The oligomerisation of alpha olefins with a C10 olefinic methyl ester creates a PAO structure but with a covalently bonded ester. This branching has decene length and the methyl ester is at the terminal carbon. The oligomerisation technology has synthesis tools for tuning properties, such as changing the reactants, ratios and conditions. These synthesis tools can also be used to efficiently manage polarity, which can be predictive of additive solubility and metal affinity. Some formulators and additive producers consider group I and group II polarity as ideal, therefore improving PAO polarity to match these two base stocks was targeted, aiming for an aniline point of around 100. This oligomer with covalently bonded ester base stock, developed for use in synthetic lubricants, has a number of unique attributes (Table 1), which include inherent additive solvency and deposit control in a finished lubricant. Its low pour point and high viscosity index improves performance across a broad range of temperatures and lubrication regimes, clean operation and better lubricity, leading to equipment durability and extended lubricant life. Because of its high viscosity, this high-performance molecule is especially designed to formulate high viscosity grade lubricants or be used as a high viscosity blend stock.
Figure 2. MTM data of neat Elevance Aria™ WTP 40 vs PAOs.
Additionally, wear performance can also be acquired following the coefficient of friction analysis. Utilisation of the MTM in stationary ball-on-disk mode allows for an accurate wear scar measurement. The figure below shows the wear scar diameter data of Elevance Aria™ WTP 40 vs PAO 40 and mPAO 40. Again, Elevance Aria™ WTP 40 shows better performance and lower wear against the commercial products, Figure 3.
Table 1. Physical properties of Elevance Aria™ WTP 40. Performance Data
Gear wear is a continuous, abrasive process where the material is removed from the mating gear teeth. This occurs either due to abrasive particles in the oil or rubbing of the surfaces with the scuffing-derived transferred material. Continual wear of tooth roots weakens the gear until it breaks. Wear typically occurs under boundary and mixed lubrication conditions, where the lubricant film thickness is not suitable to separate the tooth surfaces. The presence of the anti-wear additives will help prevent this type of damage by forming a durable chemical protective film.
To evaluate friction and wear performance of the materials, a mini-traction-machine (ball-on-disk) apparatus by PCS Instruments is utilized. A 19 mm diameter steel ball (AISI 52100) is loaded and contra-rotated against the flat surface of a disk (AISI 52100). The disk is held in a bath containing a test lubricant so that the contact between the ball and flat is fully immersed at a controlled temperature. The initial data depicted below shows the coefficient of friction at various slide-to-roll ratios at 100o
C
(Contact pressure = 1 GPa, Rolling Speed= 2 m/sec). Elevance Aria™ WTP 40 showed lower friction in all slide / roll ratio conditions compared to both conventional PAO 40 (PAO 40) and metallocene-based PAO 40 (mPAO 40) at 100o
C. This positive
lubricity performance can be attributed directly to the unique structural character of Elevance Aria™ WTP 40, Figure 2. The performance trend seen in pure base stock comparisons shown in Figure 2 remain the same in demonstration gear lubricant formulations prepared by Elevance (data not shown here).
Figure 3. Wear scar data of neat oils.
Gear lubricants are designed to perform in all three types of lubrication environments: boundary, mixed film and full film. Boundary lubrication occurs when the gear sets start or stop. When the gears are operating at slow speeds, they are in mixed lubrication regime and when they are operating at high speeds, they are in full film lubrication regime. Of course, the introduction of pressure or load into this equation alters the nature of the lubrication. For example, high loads on gears operating in full film regime will change the lubrication regime to mixed film and the higher loads for those operating in the mixed film regime will alter the regime to boundary.
Micropitting and EHL Film Thickness One mechanism of gear failure is micropitting. Micropitting is a form of localized surface damage that is prevalent in wind turbine gearboxes, and it occurs on both gear teeth and bearings. It is especially detrimental to bearing function because it alters the geometry of rollers, raceways or both. This phenomenon typically occurs under mixed-film elastohydrodynamic lubrication (EHL) where oil film thickness is of the same order as surface roughness average (Ra) and load is borne by surface asperities and lubricant. When asperities carry a significant portion of load, collisions between asperities on opposing surfaces cause elastic or plastic deformation. Under joint-development with the USDAiv
, we evaluated the EHL film thickness of Elevance Aria™ WTP 40 versus PAO, Figure 4. Continued on page 8
LUBE MAGAZINE NO.133 JUNE 2016
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