Using synthetic oils instead of mineral oils has numerous advantages for gear and gearbox lubrication. Synthetic oils are generally more resistant to breakdown under high loads, less vulnerable to oxidation and provide longer service life. Nevertheless, there are substantial differences in synthetic base oil quality, and carefully formulated additive chemistries, that can cause significant variations in lubricant performance. This discussion examines how those differences affect four critical areas: micropitting (fatigue failure), low-temperature performance, service life and the effect of base oil purity on shear stability. This discussion provides highlights of test results, which are detailed in an available Appendix.
I. MICROPITTING PROTECTION
Understanding the issue
Micropitting is a type of fatigue failure occurring on hardened tooth flanks of highly loaded gears. Figure 2 shows typical limits of the load-carrying capacity for case-hardened gears. This failure consists of very small cracks and pores on the surface of tooth flanks. Micropitting looks greyish and causes material loss and a change in the profile form of the tooth flanks, which can lead to pitting and breakdown of the gears. A typical micropitting gear failure of an industrial gearbox is shown in Figure 3. In this case, misalignment was the reason for micropitting formation.
Advantages of advanced lubricant formulations
Synthetic gear oil formulations that use advanced additives can actually stop micropitting formation. These advanced formulations can induce beneficial reactions on gear and tooth flank surfaces.
With common gear oil formulations in highly loaded gearboxes, the tooth flanks of the gears are not fully separated by the lubricant film. In that case, the lubricant’s additives serve to protect the tooth flanks against micropitting formation.
Important conditions and factors
While lubricant selection plays a role in micropitting protection, operating conditions and material, surface roughness and
geometry of the tooth flanks are also factors (Figure 4). Along with micropitting, wear is an issue when abrasive material removal occurs on the tooth flanks of gears. This failure proceeds continuously at a rapid rate and causes material loss and a change in the profile form of the tooth flanks, which can lead to breakdown of the gears. Typical wear on the tooth flanks of an industrial gear is shown in Figure 5.
Testing to validate solutions
To determine the level of micropitting and wear protection provided by different kinds of oils, three tests were performed:
• An FZG micropitting test at high load-carrying capacity
• An FZG slow-speed wear test
• An FAG FE8 wear test was performed to show bearing protection.
(This test was done at normal standard temperatures and modified for lower oil temperatures.)
Tests were performed on mineral oil, polyalphaolefin and polyglycol oils.
Test results showed that the advanced additive technologies used in high-performance synthetic gear oils can react at the surface of the tooth flanks. Even under high loads and slow speeds, micropitting formation and wear failure were substantially reduced (Figure 6).