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"About a year ago we ran a lubricant too long in a high-duty gearbox, and it oxidized and threw sludge. Since then, we've been watching the oil more closely with oil analysis. The problem is that acid numbers rise and the oil darkens after only one month of service. The lubricant used to last a full year. We keep changing the oil, but the problem just repeats. Why does our gear oil have such a short life?"
It sounds like the gearbox was not thoroughly flushed after the oil oxidized the first time. Typically, a simple drain will leave more than 15 percent of the old oil behind, occluding to machine surfaces and trapped within the casing. This also leaves a host of reactive chemicals (pro-oxidants) behind that rapidly deplete antioxidant additives, leaving the base oil unprotected.
You refer to the gearbox as high-duty, which probably means high temperature and high wear metal production. The temperature and wear particles also accelerate the rate of oxidation, especially when sludge and other pro-oxidants are in the mix. I suggest you perform a thorough flushing of the gearbox.
There are different levels of system flushing that are practiced, depending on the machinery internal conditions and type of contaminants compromising the system.
Recirculation cleaning– The recirculation of clean fluid at a high velocity to achieve a turbulent flow helps remove contamination from the fluid system.
Power flushing– A variation of recirculation, where the oil level in the sump is reduced and a high-velocity fluid is applied to mechanically dislodge, lift and entrain particulate debris. Power flushing suspends and transports particles; absorbs air, chemical products and water from the system; and releases the contaminants to a filter.
Wand flushing– A wand attached to one of the cart hoses is used to discharge at high pressure (kicking up adherent debris). The flow is then reversed, and the wand vacuums the sediments.
Solvent cleaning– This involves the use of solvents to remove organic deposits that cannot be removed by recirculation. Solvent cleaning may incorporate the use of organic (hydrocarbon-based) halogenated or non-halogenated solvents (type A-1 cleaners such as kerosene or A-2 cleaners such as naphtha) to dissolve heavily crusted or layered carbon residues. Organic solvents tend to be blends of aliphatic and aromatic hydrocarbons and dissolve soil as opposed to emulsifying soil. These materials may be warranted if evidence of heavy carbonaceous residue exists.
Chemical cleaning– Using chemicals to dissolve inorganic components, chemical cleaning may incorporate the use of aqueous alkali or acid solutions to accomplish the desired result.
Regardless of the flushing compound/fluid selected, unless it is identical to the lubricant used following the flush, it is important that all of the flushing fluid be removed from the sump prior to the final fill. Some petroleum solvents with a concentration of 5 percent can create an appreciable thinning effect on the lubricant viscosity.