"I notice that the oil cleanliness level doesn’t change much after changing the oil in my gearboxes. Shouldn't the oil be cleaner after the change?"
Most gearboxes are drained on a quarterly, semi-annual or annual basis - usually to eliminate contaminants. Typically, five percent or more of the old lube is left in the gearbox. If the oil is not drained shortly after shutdown, the sludge and contaminants will accumulate in the bottom of the sump and remain with the residual oil. When the gearbox is refilled with lubricant and restarted, the contaminant is re-suspended, and the oil change fails to achieve its objectives. Also, the new oil may not be clean if it is not pre-filtered.
Consider the following alternatives:
1. Drain the oil within 15 minutes of shutdown and pre-filter the new oil.
2. Instead of draining the oil, periodically filter the oil with a portable filtration cart while the machine is operating. Sample and analyze the oil periodically to determine if it needs to be changed. This strategy will reduce your overall cost of maintenance and extend the life of the gearbox, and requires little upfront investment.
3. Install full-time filtration on the gearbox and sample and analyze the oil periodically to determine if it needs to be changed. This strategy will also reduce your overall cost of maintenance and extend the life of the gearbox, but requires some upfront investment.
Alternative 1 helps, but alternatives 2 and 3 are real winners. Noria teaches the specifics of these methods in it's lubrication seminars.