Lubricating greases are typically known for their use in a wide variety of applications such as rolling element bearings, automotive transmission joints as well as heavy duty chassis components; but can also be found very useful in the lubrication of gears. There are two types of gear families: the primary difference being open and enclosed gearboxes. The gears within a closed system are lubricated with grease within the system. Open gears are lubricated by grease or fluid separate from the piece of equipment. In 1938, AGMA published a tentative draft specification covering the lubrication of both open and closed gears (Ref. 1). This was approved by the AGMA Lubrication Committee in 1946. The most recent version of this standard was from 2016 and was re-issued in 2021 (Ref. 2). Independently of AGMA, there are now multiple specifications for lubricants, both liquid and lubricating greases for enclosed gearboxes. This paper will focus on the lubrication of open gears, utilizing both grease and fluid Open Gear Lubricants (OGL). Mining, sugar, paper, textile cement and food production are just some of the applications that utilize OGLs that provide to meet stringent lubrication requirements that include increased base oil kinematic viscosity, high load-carrying, good antiwear properties, pumpability, environmental acceptability, low consumption rates, adequate corrosion protection amongst other secondary requirements (Ref. 3). Since industrial applications demand stringent requirements for these lubricants, OGLs have evolved through time since their conception in the 1950s (Figure 1). Since their introduction, the traditionally applied OGLs were asphaltic type products which evolved into sprayable asphaltic cut back products. These OGLs were typically formulated with high kinematic viscosity mineral oils that contained high levels of asphalt or bitumen combined with a volatile solvent diluent.

These lubricants were typically applied by spray systems and as the meshing action of gears began, the solvent would evaporate off, leaving behind a viscous lubricant. Cutback solvents were predominantly chlorinated such as 1,1,1-trichloroethane because they were rapid to evaporate, had high flash points and improved the efficacy of spray application. This type of OGL was popular from the 1950s until 1995 when the Montreal Protocol put a global ban on ozone depleting substances in commerce (Ref. 4). These OGLs also caused housekeeping problems because at higher temperatures they would readily oxidize, harden and cause lubricant buildup. At lower temperatures, these lubricants became difficult to dispense and would stiffen, crack and peel-off, leaving gears unprotected (Figure 2).