Of course the VI can be boosted using viscosity index improvers, but those improvers may be impacted by mechanical shear in the gearbox leading to viscosity loss. High viscosity metallocene PAOs (mPAO) provide a solution to mechanical shear due to their narrow molecular weight distribution, which limits shear loss as the viscosity increases. Figure 3 shows the viscosity loss after 40 hours (twice the standard test cycle) in the tapered rolling bearing test for 75W-90 gear oils formulated with different high viscosity components. The mPAO components offer good shear stability which, combined with their excellent low temperature fluidity and high VI, make them an ideal component for high performance gear oils.
gear mesh, creating a solid-like, protective film that keeps the surfaces apart.
Several factors, including the geometry and material of the gears, the load, the rotational speed and the lubricant properties will ultimately determine the EHD film thickness generated. However, given that the gearbox design is fixed, and the load has negligible effect on EHD film thickness, we can influence the film thickness by changing the lubricant parameters to suit the expected speed ranges. For Newtonian lubricants, the effective dynamic viscosity of the oil in the load zone is a function of the dynamic viscosity at entry at ambient pressure, along with the relationship between pressure and viscosity.
Figure 2. Energy savings of an ISO 460 synthetic gear oil over an ISO 460 mineral oil at low temperature (Source: ExxonMobil data - David Brown Radicon, single reduction 30:1 gearbox).
Unlike plain bearings which operate with a continuous hydrodynamic oil film, gear teeth need to form a new oil film during every gear mesh. The degree of sliding and rolling that occurs depends on the gear type and strongly influences how the oil film develops. Hypoid and worm gears, where there is significant sliding, are the most difficult to lubricate.
Figure 4. ElastoHydrodynamic (EHD) contact between gear teeth.
For most of the meshing cycle, the surfaces move at different speeds. The resulting sliding shears the near solid oil film, which is in contact with both surfaces. A traction force is generated, which is a function of the shear strength of the lubricant, which, in turn, is a function of the molecular structure of the base stock used. Synthetic base stocks typically have lower traction coefficients than mineral oils and allow gears to operate more efficiently. Depending on the degree of sliding, energy savings could be anywhere between 0.5% per stage in multiple stage helical gears and 10% or more in single stage high ratio worm gears [3].
Figure 3. 75W-90 part synthetic gear oils viscosity loss after 40 hours (source: ExxonMobil data).
The thin lubricant film which is formed in the gear mesh is the well-known elastohydrodynamic (EHD) film created between non-conforming surfaces (Figure 4). Because of the non- conformity, the load is carried over a very small contact area, giving rise to very high pressures (e.g. 0.2 – 3.0 GPa). Meshing of the gears draws the lubricant towards the converging gap between the two teeth. The lubricant is forced back on itself, and with increasing pressure, the lubricant viscosity rises exponentially. The high contact pressure causes elastic deformation of the contact surfaces which not only increases the contact area to help carry the load, but also helps to increase the gap between the teeth. As a result, the oil gets dragged into the
Figure 5. Worm gear efficiency benefits with mPAO gear oils (source: ExxonMobil data). Continued on page 8
LUBE MAGAZINE NO.129 OCTOBER 2015 7
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