The tribological performance of gas turbine lubricants

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Airey, Jake Christopher ORCID: https://orcid.org/0000-0001-9741-6157 (2020). The tribological performance of gas turbine lubricants. University of Birmingham. Eng.D.

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Abstract

This thesis is a study of the tribological properties of gas turbine lubricants with the aim of understanding how variations in the formulation of this product can affect friction and wear under different operating conditions. In particular, the focus is on understanding whether current aviation lubricants are capable of protecting and lubricating in new highly loaded gear environment inside the Power GearBox (PGB) in the new Rolls-Royce engine; the UltraFan©. This will be the largest planetary gearbox in the aviation industry to date and hence, introduces a new tribological challenge for the lubricant that needs to be investigated.

Modern gas turbine lubricants are an entirely synthetic formulated product and each component is tailored to optimise the performance within the engine. The majority component: the base oil usually comprises ~95 % of the lubricant formulation and as a result, it governs most of the viscometric properties and acts as the carrier for the other additives in the formulation. Work was done to understand how variation in the molecular structure of base oils affects friction across different operational conditions. It was found that for polyol ester base oils, increasing the chain length, the number of ester groups and decreasing the amount of molecular branching decreases friction across all lubrication regimes. This effect was also seen independent of viscosity by conducting tests for each oil at the same kinematic viscosity by varying the temperature. This effect was attributed to larger and more linear molecules possessing an improved packing efficiency, thus enabling an improved load bearing capability.

Another component of the lubricant is the anti-wear additive, this is responsible for protecting machine element surfaces in the absence of a sufficient lubricant fluid film. The introduction of a new gear environment motivated research into understanding whether current anti-wear additives in aviation could influence micropitting like other automotive anti-wear additives; this is the prevalent wear mechanism commonly found on the dedenda of gear teeth. It was demonstrated that aviation anti-wear wear additives promote micropitting much less than a common automotive anti-wear additive Zinc Dialkyl DithioPhosphate (ZDDP). The ashless anti-wear additives allow the “running-in” of a surface due to their slower tribofilm formation than ZDDP and this prevents high asperity contact pressures being maintained that accelerate micropitting.

Finally, current fully formulated gas turbine lubricants were evaluated with respect PGB conditions and four commercially available lubricants were tested. It was found that the lowest viscosity candidate resulted in the best protection giving rise to lower friction and less micropitting. This has the potential of offering other efficiency benefits, however, its promising tribological properties will need to be balanced with its thermal capability. It would also have to challenge current industry specifications that require kinematic viscosity in the range 4.9 cSt to 5.4 cSt at 100 ℃, and this lubricant is ~ 3 cSt at 100 ℃. The next best lubricant was 5 cSt lubricant with high thermal capability, this showed marginally worse micropitting performance with higher friction but was still better than the other two lubricants that overall performed more poorly. A gear lubricant containing an EP additive was found to promote micropitting more slowly but introduced another wear mechanism; polishing wear that still resulted in substantial material loss most likely through a softening mechanism. The highest viscosity lubricant also gave rise to higher friction and more micropitting than the other candidates. This study showed that viscosity has limited effect in boundary lubrication regimes and confirmed the importance of a specific anti-wear additive package.

Type of Work:

Thesis (Doctorates > Eng.D.)

Award Type:

Doctorates > Eng.D.

Supervisor(s):