Initially designed to resolve high-speed spindle bearing failures that would not respond to conventional oil and grease lubrication tactics, oil mist first found favor in the North American market in the 1960/70s with the oil and gas industry, which adopted these systems to successfully lubricate large pump- and motor-bearing assemblies.
As oil mist became more mainstream and moved into the large manufacturing industries, however, its open-vent system design—combined with the ability for operators and maintainers to easily adjust the air-flow pressure at will—often created oily workplaces. An increased awareness of environmental safety eventually resulted in a ban on both well-designed and poorly designed oil mist systems in many plants. That’s not the case these days.
An environmentally friendly descendant of the original oil mist systems, oil/air mist technology is one of the most efficient automated lubrication delivery systems available today, and one of the least expensive. (Click above image to enlarge)
Things have really changed: With the introduction of micro-process control, vortex-style mist generation and closed-loop system design, coupled with airborne-mist- detection systems, one of the most effective lube-system designs has experienced a dramatic resurrection.
How the System Works
In the original oil mist system design, a pressure-controlled, filtered compressed air supply is passed through a venturi. Oil is then siphoned from a reservoir by the quickened airflow directly following the venturi and is directed at a baffle plate, causing the oil to atomize into very fine droplets known as “dry mist.” Anything less than a ratio of 1-part-oil to 200,000-parts-air ratio (approximately 1.5 microns or 0.00006” diameter) falls back to the reservoir as heavy oil particles. Mist is then piped into a 2” diameter scheduled pipe header at between 5-40” water pressure, creating a velocity of up to 24 ft/sec.
As it approaches the bearing point, the mist is passed through a series of mist-metering fittings that allow an engineered amount of oil to enter the bearings. Depending on the fitting type used, the “dry mist” or partially reclassified mist then “envelops” and “wets” the entire bearing surface area with a thin lubricant film while imparting a partial positive pressure within the bearing housing that works to prevent contamination influx.
In the original design, the mist was allowed to vent to atmosphere. New system designs (now characterized as oil/air mist types) employ sloped lines to carry reclassified oil within the lines directly back to the reservoir, and more important, reflect a closed-loop design with drain legs and components designed to capture coalescent waste and eliminate open venting. An improved vortex air-chamber design to replace the old venturi design has led to more efficient mist generation that can now carry over distances of 600 feet—three times that of the original systems. This translates into the ability to lubricate substantially more points from a single mist-generation unit. The coalesced-oil-return improvements have turned a once total-loss system design into a much more effective partial-recovery system using less lubricant.
Pros & Cons
Improved system design, micro-process control and access control to the mist generator have made oil/air mist systems virtually tamper-resistant—and provided the ability to generate mist with finite control based on ambient conditions. With hermetic bearing contact seals to eliminate stray mist, this has resulted in an environmentally sound system and the elimination of any stigma associated with the old mist delivery systems.
Oil/air mist is not to be confused with air/oil mist (which we’ll discuss in January/February). Oil/air mist is well-suited for lubricating both slow-moving and high-speed rotating equipment. Because a very small amount of lubricant is being introduced over the entire bearing surface on a continual basis—lubricating and cooling the bearing—oil/air mist is one of the most efficient automated delivery systems available, and one of the least expensive.
In the November/December issue, we’ll be looking at Single-Point Lubricator systems. LMT