This paper outlines the second part in a series on the effect of polymeric additives commonly known as viscosity modifiers (VM) or viscosity index improvers (VII) on gear oil efficiency and durability. The main role of the VM is to improve cold temperature lubrication and reduce the rate of viscosity reduction as the gear oil warms to operating temperature. However, in addition to improved operating efficiency across a broad temperature range compared to monograde fluids the VM can impart a number of other significant rheological improvements to the fluid . This paper expands on the first paper in the series , covering further aspects in fluid efficiency, the effect of VM chemistry on these and their relationship to differences in hypoid and spur gear rig efficiency testing.Numerous VM chemistry types are available and the VM chemistry and shear stability is key to fluid efficiency and durability. The trend of increased drivetrain power density and reduced sump volume places even more burden on the fluid film protection with increasing load in the contact and increased number of duty cycles per volume of fluid further increasing shear loss severity. Simply dropping viscosity in gear lubricants does not necessarily yield efficiency benefits and reducing viscosity too far can compromise both efficiency and durability through reduced fluid film protection although it is typically hard to determine these viscosity limits empirically. However, a number of rheological properties can be readily measured and related to key lubrication regimes in which VM systems can be differentiated on operational efficiency whilst maintaining fluid film protection. This paper builds on the differentiation of VM technologies with particular focus on the impact of speed and load on energy losses associated with the fluids and elaborates further on the contribution of viscosity to efficiency.
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