All about Grease
Following their exact definition, greases are oils prevented from liquefying by thickening agents. The used thickener has a great influence on the operating temperature range of the grease and its baviour in the presence of water.
For the lubrication of vehicles, greases are used when oils would be too thin or too fluid. They are classified according to their consistency, similar to the viscosity classification of lubricating oils. Subject to DIN 51818, the numbering of the NLGI grade for greases begins with 000 where the thickest oil stops flowing. Greases of NLGI grades 00, 0 and 1 are very soft up to soft. NLGI consistency number 2 is like ointment, no. 3 is nearly firm, no. 4 very firm and no. 5 and 6 are hard to very hard. Grease of NLGI grade 6 are so-called block greases.
Greases of conisistency grade 2 are used for the lubrication of plain and rolling bearings. Softer greases are often used as gear grease or fluid gear grease.
In contrast to lubricating oils, greases are not suitable for cooling, cleaning and cannot be used in a circulating system. It is more work to clean gears or components lubricated with grease than those lubricated with oil.
But greases also have advantages over lubricating oils. Greases can better withstand unfavourable temperature and environmental conditions. They protect bearings against ingress of dirt and water. At more difficult lubricating points, greases offer better wear and corrosion protection than oils. And greases adhere better than oils.
Concerning vehicle lubrication, it is also important to know if greases contain solids. DIN 51813 prescribes the corresponding test procedure. The lubricating effect of greases with solid content as molybdenum disulphide or micro-ceramic in many fields is even better. They often show outstanding emergency operating features and avoid a gap in lubrication when overloaded by temperature, pressure or engine speed.
As a precaution, such greases with solid content should not be used for the grease filling of universal joints. The reason is the centrifugal force. In a fast turning grease supply, the otherwise helpful solids have a boomerang effect: Due to the centrifugal force, the solids settle at the exterior edge of the grease supply which can cause starved lubrication conditions at the actual lubricating point.
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