0 355~385 1 310~340 2 265~295 3 220~250 4 175~205 5 130~160 6 85~115