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Oil Systems Need to Breathe

Ken Bannister | February 21, 2018

Properly maintained breathers make it possible for lubrication systems to effectively protect moving parts.

To breathe is to allow air to freely enter and exit a fully enclosed space. We humans are accustomed to breathing automatically without thinking about the process. When it comes to machines, however, the design engineer must be cognizant of enclosed spaces and mechanisms that can internally create air-pressure buildup. Any machine or system design must be able to relieve, or ventilate, the excess air pressure at a controlled rate to return or maintain the space to a neutral or positively pressurized state.

For more information on how to specify and maintain breathers, listen to the podcast with author Ken Bannister above.

The ability of internal mechanisms to breathe and equalize pressure has a profound effect on a machine’s ability to perform work efficiently, and its component(s) lifecycle. A perfect example of this is found in early combustion-engine design in which crude piston-and-ring technology inevitably allowed combustion gases to leak past the piston rings into the crankcase, a process known as “blow-by.”

With no engineered provision for venting the enclosed crankcase, pressure would build up and compromise seals and gaskets, allowing crankcase oil to diffuse to atmosphere. This would result in engine power losses, oil contamination, and constant leaks. The problem was eventually solved through improved piston and ring design and the introduction of a crankcase ventilation system that introduces fresh air through the filler cap. The air mixes with combustion gases and drafts out through the road-draft tube, which is connected to the crankcase. The system was further refined by introducing the pressurized crankcase ventilation (PCV) system in use today.

An automotive crankcase is, of course, a reservoir similar to any gearbox or hydraulic-oil reservoir. In all cases, the reservoir is an enclosed container used to house oil that is pumped through the machine to lubricate bearing surfaces and return to the reservoir within the closed-loop system. When designed correctly, the reservoir will always have an air space (headspace) above the oil level designed to permit thermal expansion of the oil and allow the fluid to de-aerate (aerated fluids cause pump cavitation). To equalize the internal pressure build up created through the resulting changes in the oil level as the machine moves from rest to full operation, and vice versa, air must be allowed to enter and exit the reservoir through a device known simply as a breather.

If air is allowed to freely move in and out of the reservoir through the breather, so is everything contained within that air exchange. This can include contaminants and moisture, both detrimental to the oil and the very bearings surfaces oil is designed to protect. This now requires the reliability engineer and/or maintainer to recognize the ambient working conditions and choose the appropriate breather style and type for the conditions and, furthermore, exercise diligence, through preventive-maintenance procedures, to ensure that the breather is always in place in the reservoir, is clean, and is unencumbered, allowing it to work as designed. 

Desiccant breathers use a see-through polycarbonate body filled with a silica-gel absorbent that holds as much as 40% of its weight in moisture. Replace the breather when the gel turns pink. Photo courtesy Des-Case Corp., Goodlettsville, TN,

Desiccant breathers use a see-through polycarbonate body filled with a silica-gel absorbent that holds as much as 40% of its weight in moisture. Replace the breather when the gel turns pink. Photo courtesy Des-Case Corp., Goodlettsville, TN,

Anatomy of a Breather

Breathers come in all configurations, shapes, and sizes, and different styles accommodate different airflow, particulate size, and working conditions. Most breathers are consumable devices. As such, they must be changed regularly as part of the lubrication system preventive-maintenance program. The change-out schedule is based on application and ambient conditions. If the breather is a less-sophisticated design that does not display its condition to the operator or maintainer, the rule of thumb is change out every three months in dirty environments, such as a foundry, and every six months in cleaner environments.

Filler/Breather Cap

The most common breather in service is the combination filler/breather design that allows a single reservoir opening to serve two purposes. The device looks like a typical fill port with its screw-on cap and tubular mesh basket designed to prevent large debris from falling into the reservoir during the filling process (see image, right). The difference from a regular fill port is found in the combination-unit cap design, which contains a filter element to keep out contaminants. Filters can be made from different media based on the filter size and airflow restriction requirements. For example, a polyurethane filter medium is good for >10-micron particulates and allows an airflow exchange of 140 gpm (19 cfm), whereas an impregnated-paper medium will capture >3 micron particulates and allow an air exchange of 110 gpm (15 cfm).

Standard breathers

Standard breathers look almost identical to the filler/breather cap and is usually screwed onto a threaded pipe that provides air exchange through the top of the reservoir. Other styles can look similar to an automotive spin-on oil filter.

In environments that experience large shifts in ambient and working conditions, or high humidity, breather caps designed with a filter and a pressure valve can be specified. The pressure-relief and vacuum-breaker capability is designed to limit air exchange and provide a positive suction head at the pump inlet.

Desiccant breathers

Desiccant breathers are recent additions to the breather family and differ in that they offer superior air exchange and condition control and are designed to visually indicate to the operator when replacement is required. Dessicant units are designed with a see-through polycarbonate body filled with a silica-gel absorbent designed to hold as much as 40% of its weight in absorbed moisture. The gel changes from a blue to light pink color when saturated. The unit also contains regular polyester filters designed to capture particulate as small as 3 micron.

Word of Caution

Note that a breather will only work when it’s in place. Breathers taken off to fill reservoirs, or for checking purposes, must be replaced or refitted immediately if the reservoir environment is to stay protected from outside contamination. The opening photo on p. 28 shows an example in which the filler breather cap has not been refitted correctly, allowing contamination into the reservoir. Breathers are an important and integral part of any reservoir-based lubrication system—and they require their own maintenance schedule. EP

Contributing editor Ken Bannister is co-author, with Heinz Bloch, of the book Practical Lubrication for Industrial Facilities, 3rd Edition (The Fairmont Press, Lilburn, GA). As managing partner and principal consultant for Engtech Industries Inc., Innerkip, Ontario, he specializes in the implementation of lubrication-effectiveness reviews to ISO 55001 standards, asset-management systems, and training. Contact him at, or telephone 519-469-9173.




Ken Bannister

Ken Bannister

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