SECTOR FOCUS: AUTOMOTIVE GREASES Continued from page 14
One of the major root causes for false brinelling is a lack of lubricant directly in the roller to raceway contact. At temperatures below -20 °C greases with mineral base oils are not able to supply the relevant contact areas in the bearing with enough oil. Finally this leads to an almost dry contact, to wear and corrosion. This critical supply situation is intensified by the fact that the grease in a new wheel bearing needs a certain operation time to get fully distributed inside the free space of the bearing. But when being shipped on trailers or by rail, new cars have an operation time of some minutes only. As a result the bearing grease has still a quite uneven distribution – which in worse case leads to poor performance if the highest loaded rollers have to face a dry friction contact.
The obvious conclusion to switch to synthetic base oils for such greases is the easier part of the solution. After a longer testing phase it turned out that also the thickener structure had to be optimised. On the one hand the grease has to release enough base oil during car transport under low temperature conditions. On the other hand the thickener must be stable enough to meet the life time requirements – also under high speed and high temperature conditions where a high oil release is definitely not desired. These contradictory demands could finally be met with a grease which also provides a significantly lower friction in the bearing.
due to false brinelling markings of about 10 µm and more. The grey column of a more modern solution with a poly-urea grease and a fully synthetic base oil shows that synthetics are necessary but not enough to meet the demands. A well-balanced formulation of lithium-complex thickener, PAO base oil and a specific additivation finally showed best results under these extreme conditions. Moreover, the base oil viscosity did not play a major role for the false brinelling behaviour. A lower viscosity may contribute to some improvement, but it cannot replace the right formulation.
Fig 3. Rolling Contact Friction Test in DGBBs with different Grease Types and Base Oil Viscosity
In contrast to the false brinelling tests the base oil viscosity was much more decisive for the rolling contact friction. The tests were performed without any sealing of the bearings (DGBBs). For achieving lowest values also a low base oil viscosity was crucial – 50 mm²/s in this comparison at room temperature. But also here it can be seen exemplary with the blue samples right side (PU/Syn (50)1 and PU/Syn (50)2) that optimisation of a single parameter is not enough to meet complex requirements. Both greases have a promising basis for good performance – but only one formulation can really convince at the end.
Fig 2. False Brinelling Test Results at -30 °C with different Grease Types. One Test with six Bearings results in totally 60 Markings to be evaluated
Results of false brinelling tests in passenger car wheel bearings are shown in Fig.2. The red column represents a conventional lithium-complex grease with mineral base oil (viscosity in brackets). At -30 °C this grease is not able to protect the bearing from severe damage
Solutions for low temperature anti-false-brinelling greases with excellent low friction properties can be found. One key factor on this way is definitely the test methodology which must be precisely adapted to the expected operating conditions.
LINK
www.schaeffler.com
EXCERPT FROM A PRESENTATION MADE AT THE 21ST INTERNATIONAL TRIBOLOGICAL COLLOQUIUM, TAE OSTFILDERN, GERMANY, JANUARY 2018
16
LUBE MAGAZINE NO.144 APRIL 2018
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